U.S. patent application number 09/875472 was filed with the patent office on 2002-12-12 for method for the rapid fabrication of mold inserts.
Invention is credited to Amaya, Herman Ernesto, Crounse, Dennis Kent.
Application Number | 20020187065 09/875472 |
Document ID | / |
Family ID | 25365871 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020187065 |
Kind Code |
A1 |
Amaya, Herman Ernesto ; et
al. |
December 12, 2002 |
Method for the rapid fabrication of mold inserts
Abstract
A method for the rapid fabrication of mold inserts for molds and
molds c is disclosed; wherein high machinability rates and time and
cost savings along with increased tool life and material savings
are obtained through the use of blank die inserts formulated from
material commonly used in the metal injection molding process of
complex shaped parts. The method involves first the creation of
cutting path programs developed from CAD files of the part; direct
machining of the cavity and core inserts as described to
predetermined sizes; processing the cavity and core inserts to
convert the soft material--initially consisting of fine metal
powders in a matrix of binder compounds--into a dense, fully
hardenable material comparable to the material used in conventional
toolmaking; and performing any necessary finishing and fitting
operations to fit the resulting dies into a base that can be used
as part of an injection molding tool.
Inventors: |
Amaya, Herman Ernesto;
(Vernon Hills, IL) ; Crounse, Dennis Kent;
(Harvard, IL) |
Correspondence
Address: |
LEON I. EDELSON, ESQ.
LEVENFELD PEARLSTEIN
P.O. BOX 0212
CHICAGO
IL
60690-0212
US
|
Family ID: |
25365871 |
Appl. No.: |
09/875472 |
Filed: |
June 6, 2001 |
Current U.S.
Class: |
419/8 |
Current CPC
Class: |
B22F 2998/00 20130101;
Y02P 10/25 20151101; B22F 2998/10 20130101; B33Y 80/00 20141201;
B29C 45/2673 20130101; B22F 2999/00 20130101; B29C 33/306 20130101;
B22F 2003/247 20130101; B22F 7/06 20130101; B22F 3/225 20130101;
C04B 2235/6026 20130101; B22F 5/007 20130101; B22F 5/10 20130101;
B22F 2998/00 20130101; B22F 3/225 20130101; B22F 2998/10 20130101;
B22F 3/22 20130101; B22F 2202/11 20130101; B22F 3/10 20130101; B22F
2999/00 20130101; B22F 10/10 20210101; B22F 2202/11 20130101; B22F
2999/00 20130101; B22F 10/10 20210101; B22F 2202/11 20130101 |
Class at
Publication: |
419/8 |
International
Class: |
B22F 007/06 |
Claims
We claim:
1. A method of fabricating mold inserts comprising the steps of:
a-) creating a blank die block from a metal alloy material,
consisting of fine metal powders bound together by any combination
of appropriate binders that form a material compound commonly used
in powder injection molding processes. b-) applying a cutting path
program developed from the CAD files or similar of the part
application, and directly machining said die block by milling or
related means corresponding cavity and core die inserts. c-)
processing said cavity and core inserts respectively to consolidate
the fine metal powders including a means to remove said binder of
said metal alloy material and forming a sintered metal core and/or
cavity insert. d-) finishing said sintered metal insert to fit into
a mold base or master mold for molding of metal, ceramics, plastic,
die casting and related processes.
2. The method of claim 1, wherein the material used to produce the
die blocks that result in cavity and core inserts are from the
group of tool steels, carbon steels, stainless steels and other
ferrous powders and alloys that can be processed to near full
density and heat treated.
3. The method of claim 1, wherein the material used to produce the
die blocks that result in cavity and core inserts are form the
group of any non-ferrous powder such as copper and bronze, that can
be processed to near full density.
4. A method for the fabrication of mold inserts comprising the
following steps: a-) creating a CAD or computer model of the
tooling die inserts from a part configuration. b-) scaling the said
model to allow for shrinkage resulting from subsequent processing
plus expected shrinkage for the tooling application. c-) developing
cutting path programs for CNC, milling or other related machining
processes for the corresponding cavity and core die inserts. d-)
fabricating blank die blocks from a metal alloy material consisting
of fine metal powders bound together by any combination of
appropriate binders that form a material compound commonly used in
powder injection molding processes. e-) applying a cutting path
program developed from the CAD files or similar of the part
application, and directly machining said die block by milling or
related means said cavity and core die inserts. f-) processing said
cavity and core inserts respectively to consolidate the fine metal
powders including a means to remove said binder of said metal alloy
material and forming a sintered metal core and/or cavity insert.
g-) finishing said sintered metal inserts by grinding, polishing,
assembling of mold components and related activities to fit into a
mold base or master mold for molding of metal, ceramics, plastic,
die casting and/or related processes.
5. the method of claim 2, wherein the machining step (e) includes
the addition of ejector hole locations, coordinate reference
points, water cooling channels and other features normally required
in an injection molding or die casting tool.
6. the method of claim 2, wherein the cooling channels are included
in a component refered to as a yoke.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Not Applicable
BACKGROUND--FIELD OF THE INVENTION
[0002] This invention relates to the field of rapid tooling
manufacturing, particularly for mold inserts that can be
incorporated into injection molds, die cast molds or tools for
related forming methods, and to the rapid process for manufacturing
same.
BACKGROUND OF THE INVENTION--PRIOR ART
[0003] Metal molds for forming processes such as injection molding,
blow molding, die casting, sheet metal forming and die casting are
made from conventional machining techniques, EDM, casting and
electroforming.
[0004] The standard method for the fabrication of a molding tool
begins with the splitting of a three dimensional CAD representation
into the two above mentioned cavity and core halves, and proceeding
to create a positive or male version of the parts. The positive
version of the part is normally machined or ground as a set of
carbon electrodes for each mold half, and these set of carbon
electrodes are then used to burn a negative or female
representation of the parts into a block of steel, one for the
cavity half of the tool, the other for the core.
[0005] These two tooling halves can then be mounted on on a
standard injection molding machine to mold the part the actual
parts from plastic, metal, ceramic or composite material
formulations. Hard tooling for injection molding such as described
above, is also used to produce patterns for the investment casting
process as well as several powder metallurgy processes.
[0006] The present state of the art in moldmaking demands skilled
labor and the use of fully automated equipment which can cost
upwards of $100,000 per unit. Tool shops generally have a multitude
of cutting, milling and grinding equipment to deal with the
different tool materials that are cut into dies and molds. It is
because of that that the moldmaking industry is both a capital and
labor intensive process, that has been experiencing pressure from
intense international competition.
[0007] What this means in terms of technology is a greater emphasis
on the development of computer driven applications and less
emphasis on the artisan skills demanded of toolmakers in the
conventional tooling industry. It is important to consider that of
the $3.2 billion in sales that the moldmaking industry reported for
1999 in the US, fully a third of that or $1 billion was spent in
the skilled labor cost area. International competition addresses
this equation by radically lowering the cost of labor while using
conventional tooling methods and equipment.
[0008] The largest time factor in mold construction is the time
that must be taken to cut, mill, grind or EDM steel. Molds must be
durable enough to last for the production of hundreds of thousands
of parts with minimal maintenance, and for that performance the
types of steel that must be used are the tool steel alloy grades
that have great wear and impact resistance. The problem is that
because of their desirable mechanical properties, these tool steels
are often the hardest to cut, mill or grind and hence take more
time to process.
[0009] Softer materials such as aluminum or prehardened steels are
easier to cut, and this ease translates into a time factor
reduction and hence a lower cost. Aluminum for example, cuts 50%
faster than an S-7 or D-2 tool steel. Clearly aluminum tools do not
last as long as the corresponding tool steels.
[0010] The reduction in process time for the cutting or milling
operations is then a recognized bottleneck for cost improvements in
moldmaking. In a high volume moldmaking operation, this means that
a great many machines performing these operation must be used, and
this then translates into a higher overhead structure which can
buckle a company if the high volume demand disappears. While there
have been advances made in developing high speed milling machines
to directly cut into steel and to form the carbon electrodes, the
high cost of the equipment and the concomitant increase in the
overhead cost remains. In addition to that is the increased cost of
the tooling itself, that must still withstand the abrasiveness and
wear of the tool steel that it is cutting.
[0011] One advantage of the instant invention is that it takes
advantage of the heretofore unexploited conversion characteristic
of the material commonly used for the metal injection molding, from
soft, pliable and hence machinable, to dense and fully hardenable
upon full processing. As a reference point, while aluminum cuts
perhaps 50% faster than tool steels, MIM block material cuts 50%
faster than aluminum, and all without the use of cutting fluids
which in standard operations are required to cool the steel as it
is being cut. Softer more machinable material translates to
increased rates as well as increased tooling life.
[0012] One way to analyze the conventional moldmaking process is to
break it down into cost factors, where 15% of the cost and time is
invested in the design phase of the tool, 15% of the is invested in
the cost of materials, and the remaining 70% of the cost is
invested in the machining and labor. Increasing machinability of a
material reduces cost in three ways; firstly by reducing the time
required to complete the operation, secondly by reducing the
overhead cost of the operation by using less capital intensive
equipment and thirdly by reducing the tool replacement cost for
milling cutters and related items.
[0013] Due to the recognized limiting time and cost factors in
conventional moldmaking technology as described, several rapid tool
manufacturing technologies have been developed. There are three
generally recognized processes used. The first uses some of the
established rapid prototyping technologies to directly develop
molds. The second copies a rapid prototype form into metal for
instance by investment casting. The third directly manufactures
hard metallic molds directly with adapted prototyping systems.
[0014] An example of the first type of rapid tooling system is U.S.
Pat. No. 5,458,825 which describes the use of stereolithography to
directly produce blow molding tooling for rapid container
prototyping. This method of direct tooling, so called because a
pattern is not required in the building of a mold, can produce
tools of high accuracy but limited durability, so the volume runs
are short. One of the issues is that the choice of materials for
the stereolithographic process, referred to as
SLA(stereolithography apparatus), is limited, and these materials
have to be able to withstand higher molding temperatures to
accommodate a wider range of plastics for sampling. U.S. Pat. No.
5,641,448 takes the "soft" tooling produced by the any of the solid
modeling technologies such as SLA, and selectively deposits layers
of nickel around the inner mold surfaces. The nickel coated mold is
then fitted into a base for the injection molding operation. This
process does harden the tool to increase the tooling life,
nonetheless, molding parameters must be controlled towards the
lower end of the molding pressures to maintain the nickel deposited
layers intact.
[0015] An example of the second process is described in U.S. Pat.
No. 4,220,190, where the investment cast shell serves as a means to
form the functional cavity surfaces when the metal alloy is cast.
In a variation on this, SLA patterns are being used for the direct
casting of the injection tooling molds. The main issue with this
fabrication method has been the inherent surface quality of the
casting and the amount of work required to bring the cast mold or
die halves to specifications for use in injection molding
tools.
[0016] The third type of process has several variations beginning
with the use thermal spraying, 3D printing technologies, laser
sintering of powder metals, hot isostatic processing and other
variations of the use of powder metallurgy technologies including
the use of metal or powder injection molding.
[0017] Methods of thermal spraying of metal have been developed to
directly produce prototype parts and more recently to form "hard"
molds, die and tools as described in U.S. Pat. No. 5,609,922. The
patterns in this case are support members constructed not only to
form the desired shape of a cavity or a core, but also to promote
optimum heat exchange properties for the thermal spray deposition
process. in one recent variation disclosed by U.S. Pat. No.
6,074,194, the liquid material consisting not only of molten
metallic alloys but also polymeric compounds are atomized into fine
droplets by a high temperature, high velocity gas and deposited
onto a pattern The tools have the same issues of fragility as the
other thermal spray method that uses patterns from
stereolithography to serve as a base for the thermal spray
deposition process.
[0018] The use of powder metallurgy takes advantage of the fact
that powders can conform to the shape of any given pattern when
they are flowed in. Variations in the application of the process
can be identified by the way the powders are consolidated so they
can maintain the desired shape. For the purposes of forming complex
metal molds, the advantages of powder metallurgy lie not only in
the forming of complex shapes facilitated by the flow of powders,
but also by the fact that a great deal of material waste can be
avoided by processing net shape or near net shape molds when
compared to the other metal working processes.
[0019] A means of forming the die cavities through the use of
conventional powder metallurgy is described in U.S. Pat. No.
4,327,156. The practice of this invention involves flowing in
refractory powders around a flexible rubber mold that has been
previously conformed from a replicating master. To keep the powders
in place, a binder is mixed with the powders and molded or
compressed into shape, followed by a curing period to allow the
binder to harden and hold shape. The next step is remove the cavity
or core mold and to burn off the binder once it has accomplished
its purpose in an oven, thereby leaving a porous metal skeleton
that can be closed off by infiltrating a low melting point metal
such as copper. This method does provide "hard" tooling that will
last longer than the "soft" tooling of the other rapid prototyping
technologies and introduces the use of powder metallurgy as a means
to form the "hard" tooling.
[0020] A variation of this process as described in U.S. Pat. No.
5,507,336, casts a ceramic compound over a pattern to form the
cavity or core half. The procedure is to take the cavity impression
on the ceramic casting and place in a tubular container so that
loose metal powder can be poured into the container. Instead of
binding the powders together with a binder as in U.S. Pat. No.
5,507,336, the whole tubular container is placed in an oven and a
low melting metal such as copper is melted over the powder to bind
the whole shape. The next step is to remove the original ceramic
pattern to leave exposed the desired cavity or core mold half,
which can then be assembled into a complete tool for injection
molding.
[0021] Improvements have been commercially incorporated into this
methodology by coating the fine metal powders by a proprietary
polymer and selectively laser sintering the coated powders around a
given pattern. In this case the laser serves to fuse the polymer
and holds the shape of the part, thereby eliminating the need for
any tubular shaped container to hold the powders together. This
"green" part is subsequently impregnated with a low-melt binder
system and heated in an oven before sintering at higher
temperatures to provide a metal skeleton, that in the final steps
is infiltrated with copper. This process is know as "RapidTool-Long
Run(LR)" and is practiced by DTM Corporation in Austin, Tex. Some
of the patents covering this process are U.S. Pat. Nos. 5,648,450,
5,733,497, 5,749,041.
[0022] A variation of the use of laser sintering uses a 200 W laser
known as Direct Laser Sintering, to act directly on metal powder.
This metal powder consists of a mixture of bronze and nickel and
some additives and as a result has the unique property that it
shows very limited shrinkage during sintering. Some of the patents
by EOS covering this method are U.S. Pat. Nos. 5,876,767 and
5,908,569.
[0023] The three dimensional printing process developed by MIT
works much like an ink-jet printing by spreading a thin layer of
powder over a platform. Directed by a computer file, the
electrostatic ink jets are selectively sprayed with a colloidal
acrylic binder onto stainless-steel powder to create the green
part. Debinding, sintering and infiltration follow the printing
process to make the part more robust. Like the Rapidtool process,
the problem lies in the unpredictability of the shrinkage and
infiltration process, resulting in poor surface finishes and
propensity for warpage.
[0024] The above mentioned approaches have addressed the issues of
tool longevity by using powder metals to form "hard" metal dies.
Though the resulting molds are more permanent in nature, there are
two main issues which prevent these tools from being considered
permanent hardened tools. The first is that the tools are difficult
to polish due to the coarse nature of the base powders. This means
that the surface finish on parts produced from these tools may not
be adequate. The second issue is that the high copper
content--necessary to close the porosity in the initial metal
skeletons--reduces the attainable hardness of the composite to
about Rockwell B75, which is softer that similar tools machined
from aluminum. Tool life and wear resistance remains a major issue
when compared to tools manufactured from conventional methods that
can be hardened above Rockwell C60.
[0025] A recent application of powder metallurgy as a method for
producing dies is described in U.S. Pat. No. 5,435,824. It applies
hot isostatic compacting to develop a fully dense mold and die
block that does not need to be copper infiltrated to achieve full
density. Hot isostatic compacting consists of using a rubber
container which has the general shape desired, to hold the powders
together while they are compacted into shape by high pressures. The
process includes removing the rubber container once the mold can
hold its shape, and then heating or sintering the "green" article
in a furnace to consolidate the metal powders. Several alloys can
be processed from this method that can attain harnesses equivalent
to those of the wrought materials commonly applied in the
toolmaking process.
[0026] Another related application disclosed in U.S. Pat. No.
5,937,265 also uses the combination of cold and hot isostatic
pressing, with the difference of using master parts produced by
stereolithography, followed by the creation of a flexible mold from
these master parts, which are then filled with metal powders that
are first cold isostatically pressed and then hot isostatically
pressed. The main issue with both of these processes for the
construction of molds and mold components, is that the methods are
inherently limited in the complexity of components that it can
reproduce as well as issues having to do with dimensional accuracy,
since compaction and forming of the molds and dies occurs in a
directional basis.
[0027] A variation of the use of powder metallurgy as a forming
method for tool inserts is described in U.S. Pat. Nos. 5,976,457
and 6,056,915 respectively. Both patents disclose a method that
takes advantage of the forming capabilities of the metal or powder
injection molding process. In each case material is molded around
master cores, machined from aluminum or other materials to produce
a version of the cavity and core that can be later processed using
standard powder injection molding parameters, to produce a final
sintered or fused steel part with all the properties and
performance of a wrought or standard tool material. One potential
drawback of these methods, is that the dimensional accuracy depends
on the compounded tolerances of producing master cores and cavities
through one method, on top of the inherent process variation of
sintering and shrinking the parts to attain the final part sizes
and properties. The instant invention also uses less process steps
to accomplish the end result of obtaining a mold insert.
[0028] Each of the above mentioned inventions has improved the
development process by reducing the elements of time and cost, yet
each has issues that detract from its adoption as production and
extended run tools.
[0029] Most of the rapid tooling methods use variations of the
powder metallurgy process, however many of these have issues
relating to surface finish that may detract from form and function
evaluations on certain parts applications. In addition to this
dimensional tolerances of the resulting tools may vary because in
some methods the copper infiltration process causes some expansion
of the mold or the method of compaction provides a directional
bias, as in hot isostatic compacting, or in others, the final
shrinkage after sintering is not reproducible.
[0030] Continued improvement of the rapid tooling methods has to
rely on reduction of processing times, increased compliance with
cosmetic and surface finish requirements, as well as developing
dies that have comparable dimensional reproducibility and
hardenability as the materials used in conventional moldmaking. The
instant invention reduces processing times, meets cosmetic
requirements and can be finished to match the dimensional
requirements of the conventional tools.
BRIEF SUMMARY OF THE INVENTION--OBJECTS AND ADVANTAGES
[0031] The instant invention provides a method for the rapid
fabrication of mold tooling inserts that can be incorporated in a
mold base for use in forming processes such as plastic injection
molding, metal injection molding, ceramic injection molding, metal
die casting and other related forming processes, wherein high
machinability rates and time and cost savings along with increased
tool life and material savings are obtained through the use of
blank die inserts forumlated from material commmoly used in the
methal injection molding process of complex shaped parts.
[0032] Accordingly several objects and advantages of the instant
invention are:
[0033] 1-) To provide a method for the rapid fabrication of metal
die inserts used in injection molding or die casting tools with a
minimum of production steps to meet and exceed the time & cost
requirements of rapid prototype tooling.
[0034] 2-) To take advantage of the soft nature of mold insert
blocks molded or cast from material commonly used for powder
injection molding, in order to obtain increased machining rates,
and to then exploit the ability of the material to be converted
into a heat treatable tool steel metal inserts that last longer
than the present state of the art "soft" tooling, thereby allowing
greater flexibility in time and cost for the production of hard
tooling.
[0035] 3-) To increase tooling life in the machining process as a
result of using a soft machinable material not requiring the need
of coolants during the machining. The material chosen for machining
has a self-lubricating nature.
[0036] 4-) To reduce material waste by recycling the raw material
being machined.
[0037] 5-) To produce single or multiple tool steel or related
alloy metal inserts that can meet the dimensional and surface
finish requirements of permanent tooling--which the present state
of the art rapid fabrication "hard" tooling cannot.
[0038] 6-) To add additional flexibility in the rapid prototyping
tool manufacture by choosing related metal alloys such as
hardenable stainless, carbon steel or other ferrous or non-ferrous
powder materials that can be premixed with the appropriate binders
to provide cost & time savings advantages.
[0039] 7-) To add features to the die that can facilitate
fabrication and assembly. These could include water channels and
coordinate referencing features, ejector locations etc., which
would normally have to be machined into a conventional tool.
[0040] These together with other objects and advantages of the
invention will become more readily apparent to those skilled in the
art when the following general statements and descriptions are read
in the light of the appended drawings and claims.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0041] The invention will now be described in connection with the
accompanying drawings, FIGS. 1-6, which include a flowchart and
preferred embodiments of the invention.
[0042] FIG. 1 is a process flow chart for the fabrication of the
hardened steel inserts, from the machining of the soft die blocks
to the sintering and assembling of the die inserts.
[0043] FIG. 2A to 2B show a representative 3-D CAD drawing of a
part with a view of the core or interior features of the part, and
the cavity or external features of the part.
[0044] FIG. 3A & FIG. 3B shows the machining or milling of the
core pattern into a die block made from soft machinable MIM
material, in the initial and final stages respectively.
[0045] FIG. 4A to 4B shows the machining or milling of the cavity
pattern into a die block made from soft machinable MIM material, in
the initial and final stages respectively.
[0046] FIG. 5A to 5B show a representative mold base where the
resulting cavity and core inserts will be assembled.
[0047] FIG. 6A to 6B show a fully assembled mold half and a view of
the mold assembly showing the cooling water lines.
DETAILED DESCRIPTION OF THE INVENTION
[0048] The instant invention describes a novel method shown in FIG.
1, with identified steps 10-14 for the rapid fabrication of
hardened tool steel or comparable alloy molding dies and mold
components. These tools would have the added advantage of being
incorporated as production tools after any key design and
functional advantages were incorporated into any particular parts,
hence it also addresses the time and cost factors involved in
manufacturing tooling through the conventional means that have been
described.
[0049] The method of forming the mold die inserts as shown in FIG.
1, involves creating cutting path programs from CAD files; applying
these programs to a blank die block that has been formulated from
material commonly used for in the powder injection molding process;
processing the machined blocks--in this case consisting of the
cavity and core halves--first through a debinding process to remove
the binder constituents of the material, and then though a high
temperature sintering process to convert the powders into a dense
fully hardenable tool steel or similar alloy, followed in the last
step by performing secondary operations on the mold inserts to fit
them into a master mold base as part of a complete molding
tool.
[0050] The first step referenced as step 10 in the flowchart of
FIG. 1, is the starting point for all of the rapid tool
manufacturing processes as well as for the conventional toolmaking
technologies. One difference however, with the rapid tool
manufacturing processes is that cutting path programs are developed
for direct machining of the cavity and core halves, instead of
going through an intermediate step of developing master cores and
cavities in order to form the actual core and cavity inserts. The
process is similar to conventional toolmaking in that cutting path
programs are often applied directly to blank tool steel mold
inserts, but differs in the important respect that the material
that the machining is being performed on is in a soft state when it
is being machined, but can be subsequently converted to a fully
dense and hardenable tool steel or similar alloy. In this way, the
process realizes the immediate advantage of time and cost savings
by increasing machinability, while achieving in the end the
manufacturing of tool steel mold inserts that are indistinguishable
from the tool materials used in the conventional tooling
process.
[0051] The blank mold insert blocks upon which the cutting path
programs are applied are formed as referenced in steps 11 & 12
of the flowchart in FIG. 1, by either casting onto a die block mold
or by directly molding in a molding machine. The material used for
casting or molding the insert blocks is formulated typically from a
material combination of extremely fine metal powders in the size
range of 2-30 microns and binder compounds known to the art of
powder injection molding. In their standard application, the
material and process of powder injection molding is used for the
manufacturing of small and intermediate sized components to near
net shape parts. The soft nature of the molded material and the
machinability advantages is a side benefit that has heretofore not
been applied towards the production of mold inserts. As a reference
point, while aluminum cuts from 30-50% faster than the tool steels
used in conventional moldmaking, the powder injection molding
material cuts in turn from 30-50% faster than aluminum without the
need of using cutting fluids, which in standard operations are
required to keep the material cool as it is being cut. The reason
for this, is that the lubricants of the powder injection molding
material, such as wax, are reducing the heat effects of friction.
Cutting or machining rates of the flowchart shown in FIG. 1, is
therefore very efficient.
[0052] Because the mold inserts undergo a conversion process, the
cutting path programs referenced in step 10 of the flowchart shown
in FIG. 1, have to compensate for the shrinkage experienced during
the sintering/consolidation steps. This means that the cutting path
programs are made to a specific oversize shrink factor that depends
on the type of alloy powder being processed, the amount of binder
that is required to be mixed with the powders to obtain a moldable
material, and the degree of densification the parts experience
during sintering. While in the normal practice of powder injection
molding, molds are cut to an oversize factor ranging from 15-20%,
application of the instant invention will require programming to
compensate both for the expected shrink of the material being
processed as well as the material that will be molded into the
final core and cavity halves. For example, if the part is being
tooled for use in plastics, then the 2-6% shrink rate normally
experienced upon molding will have to be added onto the standard
15-20% shrinkages experienced by the PIM blocks when they are
sintered or fused to steels. In another application, if the mold
inserts are being developed for use to manufacture a powder
injection molded part, then the oversize factor would be 15-20% for
the material plus an additional 15-20% for the part, for a total
oversize range depending on the material of 30-40%.
[0053] One example of powder injection formulation used in the
instant invention, would be to use M-4 high speed tool steel
powders. These are spherical gas atomized powders produced and
sieved by Anval/Carpenter Corporation to a particle size less than
30 microns, and mixed with a polymer/wax binder so that the
premixed material has a 94% by weight powder loading with the
remaining 6% by weight being binder. Other metal powders such as
carbon steel, stainless steel, copper, or bronze, will have
different binder requirements depending on their size and shape
characteristics, as well as the type of binder chosen by those
versed in the art. The shrink rate for the M-4 material with the
powder characteristics and size as defined above, would be
1.15.
[0054] Conventional tool making practices incorporated the
intermediate use of carbon electrodes for use in a process called
electrode discharge machining, to compensate for a known problem in
direct machining of wrought steel molds, of not being able to
produce sharp comers. The milling or cutting action occurs normally
in a radiused manner, and this problem would be expected in the
instant invention, however, due to the high machinability of the
material used in the instant invention, additional cutting
processes such as honing can be programmed to adjust for these
desired features. Cutting rates can also be adjusted as well as the
design of the cutting tools themselves.
[0055] Processing of the machined core and cavity insert halves as
referenced in step 13 of the flowchart in FIG. 1, is accomplished
within the standard process parameters known to the art of powder
injection molding. A two step debinding process to remove the low
melting portions of a typical binder compound is accomplished
through the combination of solvent or thermal means leaving only
the presintered powder skeleton that is later consolidated in a
high temperature sintering step.
[0056] In the example for an M-4 tool steel molded material using a
thermoplastic polymer/wax binder, the wax can be eliminated using a
heated solvent in liquid or vapor form. This would be followed by
the removal of the remaining polymer in an inert atmosphere
furnace, thereby producing a debound mold or part consisting only
of the metal powders. This debound part will continue to hold its
shape due to the fact that the elimination of the polymer portion
of the binder, will also allow the powders to presinter or weld
together.
[0057] The debound mold is finally put through a sintering or high
temperature consolidation to produce the final near full density
mold article. The sintering, can be carried out in any high
temperature vacuum or atmosphere furnace. To process M-4 tool steel
in the reference example, the preferred mode of sintering would be
in a vacuum furnace at a maximum temperature of 2240.degree.
F.(1220.degree. C.) for 10 minutes. This will yield a mold or die
with a sintered density of approximately 8.0 g/cc, which is about
99% of the theoretical density of 8.1 g/cc. In achieving this high
density, the part will have shrunk as noted approximately 15% from
the green molded state. Different materials such as carbon steel,
bronze, copper or stainless steel, will have different sintering
temperatures and hence different shrink rates.
[0058] The sintered mold or die can be finished by any number of
secondary operations and fitted into an injection molding frame in
the final step 14, as referenced in the flowchart in FIG. 1.
Secondary operations can include heat treating, polishing, and the
addition of slide, ejection pins, and other mold accessories that
will create a functional mold tool for injection molding any
materials.
[0059] FIGS. 2-11 expand and exemplify the results of the process
steps outlined in the overall flow diagram shown in FIG. 1.
[0060] A representation of a 3-D CAD file drawing is shown in FIGS.
2A & 2B. The goal at this stage of the process is to split the
part into two corresponding halves, the interior or core 20 half
shown in FIG. 2A, and an exterior or cavity 21 half shown in FIG.
2B. These core 20 and cavity 21 representations will form the basis
for the design and manufacture of the mold cavity/core patterns,
which will be the mirror images of the CAD representations. Once
the split of the part has been defined, it is now possible to use
the same database to generate a cavity/core sets of CAD
representations, which are the inverse of the original
representations shown in FIG. 2. These representations are then
translated into cutting path programs after adjusting for the
desired size dimensions that are to be cut. In the example of a
mold insert being developed for use in plastics molding, an
allowance for shrinkage of the plastic part--for example 2%--would
be given on top of the expected shrinkage of the blank block, which
can vary from 15-20%.
[0061] FIGS. 3A shows the application of the cutting path programs
to the core half 33 by machining with a CNC type milling cutter 32
on a soft die block 30 cast or molded using metal injection molding
material. FIG. 3A is a representation of the insert as it is being
machined or milled, while FIG. 3B shows the completed insert
31.
[0062] FIGS. 4A shows the corresponding process for the cavity half
with the milling cutter 42 machining on the cast or molded die
block 40. FIG. 4B shows the completed cavity insert 41.
[0063] The blank blocks cast or molded from the powder injection
molded material are normally squared and ground. The goal is to cut
the block to conform to the general dimensions of an insert pocket
shown in 51 of FIGS. 5A & 5B without having to arrive at the
exact dimensions as finishing and fitting operations can guarantee
that that will occur. Cutting, grinding or milling can occur with a
variety of tools and at rates dictated by the geometry of the part.
Speeds can be adjusted to obtain rough cuts, followed by finer cuts
that will give the tool a better surface finish. Secondary
finishing operations, after the cavity and core blocks are
sintered, can render a tool with the required surface finish
requirements.
[0064] FIG. 5A shows a standard mold base 50 with the insert
pockets 51 to accept the processed inserts 53, shown in FIG. 5B .
FIG. 5B show the inserts fitting into a yoke 53 that holds the
inserts to facilitate the location of water lines around the
inserts. The actual dimensions of the base 50 depend on the size of
the part being replicated, as the die pocket 51 and/or yoke 52 can
be easily varied to allow the insert 53 to fit in.
[0065] FIG. 6A shows the assembled insert mold in this case for the
core half of the mold halve with the insert 53 assembled into a
yoke 52. This exemplification is for a four cavity tool. FIG. 6B is
a cross-sectional representation of this tool to highlight the
assembly of the different components as well as to demonstrate the
location of the water lines 61 within the yoke 52 containing the
core insert 53. This core die 53 will undergo a number of secondary
operations that will include heat treating to harden the metal, and
grinding and polishing, to assure a tight fit in the die pocket
51.
[0066] The major advantage, however, is that this method of
manufacture can yield, with a finalized part design, a fully
production ready multi-cavity tool at a fraction of the time and
cost to produce a comparable tool using carbon electrodes, CNC
machining and other standard tooling and rapid fabrication
practices.
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