U.S. patent application number 09/901929 was filed with the patent office on 2002-02-07 for mold for optimizing cooling time to form molded article.
Invention is credited to Baresich, Frank J..
Application Number | 20020014722 09/901929 |
Document ID | / |
Family ID | 27414626 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014722 |
Kind Code |
A1 |
Baresich, Frank J. |
February 7, 2002 |
Mold for optimizing cooling time to form molded article
Abstract
Mold cycle time is accelerated by employing thermally insulating
surface temperature boosters, which are of a minimum thickness to
promote cooling by heat transfer through the boosters. According to
the thermal transfer properties of the insulating boosters and the
respective temperatures of the molten material and the dies, the
temperature of the cavity surface is raised by contact with the
molten material to equal or exceed the temperature required to
produce a molded article, preferably just until the time that the
mold is fully filled. Heat transfer through the boosters to the
dies then cools and solidifies the molded article until it can be
removed from the mold. The temperature boosters result in increased
cavity surface temperatures, such that the mold dies can be kept at
substantially lower temperatures. The overall result is a reduction
in mold cooling time and therefore acceleration of mold cycling.
When the molded article is an optical disc, where the digital
information is transfered to at least a part of a surface of the
optical disc from a stamper that forms at least a part of the
cavity surfaces, a stamper heating means can be used to improve the
quality of optical performance. Typically, the stamper may contact
high thermal conductivity materials at or beyond the outside
diameter of the mold cavity, creating a path for the heat to flow
from and cool the outer edge of the optical disc excessively.
Transfer of the pits from the stamper is more difficult in the
cooler material at the outer edge of the disc, reducing pit
quality, and stresses are also created at the outer edge of the
disc which cause birefringence, resulting in reduced optical
performance. The stamper heating means increases the temperature of
the stamper at the area of contact enough to reduce, stop, or even
reverse the direction of the heat flow.
Inventors: |
Baresich, Frank J.;
(Melbourne Beach, FL) |
Correspondence
Address: |
WILLIAM H. MURRAY
DUANE MORRIS & HECKSCHER LLP
ONE LIBERTY PLACE
PHILADELPHIA
PA
19103-7396
US
|
Family ID: |
27414626 |
Appl. No.: |
09/901929 |
Filed: |
July 10, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09901929 |
Jul 10, 2001 |
|
|
|
09421189 |
Oct 19, 1999 |
|
|
|
6276656 |
|
|
|
|
09421189 |
Oct 19, 1999 |
|
|
|
08857762 |
May 15, 1997 |
|
|
|
6019930 |
|
|
|
|
08857762 |
May 15, 1997 |
|
|
|
08516100 |
Aug 17, 1995 |
|
|
|
08516100 |
Aug 17, 1995 |
|
|
|
07913136 |
Jul 14, 1992 |
|
|
|
Current U.S.
Class: |
264/327 ;
249/111; 249/78; 264/328.16 |
Current CPC
Class: |
B29C 45/73 20130101;
B29C 45/37 20130101; B29C 45/2642 20130101; B29L 2011/0016
20130101 |
Class at
Publication: |
264/327 ;
264/328.16; 249/78; 249/111 |
International
Class: |
B29C 033/02 |
Claims
What is claimed is:
1. A mold for optimizing molding time to form a molded article,
said mold containing a plurality of mold portions forming a mold
cavity having cavity surfaces in a shape of said molded article,
said mold portions comprising: (1) at least one mold die having at
least one primary booster adjacent to and in thermal communication
therewith, said mold die providing structural support for said
primary booster; (2) said primary booster being disposed in the
mold cavity and forming at least a part of the cavity surfaces, the
primary booster being made of material whose mathematical product
of thermal conductivity, density, and specific heat is no more than
2.0.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 at
room temperature, and having predetermined thicknesses (W.sub.b) as
calculated from the equation 5 W b = Y k b t f b c b 0.25 Y 4.0
where t.sub.f is a time to fill the mold, k.sub.b is thermal
conductivity, .rho..sub.b is density, and C.sub.b is specific heat
of the primary booster; and, (3) thermal control means for applying
temperature control stimuli to the mold die.
2. The mold of claim 1, wherein the primary boosters vary in
thickness at different locations on the cavity surfaces.
3. The mold of claim 1, further comprising edge temperature
boosters on the cavity surfaces, the edge temperature boosters
being made of materials whose mathematical product of thermal
conductivity, density, and specific heat is no more than
2.0.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 at
room temperature.
4. The mold of claim 1, wherein the molded article is an optical
disc.
5. The mold of claim 1, further comprising secondary boosters, the
secondary boosters being located between at least a part of said
primary boosters forming the cavity surfaces, and said mold dies,
the secondary boosters being in thermal communication with both the
primary boosters and the mold dies, the secondary boosters being
made of materials whose mathematical product of thermal
conductivity, density, and specific heat is less than that of the
adjacent primary boosters, whereby the secondary boosters restrict
heat flow from the primary boosters for improving build-up of heat
in the primary boosters, the secondary boosters having thicknesses
(W.sub.sb) as calculated from the equation 6 W sb = Z k sb t f sb c
sb 0.025 Z 4.0 where t.sub.f is the time to fill the mold, k.sub.sb
is the thermal conductivity, .rho..sub.sb is the density, and
C.sub.sb is the specific heat of the secondary booster.
6. The mold of claim 5, wherein said primary and secondary boosters
have differing thicknesses at different locations, causing
different heat flow from the cavity surfaces to the mold dies at
the different locations.
7. The mold of claim 5, further comprising a stamper forming at
least a part of the cavity surfaces, said stamper being in thermal
communication with at least one said primary booster.
8. The mold of claim 7, further comprising a stamper heating
means.
9. A mold for optimizing molding time to form a molded article,
said mold containing a plurality of mold portions forming a mold
cavity having cavity surfaces in a shape of said molded article,
said mold portions comprising: (1) at least one mold die having at
least one primary booster adjacent to and in thermal communication
therewith, said mold die providing structural support for said
primary booster; (2) said primary booster being disposed in the
mold cavity and forming at least a part of the cavity surfaces, the
primary booster being made of material whose mathematical product
of thermal conductivity, density, and specific heat is no more than
2.0.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 at
room temperature, and having predetermined thicknesses (W.sub.b) as
calculated from the equation 7 W b = Y k b t f b c b 0.25 Y 4.0
where t.sub.f is a time to fill the mold, k.sub.b is thermal
conductivity, .rho..sub.b is density, and C.sub.b is specific heat
of the primary booster; (3) thermal control means for applying
temperature control stimuli to the mold die; (4) a stamper forming
at least a part of the cavity surfaces, said stamper being in
thermal communication with at least one said primary booster; and,
(5) a stamper heating means.
10. The mold of claim 9, wherein the heating means is at least
partially thermally insulated from the mold die.
11. The mold of claim 9, wherein the heating means is electrical
resistive heating.
12. The mold of claim 9, wherein the stamper heating means is
located in the mold die substantially adjacent to a periphery of
the primary booster and in thermal communication with the
stamper.
13. The mold of claim 9, wherein the molded article is an optical
disc.
14. The mold of claim 9, wherein the stamper heating means is
located in the mold die of an optical disc mold in a vicinity of an
outer diameter of the cavity and in thermal communication with the
stamper.
15. A stamper heating means for uniformly cooling a molded article
during a molding process, wherein the stamper heating means is
located in a mold die substantially adjacent to a periphery of a
primary booster, said stamper heating means being used to locally
increase the temperature of a molding stamper to substantially
reduce heat flow from a mold cavity through a stamper to portions
of a mold beyond the outer diameter of the mold cavity.
16. The stamper heating means of claim 15, wherein the heating
means is at least partially thermally insulated from the mold
die.
17. The stamper heating means of claim 15, wherein the heating
means is electrical resistive heating.
18. A stamper heating means for uniformly cooling a molded article
during a molding process, wherein the stamper heating means is
located in the mold die of an optical disc mold in a vicinity of an
outer diameter of a mold cavity, said stamper heating means being
used to locally increase the temperature of a molding stamper to
substantially reduce heat flow from the mold cavity through a
stamper to portions of a mold beyond the outer diameter of the mold
cavity.
19. The stamper heating means of claim 18, wherein the heating
means is at least partially thermally insulated from the mold
die.
20. The stamper heating means of claim 18, wherein the heating
means is electrical resistive heating.
21. A method of uniformly cooling a molded article during the
molding process, wherein a stamper heating means is used to locally
increase the temperature of a molding stamper to substantially
reduce heat flow from a mold cavity through a stamper to portions
of a mold beyond the outer diameter of the mold cavity.
22. The method of claim 21, wherein the stamper heating means is at
least partially thermally insulated from the mold die.
23. The method of claim 21, wherein the stamper heating means is
electrical resistive heating.
24. The method of claim 21, wherein the stamper heating means is
located in the mold die substantially adjacent to a periphery of
the primary booster and in thermal communication with the
stamper.
25. The method of claim 21, wherein the molded article is an
optical disc.
26. The method of claim 21, wherein the stamper heating means is
located in the mold die of an optical disc mold in a vicinity of an
outer diameter of the cavity and in thermal communication with the
stamper.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation in part of application Ser. No.
08/857,762, filed May 15, 1997, now issued as U.S. Pat. No. ______,
which is a continuation in part of pending application Ser. No.
08/516,100, filed Aug. 17, 1995, which is a continuation of
application Ser. No. 07/913,136, filed Jul. 14, 1992, now
abandoned.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to the field of methods and apparatus
for molding molten material, and in particular provides a mold
structure and method for reducing and preferably minimizing the
time needed to cool the molten material to a temperature at which
the molded article is rigid enough for removal from the mold. This
is accomplished by controlling the rate of heat transfer from the
molten material to the mold body using thermally insulating surface
temperature boosters. According to the invention, thermal
insulation is embodied according to its thickness and heat transfer
properties to maximize the rate of cooling within certain limits
such that the material cools substantially to its solidifying
temperature promptly upon completion of filling of the mold.
[0004] 2. Prior Art
[0005] Production molds and molding processes may produce hundreds
of molded articles an hour. Most of these processes apply pressure
to cause a hot molten material to flow into and fill a mold cavity.
The material cools in the shape of the cavity until it is rigid
enough for removal before the mold opens and the operator or
special equipment removes the article from the mold.
[0006] Production efficiency dictates that each molding operation
should be completed in as short a time as possible in order to make
the mold available for another molding cycle. For this reason,
molders normally try to cool the molten material as fast as
possible. Often heat transfer fluid is circulated through passages
in the mold dies to control the cavity surface temperature. U.S.
Pat. Nos. 4,275,864, 4,655,280, 4,703,912, and 4,934,918 describe
some ways to provide flow passages. Heat flows from the molten
material through the mold die to the heat transfer fluid. Metals
such as H-13, H-23, P-1, P-2, P-4, P-5, P-6, P-20, and S7 tool
steels, 420 stainless steel, beryllium copper, brass, and aluminum
are common mold materials whose high thermal conductivities cause
heat to flow at a high rate. Irvin I. Rubin on page 156 of
"Injection Molding Theory and Practice," explains why high thermal
conductivity materials should be chosen for molds.
[0007] To remove heat from the molten material rapidly, molders
normally keep cavity surfaces much colder than the molten material
throughout the molding cycle. However, if cavity surfaces are kept
too cold while the mold is filling, the mold may not fill
completely (short shot) or unacceptable surface defects or stresses
may develop in the molded article. In addition, locations distant
from the location where the molten material enters the cavity may
get less material than closer locations. This causes uneven density
distribution and molded-in stresses.
[0008] The molten material tends to heat the mold. However, if the
mold is kept at a relatively higher temperature, the mold generally
can be filled more dependably because the molten material is more
flowable, and the quality of the molded article is improved. The
need for additional cooling can add to the time spent in molding
each article as compared to keeping the mold cooler initially. What
is needed is a method to optimize temperature control to balance
the interests of quality and time for a given molding process.
[0009] The minimum cavity surface temperature required during mold
filling depends on the particular molten material and the surface
quality and dimensional stability required of the molded article.
Processing temperature ranges for molten material and for mold dies
are specified by the equipment and material manufacturers. For
plastics, the recommended temperature ranges for mold dies are
below the solidifying temperatures of the plastics. For many
materials recommended temperature ranges can also be found in
sources such as "Modern Plastics Encyclopedia," MIL-HDBK-700A
"Plastics," the annual "Materials Selector" issues of "Materials
Engineering Magazine," "Metals Handbook," "Glass Engineering
Handbook," and "Kirk-Othmer: Encyclopedia of Chemical Technology
Volume 11, third edition" (for glass see pages 825-832 &
855-857).
[0010] Because defects that may be acceptable for one type of
molded article may not be acceptable for another, and because mold
heating and cooling configurations vary, the optimum temperature
for molding a specific article normally is determined in part by
analysis and in part by experiment and experience (i.e., trial and
error). Most often the optimum temperatures fall within the
temperature ranges recommended by the material manufacturer.
[0011] When a molten material contacts surfaces of a cavity, heat
flow from the molten material causes a rapid increase of the cavity
surface temperatures. For example, molten polycarbonate at
600.degree. F. and cavity surfaces initially at 195.degree. F. will
produce the following approximate temperature increases for cavity
surfaces made of common mold die metals. These increases are
approximate because convection, radiation, thermal contact
resistances, changes in thermal physical properties with
temperature, and initial temperature gradients can vary.
1 Cavity Surface Material Temperature Increase 420 stainless steel
29.degree. F. H-13 tool steel 26.degree. F. brass 14.degree. F.
aluminum 12.degree. F.
[0012] The small temperature increases at cavity surfaces of common
metal cavities demand that molders maintain the surfaces of the
cavity close to the mold filling temperatures throughout the
molding cycle. Otherwise, the required temperatures cannot be
reached at the cavity surfaces while the mold is filling. However,
keeping the cavity surfaces so close to the mold filling
temperatures after the cavity is full slows heat transfer from the
molten material into the dies and delays solidifying of the
workpiece. If die temperature is cycled instead, heating and
cooling the entire mass of dies requires additional time that also
slows the molding process.
[0013] The cavity surface temperature and the rate of cooling
affect the finished workpiece. It is know, for example,
deliberately to increase cooling time to improve surface qualities
of the molded article such as smoothness, gloss, and replication of
cavity surface finish.
[0014] DuPont Company developed a method for cycling the
temperature of mold dies to improve the smoothness of molded
surfaces. It is described in the article, "Class "A" Blow Molding:
How It's Done," Plastics Technology, June, 1988. The method reduces
the thermal mass of the mold dies then alternately circulates
heating and cooling fluid through passages in the dies. This
requires extensive structural analysis and machining in addition to
computer controlled dual fluid circuits. DuPont reports that the
process meets its goal to improve the surface of molded automobile
spoilers. However, DuPont also states it increases the cycle
time.
[0015] Others improve the surface quality of plastic articles by
heating a thin layer of the cavity surface rather than the entire
die. U.S. Pat. No. 4,340,551 discloses high-frequency induction to
heat a superficial layer of the cavity surface before injecting
plastic resin. Steps to insert the induction heater, close the
mold, heat the cavity surface, open the mold, and remove the
induction heater increase the cycle time. U.S. Pat. Nos. 3,734,449,
4,285,901, 5,041,247, and 5,064,597 locate a thin layer of metal
backed by a thermal insulation layer at the cavity surface. The
insulation layer reduces heat flow from the metal layer. The result
is a substantially higher temperature of the metal cavity surface,
so that it is above the point at which the resin solidifies while
the mold cavity is filling. The high temperature and restricted
heat transfer keep the surface of the plastic article fluid and
improve transfer of the finish of the very hot surface of the
cavity to the plastic article. The increased cavity surface
temperature and restricted heat transfer increase cooling time. M.
Liou and N. Suh in their article, "Minimizing Residual Stresses in
Molded Parts," pages 524 through 528, ANTEC '88, report that
coating a cavity surface with 0.01 centimeter of Teflon caused
higher cavity surface temperatures that increased cooling time
almost twenty percent.
[0016] To reduce cooling time, inventors have applied cooling fluid
directly to the surface of a mold cavity. U.S. Pat. Nos. 4,139,177
and 4,164,523 reduce the cooling time of thick foam articles by
flowing low boiling point liquid between the article and the cavity
surface. The compressibility of foam allows space for the liquid to
flow. The method is limited to molding foamed plastic articles, and
at least two of the preferred liquids, namely carbon dioxide and
liquid nitrogen, can pose safety hazards. The extreme cold
temperatures of these liquids can cause skin damage, and the gases
from the boiling liquid can displace air in the work place. U.S.
Pat. No. 4,208,177 flows cooling fluid at or about five pounds per
square inch pressure into a porous layer at the cavity surface. A
vacuum is then pulled on the side of the porous layer that is away
from the resin. The vacuum causes the fluid to boil, which draws
heat from the porous layer and resin. However, the pores limit the
quality of surface finishes on the molded article. Methods that
rely on direct contact of low boiling point fluid with the cavity
surfaces require expensive equipment and complicated controls.
[0017] U.S. Pat. No. 5,458,818 ('818 patent) uses a varying density
insulating insert behind a stamper. The '818 patent claims to
improve the surface smoothness of molded part by bringing the
temperature of the cavity surface formed by the stamper above the
solidifying temperature of the resin while the mold is filling. The
'818 patent also claims to reduce residual stresses and pit
replication. However, there is no suggestion in the '818 patent to
lower the temperature of the mold die. Rather, the cooling time
will increase due to the higher cavity surface temperature.
[0018] U.S. Pat. No. 5,041,247 ('247 patent) teaches the method of
blow molding and shows a mold having a blow pin, a circular shaped
die, a hard skin, an insulating layer, and a base. The hard skin is
1-20 mils thick and has a high thermal conductivity (1.times.10-2
to 1.times.10-1 cal/sec-cm-C). Materials such as stainless steel,
nickel, aluminum, and brass are used which are contrary to the
primary booster materials used in my invention. The thickness of
the insulating layer is determined by an equation where the only
variable is thermal diffusivity while the thickness of my primary
booster is determined by an equation which has both thermal
diffusivity and cavity fill time as its variables. In the '247
patent, the skin surface is heated by the melt to above the glass
transition (solidifying) temperature of the melt to duplicate the
mold surface. Increases in the thickness of the insulating layer
are shown to increase cooling time. There is no suggestion to
reduce the mold die coolant temperature. The cooling time will be
increased compared to a mold without an insulating layer. My mold
selects booster thicknesses and mold die temperatures which shorten
cooling time.
[0019] U.S. Pat. No. 5,064,597 ('597) teaches the method of
compression molding, showing a hard skin layer, an insulating
layer, and a base containing cooling means. The hard skin layer has
the same thermal conductivity requirements as the '247 patent. The
insulation layer thickness is determined using the same equation as
the '247 patent. The thermal conductivity range for the insulating
layer is 5.times.10-4 to 5.times.10-3 cal/sec-cm-C. However, some
of the primary boosters of my invention can be made of materials
such as sapphire and zirconia which have higher thermal
conductivity. In the '597 patent, the surface of the cavity is
brought to a temperature that keeps the plastic molten while the
press closes when the mold is filled. It discloses that in one
case, the cycle time is increased from 45-60 seconds to 2.2
minutes.
[0020] U.S. Pat. No. 5,124,192 ('192 patent) shows a hard
continuous skin layer over an insulating layer and covers the mold
design and construction to avoid delamination between the layers.
The '192 patent operates similarly to the '247 and the '597
patents.
[0021] U.S. Pat. No. 5,176,839 ('839 patent) uses an insulating
layer of varying density across its thickness. In the '839 patent,
the insulating layer causes the skin layer to reheat in the same
manner as the '192, '247 and the '597 patents.
[0022] In summary, the teachings of the prior art:
[0023] a. maintain the cavity surfaces close to the temperatures
required during mold filling throughout the molding cycle, which
prolongs cooling; or,
[0024] b. to improve surface quality of a molded article, increase
the temperature of cavity surfaces above the solidifying
temperature of a plastic being molded while the cavity is filling,
which lengthens cooling time even more; and/or,
[0025] c. cycle the temperatures of entire mold dies, which
requires expensive and complicated equipment; or,
[0026] d. accelerate cooling by bringing fluid into direct contact
with cavity surfaces which requires complex equipment and can
degrade the molded article. The extremely cold liquids used for one
of these methods present safety hazards and the method is limited
to foamed resin.
SUMMARY OF THE INVENTION
[0027] It is an object of this invention to provide molding methods
and molding apparatus that reduce the time required to cool
material from a molten state into a molded article that is stiff
enough that it can be removed from a mold. This invention can
produce molded articles that are as good or better than those
produced by conventional methods. It can be used to mold both
foamed and unfoamed materials, and to mold thermoplastic, ceramics,
glasses and metals. The invention provides a method and apparatus
to optimally cycle cavity surface temperatures, without expensive
and complicated equipment and controls. While the mold is filling,
dies with temperature boosters at the cavity surface cause heat
flowing from the molten material to the boosters to bring the
cavity surfaces to or above temperatures required to produce a
molded article. Dies with temperature boosters increase cavity
surface temperatures so much more than the same dies without
temperature boosters that the mold dies can be kept at
substantially lower temperatures. According to an aspect of the
invention, the boosters are made to a minimum thickness. The
thickness preferably is chosen such that, starting when the cavity
is approximately full, heat flowing from the boosters to the cooler
dies causes cooling of cavity surfaces sufficient to cool, stiffen
and solidify the molten material into an article rigid enough for
removal from a mold, in a minimum time.
[0028] A method of this invention for accelerating cooling of
molten material into a molded article comprises the steps of:
[0029] (a) providing a mold containing a plurality of mold portions
which are brought together to form a mold cavity in the shape of
said molded article, said mold portions comprising:
[0030] (1) at least one die having at least one primary booster
adjacent to and in thermal communication therewith, said die
providing structural support for said booster;
[0031] (2) said primary boosters forming at least a part of the
surfaces of said mold cavity, the primary boosters being made of
materials whose mathematical products of thermal conductivity,
density, and specific heat are no more than 2.0.times.10.sup.-6
BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 at room temperature, and
having predetermined thicknesses (W.sub.b) as calculated from the
equation 1 W b = Y k b t f b c b 0.25 Y 4.0
[0032] where t.sub.f is the time to fill the mold, k.sub.b is the
thermal conductivity, .rho..sub.b is the density, and c.sub.b is
the specific heat of the booster;
[0033] (3) controlling a temperature control stimuli to the
die;
[0034] (b) applying substantially constant temperature control
stimuli to said mold dies such that surfaces of the mold cavity are
at predetermined temperatures that are initially below the
temperatures required to produce a molded article and which will,
upon contact with molten material to be introduced into the mold
cavity, increase to or above the temperatures required during mold
filling to produce a molded article, and wherein, because of the
mathematical products of thermal conductivity, density, and
specific heat of the primary boosters the predetermined cavity
surface and die temperatures are lower than when materials with
higher corresponding products are used for cavity surfaces, such as
when the same die is used without the primary boosters;
[0035] (c) introducing molten material into the mold cavity,
whereupon the molten material heats the primary boosters and
temperatures of the surfaces of the cavity increase from the
predetermined temperatures to or above the mold filling
temperatures required to produce a molded article;
[0036] (d) while the cavity is filling with molten material,
maintaining said mold cavity surfaces at or above the temperatures
required to produce a molded article;
[0037] (e) after the cavity is approximately full, allowing heat
flowing from the boosters to the cooler dies to cool cavity
surfaces of the primary boosters, thereby cooling, stiffening, and
solidifying the molten material in an accelerated manner, until it
is rigid enough for removal from the mold.
[0038] Further methods and apparatus of the present invention
utilize secondary boosters between primary boosters and dies where
they are in thermal communications with both. The primary and
secondary boosters cooperatively bring the temperatures of cavity
surfaces to or above the temperatures required to produce a molded
article. The secondary boosters are made to predetermined
thicknesses of materials whose mathematical products of thermal
conductivity, density, and specific heat are less than those of the
primary boosters, whereby they restrict heat flow from the primary
boosters causing heat flowing from the molten material to build up
within the primary boosters. This increases temperatures of cavity
surfaces.
[0039] When the molded article is an optical disc, where the
digital information is transfered to at least a part of a surface
of the optical disc from a stamper that forms at least a part of
the cavity surfaces and is in thermal communication with at least
one primary booster, a stamper heating means can be used to improve
the quality of optical performance. The stamper may extend beyond
the primary temperature booster. It may also contact high thermal
conductivity materials at or beyond the outside diameter of the
mold cavity, creating a path for the heat to flow from and cool the
outer edge of the optical disc excessively. Transfer of the pits
from the stamper is more difficult in the cooler material at the
outer edge of the disc reducing pit quality, which is detrimental
to optical performance. Stresses are also created at the outer edge
of the disc which cause birefringence that is also detrimental to
optical performance. The stamper heating means overcomes this
problem.
[0040] The stamper heating means located in the die and in the
vicinity adjacent to the periphery of the primary temperature
booster, and in thermal contact with the stamper increases the
temperature of the stamper at the area of contact enough to reduce,
stop, or even reverse the direction of heat flow. Alternatively,
the stamper heating means may be located in the mold die of an
optical disc mold in a vicinity of an outer diameter of the cavity
and in thermal communication with the stamper. Thermal contact
between the heater and stamper may be accomplished directly or
through an intermediary material of high thermal conductivity.
Preferably, the heating means should have some thermal insulation
from the mold die. The stamper heating means could also be of the
electrical resistive heating type. By utilizing the stamper heating
means, the birefringence at the outer edge of the optical disc is
thus brought into acceptable limits.
[0041] Further objects and advantages of this invention will become
apparent from a consideration of the drawings and ensuing
descriptions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] There are shown in the drawings certain exemplary
embodiments of the invention as presently preferred. It should be
understood that the invention is not limited to the embodiments
disclosed as examples, and is capable of variation within the scope
of the appended claims. In the drawings,
[0043] FIG. 1 is a cross-section of a mold according to the first
embodiment of the invention.
[0044] FIG. 2 is a cross-section of a mold according to a second
embodiment of the invention.
[0045] FIG. 2A is a cross-section of a mold according to a second
embodiment of the invention, showing stamper heating means.
[0046] FIG. 3 is a cross-section of a mold according to a third
embodiment of the invention.
[0047] FIG. 4A shows temperature histories for Example 1, which
uses the mold of FIG. 1.
[0048] FIG. 4B shows temperature histories for Example 2, which
uses the mold of FIG. 1.
[0049] FIG. 4C shows temperature histories for Example 3, which
uses the mold of FIG. 1.
[0050] FIG. 4D shows temperature histories for Example 4, which
uses the mold of FIG. 1.
[0051] FIG. 5 Shows temperature histories for Example 5, which uses
the mold of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] FIG. 1 shows a simplified cross-section of a first
embodiment mold 42 for a molded article.
[0053] A left-side element 38 comprises left-side primary
temperature booster 14, edge temperature booster 20, and left-side
die 18. Boosters 14 and 20 define at least a part of the surfaces
of cavity 10.
[0054] Boosters 14 are made of materials that have the required
durability at the temperatures and pressures at which they must
operate, and whose mathematical products (k.rho.c) of thermal
conductivity (k), density (.rho.), and specific heat (c) are no
more than 2.0.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degree.
F..sup.2 at room temperature, and preferably no more than
1.6.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degre- e. F..sup.2.
This is much less than corresponding k.rho.c products for the
common mold metals discussed earlier. For example, the
corresponding k.rho.c product is about 9.2.times.10.sup.-6
BTU.sup.2/sec/in.sup.4/.degr- ee. F..sup.2 for 420 stainless steel,
and 55.6.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degree. F..sup.2
for an aluminum. The purpose of boosters 14 is to permit the
temperature of the cavity surface to cycle, but the surface is only
as hot as necessary to allow complete filling of the cavity and
shaping of the molten material while the cavity is being filled.
Promptly after filling is substantially complete, and no additional
heat is being introduced into the cavity by the molten material,
heat transfer from the boosters to the relatively cooler dies cools
the cavity and its contents.
[0055] The booster allows the die and the cavity surfaces to be
maintained at initial temperatures just before mold filling which
are significantly below the temperatures that would be required of
the same die and cavity surfaces without a booster. The die with
the boosters still reaches and maintains a sufficient mold cavity
surface temperature during filling of the cavity to achieve proper
molding and complete filling of the cavity to form the molded
article at the pressure being employed. By minimizing the thickness
of the boosters as provided herein, molding time is also minimized
at the same time.
[0056] When the temperatures are expressed in degrees fahrenheit,
the initial cavity surface temperature with a booster is at least
20% lower than the temperature required at the mold surface during
filling of the cavity. I have also discovered that this percentage
increases when the k.rho.c of the molten material is larger, when
the k.rho.c of the booster is smaller, and when the ratio of the
temperature of molten material to the temperature of the cavity
surface during mold filling is larger. The boosters cause heat from
the molten material entering the cavity to substantially
instantaneously raise the surface temperatures of the cavity to or
above that required to properly fill the mold and to form the
article. Then after the cavity is approximately full, the
relatively cooler die temperature used in the present method
accelerates cooling, stiffening, and solidifying of the molten
material in the form of the article. Thus the time required for
cooling of the cavity before the article can be removed is
significantly reduced by the method of the invention as compared to
the use of dies without boosters or dies with boosters whose
thermal transfer properties are not optimized, in particular being
too insulating and unduly limiting the thermal gradient that
determines the rate of cooling. Also, the prior art, e.g.,
Yotsutsuji (U.S. Pat. No. 4,285,901), which employs a metal coated
thermal insulating liner over the surface of the mold cavity does
not teach or recognize the advantage of maintaining a relatively
low die temperature to accelerate cooling after the mold cavity is
full. The manner in which the insulating layers of the prior art
are used is to continue to employ relatively high die temperatures
and increase the temperature of the cavity surface to above the
solidifying temperature of the plastic being molded in an effort to
improve the surface quality of the finished article. This lengthens
the cooling cycle as compared to the quicker cooling cycle of the
present invention.
[0057] The invention cools the cavity below the softening
temperature when the cavity is "approximately" full, which reflects
certain differences between molding operations and between the
manner in which a given molder may choose to operate. The object is
to shorten the overall cycle time, but a given molder may be
willing to have a relatively longer cycle time, for example to
improve the quality of the part. Some molding operations employ a
hold period during which pressure is maintained on the material.
For example with injection molding, inlet pressure can be held on
the material for a time as the molten material cools and shrinks,
with additional molten material flowing into the cavity to
compensate for the shrinkage. The invention is applicable if the
mold cavity temperature remains above the molding temperature until
slightly before all the material is in the cavity or slightly
after, or similarly until after a required hold period has elapsed,
in each case achieving advantages over known molding methods
operated in a comparable manner.
[0058] The invention departs from lessons taught by Rubin, supra,
and from the common wisdom of the molding industry, in using
temperature boosting or insulating materials to reduce cooling time
(the time from completion of filling to solidification), whereas
insulating materials can normally be expected to extend cooling
time. This is accomplished with booster materials having low
thermal conductivity (i.e., low k.rho.c products), but the boosters
are not defined by thermal conductivity alone. It is possible to
choose a material for an insulation layer that has a lower thermal
conductivity than another material but to choose or configure it
such that the chosen insulation material raises the temperature at
the surface of the cavity less. This occurs when the density and/or
specific heat is sufficiently greater and/or by limiting its
thickness. Relatively greater density and/or specific heat require
more heat energy to achieve a given temperature increase on the
cavity surfaces due to the nature of the material. Given a
temperature difference, relatively thinner booster material
produces a steeper temperature gradient across the booster, which
leads to increased heat conduction from the surface into the dies.
These contrast with the effect of reduced thermal conductivity to
slow heat flow through the insulation layer and cause higher
temperatures at the cavity surfaces.
[0059] For example, Xydar G-430 brand liquid crystal polymer has a
lower thermal conductivity than polyphenylene sulphide
(2.9.times.10.sup.-6 versus 3.85.times.10.sup.-6
BTU/sec-in-.degree. F.). But Xydar G-430 causes a smaller
temperature increase than polyphenylene sulphide. This is because
Xydar G-430 has a higher product of thermal conductivity, density,
and specific heat than polyphenylene sulphide(6.87.times.10.sup.-
-8 versus 5.56.times.10.sup.-8 BTU.sup.2/sec-in.sup.4-F.sup.2).
[0060] Because while the mold is filling, heat flowing from the
molten material increases the temperature of the cavity surfaces so
much more than with dies without temperature boosters, the initial
temperature of die 18 can be lowered. For example, to bring cavity
surfaces to 270.degree. F. when contacted by 600.degree. F. molten
polycarbonate, the approximate predetermined temperature (T.sub.P)
of cavity surfaces just prior to contact by molten material is:
2 Cavity Surface Material Predetermined Temperature Aluminum
260.degree. F. H-13 tool steel 248.degree. F. 420 stainless steel
244.degree. F. sapphire booster 191.degree. F. quartz booster
134.degree. F.
[0061] The above values for predetermined temperature (T.sub.P)
where calculated from the equations 2 T p = T - aT M 1 - a = 270 -
a ( 600 ) 1 - a where : a = ( k c ) M ( k c ) M + ( k c ) S
[0062] T is the temperature of the cavity surface required during
mold filling to produce a molded article and is determined from
manufacturers recommendations, analysis, experiment and/or concerns
as to the surface quality; T.sub.M is the initial bulk (processing)
temperature of the molten material as it enters the cavity and is
determined in the same ways as temperature T; k is the thermal
conductivity of a material; .rho. is the density of the material; c
is the specific heat of the material; and subscripts M and S
distinguish the molten material and the cavity surface material,
respectively.
[0063] The above equations produce approximate temperatures and are
subject to other variables such as convection, contact resistance,
changes in thermal physical properties with temperature, and
initial temperature gradients. Nonetheless, they show that the
mathematical products of thermal conductivity, density, and
specific heat (k.rho.c) of primary boosters result in substantially
lower predetermined cavity surface and die temperatures than when
materials with higher corresponding k.rho.c products such as common
mold metals are used for cavity surfaces. Due to heat storage, the
die temperature is lower than the predetermined cavity surface
temperature. The amount it is lower depends on the design of the
die and the molding cycle. The difference between the predetermined
temperature and the die temperature is normally less when the
temperature stimuli, such as coolant passages, are closer to the
cavity surfaces and when the molding cycle is longer.
[0064] The thickness (W.sub.b) of booster 14 is calculated from the
following equation, such that the booster is at least thick enough
to hold the temperatures of cavity surfaces at or above
temperatures required to produce a molded article until the cavity
is approximately full. However, subject to variations discussed
below, the booster is preferably kept as thin as practicable so as
to provide good heat transfer from the molten material into the
die. Thus, after the cavity is approximately full, heat flowing
from the boosters to the cooler dies cools the cavity surfaces to
minimize cooling time. 3 W b = Y k b t f b c b 0.25 Y 4.0
[0065] where t.sub.f is the time to fill the mold, k.sub.b is the
thermal conductivity, .rho..sub.b is the density, and c.sub.b is
the specific heat of the booster;
[0066] The mold can have boosters that are different in material or
thickness in order to achieve a similar result at different
portions of the molded article, such as to accommodate thicker and
thinner areas. For molding a thin article with a peripheral edge
such as a polycarbonate disk, edge booster 20 can be disposed at
the peripheral edge of the cavity to reduce heat flow from the edge
as compared to areas spaced from the edge. For edge boosters, the
mathematical product of thermal conductivity, density, and specific
heat is no more than 2.0.times.10.sup.-6
BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 at room temperature and
preferably the product is smaller than it is for boosters 14. The
purpose of booster 20 is to cool the molded article more uniformly
by reducing heat flow at its edges, which otherwise would cool more
quickly than other portions.
[0067] Although not shown in FIG. 1, booster thicknesses can be
different at different locations in other respects to promote more
uniform cooling. Also, boosters may cover only part of the cavity
surfaces. Generally, boosters preferably are thicker where the
molded article is thinner (due to closer proximity of the material
of the workpiece to the dies) and are thinner where the molded
article is thicker. This can be used to determine the particular
thickness of the boosters within the range stated above. In
addition, manufacturing variations may dictate foregoing time
savings possible by use of boosters that are as thin as possible in
the range, in order to make the boosters and dies more durable or
to extend the time that the cavity surfaces exceed the molding
temperature to achieve a more precise imprint of the mold surface
contours on the molded workpiece (for example to better replicate
cavity surfaces).
[0068] The particular value for factor "Y" can be determined by
finite element analysis, namely the modelling of temperature over
time at particular points, after defining heat transfer
characteristics of a system. An efficient way to select a specific
value of factor "Y" is to employ a finite element model of one or
more slices through the mold, extending from the temperature
control stimuli on one side of the mold to the temperature control
stimuli on the other side, the slice extending through the cavity
and the workpiece to be molded. An example of the finite element
modelling approach is provided under the description for FIG. 2.
Specific design cases are described below.
[0069] When the thickness of the molded article is substantially
uniform, the temperature control stimuli is substantially the same
distance from the surface on both sides of the mold cavity, and the
molded article is removed in a small fraction of the time the mold
is open, the value of "Y" can be chosen at an intermediate point in
the range, preferably between 1.0 and 3.0, namely at 1.0 if molding
speed is an overriding concern and at 3.0 if surface condition is
more important than speed.
[0070] The thickness of the molded article may vary significantly,
and if the factor "Y" is set at 1.0 in the range, the factor (i.e.,
the thickness of the primary booster) can be increased by a factor
as large as four where the molded article is thinner, or decreased
by a factor of 0.25 where the molded article is thicker. A range of
0.25 to 4.0 has been found to provide sufficient versatility to
account for thickness variations and compromises between speed and
surface quality and/or mold durability concerns, while retaining
the advantage of providing surface temperature boosters but
limiting them so that the surface temperature is above molding
temperature only as long as is required. Otherwise, thermal
transfer through the boosters is maximized to minimize molding
time.
[0071] In a finite element analysis when the temperature control
stimuli is not substantially the same distance from the surface on
both sides of the mold cavity, or if the molded article is not
removed in a small fraction of the time the mold is open, more heat
will be stored in the side with more material between the cavity
surface and temperature control stimuli or on the side where the
molded article contacts the cavity surface for a substantially
longer time period. This will cause the cavity surfaces of one side
of the mold to heat and cool at a different rate than the cavity
surfaces of the other side of the mold. Lowering the temperature of
control stimuli at the side of greater heat storage relative to the
other side can cause the cavity surfaces to heat and cool at
substantially the same rate. However, it is an aspect of the
invention that molding can be accomplished using low die
temperatures. Further temperature reduction could cause problems
(e.g., condensation could form and ruin the surface of the molded
article). Also, if the precisely machined die parts differ too much
in temperature, differential thermal expansion may cause the mold
parts to bind. Therefore, it may be preferable to heat and cool
cavity surfaces uniformly and adjust their temperatures by making
the primary booster thinner on the side of greater heat storage,
where values of "Y" between 0.25 and 2.0 would be typical.
[0072] The value of "Y" may have to be less than optimum (i.e.,
greater than the theoretical minimum) because current technology
limits the minimum or maximum thickness of a desired booster
material. Zirconia of adequate structural integrity can be plasma
sprayed onto a metal surface until it is 0.010 to 0.035 inch thick.
Many brittle materials such as sapphire and quartz that are very
thin, such as 0.030 inch or less, may be difficult to handle, bond,
braze or clamp without damage. Other materials such as Kapton brand
polyimide film are only produced in a few thicknesses. It is
sometimes appropriate to vary the value of "Y" in its range to
accommodate these limiting conditions.
[0073] Depending upon the specific requirements for cavity surface
characteristics such as smoothness, hardness, stiffness, scratch
resistance, chip resistance, chemical resistance, etc., inorganic
materials such as borosilicate glass, quartz, glass ceramic,
titanium, and sapphire are examples of materials that can be used
as surface temperature boosters. For less demanding applications
such as edge boosters, materials such as polyimide, liquid crystal
polymer, polyphenylene sulphide, and mica may also be suitable.
[0074] Mold makers can pre-shape boosters by machining, molding,
grinding, etc., then attach them in place using bolts, bonds,
brazing or clamps, etc. Alternatively, booster material can be
attached in such manners and then shaped to form the mold cavity by
machining, grinding, etc. Booster material can be applied directly
in layers by means such as sputtering, spin coating, or Sol-Gel
coating (Geltech, Alachua, Fl and others).
[0075] In the embodiment shown, left-side die 18 provides
structural support to boosters 14 and 20. Die 18 can be made from
various materials. The chosen material should have high thermal
conductivity, e.g., common mold metals, and must have thermal
coefficient of expansion properties compatible with booster
materials so that differences in expansion cannot damage boosters
14 and 20 during temperature cycling. Fluid passages 12 and 22 can
be provided to carry heated or cooled heat transfer fluid through
die 18. Other known thermal means for temperature control stimuli
may be used.
[0076] FIG. 1 also shows a right-side element 36. Element 36
comprises right-side primary temperature boosters 24, and
right-side die 28. Boosters 24 define at least a part of the
surfaces of a mold cavity 10. Right-side primary temperature
boosters 24 have the same purposes and requirements as left-side
primary temperature boosters 14 and can be made of the same or
different materials. They can be installed in any of the ways
already described for boosters 14. A right-side die 28 supports
boosters 24 in the same way left-side die 18 supports boosters 14
and has the same requirements. Fluid passages 12 and 22 are also
located in right-side die 28. Other thermal means for temperature
control stimuli may be used. An entrance 40 provides a way for the
molten material to enter mold cavity 10. Other means for
introducing molten material into the mold cavity may be used.
[0077] The invention is applicable to molding various articles but
is particularly useful for molding optical discs such as audio
compact discs and CD-ROMs. If mold 42 of FIG. 1 is used to make
such an optical disc, one of boosters 14 or 24 contains the digital
information to be transferred to the disc by molding using the
cavity surface. The information can be added to the booster surface
by etching or by other methods. Alternatively, the information can
be provided in a very thin coating of another material such as
quartz, titanium nitride, or nickel which is itself applied to the
cavity surface of booster 14 or 24. The coating must be
sufficiently thick to carry audio disc pits, which are
approximately 0.11 micron deep, but not so thick as to
significantly influence thermal behavior of the temperature
boosters. The thickness of the coating should be less than one
micron.
[0078] The mold 42 of FIG. 1 is operated by the steps:
[0079] (a). Provide mold 42.
[0080] (b). Apply temperature control stimuli to dies 18 and 28.
Preferably, circulate heat transfer fluid at approximately constant
temperatures through fluid passages 12 and 22 located in dies 18
and 28 adjusting fluid temperatures so that after many molding
cycles, temperatures cycle repeatably so that the surfaces of the
cavity are at predetermined temperatures just prior to contact by
the molten material to be introduced into the mold cavity. The
predetermined cavity surface temperatures are the temperatures that
will upon contact by the molten material, increase to the
temperatures required while the mold is filling to produce a molded
article. Because of the low k.rho.c product of the boosters, the
predetermined temperatures are substantially lower than when cavity
surfaces are formed by materials with higher k.rho.c products, such
as when the same die is made of common mold metals and does not
have any boosters. When molding thermoplastics, predetermined
temperatures are also substantially lower than when the
temperatures of cavity surfaces during mold filling are above the
solidifying temperature of the thermoplastic, as is done in some of
the prior art, e.g., Yotsutsuji, supra.
[0081] (c). Introduce molten material into cavity 10 through
entrance 40 and into contact with cavity surfaces formed by
boosters 14, 20 and 24 where the product of thermal conductivity,
density, and specific heat of the boosters cause heat flowing from
the molten material to increase cavity surface temperatures from
the predetermined temperatures (T.sub.P), which are significantly
below the temperatures required during mold filling, to or above
the required mold filling temperature. Representative temperature
increases from the initial predetermined temperatures (T.sub.P) to
a mold filling temperature (T) of 270.degree. F. when a cavity
surface is contacted by polycarbonate at 600.degree. F. are listed
below:
3 Cavity Surface Material Temperature Increase Aluminum 10
F.degree. H-13 tool steel 22 F.degree. 420 stainless steel 26
F.degree. sapphire booster 79 F.degree. quartz booster 136
F.degree.
[0082] This list shows that the temperature increases for cavity
surfaces of the boosters of this invention are substantially
greater than for cavity surfaces of dies made entirely from common
mold metals. The large temperature increases caused by the boosters
make it possible to bring cavity surfaces to the required
temperatures during mold filling from the low predetermined cavity
surface and die temperatures.
[0083] When this invention is used to mold a thermoplastic, the
temperatures (T) of cavity surfaces required during mold filling
are below the solidifying temperatures of the thermoplastic.
[0084] (d). While the cavity is filling with molten material, the
cavity surfaces are held at or above the temperatures (T) required
to produce a molded article by cooperatively using heat flowing
from the molten material to boosters 14, 20, and 24 and the
combined effect of the thermal conductivities, specific heats,
densities, and predetermined thicknesses of these boosters. The
thicknesses of the boosters being selected to be enough to hold
cavity surface temperatures at or above the temperatures required
to produce a molded article until the mold cavity is approximately
full yet optimally thin for the next step. The equation for
selecting booster thicknesses was provided above.
[0085] (e). The thicknesses of boosters 14 and 24, having been
selected in the preceding step, preferably to be as thin as
practicable, optimize the thermal resistances and capacity of the
boosters, which work in cooperation with the relatively cool die
temperatures and decreasing heat flow from the molten material.
Starting when the cavity is approximately full, heat flowing from
the boosters to the cooler dies, reduces the temperature of the
booster/cavity surfaces from or above the mold filling temperatures
(T) towards the predetermined cavity surface temperatures
(T.sub.P). Because the predetermined cavity surface temperatures
for this invention are substantially lower than they could be for
dies made entirely of common mold metals, the cavity surfaces can
be cooled more than they could be for common metal molds. In fact
they should be cooled more, because otherwise the temperature
increase caused by the molten material, which increase is large due
to the boosters, would cause the cavity to be above the
predetermined temperatures (T.sub.P) when the mold is available for
a next molding cycle. Waiting to cool further would lengthen the
molding cycle unnecessarily.
[0086] Keeping the dies relatively cool, and also regulating the
temperature such that the cavity surfaces are relatively cool after
the mold is full, increases the rate of heat transfer from the
molten material, because heat transfer is due in part to
temperature gradient. Thus cooler temperatures for the mold tend to
cool, stiffen, and solidify the workpiece faster. I have found that
the time to cool material from the molten state into an article
rigid enough for removal from a mold can be reduced at least twenty
percent by the technique described herein.
[0087] In contrast to the present invention, U.S. Pat. Nos.
3,734,449, 4,285,901, 5,041,247, and 5,064,597, the experiment of
Liou, supra, and my cross-referenced patent for molding
thermoplastics, operate at much hotter die temperatures. When the
mold is filling, the cavity surfaces exceed the solidifying
temperature of the thermoplastic being molded. The increased
temperatures of the cavity surfaces and dies reduce the rate of
heat flow from the thermoplastic during and after mold filling,
which increases the time to solidify the article. Liou reported
approximately twenty percent increase in cooling time in contrast
to at least a twenty percent decrease in cooling time afforded by
the methods and apparatus of this invention.
[0088] DuPont, supra, and U.S. Pat. Nos. 4,340,551, 4,338,068 and
3,619,449 employ complicated and expensive equipment to add heat
and remove heat. This causes the cavity surface temperature to
cycle, that is to increase for mold filling and to decrease after
the mold is full. This technique is useful but complex. The present
invention causes the desired cavity surface temperature cycling
without first adding and then removing heat at the dies and without
moving parts and complex controls.
[0089] I have found finite element transient heat transfer analysis
is a suitable way to select materials, evaluate thicknesses for
boosters 14, 20, and 24, and select temperatures for molten
material and dies. One dimensional models are fast and adequate for
preliminary evaluation and selection. Two or three dimensional
models and molding experiments can be used for final selections. An
example is provided in the description for FIG. 2.
[0090] FIG. 2 shows a simplified cross-section of mold 44, a second
embodiment that is useful for molding optical discs. A left-side
element 38" comprises stamper 32, left-side primary temperature
boosters 14", edge temperature booster 20", and left-side die 18".
Stamper 32 and edge booster 20" define at least a part of the
surfaces of a cavity 10".
[0091] The purpose of stamper 32 is to provide a surface that
contains the digital information that will be transferred to an
optical disc when it is molded. The stamper is made of material
that has the required durability at the temperatures and pressures
at which it must operate, and whose mathematical product of thermal
conductivity, density, and specific heat is greater than those of
boosters 14". Suitable materials can be selected from the group
consisting of metals, glasses, and ceramics. A thin nickel layer
0.010 to 0.013 inch thick is typically used for the stamper, but
the thickness may be 0.005 to 0.025 inch. The stamper increases the
demands on boosters 14", because the boosters must restrict heat
flow from the stamper to cause some of the heat from the molten
material to build up within the stamper to raise its temperature
and thereby the cavity surface temperatures. More heat is required
to raise the temperature of the stamper when the thickness, and the
mathematical product of density and specific heat of the stamper
are greater. I have also discovered that the greater the thickness,
and product of density and specific heat of the stamper, the more
time it takes to raise the cavity surface temperature from the
predetermined temperature to the temperature required during mold
filling. Although the time is short, it is significant compared to
the substantially instantaneous increase in temperature provided by
boosters, which may require higher predetermined temperatures, or a
layer thermally similar to the stamper on the opposite cavity face,
or boosters with lower k.rho.c products on the stamper side to
compensate. The performance penalties of the stamper are overcome
by two of its benefits compared to boosters: (1) the technology for
incorporating digital information into part of the cavity surface
of a metal stamper is in common use, and (2) the large investment
in stamper making equipment has already been made by stamper
manufacturers.
[0092] In addition to the demands imposed on the booster described
above, the purposes, requirements, and suitable materials for
boosters 14" and 20" are the same as those for boosters 14 and 20
of FIG. 1, mold 42. Although not shown in FIG. 2, booster
thicknesses can be different at different locations to promote more
uniform cooling, and boosters can be located behind only part of
the stamper. To more closely match the temperature history curve of
the cavity surface on the opposite side, the thickness of the
primary booster behind the stamper can be reduced. A value of "Y"
between 0.25 and 1.0 is typical. Except for the stamper, the mold
can be made using the same methods as used for mold 42 of FIG.
1.
[0093] Left-side die 18" provides structural support to stamper 32,
and boosters 14" and 20". The purposes, and requirements for die
18" and fluid passages 12" and 22" are the same as those for die 18
and passages 12 and 22 of mold 42 of FIG. 1.
[0094] FIG. 2 also shows a right-side element 36". Element 36"
comprises right-side primary temperature boosters 24", and
right-side die 28". Boosters 24" can be made of the same or
different materials as boosters 14", and have the same
requirements. The boosters can be installed in any of the ways
already described for left-side element 38". A right-side die 28"
supports boosters 24" in the same way left-side die 18" supports
boosters 14" and has the same requirements. Fluid passages 12" and
22" are also located in right-side die 28". Other thermal means for
temperature control stimuli for the die may be used. An entrance
40" provides a way for molten material to enter mold cavity 10".
Although not shown in the figures, entrance 40 can be on the left
side rather than the right side of the mold. Other means for
bringing molten material into the mold cavity may be used.
[0095] A computerized finite element analysis was performed for
Example 5, which simulates molding an optical disc using this
embodiment of the invention. For transient thermal analysis the
geometry, thermal conductivity, density, and specific heat of the
materials are input. The boundary temperatures, initial temperature
distribution, calculation time steps, and total heat transfer time
must also be provided. Five models were used to properly simulate a
molding cycle. The first (the cooling mode) depicts the mold closed
from the time hot molten polycarbonate contacts the cavity surface
until the mold is opened. The second depicts the mirror-side (the
side opposite from the stamper) while the mold is open and the
cavity surface is exposed to air. The third and fourth depict the
stamper-side first with the disc still in contact, then with the
disc removed and the stamper exposed to air. The final model (delay
model) depicts the mold closed before the molten polycarbonate
contacts the cavity surfaces. Room temperature was used as the
initial temperature for the cooling model. The output temperatures
from each model are used as the input temperatures for the
appropriate model that follows. That is the output temperatures
from the cooling model are the input temperatures for the mirror
and stamper with disc models, the output from the stamper with disc
is the input for the stamper exposed to air model, the output from
the mirror and stamper exposed to air models are the input for the
delay model, and the output from the delay model is the input for
the cooling model for the next cycle. The cycle is repeated until
the input and output temperatures are repeatable each cycle. This
can be as little as two cycles or more than forty cycles depending
upon the mold design and molded article. Thirty cycles were
required for the final run of this set of models.
[0096] The cooling model is 0.02 inch wide by 3.078 inches long. It
simulates a cut through the mold from heat transfer fluid passages
on the mirror-side to heat transfer passages on the stamper-side.
The distance from the heat transfer fluid passages to the stamper
is 1.513 inches, the stamper is 0.010 inch thick, the polycarbonate
optical disc is 0.048 inch thick, and the distance from the
polycarbonate to the heat transfer fluid passages on the
mirror-side is 1.510 inches. The temperatures of the heat transfer
fluid are provided as specified constant temperatures at the
appropriate locations in the model geometry. Substituting 0.5
seconds mold filling time and the thermal conductivity, density and
specific heat for sapphire into the equation for the primary
booster thickness, the calculated thickness for a sapphire booster
is 0.026 where "Y" equals 1.0. Similarly the thickness calculated
for a Kapton brand polyimide booster is 0.008. The actual
thicknesses is determined by trial and error through several finite
element model analysis runs. The distance from the fluid passages
to the stamper was initially divided into 1.505 inches consisting
of 12 elements of 420 stainless steel, and 0.008 inches consisting
of eight elements of Kapton. The distance from the polycarbonate to
the fluid passages on the mirror-side were divided into 0.080 inch
consisting of 8 elements of sapphire, and 1.430 inches consisting
of 6 elements of 420 stainless steel. The polycarbonate was
modelled as 16 elements starting at 0.0005 inch thick at each
cavity surface with each succeeding element thicker than the
preceding one as they approach the center of the polycarbonate. The
mirror model is made by deleting from the cooling model all
elements from the stamper-side fluid passages through to the first
sapphire element, which forms the cavity surface of the
mirror-side. The cavity surface is identified as a convection
boundary and the temperature and corresponding heat transfer
coefficients, which are calculated from formulas in standard
textbooks, are applied at these surfaces. A similar approach is
used for the remaining models. That is, delete all elements that do
not depict the desired model and add boundary conditions. This is a
common technique for developing a new model from an existing model
when applying the widely used finite element analysis method.
[0097] The initial time step for the transient heat flow analysis
was 1.5E -6 second with the time steps increasing in magnitude each
calculation step to a maximum of 0.25 seconds. The extremely small
initial time step was chosen to ensure mathematical stability since
the first polycarbonate element is only 0.0005 inch thick. It may
be possible to use a first time step as much as 60 times larger.
Trial computer runs were made adjusting the fluid temperatures and
booster thicknesses until the cavity surfaces were above the
temperatures required to mold an optical disc until the cavity was
approximately full. The final booster thicknesses were 0.002 inch
Kapton (Y=0.250) and 0.040 inch sapphire (Y=1.5).
[0098] The mold 44 of FIG. 2 is operated by the steps:
[0099] (a). Provide mold 44.
[0100] (b). This step is the same as step (b) for operation of mold
42 of FIG. 1.
[0101] (c). The right side of this mold operates in the same way as
step (c) for the operation of mold 42 of FIG. 1. For the left side,
molten material being introduced into cavity 10" through entrance
40" comes into contact with stamper 32 where heat flowing from the
molten material to said stamper causes temperatures of cavity
surfaces formed by the stamper to increase. Cavity surfaces reach
temperatures determined by the products of thermal conductivity,
density and specific heat of the molten material, and of the
materials used for the stampers, and by the temperatures of each
just before they contact. For example, if a nickel stamper
initially at 165.degree. F. is contacted by 600.degree. F. molten
polycarbonate the temperature at the cavity surface increases about
15.degree. F. to 180.degree. F. The cavity surfaces remain at these
temperatures as though boosters 14" do not exist until heat starts
flowing through the stamper into the boosters. Higher thermal
diffussivities, and less thicknesses of stamper 32, reduce the time
until heat flows through the stamper into the boosters. The thermal
diffussivity of a material is the thermal conductivity of the
material divided by its density and specific heat. When heat starts
flowing into boosters 14" the thermal conductivities, densities,
and specific heats of the booster (k.rho.c product) act
cooperatively to restrict the heat flow from the adjacent stamper
32. This causes the heat flowing from the molten material to build
up within stamper 32 such that temperatures of cavity surfaces of
the stamper increase to or above the temperatures (T) required
during mold filling to produce a molded article. When molding a
thermoplastic into an optical disc, the temperatures (T) of cavity
surfaces required during mold filling are below the solidifying
temperature of the thermoplastic. Polycarbonate is the current
material of choice of most, if not all, optical disc
manufacturers.
[0102] Steps (d) and (e) are the same as steps (d) and (e) for the
operation of mold 42 of FIG. 1.
[0103] FIG. 2A shows a simplified cross-section of mold 44, a
second embodiment that is useful for molding optical discs. Stamper
heating means 47 are also shown in FIG. 2A. The stamper 32 may
extend beyond the primary temperature booster 14" as shown in FIG.
2A. It may also contact high thermal conductivity materials at or
beyond the outside diameter of the mold cavity. When it does, it
creates a path for heat to flow from and cool the outer edge of the
optical disc excessively. Transfer of the pits from the stamper is
more difficult in the cooler material at the outer edge of the disc
reducing pit quality, which is detrimental to optical performance.
Stresses are also created at the outer edge of the disc which cause
birefringence that is also detrimental to optical performance. The
stamper heating means 47 overcomes this problem. The heating means
located in the mold die and in the vicinity adjacent to the
periphery of the primary temperature booster, and in thermal
contact with the stamper, increases the temperature of the stamper
at the area of contact enough to reduce, stop, or even reverse the
direction of heat flow. Alternatively, the stamper heating means
may be located in the mold die of an optical disc mold in a
vicinity of an outer diameter of the cavity and in thermal
communication with the stamper. Thermal contact between the heater
and the stamper may be accomplished directly or through an
intermediary material of high thermal conductivity. Preferably, the
heating means should have some thermal insulation from the die. The
stamper heating means could also be of the electrical resistive
heating type. By utilizing the stamper heating means, the
birefringence at the outer edge of the optical disc is thus brought
into acceptable limits.
[0104] An axisymmetric finite element model of an optical disc mold
with a 4.2 mm.times.2.2 mm cross section heater was made. The 2.2
mm surface of the heater contacted the stamper. The heater was
bonded in place with adhesive that thermally insulated it from the
die. The hold and cooling portion of the molding cycle was 2.0
seconds for the mold of the invention. With the heater off, the
temperature of the surface of the disc reached 201 F..degree. at 40
mm radius and 182 F..degree. at 59 mm radius by the end of the hold
period. When the mold opened the temperatures had cooled to 166
F..degree. at 40 mm radius and 152 F..degree. at 59 mm radius. With
190 watts applied to the model of the heater in the form of
energy/(time-unit volume), the temperature of the surface of the
disc reached 205 F..degree. at 40 mm radius and 203 F..degree. at
59 mm radius by the end of the hold period. When the mold opened
the temperatures were 169 F..degree. at 40mm radius and 176
F..degree. at 59 mm radius. The temperature of the heater ranged
between 167 F..degree. and 195 F..degree. during the cooling period
of the molding cycle and was higher when the stamper temperature
was higher. The mold without temperature boosters or stamper
heaters running at 3.1 seconds for hold and cooling is known to
produce acceptable compact discs. Using it, the surface of the disc
reaches 198 F..degree. at 40 mm radius and 186 F..degree. at 59 mm
radius by the end of the hold period. When this mold opens the
temperatures had cooled to 177 F..degree. at 40 mm radius and 166
F..degree. at 59 mm radius. The heater successfully caused the
maximum temperature of the disc surface to be very uniform from 40
mm to 59 mm radii at temperatures high enough to produce acceptable
discs. In fact, the maximum temperature at 59 mm is 17 F..degree.
higher than achieved by the mold without temperature boosters or
stamper heaters, which improves pit replication in this area.
[0105] An experiment was performed using a rectangular cross
section 350 watt eletrical heater, commonly known as a cable
heater, for the stamper heating means. The cable heater was bonded
to the die 18" with adhesive that also thermally insulated it from
the die 18". It was installed and machined so that one face of the
heater was flush with the primary booster 14". When the stamper was
installed it contacted the heater directly. For the purpose of the
experiment, a temperature controller was used to adjust the power
to and the temperature of the heater. During the experiments, the
mold was operated at various cooling times, and with various mold
coolant temperatures. In every case, the birefringence at the outer
edge of the disc was reduced to acceptable limits by adjusting the
heater temperature. Pit quality throughout the data area of the
disc was excellent. Without the heater the birefringence exceeded
acceptable limits.
[0106] For example, for 2.3 seconds hold and cooling, 75 F..degree.
stamper die coolant, 70 F..degree. opposing die coolant, and 175
F..degree. heater temperature, the birefringence and skew
(flatness) in the discs was unacceptably high at 59 mm radius. When
the stamper heater temperature was increased to 190 F..degree., the
molded discs met all quality requirements. Another example, for 2.5
seconds hold and cooling, 80 F..degree. stamper die coolant, 70
F..degree. opposing die coolant, and 190 F..degree. heater
temperature, the birefringence and skew in the discs were
unacceptably high at 59 mm radius. When the stamper heater
temperature was increased to 200 F..degree., the molded discs met
all quality requirements.
[0107] To ensure excellent pit quality at the outer edge of the
data area a molder may choose to bring the maximum temperature at
59 mm radius above the temperature midway through the data area (40
mm radius) by increasing the temperature of the stamper heater.
This would reverse the heat flow through the heater towards the
cavity.
[0108] By using a temperature controller the temperature of the
heater is quickly and easily adjusted to control the quality of the
outer edge of optical discs. The temperature and power requirements
are easily determined by finite element analysis or testing. A 350
watt 4.4 mm.times.2.2 mm cable heater operating in the range of 175
F..degree. to 230 F..degree. will cover most of the compact audio
disc molding conditions for a mold using temperature boosters of
this invention. The preferred starting temperature is 190
F..degree.. The stamper heater can also be used to make the maximum
temperatures of a disc being molded with an existing mold more
uniform. The same heater could be used, and the heater temperature
can easily be determined by finite element analysis or testing.
[0109] FIG. 3 shows a simplified cross-section of mold 46, a third
embodiment that is useful for molding an article. A left-side
element 38' comprises left-side temperature booster 14', left-side
secondary temperature booster 16, edge temperature booster 20', and
left-side die 18'. Boosters 14' and 20' define at least a part of
the surfaces of cavity 10'. Boosters 14' are made of materials that
have the required durability at the temperatures and pressures at
which they operate and whose mathematical products (k.rho.c) of
thermal conductivity (k), density (.rho.), and specific heat (c)
are no more than 2.0.times.10.sup.-6
BTU.sup.2/sec/in.sup.4/.degree. F..sup.2 and preferably no more
than 1.6.times.10.sup.-6 BTU.sup.2/sec/in.sup.4/.degre- e. F..sup.2
at room temperature. For booster 16, the k.rho.c product must be
smaller than it is for booster 14'. Edge booster 20' has the same
requirements as edge booster 20 of mold 42 of FIG. 1. Primary
booster materials identified under the description for mold 42 of
FIG. 1 can be used for boosters 14', 16 and 20'. In addition,
polyimide, liquid crystal polymer, and mica are examples of other
materials that can be used to make boosters 16 and 20'. Primary
boosters 14' can be bonded to dies 18" using an adhesive such as
acrylic, epoxy, or silicone that also functions as a secondary
booster 16. Secondary booster 16 is needed when the materials used
for primary boosters are selected for their desirable
characteristics such as those listed above, but their temperatures
do not increase sufficiently when contacted by the molten material
such that predetermined cavity surface and die temperatures are low
enough to accelerate cooling the amount desired. The secondary
boosters restrict heat flow from adjacent primary boosters causing
heat flowing from the molten material to build up within the
primary booster such that temperatures of their cavity surfaces
increase to or above that required during mold filling. Boosters
14' are shown thinner towards the cavity edges and booster 16 is
shown thicker towards the cavity edges to illustrate a design
suitable for molding a double convex lens. For articles other than
double convex lenses, the thicknesses may be uniform or thinned or
thickened at various locations to tailor heat flow for the specific
molded article. The thicknesses (W.sub.sb) of secondary boosters
fall within a range as calculated from the equation 4 W sb = Z k sb
t f sb c sb 0.025 Z 4.0
[0110] where t.sub.f is the time to fill the mold, k.sub.sb is the
thermal conductivity, .rho..sub.sb is the density, and c.sub.sb is
the specific heat of the secondary booster.
[0111] When the thickness of the molded article is substantially
uniform, the temperature control stimuli is substantially the same
distance from the surface on both sides of the mold cavity, and the
molded article is removed in a small fraction of the time the mold
is open, value of "Z" should be between 0.1 and 0.5. The primary
booster should be as thin as possible to allow secondary booster
effects to be felt as early as possible. A value of "Y" between
0.25 and 1 is desirable.
[0112] When the thickness of the molded article varies
significantly, The thickness of the secondary booster should be
increased where the molded article is thinner and decreased where
the molded article is thicker. A value of "Z" as low 0.025 may be
necessary where the molded article is thickest and as high as 4.0
where the molded article is thinnest. Generally, the thickness of
the primary booster should be decreased where the molded article is
thinner. A value of "Y" as low 0.25 may be necessary where the
molded article is thinner.
[0113] When the secondary booster is made of adhesive, availability
and processing characteristics may lead to less than optimum
adhesive thickness. For example, economics and available equipment
may dictate that a transfer film adhesive be used to ensure more
uniform adhesive (secondary booster) thickness, but the adhesive is
only available in thicknesses different from the desired thickness.
A value of "Z" between 0.025 and 2.0 may be selected. Specific
values of "Z" in the stated ranges can be varied for the same
reasons discussed above with respect to "Y" and finite element
temperature analysis modeling can be used as described.
[0114] Left-side die 18' provides structural support to boosters
14', 16 and 20'. The purposes, and requirements for die 18' and
fluid passages 12' and 22' are the same as those for die 18 and
passages 12 and 22 of mold 42 of FIG. 1.
[0115] FIG. 3 also shows a right-side element 36'. Element 36'
comprises right-side primary temperature boosters 24', right-side
secondary temperature booster 26, and right-side die 28'. Boosters
20', and 24' define at least a part of the surfaces of a mold
cavity 10'. Boosters 24' and 26 can be made of the same or
different materials as boosters 14' and 16, respectively. They also
have the same purposes and requirements as boosters 14' and 16. A
right-side die 28' supports boosters 20', 24' and 26 in the same
way left-side die 18' supports boosters 14'and 16 and has the same
requirements. Fluid passages 12', and 22' are also located in
right-side die 28. Other thermal means for temperature control
stimuli for the dies may be used. An entrance 40' provides a means
for the molten material to enter mold cavity 10'. Other means for
bringing molten material into the mold cavity may be used. The mold
can be made using the same methods used for mold 42 of FIG. 1.
[0116] Although not shown in FIG. 2 or FIG. 3, stamper 32 can be
used with molds that have both primary and secondary boosters.
[0117] The mold 46 of FIG. 3 is operated by the steps:
[0118] (a). Provide a mold 46.
[0119] (b). This step is the same as step (b) for the operation of
mold 42 of FIG. 1.
[0120] (c). Introduce molten material into cavity 10' through
entrance 40' and into contact with cavity surfaces formed by
temperature boosters 14', 20', and 24' where the mathematic
products of thermal conductivity, density, and specific heat of
these boosters cause heat flowing from the molten material to these
boosters to increase temperatures of cavity surfaces. Cavity
surfaces reach temperatures determined by the mathematical product
of thermal conductivity, density, and specific heat of the molten
material, and of the materials used for these boosters, and by the
temperatures of each just before they contact. The cavity surfaces
remains at these temperatures as though secondary boosters 16 and
26 do not exist until heat starts flowing through the primary
boosters into the secondary boosters. The thinner boosters 14' and
24' are, and the higher their thermal diffussivities, the sooner
heat starts flowing into the secondary boosters 16 and 26. When
heat starts flowing into secondary boosters 16 and 26, the thermal
conductivity, density, and specific heat of these boosters act
cooperatively to restrict the heat flow from the adjacent booster
14' or 24'. This causes heat flowing from the molten material to
build up within primary boosters 14' and 24', thereby further
increasing temperatures of cavity surfaces to or above the
temperatures required during mold filling to produce a molded
article. When molding a thermoplastic, the temperatures (T) of
cavity surfaces required during mold filling are below the
solidifying temperatures of the thermoplastic. By thinning primary
boosters 14' and 24', and thickening secondary boosters 16 and 26
as they approach edges of cavity 10' as shown in FIG. 3, heat flow
decreases towards the edges of the molded article where the heat
flow is normally greatest. Boosters 14', 16, 24', and 26 can be
thinned or thickened as necessary to tailor heat flow for a
specific molded article.
[0121] (d). While the cavity is filling with molten material, hold
the cavity surfaces at or above the temperatures (T) required to
produce a molded article by cooperatively using heat flowing from
the molten material to boosters 14', 16, 20', 24', and 26' and the
mathematical product of the thermal conductivity, specific heat,
and density, and the predetermined thicknesses of each of these
boosters.
[0122] (e). This step is the same as step (e) for the operation of
mold 42 of FIG. 1.
[0123] Transient heat transfer analysis using finite element
techniques is a suitable way to select the materials and determine
the final thicknesses of boosters 14', 16, 24', and 26,
particularly using available computer programs. Depending on the
geometry, one or two dimensional models are adequate for material
selection and evaluating preliminary sizing. Three dimensional
models or molding experiments may be appropriate for final
design.
EXAMPLES
[0124] Using NISA II, I performed transient thermal analyses of
all-metal molds and molds and methods according to this invention.
NISA II is a commercially available family of finite element
analysis software programs. The computer models are one
dimensional.
[0125] The examples illustrate the advantages of the invention.
They show where 155.degree. F. dies are normally required,
105.degree. F. dies can now be used. Where cooling normally takes
34 seconds, it can now be done in 25.5 seconds. The methods and
apparatus of this invention, although they employ thermally
insulating surface temperature boosters that could be expected to
extend cooling time, are nevertheless shown to be effective to cool
mold surfaces after the cavity is full, significantly faster than
comparable methods using uninsulated (e.g., common metal) dies.
Consequently, at least the surface skin of the molded article is
cooler and more rigid at the time the article is ejected from the
mold than it would be if a common metal mold were used.
[0126] In many cases, a molded article is rigid enough to be
removed from the mold before the center of the molded article is
solid. In every case it will be more rigid than an article molded
in a common metal mold and can be removed sooner. If the article
can be removed when the center material is not yet solid, the molds
and methods of the invention will reduce cooling time more than
indicated by the finite element examples, which show reductions of
cooling time of at least twenty five percent.
Example 1
[0127] Thermal analysis of a mold 42 according to FIG. 1 was
compared with that for an all-metal mold. The molded article is
0.105" thick polystyrene. The temperature boosters 14 and 24 are
0.040" thick sapphire. The dies 18 and 28 are 420 stainless steel.
The all-metal molds are also 420 stainless steel. At least thirty
mold cycles were simulated for each mold. The time assumed to open
the mold, remove the molded article and close the mold was 4
seconds.
[0128] For the all-metal mold, the heat transfer fluid temperature
applied to the die was kept at 155.degree. F. The predetermined
cavity surface temperature was 179.degree. F. and was repeatable
within thirty cycles. The cavity surface temperature increased to
198.degree. F. upon contact by 450.degree. F. polystyrene, which
also was repeatable. The center of the polystyrene cooled to the
solidifying temperature of 212.degree. F. in 34 seconds.
[0129] For mold 42 of FIG. 1, the heat transfer fluid temperature
at the die was kept at 103.degree. F. left-side and 105.degree. F.
right-side. The predetermined cavity surface temperature was
repeatable at 139.degree. F. for both sides after thirty cycles.
The cavity surface temperatures repeatedly increased to 198.degree.
F. upon resin contact. The 198.degree. F. temperature was held for
about 0.5 seconds, which is the time assumed to fill the mold. The
center of the polystyrene cooled to the solidifying temperature of
212.degree. F. in 25.5 seconds. The invention reduced the cooling
time 25 percent as compared to the all-metal mold control example.
FIG. 4A shows the temperature histories for the center of the resin
and the cavity surface for both molds.
Example 2
[0130] The thermal analysis of Example 1 was repeated using 0.033
inch thick borosilicate glass temperature boosters. The temperature
of the heat transfer fluid at the die was held at 40.degree. F.
left-side and 42.degree. F. right-side. After thirty cycles, the
predetermined cavity surface temperature became repeatable at
86.degree. F. left-side and 87.degree. F. right-side. Upon contact
by 450.degree. F. polystyrene, the surface temperatures increased
repeatedly to 200.degree. F. The cavity surface temperatures
remained above 198.degree. F. about 0.5 seconds, which is the time
assumed to fill the mold. The center of the polystyrene decreased
to the solidifying temperature of 212.degree. F. in 21 seconds. The
invention shortened the cooling time 38 percent compared to the
all-metal mold of Example 1. FIG. 4B shows the temperature
histories for the center of the resin and for the cavity surface
for the invention and for the all-metal mold.
Example 3
[0131] Another thermal analysis was performed for mold 42, this
time for a 0.048" thick polycarbonate molded article. The
temperatures of the heat transfer fluid for the all-metal dies were
170.degree. F. left-side and 175.degree. F. right-side. After over
30 cycles, the predetermined cavity temperatures were repeatable at
262.degree. F. left-side and 265.degree. F. right-side. Upon
contact by 620.degree. F. polycarbonate, the left-side temperature
increased to 294.degree. F. and the right-side temperature to
297.degree. F., which were repeatable. The cavity surfaces were
above 294.degree. F. at 0.25 seconds, which is the time assumed to
fill the cavity. The center of the resin cooled to 295.degree. F.
in 5 seconds, which is 10.degree. F. below the solidifying
temperature of this polycarbonate. The time to open the mold,
remove the article, and close the mold was 3.75 seconds.
[0132] For the invention, the boosters were changed to 0.030" thick
sapphire. The temperatures of the heat transfer fluid applied to
the dies were 65.degree. F. left-side and 1 10.degree. F.
right-side. After 30 cycles, the predetermined cavity temperatures
were repeatable at 206.degree. F. left-side and 203.degree. F.
right-side. Upon resin contact, the left-side increased to
302.degree. F. and the right-side to 299.degree. F. and was
repeatable. The cavity surfaces were above 298.degree. F. at 0.25
seconds, which is time assumed to fill the cavity. The center of
the resin cooled to 295.degree. F. in 3.25 seconds. The cooling
time was reduced 35 percent.
[0133] Temperature histories for the center of the resin and the
cavity surfaces for both molds are shown in FIG. 4C.
Example 4
[0134] A thermal analysis of mold 42 was also done for a 0.105"
polycarbonate article. The temperature of the heat transfer fluid
for the all-metal dies was 205.degree. F. for both left-side and
right-sides. After 30 cycles, the predetermined cavity temperatures
were repeatable at 258.degree. F. left-side and 257.degree. F.
right-side. Upon contact by 620.degree. F. polycarbonate, the
left-side increased to 290.degree. F. and the right-side to
289.degree. F., which was repeatable. The cavity surface was
slightly under 289.degree. F. at 0.5 seconds, which is the chosen
time for the cavity to fill. The center of the resin cooled to
295.degree. F. in 18 seconds.
[0135] Boosters made of 0.035" sapphire were chosen according to
the invention. The polycarbonate temperature was reduced from
620.degree. F. to 580.degree. F. The temperatures of the heat
transfer fluid applied to the dies were 152.degree. F. left-side
and 158.degree. F. right-side. After more than 30 cycles, the
predetermined cavity temperatures were repeatable at 203.degree. F.
left-side and 202.degree. F. right-side. Upon resin contact, both
the left-side and right-sides increased to 290.degree. F. The
cavity surfaces were above 286.degree. F. at 0.5 seconds. The
center of the resin cooled to 295.degree. F. in 13.5 seconds. The
cooling time was reduced 25 percent as compared to the all-metal
mold control example.
[0136] FIG. 4D shows the temperature histories for the center of
the resin and the cavity surfaces for both molds.
Example 5
[0137] This example simulates molding a 0.048 inch thick
polycarbonate optical disc in mold 44 of FIG. 2. Item 32 is a 0.010
inch thick nickel stamper that has digital information in the
cavity surface for transfer to the optical disc. The optical disc
can be a compact audio disc or a CD-ROM, for example. Boosters 14"
and 24" are 0.002 inch thick Kapton brand polyimide film, and 0.04
inch thick sapphire respectively. The time to open the mold, remove
the disc, and close the mold is 3.75 seconds.
[0138] The all-metal mold has a 0.010 inch thick nickel stamper on
the left-side. It carries digital information for transfer to the
optical disc. The heat transfer fluid at the die on the stamper
side is held at 165.degree. F. The heat transfer fluid is held at
170.degree. F. for the right-side. After over 30 cycles, the
predetermined temperatures are repeatable at 253.degree. F. on the
left-side and 254.degree. F. on the right-side. Upon contact by
620.degree. F. polycarbonate, the temperatures increase to
282.degree. F. on the left-side and 285.degree. F. on the
right-side at 0.5 seconds when mold filling has just completed.
This also is repeatable. The center of the resin cools from
620.degree. F. to 273.degree. F. in 5 seconds.
[0139] For the mold of the invention, the heat transfer fluid is
held at 65.degree. F. on the left-side and 75.degree. F. on the
right-side. After over 30 cycles, the predetermined temperatures
are repeatable at 189.degree. F. on the left-side and 188.degree.
F. on the right-side. Upon contact by 620.degree. F. polycarbonate,
temperatures increase to 286.degree. F. and 285.degree. F. on the
left and right side respectively. At 0.5 seconds, when mold filling
has just completed, they are at 285.degree. F. and 284.degree. F.
at the left and right sides. The center of the resin cools to
273.degree. F. in 3.25 seconds. In this example the invention
reduced the cooling time 35 percent.
[0140] FIG. 5 shows the temperature histories for the center of the
resin and the cavity surfaces for both the all-metal mold and the
mold of the invention.
[0141] While several embodiments of the invention have been shown
and described, it is to be understood that the invention is not
limited to the specific examples, but is susceptible to changes and
modifications known to persons skilled in the art. For example, the
mold and methods can be adapted for use in casting, pressing,
compression molding, injection molding, blow molding, and
combinations of these. Plastics, metals, glass and ceramics may be
molded according to the invention. The initial bulk temperature of
the molten material can be increased compared to methods using
common metal molds and the predetermined temperatures lowered even
more than described in the examples. Whereas the invention is
intended to encompass these and other variations, reference should
be made to the appended claims to determine the scope of the
invention in which exclusive rights are claimed.
* * * * *