U.S. patent number RE39,354 [Application Number 10/282,191] was granted by the patent office on 2006-10-17 for sinterable semi-crystalline powder and near-fully dense article formed therewith.
This patent grant is currently assigned to 3D Systems, Inc.. Invention is credited to Elmer Douglas Dickens, Jr., Paul F. Forderhase, Biing-Lin Lee, Angelo Joseph Magistro, Kevin P. McAlea, Hendra Ng, Glenn Alfred Taylor.
United States Patent |
RE39,354 |
Dickens, Jr. , et
al. |
October 17, 2006 |
Sinterable semi-crystalline powder and near-fully dense article
formed therewith
Abstract
A laser-sinterable powder product has been prepared having
unique properties which allow the powder to be sintered in a
selective laser sintering machine to form a sintered part which is
near-fully dense. For most purposes, the sintered part is
indistinguishable from another part having the same dimensions made
by isotropically molding the powder. In addition to being freely
flowable at a temperature near its softening temperature, a useful
powder is disclosed that has a two-tier distribution in which
substantially no primary particles have an average diameter greater
than 180 .mu.m, provided further that the number average ratio of
particles smaller than 53 .mu.m is greater than 80%, the remaining
larger particles being in the size range from 53 .mu.m to 180
.mu.m. A powder with slow recrystallization rates, as evidenced by
non-overlapping or slightly overlapping endothermic and exothermic
peaks in their differential scanning calorimetry characteristics
for scan rates of on the order of 10.degree. C. to 20.degree. C.
per minute, will also result in sintered parts that are near-fully
dense, with minimal dimensional distortion.
Inventors: |
Dickens, Jr.; Elmer Douglas
(Richfield, OH), Lee; Biing-Lin (Cranston, RI), Taylor;
Glenn Alfred (Bremerhaven, DE), Magistro; Angelo
Joseph (Quaker City, OH), Ng; Hendra (Cleveland, OH),
McAlea; Kevin P. (London, GB), Forderhase; Paul
F. (Austin, TX) |
Assignee: |
3D Systems, Inc. (Valencia,
CA)
|
Family
ID: |
26970460 |
Appl.
No.: |
10/282,191 |
Filed: |
October 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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08664356 |
Jun 17, 1996 |
5648450 |
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08298076 |
Aug 30, 1994 |
5527877 |
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07980004 |
Nov 23, 1992 |
5342919 |
|
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Reissue of: |
08892583 |
Jul 14, 1997 |
06136948 |
Oct 24, 2000 |
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Current U.S.
Class: |
528/323; 264/122;
264/125; 428/308.4; 156/62.2 |
Current CPC
Class: |
C08J
9/24 (20130101); C08J 3/12 (20130101); B29C
64/153 (20170801); B29C 41/003 (20130101); B29C
67/04 (20130101); B33Y 70/00 (20141201); B33Y
10/00 (20141201); B29C 2035/0838 (20130101); B29K
2023/06 (20130101); B29K 2077/00 (20130101); Y10S
521/919 (20130101); B29K 2995/0094 (20130101); B29K
2023/00 (20130101); B29K 2023/12 (20130101); Y10T
428/249958 (20150401); B29K 2079/00 (20130101); B29K
2059/00 (20130101); B29K 2067/006 (20130101) |
Current International
Class: |
C08G
69/14 (20060101) |
Field of
Search: |
;156/62.2 ;264/122,125
;428/308.4,411 ;528/323 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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89-065403 |
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Jul 1987 |
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JP |
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88/02677 |
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Apr 1988 |
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WO |
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93/01258 |
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Jan 1993 |
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WO |
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94/12340 |
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Jun 1994 |
|
WO |
|
Other References
"Same-day prototyping blossoms," Plastics World (Dec. 1989), p. 11.
cited by other .
"Rapid prototyping: parts without tools," Aerospace America (Aug.
1991), pp. 18-23. cited by other .
Kimble, "The Selective Laser Sintering Process: Applications of a
new Manufacturing Technology," Intelligent Design and manufacturing
for Prototyping, vol. 50 (ASME 1991) pp. 73-80. cited by other
.
"The Leader in Rapid Prototyping and Rapid Manufacturing Solutions"
brochure (DTM Corporation 1995). cited by other .
Killian, "Letter to Helmut Keidl" (Mar. 1995). cited by other .
Marcus, et al., "From Computer to Component in 15 Minutes: The
Integrated Manufacture of Three-Dimensional Objects," JOM (1990),
pp. 8-10. cited by other .
"DTM Strengthens Materials Offerings With Composite Nylon,"
Horizons, vol. 69, No. 6 (1990), pp. 1030-31. cited by other .
The Sinterstation 2000 System Selective Laser Sintering Guide to
Materials, (DTM Corporation 1992). cited by other .
The Sinterstation 2000 System Selective Laser Sintering Guide to
Materials--Polycarbonate, (DTM Corporation 1993). cited by other
.
The Sinterstation 2000 System Selective Laser Sintering Guide to
Materials--Wax, (DTM Corporation 1992). cited by other .
Sun, et al., "A Model for Partial Viscous Sintering," Solid
Freeform Fabrication Symposium Proceedings (Univ. of TX 1991) pp.
46-55. cited by other .
Xue, et al., "Models for the Prediction of the Thermal
Conductivities of Powders," Solid Freeform Fabrication Symposium
Proceedings (Unvi. of TX 1991). cited by other .
Nutt, "The SLS.TM. Selective Laser Sintering Process: An Update on
the Technology, Materials, and Applications," Proc. of the 1.sup.st
European Conference on Rapid prototyping (Jul. 6 & 7, 1992).
cited by examiner.
|
Primary Examiner: Boykin; Terressa
Attorney, Agent or Firm: D'Alessandro; Ralph Simons; William
A.
Parent Case Text
This application is a continuation of application Ser. No.
08/664,356, filed Jun. 17, 1996, now U.S. Pat. No. 5,648,450,
issued Jul. 15, 1997, which is a continuation-in-part of commonly
assigned application Ser. No. 08/298,076, filed Aug. 30, 1994, now
U.S. Pat. No. 5,527,877, which is a continuation-in-part of
commonly assigned application Ser. No. 07/980,004, filed Nov. 23,
1992, now U.S. Pat. No. 5,342,919.
Claims
We claim:
1. A method of producing a three-dimensional object, comprising the
steps of: applying a layer of a powder at a target surface, the
powder comprised of a semi-crystalline organic polymer, the powder
having a melting peak and a recrystallization peak, as shown in
differential scanning calorimetry traces, which slightly overlap
when measured at a scanning rate of 10-20.degree. C./minute.
2. The method of claim 1, wherein the polymer is selected from a
group consisting of polyacetal, polypropylene, polyethylene, and
ionomers.
3. The method of claim 1, wherein the polymer is selected from a
group consisting of branched polyethylene and branched
polypropylene.
4. The method of claim 1, wherein the powder has a melting point
below about 200.degree. C.
5. The method of claim 1, wherein the overlap of the melting peak
and the recrystallization peak is no more than about 13.degree.
C.
6. The method of claim 1, wherein the overlap of the melting peak
and the recrystallization peak is no more than about 11.degree.
C.
7. An article formed of a semi-crystalline organic polymer powder
that is laser-sintered in layerwise fashion, the powder having a
melting peak and recrystallization peak, as shown in differential
scanning calorimetry traces, which slightly overlap when measured
at a scanning rate of 10-20.degree. C./minute.
8. The article of claim 7, wherein the polymer is selected from a
group consisting of polyacetal, polypropylene, polyethylene, and
ionomers.
9. The article of claim 7, wherein the polymer is selected from a
group consisting of branched polyethylene and branched
polypropylene.
10. The article of claim 7, wherein the powder has a melting point
below about 200.degree. C.
11. The article of claim 7, wherein the overlap of the melting peak
and the recrystallization peak is no more than about 13.degree.
C.
12. The article of claim 7, wherein the overlap of the melting peak
and the recrystallization peak is no more than about 11.degree.
C.
13. A method of producing a three-dimensional object, comprising
the steps of: applying a layer of a powder at a target surface, the
powder comprised of a semi-crystalline organic polyester-based
polymer, the powder having a melting peak and a recrystallization
peak, as shown in differential scanning calorimetry traces, which
do not substantially overlap when measured at a scanning rate of
10-20.degree. C./minute; directing energy at selected locations of
the layer corresponding to the cross-section of the object to be
formed in the layer, to fuse the powder thereat; repeating the
applying and directing steps to form the object in layerwise
fashion; and removing unfused powder from the object.
14. An article formed of a semi-crystalline organic polyester-based
polymer powder that is laser-sintered in layerwise fashion, the
powder having a melting peak and a recrystallization peak, as shown
in differential scanning calorimetry traces, which do not
substantially overlap when measured at a scanning rate of
10-20.degree. C./minute.
15. A method of producing a three-dimensional object, comprising
the steps of: applying a layer of a powder at a target surface, the
powder comprised of a semi-crystalline organic .Iadd.polymer having
a two-tiered particle size distribution with at least 80% of all
particles in the powder being 1-53 .mu.m in size, .Iaddend.the
powder having a caking temperature (T.sub.c) that is greater than a
softening point temperature (T.sub.s) of the powder, as shown in
differential scanning calorimetry traces; directing energy at
selected locations of the layer corresponding to the cross-section
of the object to be formed in the layer, to fuse the powder
thereat; repeating the applying and directing steps to form the
object in layerwise fashion; and removing unfused powder from the
object.
16. The method of claim 15, wherein the polymer is a nylon.
17. The method of claim 16, wherein the nylon is selected from a
group consisting of nylon 6, nylon 11, and nylon 12.
18. The method of claim 15, wherein the polymer is a
polyester-based polymer.
19. The method of claim 15, wherein the polymer is selected from a
group consisting of polyacetal, polypropylene, polyethylene, and
ionomers.
20. The method of claim 15, wherein the polymer is selected from a
group consisting of branched polyethylene and branched
polypropylene.
21. The method of claim 15, wherein the powder has a melting point
below about 200.degree. C.
22. A method of producing a three-dimensional object, comprising
the steps of: applying a layer of a powder at a target surface, the
powder comprised of a semi-crystalline organic polymer .Iadd.having
a two-tiered particle size distribution with at least 80% of all
particles in the powder being 1-53 .mu.m in size, .Iaddend.selected
from a group consisting of nylon 6, nylon 11, and nylon 12;
directing energy at selected locations of the layer corresponding
to the cross-section of the object to be formed in the layer, to
fuse the powder thereat; repeating the applying and directing steps
to form the object in layerwise fashion; and removing unfused
powder from the object.
23. .[.An.]. .Iadd.A near-fully dense, distortion free
.Iaddend.article formed of a semi-crystalline organic polymer
powder that is laser-sintered in layerwise fashion, the powder
having a caking temperature (T.sub.c) that is greater than a
softening point temperature (T.sub.s) of the powder, as shown in
differential scanning calorimetry traces .Iadd.and having a
two-tiered particle size distribution with at least 80% of all
particles in the powder being 1-53.mu.m in size.Iaddend..
24. The article of claim 23, wherein the polymer is a nylon.
25. The article of claim 24, wherein the nylon is selected from a
group consisting of nylon 6, nylon 11, and nylon 12.
26. The article of claim 23, wherein the polymer is a polymer is a
polyester-based polymer.
27. The article of claim 23, wherein the polymer is selected from a
group consisting of polyacetal, polypropylene, polyethylene, and
ionomers.
28. The article of claim 23, wherein the polymer is selected from a
group consisting of branched polyethylene and branched
polypropylene.
29. The article of claim 23, wherein the powder has a melting point
below about 200.degree. C.
30. .[.An.]. .Iadd.A near fully-dense, distortion-free
.Iaddend.article formed of a semi-crystalline organic polymer
powder that is laser-sintered in layerwise fashion, the powder
comprised of a semi-crystalline organic polymer selected from a
group consisting of nylon 6, nylon 11, and nylon 12 .Iadd.and
having a two-tiered particle size distribution with at least 80% of
all particles in the powder being 1-53 .mu.m in size.Iaddend..
.Iadd.31. The method according to claim 13 further comprising
applying a layer of powder wherein the powder has less than 5% of
the particles greater than 180 .mu.m..Iaddend.
.Iadd.32. The method according to claim 31 further comprising
having a major portion by weight of the particles in the powder
having a sphericity of greater than 0.5..Iaddend.
.Iadd.33. The method according to claim 32 further comprising
applying a layer of powder wherein at least 90% of the particles
are smaller than 53 .mu.m in size..Iaddend.
.Iadd.34. The method according to claim 22 further comprising
applying a layer of powder wherein the powder has less than 5% of
the particles greater than 180 .mu.m. .Iaddend.
.Iadd.35. The method according to claim 34 further comprising
having a major portion by weight of the particles in the powder
having a sphericity of greater than 0.5..Iaddend.
.Iadd.36. The method according to claim 35 further comprising
applying a layer of powder wherein at least 90% of the particles
are smaller than 53 .mu.m in size..Iaddend.
.Iadd.37. The article according to claim 23 further comprising
applying a layer of powder wherein the powder has less than 5% of
the particles greater than 180 .mu.m..Iaddend.
.Iadd.38. The article according to claim 37 further comprising
having a major portion by weight of the particles in the powder
having a sphericity of greater than 0.5..Iaddend.
.Iadd.39. The article according to claim 38 further comprising
applying a layer of powder wherein at least 90% of the particles
are smaller than 53 .mu.m in size..Iaddend.
.Iadd.40. The article according to claim 30 further comprising
applying a layer of powder wherein at least 90% of the particles
are smaller than 53 .mu.m in size..Iaddend.
.Iadd.41. The article according to claim 40 further comprising
applying a layer of powder wherein the powder has less than 5% of
the particles greater than 180 .mu.m..Iaddend.
.Iadd.42. The article according to claim 41 further comprising
having a major portion by weight of the particles in the powder
having a sphericity of greater than 0.5..Iaddend.
Description
This invention is in the field of rapid prototyping, and is more
particularly directed to materials for producing prototype parts by
way of selective laser sintering.
BACKGROUND OF THE INVENTION
This invention relates to a synthetic resinous powder product to be
laser-sintered in a selective laser sintering machine, such as a
SINTERSTATION 2000 system manufactured and sold by DTM Corporation.
The laser-sinterable powder (referred to as "sinterable powder"
herein) is "designed" or "tailored" to incorporate specific
physical properties uniquely adapted to form a bed (of powder) upon
which a sintering laser in the infra-red region is directed.
Prior art sinterable powders are unable to yield a sintered part
which, for most purposes, appears to be a duplicate of one which is
isotropically molded. Moreover, conventional sinterable powders
form a bed which generally lacks the ability to provide the exigent
heat transfer characteristics which determine whether a sintered
part will be distorted, even if it is successfully completed. Since
a layer of particles typically rolled out of the feed bed and onto
the part bed of a selective laser sintering machine, is about 8
mils (200 .mu.m) such powders used had a maximum particle diameter
which was less than 200 .mu.m and whatever "fines" were generated
in the course of grinding the powder to the desired mesh size,
irrespective of the distribution of particle sizes in the
powder.
It has been observed that the selective laser sintering of
amorphous polymer powders typically results in finished parts that
are somewhat porous. Typical amorphous polymers exhibit a second
order thermal transition at a temperature that is commonly referred
to as the "glass transition" temperature, and also exhibit a
gradual decrease in viscosity when heated above this temperature.
In the selective laser sintering of amorphous polymers, the part
bed is maintained at a temperature near the glass transition
temperature, with the powder being heated by the laser at the part
locations to a temperature beyond the glass transition temperature
to produce useful parts, since viscosity controls the kinetics of
densification. While it may be at least theoretically possible to
build fully dense (i.e., non-porous) parts from amorphous polymers,
practical considerations arising from the use of high power lasers,
such as thermal control, material degradation, and growth
(undesired sintering of powder outside of the scanned regions) have
prevented the production of such fully dense parts. Further, it has
been observed that the selective laser sintering of amorphous
polymer powders is also vulnerable to "in-build curl", where
subsequent sintered layers added to the part shrink onto the solid
substrate, causing the part to warp out of the part bed.
The sinterable powders of the present invention are directed to
yielding a sintered article ("part") which, though porous, not only
has the precise dimensions of the part desired, but also is so
nearly fully dense (hence referred to as "near-fully dense") as to
mimic the flexural modulus and maximum stress at yield (psi), of
the article, had it been fully dense, for example, if it had been
isotropically molded.
In addition, the properties deliberately inculcated in the
sinterable powder are unexpectedly effective to provide the bed
with sufficient porosity to permit cooling gas to be flowed
downwardly through it, yet maintaining a quiescent bed in which the
sintered part mimics the properties of a molded article.
The term "near-fully dense" refers to a slightly porous article
which has a density in the range from 80%-95% (void fraction from
0.2 to as low as 0.05), typically from 85%-90% of the density (void
fraction 0.15-0.1) of a compression molded article which is deemed
to be fully dense.
The term "fully dense" refers to an article having essentially no
measurable porosity, as is the case when an article of a synthetic
resinous powder is compression (or injection) molded from a
homogeneous mass of fluent polymer in which mass individual
particles have lost their identity.
By a "quiescent bed" we refer to one upon the surface of which the
particles are not active, that is, do not move sufficiently to
affect the sintering of each layer spread upon a preceding slice
sintered in the part bed. The bed is not disrupted by the downward
flow of gas, so that the bed appears to be static.
To date, despite great efforts having been focused on a hunt for
the formulation of a sinterable powder which will yield a
near-fully dense part, that formulation has successfully eluded the
hunt. The goal is therefore to produce a mass of primary particles
of a synthetic resin which has properties specifically tailored to
be delivered by a roller to the "part bed" of a selective laser
sintering machine, then sintered into a near-fully dense prototype
of a fully dense article.
A powder dispenser system deposits a "layer" of powder from a
"powder feed bed" or "feed bed" into a "part bed" which is the
target area. The term "layer" is used herein to refer to a
predetermined depth (or thickness) of powder deposited in the part
bed before it is sintered.
The term "prototype" refers to an article which has essentially the
same dimensions of a compression or injection molded article of the
same material. The porous prototype is visually essentially
indistinguishable from the molded article, and functions in
essentially the same manner as the molded article which is
non-porous or fully dense. The flexural modulus, flexural strength
and flexural elongation at yield, are essentially indistinguishable
from the values obtained for a molded article. One is
distinguishable from the other only because the prototype has a
substantially lower, typically less than one-half, the ultimate
tensile elongation (%), and notched Izod impact (ft-lb/in), than a
compression molded article, though the prototype's tensile modulus,
tensile strength, and elongation at yield are substantially the
same as those of the compression molded article (see Table 1
hereinbelow). In Table 1, the values given in square brackets are
the standard deviations under the particular conditions under which
the measurements were made.
The tensile elongation, ultimate (%), and notched Izod impact are
lower for the prototype because of its slight porosity. Therefore
the energy to break, which is the area under the stress curve up to
the point of break at ultimate elongation, is also very much lower
than that for the compression molded article. As is well known, any
small imperfections in a homogeneous article will be reflected in
the ultimate tensile elongation and notched Izod impact. However,
confirmation that the molded article has been closely replicated is
obtained by a comparison of the fracture surfaces of the prototype
and of the molded article. Photo-micrographs show that these
fracture surfaces of the prototype are visually essentially
indistinguishable from fracture surfaces of an isotropically molded
non-porous part except for the presence of a profusion of cavities
having an average diameter in the range from 1 .mu.m-30 .mu.m
randomly scattered throughout said part, indicating similar creep
and fatigue characteristics. As one would expect, the cavities
provide evidence of the porosity of the prototype. Therefore it is
fair to state that, except for the lower ultimate elongation of
Izod impact of the prototype, due to its slight porosity, the
prototype fails in the same manner as the molded article.
A laser control mechanism operates to direct and move the laser
beam and to modulate it, so as to sinter only the powder disposed
within defined boundaries (hence "selectively sintered"), to
produce a desired "slice" of the part. The term "slice" is used
herein to refer to a sintered portion of a deposited layer of
powder. The control mechanism operates selectively to sinter
sequential layers of powder, producing a completed part comprising
a plurality of slices sintered together. The defined boundaries of
each slice corresponds to respective cross-sectional regions of the
part. Preferably, the control mechanism includes a computer--e.g. a
CAD/CAM system to determine the defined boundaries for each slice.
That is, given the overall dimensions and configuration of the
part, the computer determines the defined boundaries for each slice
and operates the laser control mechanism in accordance with the
defined boundaries for each slice. Alternatively, the computer can
be initially programmed with the defined boundaries for each
slice.
The part is produced by depositing a first portion of sinterable
powder onto a target surface of the part bed, scanning the directed
laser over the target surface, and sintering a first layer of the
first portion of powder on the target surface to form the first
slice. The powder is thus sintered by operating the directed laser
beam within the boundaries defining the first slice, with high
enough energy (termed "fluence") to sinter the powder. The first
slice corresponds to a first cross-sectional region of the
part.
A second portion of powder is deposited onto the surface of the
part bed and that of the first sintered slice lying thereon, and
the directed laser beam scanned over the powder overlying the first
sintered slice. A second layer of the second portion of powder is
thus sintered by operating the laser beam within the boundaries
which then define the second slice. The second sintered slice is
formed at high enough a temperature that it is sintered to the
first slice, the two slices becoming a cohesive mass. Successive
layers of powder are deposited onto the previously sintered slices,
each layer being sintered in turn to form a slice.
Repetition of the foregoing steps results in the formation of a
laser-sintered article lying in a "part bed" of powder which
continually presents the target surface. If the particles of powder
at the boundaries of each layer are overheated sufficiently to be
melted, unmelted particles immediately outside the boundaries
adhere to the molten particles within, and the desired sharp
definition of the surface of the sintered article is lost. Without
sharp definition at the boundaries, the article cannot be used as a
prototype.
Particles of powder adjacent the surfaces of the article to be
formed should resist being strongly adhered to those surfaces. When
particles are not so strongly adhered they are referred to as
"fuzz" because fuzz is easily dislodged from the surface, manually,
and the dislodged particles retain most of their individual
identities. Particles so tightly adhered to the surface as to be
removed satisfactorily only with a machining step, are referred to
as "growth". Such growth makes a sintered part unfit for the
purpose at hand, namely to function as a prototype for a
compression molded part.
A method for sintering a powder into a shaped article in a
selective laser sintering machine is disclosed in U.S. Pat. Nos.
4,247,508 to Housholder; 4,863,538 and 5,132,143 to Deckard;
4,938,816 to Beaman et al.; and, 4,944,817 to Bourell et al., the
relevant disclosure of each of which is incorporated by reference
thereto as if fully set forth herein. "Sintering" is defined as the
heating of the powder to a temperature which causes viscous flow
only at contiguous boundaries of its particles, with at least some
portion of substantially all particles remaining solid. Such
sintering causes coalescence of particles into a sintered solid
mass the bulk density of which is increased compared to the bulk
density of the powder particles before they were sintered; and, a
part formed by "slice-wise" joining of plural vertically contiguous
layers which are sintered into stacked "slices" is therefore said
to be autogenously densified. A layer of powder is confined by
vertically spaced apart horizontal planes, no more than about 250
.mu.m apart and each slice is typically in the range from 50 .mu.m
to 180 .mu.m thick.
A specific goal of this invention is to produce a sinterable powder
of a single, that is, unblended, synthetic resin which, when
exposed to the laser beam, is heated so that the outer portions of
each particle have a narrowly defined range of viscosity which
results in the fusion of successive slices.
It must be remembered that before the powder can be sintered in the
part bed, it must be delivered from the feed bed to the part bed
upon which the powder is distributed in a thin, even layer about
125.mu. thick, by the roller of the selective laser sintering
machine. Each distributed layer should be thin and evenly
distributed because the temperature gradient through the
cross-section of the sintered slice must be small, typically
<5.degree. C., more preferably <2.degree. C., and most
preferably <1.degree. C. To meet this demanding criterion, the
powder must be freely flowable from the feed bed onto the part
bed.
By "freely flowable" we refer to a mass of small particles, the
major portion of which, and preferable all of which have a
sphericity of at least 0.5, and preferably from 0.7 to 0.9 or
higher, so that the mass tends to flow steadily and consistently as
individual particles. Though such flow is conventionally considered
a characteristic of a powder which flows through an orifice
slightly larger than the largest particle, such flow (through an
orifice) is of less importance than the ability of the powder to be
picked up in the nip of a rotating roller and transported by it as
an elongated fluent mass of individual particles urged along by the
roller. A freely flowable powder has the property of being able to
be urged as a dynamic elongated mass, referred to as a "rolling
bank" of powder, by the rotating roller, even at a temperature near
T.sub.s the "softening point" of the powder.
At T.sub.s, the powder is on the verge of not being flowingly
transportable as a rolling bank against a rotating roller. By
"softening point" we refer to T.sub.s, at which a powder's storage
modulus (G'.sub.s) has decreased substantially from its value of G'
at room temperature. At or above T.sub.s the storage modulus
G'.sub.s of a sintered slice of the powder is low enough so as not
to let it "curl". By "curl" we refer to the slice becoming
non-planar, one or more portions or corners of the slice rising
more than about 50 .mu.m above the surface of the last (uppermost)
slice in the horizontal x-y-plane.
A slice will curl when there is a too-large mismatch between the
temperature of the initial slice sintered by the laser and the bed
of powder on which it lies, or, between powder freshly spread over
a just-sintered slice and the temperature at the upper interface of
the slice and the freshly spread powder. Such a mismatch is the
result of "differential heating". The importance of countering
curly is most critical when the first slice is formed. If the first
slice curls, the roller spreading the next layer of powder over the
slice will push the slice off the surface of the part bed.
If the powder is transported from the feed bed to the part bed in
which a hot slice is embedded, and the temperature at the interface
T.sub.i between the hot upper surface of the slice and the freshly
spread powder is high enough to raise the temperature of the
freshly spread powder above T.sub.s, this powder cannot be
rollingly distributed over the hot slice because the powder sticks
and smears over the hot slice. The indication is that the slice is
too hot.
If the powder in the feed bed is too cool, that is, so cool that
the equilibrium temperature on the surface of the hot, embedded
slice is such that the temperature of the freshly spread powder is
below T.sub.s, the slice will curl.
The slice will not curl when the powder spread over it reaches an
equilibrium temperature at the interface, and the equilibrium
temperature is at or above T.sub.s. The precise temperature T.sub.i
at the interface is difficult to measure, but to form successive
slices cohesively sintered together, the temperature of the powder
at the interface must be above T.sub.s, but below the powder's
"sticky point" or "caking temperature" T.sub.c at which the powder
itself will not flow.
By "sticky" we infer that the force required to separate contiguous
particles has exceeded an acceptable limit for the purpose at hand.
This caking temperature T.sub.c may be considered to be reached
when a critical storage modulus (G'.sub.c) of the powder has been
reached or exceeded. The storage modulus is a property of the
powder akin to a material's tensile strength and can be measured
directly with a Rheometrics dynamic mechanical analyzer.
To form a sintered part in a selective laser sintering machine, an
initial slice is sintered from powder held in the part bed at near
T.sub.s but well below T.sub.c. By "near T.sub.s" we refer to a
temperature within about 5.degree. C. of T.sub.s, that is
T.sub.s.+-.5, preferably T.sub.s.+-.2.
Immediately after the initial slice is formed, the slice is much
hotter than the powder on which it rests. Therefore a relatively
cool powder, as much as about 40.degree. C., but more typically
about 20.degree. C. below its T.sub.s, may be spread over the hot
slice and the interface temperature raises the temperature of the
powder to near T.sub.s. As the powder is spread evenly over the hot
slice is to remain cool enough to be spread, but soon thereafter,
due to heat transfer at the interface, must reach or exceed
T.sub.s, or the just-sintered slice will curl; that is, the
temperature of the powder preferably enters the "window of
sinterability". This window may be measured by running two DSC
(differential scanning calorimetry) curves on the same sample of
powder, sequentially, with a minimum of delay between the two runs,
one run heating the sample past its melting point, the other run,
cooling the sample from above its melting point until it
recrystallizes. The difference between the onset of melting in the
heating curve, Tm, and the onset of supercooling in the cooling
curve, Tsc, is a measure of the width of the window of
sinterability (see FIG. 6).
To ensure that the powder from the feed bed will form a rolling
bank even when it is rolled across the hot slice, the powder is
usually stored in the feed bed at a storage temperature in the
range from 2.degree. C. to 40.degree. C. below the powder's T.sub.s
and transferred at this storage temperature to the part bed, the
feed bed temperature depending upon how quickly a layer of powder
spread over a just-sintered slice enters the window of
sinterability. The T.sub.s may be visually easily obtained--when
the powder is too hot to form a rolling bank, it has reached or
exceeded its T.sub.s.
It will now be realized that the cooler the powder (below T.sub.s)
the higher the risk of curling, if the interface temperature is not
high enough to raise the temperature of the layer of powder at
least to T.sub.s. A commensurate risk accrues with a powder stored
at too high a temperature. The storage temperature is too high,
though the powder forms a rolling bank, when the powder smears or
sticks as it traverses the slice, an indication that the powder
overlying the slice has not only exceeded T.sub.s but also reached
(or gone beyond) T.sub.c.
Thus, though it is difficult to measure the interface temperature,
or to measure T.sub.c with a temperature probe, so as to measure
the width of the window, it can be done visually. When the rolling
bank of powder sticks or smears over the last-sintered slice, the
T.sub.c of powder has been reached or exceeded. Thus with visual
evidence once can determine the temperature range (T.sub.c-T.sub.s)
which is the window of sinterability or the "selective laser
sintering operating window", so referred to because the powder
cannot be sintered successfully at a temperature outside the
selective-laser-sintering-window. (see FIG. 6).
At the start of a sintering cycle it is best to maintain the
temperature of the upper layer of the part bed at T.sub.s,
preferably 0.5.degree.-2.degree. C. above T.sub.s so that the
uppermost layer is presented to the laser beam in the
selective-laser-sintering-window. After the first slice is formed,
feed is rolled out from the feed bed at as high a temperature as
will permit a rolling bank of powder to be transferred to the part
bed. The most desirable powders are freely flowable in a rolling
bank at a temperature only about 5.degree. C. below their
T.sub.s.
However, as the mass of the sintered slices accumulates in the part
bed, the sintered mass provides a large heat sink which transfers
heat to each layer of powder freshly spread over the hot mass, thus
allowing a relatively cool powder, as much as 30.degree. C., more
typically 20.degree. C., lower than T.sub.s to be transferred from
the feed bed, yet quickly come to equilibrium in the
selective-laser-sintering-window as the layer is spread over the
last preceding slice. Thus, when each layer is sintered, the
later-formed slices will not curl.
It is important that the powder be "freely flowable" from the feed
bed, preferably at a temperature sufficiently near T.sub.s to
ensure that the last-sintered slice will not curl when the powder
is spread upon it. As already pointed out above, if the first slice
formed curls, no further progress can be made. A fresh start must
be made to sinter the part.
A powder is not freely flowable when the temperature at which it is
held or distributed exceeds its softening point. The powder cakes
and does not flow at all when the caking temperature is reached.
For example, one may consider that at T.sub.c, G'.sub.s decreases
to a critical G'.sub.c, in which case the caking temperature
T.sub.c may also be referred to as the "G'.sub.c temperature".
It is possible to transfer powder from the feed bed to the part bed
at above T.sub.s if the impaired flowability allows one to do so,
and the risk of operating too close to T.sub.c is acceptable.
Generally a powder does not form a rolling bank at or above its
T.sub.s.
According to one aspect of the invention, it is preferred that the
powder used in the selective laser sintering process be sinterable
in a wide selective-laser-sintering-window. Though within narrow
limits, the `width` (in .degree. C.) of the window, varies from the
start of the cycle and at the end (particularly when a large part
is formed, as explained above). The width of the window also varies
depending upon the composition of the powder. This width ranges
from about 2.degree. C. to about 25.degree. C.; more typically, it
is about 5.degree. C.-15.degree. C. With a powder which is freely
flowable over a wide temperature range, one is able to form, in the
best mode, a solid, near-fully dense article when the powder is
sintered in a selective laser sintering machine which uses a roller
to spread the powder.
The temperature at which G'.sub.s is measured is believed to not be
critical, provided the G'.sub.c temperature offers an adequately
large selective-laser-sintering-window. Most desirable
laser-sinterable powders have an unexpectedly common
characteristic, namely that the value of their G'.sub.c is narrowly
defined in the range from 1.times.10.sup.6 dynes/cm.sup.2 to
3.times.10.sup.6 dynes/cm.sub.2.
For a crystalline powder (100% crystallinity), the softening point
is its melting point Tm. Therefore G'.sub.s and G'.sub.c are
essentially identical and there is no G'-window. For an amorphous
powder, its softening point is its initial glass transition
temperature Tg. An amorphous powder can offer a large window of
sinterability but because its viscosity decreases too slowly as
temperature increases and the G'.sub.c limit of the
selective-laser-sintering-window is approached, the viscosity is
still too high. That is, the viscosity is too high to allow
requisite interchain diffusion at the boundaries of the particles
without melting the entire particle. Therefore an amorphous powder
is difficult to sinter to near-full density, so that powders which
qualify as the product of this invention are semi-crystalline
powders such as nylon, polybutylene terephthalate (PBT) and
polyacetal (PA) which provide signs of crystalline order under
X-ray examination, and show a crystalline melting point Tm as well
as a glass transition temperature Tg. Because the crystallinity is
largely controlled by the number and distribution of branches along
the chain, the crystallinity varies, bulky side chains or very long
chains each resulting in a reduction of the rate of
crystallization. Preferred polymers have a crystallinity in the
range from 10%-90%, more preferably from 15%-60%.
To summarize, the selective laser sintering process is used to make
3-D objects, layer-upon-layer sequentially and in an additive
manner. The process is more fully described in the '538 Deckard
patent and comprises the following steps: (1) Powder from the feed
bed is "rolled out" by a roller, to a part bed where the powder is
deposited and leveled into a thin layer, typically about 125 .mu.m
(0.005'') in depth. (2) Following a pattern obtained from a two
dimensional (2-D) section of a 3-D CAD model, a CO2 laser "sinters"
the thin layer in the target region of the part bed and generates a
first slice of sintered powder in a two-dimensional ("2-D") shape.
Directions for the pattern, and each subsequent pattern for
successive slices corresponding to a desired three-dimensional
("3-D") prototype are stored in a computer-controller. It is
critical for a slice-upon-slice construction of the prototype that
the laminar, planar shape of each slice of sintered powder be
maintained, that is, "without curling". (3) A second layer of
powder from the feed bed is then deposited and leveled over the
just-sintered layer in the part bed, forming a second slice
sintered to the first slice. (4) The computer-controller makes
incremental progress to the next 2-D section, the geometry of which
is provided from the 3-D model, and instructs the laser/scanner
system to sinter the regions of the bed desired for successive 2-D
sections. (5) Still another layer of powder is deposited from the
feed bed and leveled over the just-sintered layer in the part bed.
(6) The foregoing steps are repeated, seriatim, until all layers
have been deposited and sequentially sintered into slices
corresponding to successive sections of the 3-D model. (7) The
sintered 3-D object is thus embedded in the part bed, supported by
unsintered powder, and the sintered part can be removed once the
bed has cooled. (8) Any powder that adheres to the 3-D prototype's
surface as "fizz" is then mechanically removed. (9) The surfaces of
the 3-D prototype may be finished to provide an appropriate surface
for a predetermined use.
This invention relates mainly to producing and using a powder which
is designed to satisfy the requirements of the first three steps of
the process.
Although we have experimentally processed many synthetic resinous
powders in the selective laser sintering machine, we have found
that few make near-fully dense parts. In most cases the measured
values of flexural modulus and maximum stress at yield are at least
30% lower than values obtained made by injection or compression
molding the same part. We now understand, and have set forth below,
what properties are required of a powder which can be successfully
sintered in a selective laser sintering machine, and have accepted,
at least for the time being, the many disappointing results we
obtained with amorphous polymers such as polycarbonate (PC) and
acrylonitrile-butadiene-styrene resins (ABS).
It has now become evident that a semi-crystalline or substantially
crystalline organic polymer is the powder of choice if it is to
provide the high definition of surface ("lack of growth") which a
prototype made from the tailored powder of this invention,
provides.
By a "semi-crystalline polymer" or "substantially crystalline
polymer" is meant a resin which has at least 10% crystallinity as
measured by DSC, preferably from about 15%-90%, and most preferably
from about 15-60% crystallinity.
U.S. Pat. No. 5,185,108, issued Feb. 9, 1993, incorporated herein
by this reference, teaches that to produce a sintered article of
wax having a void fraction (porosity) of 0.1, a two-tier weight
distribution of wax particles was necessary. The desired two-tier
distribution was produced by a process which directly generated a
mass of wax microspheres such that more than half (>50%) the
cumulative weight percent is attributable to particles having a
diameter greater than a predetermined diameter (100 .mu.m is most
preferred for the task now at hand) for the particular purpose of
packing at least some, and preferably a major portion of the
interstitial spaces between larger particles, with smaller
ones.
The two-tier distribution described in U.S. Pat. No. 5,185,108 was
arrived at by recognizing that the densest packing of uniform
spheres produces a void fraction (porosity) of 0.26 and a packing
fraction of 0.74 as illustrated in FIG. 1; and further, by
recognizing that the packing factor may be increased by introducing
smaller particles into the pore spaces among the larger spheres. As
will be evident, the logical conclusion is that the smaller the
particles in the pore spaces, the denser will the packed powder (as
illustrated in FIG. 2) and the denser will be the part sintered
from the powder.
As will further be evident, the greater the number of small
particles relative to the large, in any two-tier distribution, the
denser will be the part. Since the goal is to provide a near-fully
dense part, logic dictates that one use all small particles, and
that they be as small as can be.
However, a mass of such uniformly small particles is not freely
flowable. To make it freely flowable one must incorporate larger
particles into the mass, much in the same manner as grains of rice
are commonly interspersed in finely ground table salt in a salt
shaker. Therefore, the tailored powder is a mixture of relatively
very large and relatively very small particles in a desirable
two-tier particle size distribution for the most desirable
sinterable powders.
The demarcation of size in the two-tier distribution and the ratio
of the number of small particles to the number of large particles,
set forth hereinbelow, are both dictated by the requirements of the
selective laser sintering machine.
Further it was found that the rate of heat transfer into the mass
of a small particle is so much higher than that into the mass of a
large particle, that one could not know either just how large the
particles in the upper tier should be, nor how many of such large
particles could be present. If the heat transfer to small particles
in the bed adjacent the boundaries of each layer was too high,
unacceptable growth is generated. If the heat transfer is not high
enough, the large particles, namely those >53 .mu.m, in the
layer are not sintered, thus forming a defective slice. It is
because essentially all these large particles are sintered without
being melted, and a substantial number of the small particles
<53 .mu.m are melted sufficiently to flow into and fill the
interstices between sintered large particles, that the finished
sintered part is near-fully dense. Under successful sintering
conditions to form a near-fully dense part, the temperature of the
powder must exceed T.sub.s in less time than is required to melt
the large particles >53 .mu.m. If the time is too long, large
particles will melt and there will be growth on the surfaces of the
part; if the time is too short, all the large particles are not
sintered. Thus the large particles not only help form a rolling
bank, but also fill an important role to maintain the desired
transient heat transfer conditions.
It has been found that only a substantially crystalline powder
which does not melt sharply, lends itself to the purpose at hand,
and only when the powder is stripped of substantially all too-large
particles (termed "rocks") larger than 180 .mu.m (80 mesh, U.S.
Standard Sieve Series). By "substantially all" we mean that at
least 95% of the number of "rocks" in the powder are removed.
It has further been found that a laser-sinterable powder in the
proper size range of from about 1 .mu.m-180 .mu.m, may, according
to one aspect of the invention, be specified by (i) narrowly
defined particle size range and size in a two-tier distribution,
and, (ii) the "selective-laser-sintering-window".
According to another aspect of the invention to be described in
detail hereinbelow, it has now been realized that the two-tiered
particle size distribution is not absolutely necessary in order to
create a distortion-free fully dense part in the selective laser
sintering process, provided that the recrystallization rate of the
material is sufficiently low.
Referring to the first aspect of the invention noted above, the
unexpected effect of using the tailored powder with a defined
selective-laser-sintering-window is supported by evidence of the
sinterability of the powder in this window. The
selective-laser-sintering-window is directly correlatable to the
powder's fundamental properties defined by its G'.sub.c
temperature.
More surprising is that, despite the much larger number of small
particles than large in the part bed, it is possible to flow the
stream of cooling gas (nitrogen) downwardly through the quiescent
bed at low enough a pressure so as not to disturb the particles on
and near the surface of the bed sufficiently to cause movement
noticeable by the naked eye (hence referred to as "quiescent"). One
would expect the pressure drop through a bed of very fine
particles, more than 80% of which are smaller than 53 .mu.m (270
mesh) to be relatively high. But the presence of the large
particles, coupled with the fact that the powder is delivered from
the feed bed and distributed evenly by a roller, rather than being
pressed onto the bed, unpredictably provides the requisite porosity
in the range from 0.4 to 0.55 to allow through-flow of a gas at
superatmospheric pressure in the range from 103 kPa (0.5 psig) to
120 kPa (3 psig), preferably from 107-115 kPa (1-2 psig) with a
pressure drop in the range from 3-12 kPa, typically 5-7 kPa,
without disturbing a quiescent part bed 30 cm deep.
The part bed formed by the tailored powder is unique not only
because its specific use is to generate laser-sintered parts, but
because the bed's narrowly defined porosity and defined particle
size provides "coolability". In operation, the powder in the part
bed is heated by a multiplicity of hot sintered slices to so high a
temperature that the powder would reach its caking temperature
T.sub.c if the hot bed could not be cooled.
An identifying characteristic of a preheated `part bed` of powder
having a two-tiered distribution, with primary particles in the
proper size range, stripped of rocks >180 .mu.m, is that the bed
is not too tightly packed to permit the flow of cooling gas through
the bed. This characteristic allows the part bed to be maintained,
during operation sintering a part, with a specified temperature
profile which allows formation of a distortion-free sintered part
as it is formed slice-wise; and also, after the sintered part is
formed, and the part lies in the heated part bed. By
"distortion-free" is meant that no linear dimension of the part is
out of spec more than .+-.250 .mu.m, and no surface is out of plane
by more than .+-.250 .mu.m (20 mils).
Though the importance of a two-tier particle size weight
distribution was disclosed with respect specifically to wax
particles in U.S. Pat. No. 5,185,108, it was not then realized that
the ranges of particle sizes in each tier of the two-tier
distribution controlled both, the density of the sintered part and
the sinterability of the powder. Neither was it known that the
distribution of particle sizes in a two-tier distribution was as
critical as the viscosity characteristics of the material as a
function of temperature.
The ranges of sizes in the two-tier distribution of particles used
in the powder according to this aspect of the invention is
different from the ranges of the two-tier distribution of the wax
powder described in U.S. Pat. No. 5,185,108. Quite unexpectedly,
the formation of a near-fully dense sintered part requires that at
least 80% of the number of all particles in the bed are from 1
.mu.m-53.mu. and that there be substantially no (that is, <5%)
particles greater than 180 .mu.m (80 mesh) in a part bed. The
importance of the few "large particles" to maintain (i)
free-flowability near T.sub.s and (ii) a predetermined temperature
profile in a part bed while a sintered part is being formed,
irrespective of the density of the part formed, to negate
undesirable "growth" on the part, was not then known.
Because the "selective-laser-sintering-window" may be defined by
the requirements of the selective laser sintering process, the part
bed (and sometimes the feed bed) is heated to near T.sub.s to
negate the proclivity of the sintered layer to "curl". To minimize
the curling of a slice as it lies on a part bed, it has been
discovered that a preferred temperature profile is to be maintained
in the bed, with a slight but narrowly specified temperature
gradient on either side of a horizontal zone through the portion of
the bed occupied by the sintered part, referred to as the "hot"
zone.
The typical gradient in a part bed in a selective laser sintering
machine is first positive, that is, the temperature increases to a
maximum, then the gradient is negative, that is the temperature
decreases from the maximum. The upper temperature gradient in the
upper portion of the bed is positive, that is the temperature
increases until it reaches a maximum temperature T.sub.max in the
hot zone. The lower temperature gradient in the lower portion of
the bed is negative, that is the temperature decreases from
T.sub.max in the hot zone to the bottom of the bed.
More specifically, the temperature in the upper portion of the bed
progressively increases as one moves downward from the upper
surface of the bed to T.sub.max; then progressively decreases as
one moves downward from T.sub.max to the bottom surface of the part
bed, which surface is in contact with the bed-supporting
piston.
The gradient in a conventional selective laser sintering machine
without controlled gas-cooling of the part bed, in each direction
is typically greater than 2.degree. C./cm (5.degree. C./in). Such a
gradient was found to be too high to provide an acceptable risk of
distortion of the part.
These considerations lead to temperature limits in the feed and
part beds which limits define the G'-window and
selective-laser-sintering-window, namely, (i) the temperature at
which the part bed is maintained, and the temperature profile
therein, and (ii) the temperature at which the feed bed is
maintained.
In turn, the temperature at which the part bed is maintained is
defined by (a) a lower (minimum) part bed temperature below which
curling is so pronounced as to negate any reasonable probability of
effecting a slice-wise fusion of plural vertically contiguous
slices; and, (b) an upper (maximum) temperature at which
interparticle viscosity in the part bed makes it so "sticky" as to
fuzz (obfuscate) the predetermined boundaries of the part to be
made. All sintered powder between vertically spaced apart lateral
planes in the part bed is solidified sufficiently to have
mechanical strength. The remaining unsintered powder remains
freely-flowable.
The "improved" sinterable tailored powder provides not only the
specified particle size and two-tier distribution, but also a
usable and desirable selective-laser-sintering-window. The ability
of a powder simultaneously to satisfy each of the requirements,
provides a measure of how "good" the chance that a powder will be
sinterable in the selective laser sintering process to yield a
near-fully dense, but porous article.
A major practical consequence of the narrowly defined window
requires that the part bed be maintained at a specified temperature
and with a specified temperature profile so that each layer to be
sintered lies within the confines of the
selective-laser-sintering-window. A different temperature, whether
higher or lower, and/or a different temperature profile, results in
regions of the just-sintered initial slice of powder which will
either cause a sintered slice to melt and be distorted in a layer
of the part bed which has "caked"; or, will cause a sintered slice
to curl if the part bed temperature is too low. In the past this
has been an all too common occurrence with the result that an
undesirable part was made. The tailored powder and unique bed which
it forms now make production of an unacceptable part an uncommon
occurrence.
SUMMARY OF THE INVENTION
A laser-sinterable semi-crystalline synthetic resinous powder
(referred to as a "tailored powder"), having defined parameters of
particle size distribution, and crystallization characteristics is
found to overcome the disadvantages of known powders used to form a
sintered part in a selective laser sintering machine. The
unexpected effect of providing a sinterable powder which has a
defined selective-laser-sintering-window is evidenced in the
ability to predict the sinterability of the powder with a laser
generated at a wave-length which is absorbed sufficiently to heat
particles of the powder to their critical storage modulus G'.sub.c
when the outer portion of the particles have the viscosity required
to be cohesively sintered.
According to a first aspect of the invention, the two-tier particle
size distribution and the number average ratio of particles smaller
than 53 .mu.m be >80%, that is, more than 80% of all the
particles in the powder be smaller than 53 .mu.m, allow the powder
to be freely flowable onto the part bed so as to be presented to
the laser beam in the selective-laser-sintering-window, and also to
form a bed of desired porosity which (i) allows passage of a low
pressure inert cooling gas to keep the bed from overheating, and
(ii) provides the desired absorption of infra-red energy from the
laser beam to yield a near-fully dense slice. A specified
temperature profile is maintained in the part bed with the
flow-through inert cooling gas stream, but the tailored selective
laser sintering powder is sintered with a conventional selective
laser sintering protocol. The powder yields a sintered article
which is porous but so near-fully dense that the porous article has
strength characteristics which unexpectedly mimic (are
substantially the same as) those of an isotropically molded, fully
dense article of the same powder.
It is therefore a primary object of this invention to provide a
near-fully dense part in a selective laser sintering machine, the
part formed from a semicrystalline or substantially crystalline
synthetic resinous sinterable powder having tailored properties
uniquely adapted to the purpose at hand.
According to one aspect of this invention, it is a general object
of this invention to provide a bed of tailored powder of a
semi-crystalline unblended polymer having the following physical
properties: (a) a major portion by weight of the powder, and
preferably essentially all the powder having a sphericity in the
range from greater than 0.5 to 0.9, and a two-tier particle size
distribution of primary particles having an average diameter
smaller from than 180 .mu.m, with substantially no particles
>180 .mu.m, provided further that the number average ratio of
particles smaller than 53 .mu.m is greater than 80%, preferably
greater than 90%, and most preferably greater than 95%, the
remaining particles being in the size range from 53 .mu.m to 180
.mu.m; a layer of the powder no more than 250 .mu.m deep absorbs
essentially all infra-red energy at the 10.6 .mu.m wavelength
beamed therethrough, and absorbs more than 50% of that energy in a
layer no more than 180 .mu.m thick;
(b) a crystallinity in the range from 10% to 90%, preferably from
15% to 60%, and,
(c) a "selective-laser-sintering-window" in the temperature range
from 2.degree. C.-25.degree. C. between the softening temperature
T.sub.s of the powder and its "caking temperature" T.sub.c, such
that the powder has a "flow time" of <20 sec for 100 g in a
funnel test (ASTM D1895-61T) at a temperature near T.sub.s in a
range from 70.degree. C. to 220.degree. C., but below the powder's
T.sub.c; and, (d) a melt viscosity in the range from 100-10.sup.4
poise (10-1000 Pa-sec) when the temperature of the powder being
sintered exceeds T.sub.c in less time than is required to melt
contiguous large particles >53 .mu.m.
The numerical value of the storage modulus G'.sub.s for the
tailored powder is much lower than the value of G' at room
temperature, and the temperature at which G'.sub.s is measured is
in preferably the range from 5.degree. C. to 25.degree. C. below
the G'.sub.c temperature of the powder.
It is also a general object of this invention to provide a bed of
tailored powder in a laser-sintering zone, the bed having the
foregoing defined characteristics which are evidenced in: (i) a
"selective-laser-sintering-window" in the range from T.sub.s to
T.sub.c; and, (ii) a `part bed` in which the sintered part is
removably embedded while it dissipates heat to generate a
temperature profile defined by sequential positive and negative
temperature gradients, in a vertical plane through the part bed;
such a gradient occurs when the uppermost slice is less than 250
.mu.m thick, and is near T.sub.s of the powder, and the temperature
of the sintered part is near T.sub.c. Further, the gradient from
the upper surface of the part bed to the maximum temperature in the
horizontal zone in which the sintered part lies, is positive, the
temperature increasing at a rate in the range from 0.2.degree.
C./cm (0.5.degree. C./in) to 2.degree. C./cm (5.degree. C./in) of
vertical depth; and, from the maximum temperature in the horizontal
zone, to the bottom of the bed, the gradient is negative, the
temperature decreasing at a rate in the range from 0.2.degree.
C./cm (0.5.degree. C./in) to 2.degree. C./cm (5.degree. C./in).
It has also been discovered that the tailored powder which is
free-flowing at an elevated temperature below its Tg or Tm,
typically at from 30.degree. C. below T.sub.c, but with some
powders, as little as 2.degree. C., is uniquely adapted to yield,
when sintered by a laser beam, a near-fully dense, laser-sintered
article having a density in the range from 80%-95%, typically from
85%-90% of the density of a compression molded article which is
deemed to be fully dense, and the mode of failure, when fractured
in bending, is essentially identical to the mode of failure of an
isotropically molded article of the same powder, except for
cavities corresponding to the porosity of the sintered article. The
sintered article may have some unsintered particles ("fuzz")
adhering to its surface, but the fuzz is removable by lightly
abrading the surface without changing the contours of the
near-fully dense sintered article.
It is therefore another general object of this invention to produce
a laser-sinterable polymer powder consisting essentially of an
unblended polymer having substantially no particles >180 .mu.m
in a mass of particles in which the number average ratio of
particles in the range from 1 .mu.m-53 .mu.m is greater than 80%,
the remaining particles being in the size range from 53 .mu.m to
180 .mu.m; and substantial crystallinity in the range from 25% to
95%, which provides a selective-laser-sintering-window of from
2.degree. C. to 25.degree. C., and which powder when sintered in a
bed with a specified temperature profile, allows each layer of
powder, in the range from about 50 .mu.m (2 mil) to about 250 .mu.m
(10 mils) thick, to be sintered without curling.
It is a specific object of this invention to provide a
laser-sinterable unblended polymer powder tailored to have the
aforespecified two-tier distribution of primary particles which
have a sphericity in the range from greater than 0.5 to 0.9, a bulk
density of 500 to 700 g/L, and crystallinity in the range from 15
to 90%; has a "flow time" as given, at near T.sub.s but 2.degree.
C. to 25.degree. C. below the powder's caking temperature T.sub.c;
and a specified melt viscosity (shear viscosity) >10 Pa-sec,
typically in the range from 10 pa-sec to 1000 Pa-sec, when the
temperature of the powder being sintered exceeds T, in less time
than is required to melt contiguous large particles >53 .mu.m;
provided further that the pressure drop through a quiescent part
bed 38 cm deep with a gas flow of 3-10 L/min through the bed is
less than 10 kPa. The amount of gas flowed is not narrowly critical
provided it is insufficient to cause channeling in the bed, or
otherwise disrupt the bed, and sufficient to maintain the desired
temperature profile in the bed.
According to another aspect of the invention, it has been found
that the two-tiered particle size distribution is not required for
the creation of a near-fully dense part, with minimal dimensional
distortion, for materials and conditions where the
recrystallization rate is sufficiently low. In this regard, it has
been discovered that the rate of crystallization of the
semi-crystalline organic polymer is a key property in controlling
curl and achieving dimensional control in the sintered part.
Materials that recrystallize relatively slowly after melting
exhibit sufficient dimensional stability and create near-fully
dense, distortion-free parts in the selective laser sintering
process. Specifically, polymers that show little or no overlap
between the melting and recrystalltion peaks when scanned in a DSC
at typical rates of 10-20.degree. C./minute work best in the
selective laser sintering process.
It is therefore another object of this invention to provide a
laser-sinterable polymer powder that resolidifies sufficiently
slowly to eliminate in-build curl and in-plane distortion in parts
produced by the selective laser sintering process.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and additional objects and advantages of the
invention will best be understood by reference to the following
detailed description, accompanied with schematic illustrations of
preferred embodiments of the invention, in which illustrations like
reference numerals refer to like elements, and in which:
FIG. 1 is a schematic illustration of a bed of uniform spheres
packed in a bed.
FIG. 2 is a schematic illustration of a bed of large spheres and
very small ("too-small") spheres, showing that the too-small
particles fit within the interstitial spaces between larger
particles, and produce a bed of higher bulk density and
correspondingly higher pressure drop.
FIG. 3 is a graphical presentation of the number distribution of a
particular tailored powder, namely Nylon 11.
FIG. 4 is a graphical presentation of the volume distribution of
the same powder for which the number distribution is illustrated in
FIG. 3.
FIG. 5 is a schematic illustration of an elevational
cross-sectional view of a cylindrical part bed of a selective laser
sintering machine showing the position of the bed-supporting
cylinder near the top of the cylinder at the beginning of the
sintering procedure, and after the sintered part is formed; along
with indications of the temperature profile within the bed for the
tailored powder of this invention used with a conventional
selective laser sintering procedure (on the left) without
exteriorly controlling the temperature profile; and for the
tailored powder with exterior temperature control of the bad
temperature profile (right hand side).
FIG. 6 shows DSC scans for the heating and cooling curves of a
laser-sinterable PBT powder.
FIGS. 7A and 7B show heating and cooling DSC scans for wax, taken
at 20.degree. C./minute, showing the overlap between the melting
and recrystallization peaks.
FIGS. 8A and 8B show heating and cooling DSC scans for Nylon 11,
taken at 10.degree. C./minute, showing the lack of overlap between
the melting and recrystallization peaks.
FIGS. 9A and 9B show heating and cooling DSC scans for SC 912
powder, taken at 10.degree. C./minute, showing little overlap
between the melting and recrystallization peaks.
FIG. 10 shows heating and cooling DSC scans for SC 912 powder,
taken at 10.degree. C./minute, showing an alternate method of
characterizing the little overlap between the melting and
recrystallization peaks.
FIGS. 11A and 11B show heating and cooling DSC scans for Affinity
SM-1300 powder, taken at 10.degree. C./minute, showing little
overlap between the melting and recrystallization peaks.
FIG. 12 shows heating and cooling DSC scans for Affinity SM-1300
powder, taken at 10.degree. C./minute, showing an alternate method
of characterizing the little overlap between the melting and
recrystallization peaks.
FIG. 13 shows heating and cooling DSC scans for IP60 powder, taken
at 10.degree. C./minute, showing the overlap between the melting
and recrystallization peaks.
FIG. 14 shows heating and cooling DSC scans for Surlyn 8660 powder,
taken at 10.degree. C./minute, showing the overlap between the
melting and recrystallization peaks.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The selective laser sintering machine specifically referred to
herein uses a 10.6 .mu.m CO.sub.2 laser, but any other infra-red
laser-generating source may be used, as well as excimer lasers and
neodymium glass lasers which generate in the near-infra-red. A
preferred laser is a Synrad Model C48-115 CO.sub.2 laser. Powders
are sintered using a 10.6 .mu.m laser in the range from 3 to 30
watts operated at a ratio of power/scan speed in the range from
0.075 watts/cm/sec to 0.3 watts/cm/sec, preferably in the range
from 0.1-0.2 watts/cm/sec using a beam width in the range from
0.23-0.9 mm, preferably from 0.4-0.6 mm. Particularly with Nylons
6, 11 and 12, PBT and PA, the selective laser sintering machine is
preferably operated at a fluence in the range from 1 mJ/mm.sup.2 to
100 mJ/mm.sup.2, more preferably from 15 mJ/mm.sup.2 to 45
mJ/mm.sup.2, where fluence (milliJoules/mm.sup.2) is the measure of
the energy of the laser beam delivered to a defined area of powder.
The laser is typically operated with a beam width of 0.6 mm and a
power ranging from 3-22 watts, preferably from 5-10 watts, at a
scanning speed ranging from about 76.2 cm/sec to 178 cm/sec.
Among the requirements of a preferred semi-crystalline or
substantially crystalline powder uniquely tailored to yield, when
sintered, a porous but near-fully dense article, are the
following:
Free-flowing or Non-caking: The powder is freely flowable and does
not cake when stored for up to 24 hr, at a temperature in the range
from 2.degree. C. to 20.degree. C. below its caking temperature; in
particular, the powder does not cake even when held from 1-8 hr in
the part bed at T.sub.s, at a slightly higher temperature than its
storage temperature in the feed bed which latter is lower than
T.sub.s of the powder. A determination as to whether a powder will
meet the free-flowability requirement is made by running the
time-temperature ASTM flow test referred to hereinabove. Maximum
particle size and sphericity: The powder has sphericity >0.5 and
contains essentially no particles having a nominal diameter
exceeding 180 .mu.m.
Referring to FIG. 3 there is presented in graphical form the number
average particle distribution of Nylon 11. This powder was produced
from Nylon 11 (Lot #R256-A02).
The Nylon 11 was ground in a manner which produces a mixture of
relatively coarse particles having a sphericity greater than 0.5
and a wide distribution of smaller particles. The mixture was then
sieved to eliminate substantially all particles larger than 180
.mu.m, and the remaining particles were classified so as to conform
to the number distribution shown in FIG. 3. The size distribution
of the particles is obtained with a Malvern instrument which
measures the size of the particles with a laser beam.
Flowability in the selective-laser-sintering-window: The melt
viscous flow of polymer powder on the surface of a slice heated by
the laser is determined by maintaining a temperature balance. For
good interdiffusion of the polymer chains (sufficient to provide
particle-to-particle adhesion, and layer-to-layer adhesion), a low
melt viscosity is desirable. However, part definition is lost if
significant melting occurs because the melt cannot be tightly
confined near boundaries of the part being formed. Sintering is
effected without means to assure isolation of the desired part
features.
The selective-laser-sintering-window is of importance at this step
(and step 1) because the temperatures of both beds, the feed bed
and the part bed are elevated. Since the temperature of the part
bed is elevated to the softening point of the powder to minimize
curling, the wider the selective-laser-sintering-window, the
greater the processing latitude provided by the powder. Maintaining
the balance of properties in a tailored powder permits the
requisite particle-to-particle fusion within a layer, and also
layer-to-layer fusion, both of which are necessary to make a porous
but near-fully dense part.
Referring to FIG. 4 there is shown a volume distribution curve of
the same particles for which the number distribution is illustrated
in FIG. 3, to show why the powder is freely flowable and how much
of the volume in a bed of particles is occupied by "large
particles". It appears that the few large particles are mainly
responsible for rolling out the small particles with them, and also
for permitting the essentially unobstructed passage of inert gas
downwardly through the bed.
Growth: Since the finished (sintered) three-dimensional (3-D)
part(s) are formed in the part bed in which the unsintered powder
provides mechanical support for the sintered part, the part is
subject to the thermal changes in the part bed due to the presence
of the sintered part. Sequential, sudden heating of successive
slices of powder in a thermally insulated environment causes the
bed temperature to rise. The insulating environment is due to the
sintered part being surrounded by a mass of porous powder which is
a good insulator. When the temperature around the sintered part is
either not low enough, or too high, the sintered part will distort
due to thermal stresses in the bed. In addition, if the surfaces of
the hot sintered part are too hot, there are agglomerations of
fused particles adhering to and scattered as "growth" over the
surfaces of the finished part, which growth must be removed and
this can usually only be done by machining the growth away. When
some "growth" does occur with the use of a tailored powder, the
growth is so slight that it can be removed without damage to the
surfaces of the part so that the surfaces are smooth to the touch.
If there is substantial growth, the part made is scrapped.
The benefit of large particles in the two-tier distribution,
according to this aspect of the invention, will be understood when
it is realized that too-small particles, if not rollingly deposited
on the part bed, would get packed and obstruct flow of the inert
gas. The effect of being rollingly deposited layer upon layer,
referred to as "layer-wise", onto the surface of the bed results in
a "fluffy" bed which is dynamically stable but quiescent and
relatively porous. The bed densities of a powder when not rollingly
deposited are typically at least 20% higher than that of a bed of
rollingly deposited powder.
A bed of such particles, when packed, are more quickly heated and
over-heated (because of their small mass). The over-heated
particles are then easily fused to the surface of the sintered part
as "growth". The importance of controlling the top-to-bottom
temperature profile within the part bed will be better understood
by reference to FIG. 5.
The preferred crystallinity of a tailored powder which produces a
near-fully dense sintered part with minimal growth is that which is
correlatable to an observed heat of melting by DSC in the range
from 20-120 cals/gm preferably from 30-60 cals/gm.
Referring to FIG. 5 there is shown schematically, in
cross-sectional view, a cylindrical part bed referred to generally
by reference numeral 10, having sidewalls 11 and a bottom 12
through the center of which is slidably inserted a piston rod 13
having a piston 14 with a flat horizontal surface which supports a
bed of thermooxidatively degradable powder 20. A through-passage
having a relatively large diameter in the range from about 2.5 cm
to 3.5 cm has a porous sintered metal disc 15 press-fitted into it
to provide essentially free-flow of an inert gas, preferably
nitrogen or argon, through it. A typical part bed has a diameter of
30.5 cm, and the travel of the piston from the bottom 12 to the top
of the walls 11 is 38.1 cm.
A cylindrical part 30 with tapered ends, the bottom being
truncated, is formed by sintering layer upon layer of preheated
tailored powder, starting with the piston in the position indicated
by its phantom outline at 14', supporting a bed of preheated powder
about 10 cm deep, indicated by the depth d.sub.1. The powder and
walls of the cylinder are heated by infrared heating means to keep
the temperature of the bed about 10.degree. C. below the sticky
temperature of the powder. However, it is difficult to heat the
piston within the cylinder so that the piston is typically at a
slightly lower temperature than the powder. Further, the mass of
the piston provides a heat sink to which the bottom layer of powder
dissipates heat faster than any other layer. The upper surface of
the bed is in the same plane as the top of the cylinder over which
the roller (not shown) of the selective laser sintering machine
distributes powder from the feed bed (also not shown).
As layer upon layer of powder is sintered, forming sequential
horizontal slices of the sintered part 30, the piston 14' moves
downwards until finally the part is completely sintered. The
sintered part 30 is thus supported on the bed of powder on the
bottom, and the depth of this lower portion of the bed is indicated
as being b.sub.1. This bed is the same initially presented as the
target, and its depth b.sub.1 remains numerically equal to the
depth d.sub.1 when the piston 14 has moved down to a depth
indicated by d.sub.2. The sintered part 30 rests on the bed of
powder b.sub.1 thick, the bottom of the sintered part being at a
depth d.sub.3.
Referring now to the result of a conventional selective laser
sintering procedure, there is formed a hot sintered part 30
dissipating heat to the powder 20 surrounding it in unsteady state
heat transfer. The lower portion b.sub.1 forms a relatively cool
zone of powder which dissipates heat to the piston 14, and through
which powder heat from the part 30 is relatively well dissipated by
convection currents through the bed b.sub.1.
As soon as sintering is completed, the upper portion of the bed
having depth d.sub.4, particularly near the surface, begins to
dissipate heat from part 30 lying within upper portion d.sub.4.
Heat dissipated by the part 30 is transferred relatively well
mainly by convection currents through the upper portion d.sub.4 of
the powder bed 20, and less effectively through lower portion
b.sub.1.
The portion of the sintered part 30 lying in the intermediate
portion of the bed 20, that is, the portion between the bed depths
d.sub.1 and b.sub.1, is relatively well insulated by the
surrounding powder. Heat from the part 30 causes the temperature to
rise so that a maximum temperature T.sub.max is reached. The
temperature at the surface of the relatively quickly cooling upper
portion of the bed, is indicated by T.sub.min1 and the temperature
of the relatively quickly cooling lower portion of the bed b.sub.1
is indicated by T.sub.min2. It is thus seen that a temperature
profile is established in the bed, the maximum temperature being
substantially above the lowest temperatures in the profile, and
located in a horizontal zone intermediate the upper and lower
surfaces of the bed.
In the conventional selective laser sintering procedure, using the
novel tailored powder, there is no forced cooling of the heated bed
with gas so that a typical gradient between T.sub.min1 and
T.sub.max, and between T.sub.max and T.sub.min2 is more than
2.degree. C./cm in each case (on either side of T.sub.max). For
example, if T.sub.min1 at the upper surface after sintering is
175.degree. C., T.sub.max is 182.degree. C. and T.sub.min2 is about
171.degree. C. Because T.sub.max is very close to the melting point
183.degree. C. of the powder, the sintered part is exposed to a
high likelihood of being distorted. It will be evident that a large
part of this powder could not be sintered successfully in a
conventional selective laser sintering bed because T.sub.max will
exceed T.sub.c and the part will distort.
In FIG. 5, on the left hand side thereof, the straight lines drawn
connecting the temperatures at the surface and bottom of the bed,
are drawn on the assumption that the gradient is a straight line,
which it most probably is not, but the linear representation serves
as an approximation to focus the fact that the gradient is steeper
for the conventional selective laser sintering procedure than it is
for the novel procedure in which an inert cooling gas is flowed
through the bed while the part is being sintered.
In the procedure with forced cooling, using the novel tailored
powder, the porosity of the bed permits forced cooling of the
heated bed with inert gas, so that a typical gradient between
T.sub.min1 and T.sub.max, and between T.sub.max and T.sub.min2 is
less than 2.degree. C./cm in each case. For example, if T.sub.min1
at the upper surface after sintering is 175.degree. C., T.sub.max
is 177.degree. C. and T.sub.min2 is about 173.degree. C. Because
T.sub.max is not close to the melting point 183.degree. C. of the
powder, the sintered part is not likely to be distorted.
The temperature profile for the process conditions using the inert
cooling gas are shown on the right hand side of FIG. 5, where it is
seen that the gradient to T.sub.max is less, and T.sub.max itself
is lower than it was in the conventional selective laser sintering
process. Thus, the risk of part distortion and growth (on the
surface) is minimized as is the thermal degradation to the powder
surrounding the sintered part. Such thermal degradation occurs when
the powder is overheated, that is, too far past its softening
point, even if it is not heated past its G'.sub.c temperature.
To put the foregoing details in perspective, one may evoke a
physical picture of the selective-laser-sintering-window by
reference to FIG. 6 in which curve A (plotted with squares to track
heat flow) represents the cooling curve for a sample of tailored
PBT powder. The peak occurs at 193.degree. C., but supercooling
commences near the temperature 202.degree. C., a point indicated by
the arrow C (T.sub.s). Curve B (plotted with circles) represents
the heating curve for the same sample. The peak occurs at
224.degree. C., but onset of melting commences near the temperature
212.degree. C., a point indicated by the arrow M (T.sub.c). Thus,
the window W is provided by the difference in the temperatures at M
and C, which for this sample of PBT is 10.degree. C.
The following results were obtained when Nylon 11 having
G'.sub.c=2.times.10.sup.6 dynes/cm.sup.2 at 175.degree. C. was
sintered into test bars with a beam width of 0.6 mm, the laser
power set at 8 watts and a scan speed of 175 cm/sec. The values for
four sets of bars were averaged in Table 1 hereinbelow.
Other preferred semi-crystalline polymers which are successfully
tailored for use in the selective laser sintering machine are
polybutylene terephthalate (PBT); polypropylene (PP); and
polyacetal (PA). The preferred mean primary particle diameter for
each of the tailored powders is in the range from 80 .mu.m-100
.mu.m. The values for these powders are given in the following
Table 2.
TABLE-US-00001 TABLE 2 selective-laser-sintering- Powder T.sub.s
.degree. C. T.sub.c .degree. C. window, .degree. C. Nylon 11 153
170 17 PBT 195 210 15 PA 150 157 7
Each of the foregoing tailored powders was used to make sintered
bars 10 cm long, 2.5 cm wide and 3 cm thick. A statistically
significant number of bars were made from each powder and tested to
compare the sintered bars with bars of identical dimensions but
compression molded. The results with PBT are set forth in the
following Table 3:
TABLE-US-00002 TABLE 3 Comparison of Physical Properties of
Sintered and Compression Molded Test Bars of PBT Sintered Injection
Molded* Density, g/cm.sup.3 1.19 1.31 Flexural Modulus, psi 2.99
.times. 10.sup.5 3.80 .times. 10.sup.5 Max. Stress at yield, psi
8.3 .times. 10.sup.3 14.7 .times. 10.sup.3** Notched Izod, ft-lb/in
0.29 0.70 HDT, .degree. C. 206 163** *supplier's data - no
compression molded data available. **value of max stress yield for
injection molded sample would be higher because of chain
orientation; value of HDT is different because the sample
preparation and thermal history is different from applicants'
sample
The conditions for sintering test bars from several different
semicrystalline materials, each of which having a different window
of sinterability is provided in the following Table 4 hereinbelow.
In each case, the selective laser sintering machine was operated
with a laser having a beam width of 0.6 cm, at its maximum power
(22 watts) and a scan speed of from 127-178 cm/sec (50-70 in/sec),
maximum power being used so as to finish sintering test bars in the
least possible time. In each case the bars were sintered in a part
bed having a diameter of 30 cm which can hold powder to a depth of
37.5 cm. In each case, the powder was maintained in the feed bed at
below T.sub.s and the powder was transferred by a roller to the
part bed, the surface of which was near T.sub.s. In each case, the
bed was heated by an external electric heater to bring it up to
temperature. In each case, note that the density of the sintered
part is about 90% of the density of a molded, fully dense, part.
Even better physical properties are obtained when the parts are
sintered at lower power and slower scan speed (lower fluence).
According to another aspect of the present invention, it has now
been discovered that the rate of crystallization of the
semi-crystalline organic polymer is a key property in controlling
curl and achieving "in-plane" (x-y) dimensional control. In the
selective laser sintering process, the part bed temperature can
usually be maintained just below the onset of melting the
semi-crystalline powder. At the melting point, the material is
transformed from a solid to a viscous liquid over a narrow
temperature range. Only a small quantity of energy (the heat of
fusion) is required to transform the material to a state where
densification can occur. Not all semi-crystalline polymers work
well in the selective laser sintering process, however. Materials
that resolidify or recrystallize quickly after melting tend to
exhibit in-build curl, just like amorphous materials. Wax is an
example of a material that recrystallizes so quickly that it
develops in-build curl. To build flat wax parts in the selective
laser sintering process, support structures which anchor the parts
to the piston bed are required.
Some materials, however, resolidify slowly enough at the part bed
temperature (i.e., the driving force for crystallization is small
enough near the melting point) that the parts remain in the
supercooled liquid state for a significant amount of time during
the part building process. Since liquids do not support stresses,
no in-build curl is observed as long as the part is not cooled
sufficiently to induce more rapid recrystallization. Nylon 11 is an
example of a material that recrystallizes sufficiently slowly in
the selective laser sintering process to eliminate in-build curl.
During the building of Nylon 11 parts in the selective laser
sintering process, the parts remain transparent to depths of
greater than one inch. This transparency indicates that little or
no resolidification or recrystallization of the part has occurred,
since resolidified, semi-crystalline parts are opaque.
The rate of crystallization can also be characterized by DSC. While
actual rates of crystallization are often difficult to quantify
from these experiments, the difference in temperature between the
onset of melting and onset of recrystallization is directly related
to the rate of crystallization--the larger this temperature
difference, the slower the rate of crystallization. As discussed
hereinabove with respect to the "window of sinterability," to
create a DSC trace, a material is heated to above its melting point
at a controlled rate and then cooled back down, also at a
controlled rate. This observed temperature difference between
melting and recrystallizing, however, can also be affected by the
heating and cooling rates used to create the DSC data. Data must
therefore be reported in terms of scanning rate. Specifically,
polymers that show little or no overlap between the melting and
recrystallization peaks when scanned in a DSC at typical rates of
10-20 C./minute work best in the selective laser sintering
process.
FIGS. 7A and 7B show heating and cooling curves, respectively, for
wax, taken at a rate of 20.degree. C./minute. FIG. 7A shows a
heating curve for a sample of wax powder where, as the crystalline
phase melts, an endothermic peak is observed. FIG. 7B shows a
cooling curve for the same sample of wax where, when cooled, an
exothermic peak is observed as the material recrystallizes. Note
that the melting and recrystallization peaks shown in FIGS. 7A and
7B overlap significantly--from about 40.degree. C. to about
60.degree. C. FIGS. 7A and 7B thus indicate that wax recrystallizes
relatively quickly when cooled to a temperature just below its
melting point. This rapid recrystallization causes in-build curl in
the selective laser sintering process, unless special precautions
are taken.
FIGS. 8A and 8B show heating and cooling curves, respectively, for
Nylon 11, taken at a rate of 10.degree. C./minute. FIG. 8A shows a
heating curve for a sample of Nylon 11 powder. FIG. 8B shows a
cooling curve for the same sample of Nylon 11 powder. Note that the
melting and recrystallization peaks shown in FIGS. 8A and 8B do not
overlap at all. FIGS. 8A and 8B indicate that Nylon 11
recrystallizes upon cooling at a temperature significantly lower
than its melting point. Thus, Nylon 11 remains in the liquid state
relatively longer than wax at temperatures below the melting point
of the respective materials. Because liquids do not support
stresses, Nylon 11 therefore does not exhibit in-build curl in the
selective laser sintering process.
FIGS. 9A and 9B show heating and cooling curves, respectively, for
SC 912, a polypropylene copolymer powder sold by Montel. FIG. 9A
shows a heating curve for a sample of SC 912 powder as shown by DSC
measured at a scanning rate of 10.degree. C./minute. FIG. 9B shows
a cooling curve for a sample of SC 912 powder as shown by DSC
measured at a scanning rate of 10.degree. C./minute. The melting
and recrystallization peaks shown in FIG. 9A and 9B exhibit an
overlap of approximately 13.degree. C., from about 120.degree. C.
to about 133.degree. C. Such little degree of overlap indicates
that SC 912 powder recrystallizes sufficiently slowly in a
selective laser sintering process to eliminate in-build curl.
Alternatively, this sufficiently slow recrystallization rate of SC
912 powder can be expressed as a percentage ratio of the area under
the melting peak below the temperature at which SC 912 begins to
recrystallize during cooling to the total area under the melting
peak. FIG. 10 shows a heating curve and a cooling curve for a
sample of SC 912 powder as shown by DSC measured at a scanning rate
of 10.degree. C./minute. One should note that in FIG. 10, the
endothermic melting peak points downward and the exothermic cooling
peak points upward, as the display convention of the DSC apparatus
used to generate FIG. 10 was directly opposite from the convention
of the DSC apparatus used to generate FIGS. 9A and 9B. As shown by
the cooling curve of FIG. 10, the sample of SC 912 begins to
recrystallize at approximately 143.degree. C. The total area under
the melting peak is measured as approximately 74.0 Joules/gram. The
area under the melting peak below this onset of recrystallization
temperature, which is shaded and labeled with the letter A in FIG.
10, is measured as approximately 4.6 Joules/gram. Therefore,
approximately 6.2% of the total area under the melting peak is
below the onset of recrystallization temperature. Such little
degree of overlap between the melting peak and the
recrystallization peak indicates that SC 912 recrystallizes
sufficiently slowly in a selective laser sintering process to
eliminate in-build curl.
FIGS. 11A and 11B show heating and cooling curves, respectively,
for Affinity SM-1300 (hereinafter "SM-1300"), a single site,
branched polyethylene copolymer powder sold by Dow Chemical. It is
believed that SM-1300 is a copolymer of ethylene and octene. FIG.
11A shows a heating curve for a sample of SM-1300 powder as shown
by DSC measured at a scanning rate of 10.degree. C./minute. The
portion of the heating curve from about 45.degree. C. to about
55.degree. C. represents the glass transition temperature for the
powder. FIG. 11B shows a cooling curve for a sample of SM-1300
powder as shown by DSC measured at a scanning rate of 10.degree.
C./minute. The melting and recrystallization peaks shown in FIGS.
11A and 11B exhibit an overlap of approximately 11.degree. C., from
about 75.degree. C. to about 86.degree. C. Such little degree of
overlap indicates that SM-1300 recrystallizes sufficiently slowly
in a selective laser sintering process to eliminate in-build
curl.
Alternatively, this sufficiently slow recrystallization rate of
SM-1300 powder can be expressed as a percentage ratio of the area
under the melting peak below the temperature at which SM-1300
begins to recrystallize during cooling to the total area under the
melting peak. FIG. 12 shows a heating curve and a cooling curve for
a sample of SM-1300 powder as shown by DSC measured at a scanning
rate of 10.degree. C./minute. One should note that in FIG. 12, the
endothermic melting peak points downward and the exothermic cooling
peak points upward, as the display convention of the DSC apparatus
used to generate FIG. 12 was directly opposite from the convention
of the DSC apparatus used to generate FIGS. 11A and 11B. As shown
by the cooling curve of FIG. 12, the sample of SM-1300 begins to
recrystallize at approximately 99.5.degree. C. The total area under
the melting peak is measured as approximately 38.7 Joules/gram. The
area under the melting peak below this onset of recrystallization
temperature, which is shaded and labeled with the letter B in FIG.
12, is measured as approximately 1.06 Joules/gram. Therefore,
approximately 2.7% of the total area under the melting peak is
below the onset of recrystallization temperature. Such little
degree of overlap between the melting peak and the
recrystallization peak indicates that SM-1300 powder recrystallizes
sufficiently slowly in a selective laser sintering process to
eliminate in-build curl.
The preferred powders of Nylon 11, SC 912, and Affinity SM-1300 are
exemplary and not limiting of this aspect of the present invention.
In general, it is believed that certain nylons (other than Nylon
11), polyacetals, polypropylenes, polyethylenes, and ionomers
exhibit similar melting and recrystallization behavior in DSC scans
and in the selective laser sintering process, and are therefore
also preferred materials according to this aspect of the invention.
Other materials believed to exhibit similar melting and
recrystallization behavior in DSC scans and in the selective laser
sintering process, and which are therefore also preferred materials
according to this aspect of the invention, include certain
copolymers of nylons, acetals, ethylenes, and propylenes (other
than SC 912); certain branched versions of polyethylene and
polypropylene; and certain branched versions of polyethylene
copolymers (other than SM-1300) and polypropylene copolymers.
Copolymerization and branching are modifications to the molecular
structure of polymers that can be used to control the degree of
crystallinity as well as the rate of recrystallization. All of the
above-described materials preferably exhibit an overlap between
their melting and recrystallization peaks, as shown in DSC traces
measured at a scanning rate of 10-20.degree. C./minute, ranging
from 0.degree. C. (no overlap) to no more than about 13.degree. C.
More preferably, all of the above-described materials exhibit such
an overlap ranging from 0.degree. C. (no overlap) to no more than
about 11.degree. C. In addition, all of the above-described
materials preferably exhibit a percentage ratio of the area under
the melting peak below the onset of recrystallization temperature
to the total area under the melting peak, as shown in DSC traces
measured at a scanning rate of 10-20.degree. C./minute, ranging
from about 0% (no overlap) to no more than about 6.2%. More
preferably, the above-described materials exhibit such a percentage
ratio ranging from 0% (no overlap) to no more than about 2.7%.
It has been found that IP60, a particular high density polyethylene
powder sold by Dow Chemical, does not exhibit the above-described
melting and recrystallization behavior in DSC scans, and therefore
IP60 powder does not work well in the selective laser sintering
process. More specifically, FIG. 13 shows a heating curve and a
cooling curve for a sample of IP60 powder as shown by DSC measured
at a scanning rate of 10.degree. C./minute. As shown in FIG. 13,
the IP60 powder exhibits an overlap between its melting and
recrystallization peaks of approximately 24.degree. C., from about
97.degree. C. to about 121.degree. C. In addition, the cooling
curve of FIG. 13 shows that IP60 powder begins to recrystallize at
approximately 121.degree. C. The total area under the melting peak
is measured as approximately 123.1 Joules/gram. The area under the
melting peak below this onset of recrystallization temperature,
which is shaded and labeled with the letter C in FIG. 13, is
measured as approximately 26.2 Joules/gram. Therefore,
approximately 21.3% of the total area under the melting peak of
IP60 powder is below the onset of recrystallization
temperature.
Similarly, it has also been found that Surlyn 8660, a particular
ionomer powder sold by Dupont, does not exhibit the above-described
melting and recrystallization behavior in DSC scans, and therefore
Surlyn 8660 powder does not work well in the selective laser
sintering process. More specifically, FIG. 14 shows a heating curve
and a cooling curve for a sample of Surlyn 8660 powder as shown by
DSC measured at a scanning rate of 10.degree. C./minute. As shown
in FIG. 14, the Surlyn 8660 powder exhibits an overlap between its
melting and recrystallization peaks of approximately 40.degree. C.,
from about 45.degree. C. to about 85.degree. C. In addition, the
cooling curve of FIG. 14 shows that Surlyn 8660 powder begins to
recrystallize at approximately 85.degree. C. The total area under
the melting peak is measured as approximately 28.8 Joules/gram. The
area under the melting peak below this onset of recrystallization
temperature, which is shaded and labeled with the letter D in FIG.
14, is measured as approximately 10.4 Joules/gram. Therefore,
approximately 36.1% of the total area under the melting peak of
Surlyn 8660 powder is below the onset of recrystallization
temperature.
Accordingly, polymers that show little or no overlap between their
melting and recrystallization peaks when scanned at typical DSC
rates of 10-20.degree. C./minute work best in a selective laser
sintering process. Such overlap can be expressed in terms of
.degree. C. or in terms of a percentage ratio of the area under the
melting peak below the onset of recrystallization temperature to
the total area under the melting peak. For example, wax, IP60
powder, and Surlyn 8660 powder are not suitable materials by this
test, while Nylon 11 powder, SC 912 powder, and SM-1300 powder are.
Most suitable materials preferably have melting points below
200.degree. C. As noted above, suitable materials according to this
aspect of the invention include Nylon 11 powder; SC 912 powder;
SM-1300 powder; certain nylons (other than Nylon 11), polyacetals,
polypropylenes, polyethylenes, and ionomers, certain copolymers of
nylons, acetals, ethylenes, and propylenes (other than SC 912);
certain branched versions of polyethylene and polypropylene; and
certain branched versions of polyethylene copolymers (other than
SM-1300) and polypropylene copolymers.
Having thus provided a general discussion, described the
requirements of a laser-sinterable powder in detail, and
illustrated the invention with specific examples of the best mode
of making and using the powder, it will be evident that the
invention has provided an effective solution to a difficult
problem. It is therefore to be understood that the claims are not
to be limited to a slavish duplication of the invention and no
undue restrictions are to be imposed by reason of the specific
embodiments illustrated and discussed.
TABLE-US-00003 TABLE 1 Value Compression Property Laser Sintered
Molded Thermal Glass Transition (.degree. C.) Melt (onset, .degree.
C.) Heat Distortion @ 264 psi (.degree. C.) 46, 46 41, 41 @ 66 psi
(.degree. C.) 163, 167 163, 159 TGA (onset of degradation) not
measured Mechanical Tensile (5 mm/min crosshead) Modulus (psi )
[.sigma.] 201,100 [10,540] 207,700 [11,630] Elongation, ultimate
(%) 28.0 [5.3] 201.6 [151] Strength (psi) 6323 [157] 6315 [115]
Elongation, yield (%) 26.0 [3.3] 30.0 [1.3] Energy to break (lb-in)
205 [53] 2,149 [316] Tensile (50 mm/min crosshead) Modulus (psi)
221,500 [28,610] 227,800 [18,890] Elongation, ultimate (%) 27.0
[5.5] 271.8 [1463] Strength (psi) 6413 [130] 6200 [517] Elongation,
yield (%) 24.1 [32] 21.9 [9.3] Energy to break (lb-in) 203 [43]
1,995 [566] Flexural Modulus (psi) 146,800 [4147] 176,900 [4368]
Strength (psi) 7154 [159] 7044 [271] Elongation, yield (%) .091
[.002] .065 [.002] Izod Impact (notched) @ 23.degree. C. (ft-lb/in)
1.4 [2] 1.89 [24] @ -40.degree. C. (fl-lb/in) 1.03 [.2] Physical
Specific Gravity 1.0204 [.004] 1.0360 [.0004]
TABLE-US-00004 TABLE 4 Feed Bed Part Bed Part Dens. Full Dens.
Notch HDT MAX STRESS FLEX MOD Ex Polymer Temp. .degree. C. Temp.
.degree. C. gm/cm.sup.3 gm/cm.sup.3 Impact .degree. C. psi psi 1
Nylon 6 140 180 0.958 1.04 1.5 175 11510 272100 2 Nylon 11 135 165
0.919 0.987 1.67 166 8310 159900 3 Nylon 12 75 160 0.90 1.01 0.39
163 8120 150750 4 P' Acetal 130 150 1.283 1.41 0.79 149 9468 213400
5 PBT 160 195 1.19 1.31 0.29 206 8270 299700 *(ft-lb/in): Izod
impact, notched-measured at 23.degree. C.
* * * * *