U.S. patent application number 13/601359 was filed with the patent office on 2013-11-07 for injection mold having a simplified evaporative cooling system or a simplified cooling system with exotic cooling fluids.
The applicant listed for this patent is Charles John Berg, JR., Ralph Edwin NEUFARTH, Robert Lawrence Prosise, Niall D. Robinson, Rainer Scharrenberg. Invention is credited to Charles John Berg, JR., Ralph Edwin NEUFARTH, Robert Lawrence Prosise, Niall D. Robinson, Rainer Scharrenberg.
Application Number | 20130295219 13/601359 |
Document ID | / |
Family ID | 49512707 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130295219 |
Kind Code |
A1 |
NEUFARTH; Ralph Edwin ; et
al. |
November 7, 2013 |
Injection Mold Having a Simplified Evaporative Cooling System or a
Simplified Cooling System with Exotic Cooling Fluids
Abstract
An injection mold assembly for a high output consumer product
injection molding machine, the injection mold assembly having a
simplified cooling system that is an evaporative cooling system or
a cooling system including a hazardous, dangerous, or expensive
cooling fluid. The simplified cooling system has a cooling fluid
channel that is confined to a mold support plate.
Inventors: |
NEUFARTH; Ralph Edwin;
(Liberty Township, OH) ; Robinson; Niall D.;
(Newburyport, MA) ; Scharrenberg; Rainer;
(Marburg, DE) ; Prosise; Robert Lawrence;
(Cincinnati, OH) ; Berg, JR.; Charles John;
(Wyoming, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NEUFARTH; Ralph Edwin
Robinson; Niall D.
Scharrenberg; Rainer
Prosise; Robert Lawrence
Berg, JR.; Charles John |
Liberty Township
Newburyport
Marburg
Cincinnati
Wyoming |
OH
MA
OH
OH |
US
US
DE
US
US |
|
|
Family ID: |
49512707 |
Appl. No.: |
13/601359 |
Filed: |
August 31, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61641349 |
May 2, 2012 |
|
|
|
Current U.S.
Class: |
425/552 |
Current CPC
Class: |
B29C 35/16 20130101;
B29K 2905/02 20130101; B29C 33/046 20130101; B29C 45/7306 20130101;
B29K 2995/0013 20130101; B29L 2031/757 20130101; B29C 45/7337
20130101; B29C 45/7312 20130101 |
Class at
Publication: |
425/552 |
International
Class: |
B29C 45/73 20060101
B29C045/73 |
Claims
1. A mold assembly for an injection molding machine, the mold
assembly comprising: a first mold side and a second mold side, the
first mold side and the second mold side defining a mold cavity
therebetween; a first mold support plate being disposed one of
immediately adjacent to or in direct contact with the first mold
side; a second mold support plate being disposed one of immediately
adjacent to or in direct contact with the second mold side; and an
evaporative cooling system for removing heat from one of the first
and second mold sides during an injection molding process, the
evaporative cooling system including one or more cooling fluid
channels, wherein none of the cooling fluid channels of the
evaporative cooling system extends into the first mold side or the
second mold side.
2. The mold assembly of claim 1, wherein the evaporative cooling
system includes a cooling fluid that circulates in a closed-loop
cooling circuit through one or more cooling fluid channels.
3. The mold assembly of claim 2, wherein the closed-loop cooling
circuit is confined to one of the first mold support plate and the
second mold support plate.
4. The mold assembly of claim 3, wherein the one of the first mold
support plate and the second mold support plate includes a
projection and the corresponding first mold side or second mold
side includes a complimentary recess that is sized and shaped to
receive the projection.
5. The mold assembly of claim 1, wherein the evaporative cooling
system includes a condenser coupled to one of the first mold
support plate and the second mold support plate.
6. The mold assembly of claim 1, wherein the evaporative cooling
system includes a spray bar that sprays cooling fluid on a surface
of one of the first mold support plate and the second mold support
plate, the cooling fluid evaporating on the surface thereby
extracting heat from the one of the first mold support plate and
the second mold support plate.
7. The mold assembly of claim 1, wherein the first and second mold
sides are made from materials having a thermal conductivity of
greater than 30 BTU/HR FT .degree. F.
8. The mold assembly of claim 1, wherein one of the first mold
support plate and the second mold support plate has an average
thermal conductivity that is higher than the average thermal
conductivity of one of the first and second mold sides.
9. The mold assembly of claim 1, wherein at least one of the first
and second mold sides is made of a material having an average
surface hardness of less than 30 Rc.
10. The mold assembly of claim 1, wherein a mold cavity between the
first and second mold sides defines a part having a wall thickness
of less than about 2 mm.
11. The mold assembly of claim 1, wherein at least one of the first
and second mold sides is made from aluminum.
12. The mold assembly of claim 1, wherein at least one of the first
and second mold sides is formed from a material having at least one
of, a milling machining index of greater than 100%, a drilling
machining index of greater than 100%, and a wire EDM machining
index of greater than 100%.
13. The mold assembly of claim 1 disposed in a substantially
constant low pressure injection molding system.
14. The mold assembly of claim 1, further comprising a second
cooling system including a plurality of cooling channels extending
into at least one of the first mold support plate, the second mold
support plate, the first mold side, and the second mold side, and
the second cooling system including circulating cooling liquid.
15. The mold assembly of claim 1, further comprising at least one
of, a mold cavity having an L/T ratio of greater than 100; at least
four mold cavities; one or more heated runners; a balanced molten
plastic feed system; and a guided ejection system.
16. A mold assembly for an injection molding machine, the mold
assembly comprising: a first mold side and a second mold side, the
first mold side and the second mold side defining a mold cavity
therebetween; a first mold support plate being disposed one of
immediately adjacent to or in direct contact with the first mold
side; a second mold support plate being disposed one of immediately
adjacent to or in direct contact with the second mold side; a
cooling system for removing heat from one of the first and second
mold sides during an injection molding process; and a cooling fluid
disposed in the cooling system, the cooling fluid having a thermal
conductivity of about 1 W/mK or greater.
17. The mold assembly of claim 16, further comprising cooling
channels that are confined to one of the first and second mold
support plates.
18. A mold assembly for an injection molding machine, the mold
assembly comprising: a first mold side and a second mold side, the
first mold side and the second mold side defining a mold cavity
therebetween; a first mold support plate one of immediately
adjacent to or in direct contact with the first mold side; a second
mold support plate one of immediately adjacent to or in direct
contact with the second mold side; an evaporative cooling system
for removing heat from one of the first and second mold sides
during an injection molding process, the evaporative cooling system
having an evaporative cooling channel confined to the first or
second mold support plate.
19. The mold assembly of claim 18, wherein the evaporative cooling
system includes a compressor, a condenser, and an expansion valve
located outside of the first or second mold support plate, the
compressor, the condenser, and the expansion valve being fluidly
connected with one of the first and second mold sides.
20. The mold assembly of claim 19, wherein the first mold support
plate is in complete contact with the first mold side or the second
mold support plate is in complete contact with the second mold
side.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/641,349, filed May 2, 2012 which is hereby
incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to injection molds, more
particularly, to injection molds having a simplified evaporative
cooling system.
BACKGROUND
[0003] Injection molding is a technology commonly used for
high-volume manufacturing of parts made of meltable material, most
commonly of parts made of thermoplastic polymers. During a
repetitive injection molding process, a plastic resin, most often
in the form of small beads or pellets, is introduced to an
injection molding machine that melts the resin beads under heat,
pressure, and shear. The now molten resin is forcefully injected
into a mold cavity having a particular cavity shape. The injected
plastic is held under pressure in the mold cavity, cooled, and then
removed as a solidified part having a shape that essentially
duplicates the cavity shape of the mold. The mold itself may have a
single cavity or multiple cavities. Each cavity may be connected to
a flow channel by a gate, which directs the flow of the molten
resin into the cavity. Thus, a typical injection molding procedure
comprises four basic operations: (1) heating the plastic in the
injection molding machine to allow it to flow under pressure; (2)
injecting the melted plastic into a mold cavity or cavities defined
between two mold halves that have been closed; (3) allowing the
plastic to cool and harden in the cavity or cavities while under
pressure; and (4) opening the mold halves to allow the part to be
ejected from the mold.
[0004] The molten plastic resin is injected into the mold cavity
and the plastic resin is forcibly pushed through the cavity by the
injection molding machine until the plastic resin reaches the
location in the cavity furthest from the gate. The resulting length
and wall thickness of the part is a result of the shape of the mold
cavity.
[0005] The molds used in injection molding machines must be capable
of withstanding these high melt pressures. Moreover, the material
forming the mold must have a fatigue limit that can withstand the
maximum cyclic stress for the total number of cycles a mold is
expected to run over the course of its lifetime. As a result, mold
manufacturers typically form the mold from materials having high
hardness, such as tool steels, having greater than 30 Rc, and more
often greater than 50 Rc. These high hardness materials are durable
and equipped to withstand the high clamping pressures required to
keep mold components pressed against one another during the plastic
injection process. Additionally, these high hardness materials are
better able to resist wear from the repeated contact between
molding surfaces and polymer flow.
[0006] High production injection molding machines (i.e., class 101
and class 102 molding machines) that produce thinwalled consumer
products exclusively use molds having a majority of the mold made
from the high hardness materials. High production injection molding
machines typically produce 500,000 parts or more. Industrial
quality production molds must be designed to produce at least
500,000 parts, preferably more than 1,000,000 parts, more
preferably more than 5,000,000 parts, and even more preferably more
than 10,000,000 parts. These high production injection molding
machines have multi cavity molds and complex cooling systems to
increase production rates. The high hardness materials described
above are more capable of withstanding the repeated high pressure
clamping and injection operations than lower hardness materials.
However, high hardness materials, such as most tool steels, have
relatively low thermal conductivities, generally less than 20
BTU/HR FT .degree. F., which leads to long cooling times as heat is
transferred from the molten plastic material through the high
hardness material to a cooling fluid.
[0007] In an effort to reduce cycle times, typical high production
injection molding machines having molds made of high hardness
materials include relatively complex internal cooling systems that
circulate cooling fluid within the mold. These cooling systems
accelerate cooling of the molded parts, thus allowing the machine
to complete more cycles in a given amount of time, which increases
production rates and thus the total amount of molded parts
produced. However, these cooling systems add complexity and cost to
the injection molds. In some class 101 molds more than 1 or 2
million parts may be produced, these molds are sometimes referred
to as "ultra high productivity molds" Class 101 molds that run in
400 ton or larger presses are sometimes referred to as "400 class"
molds within the industry.
[0008] High hardness materials are generally fairly difficult to
machine. As a result, known high throughput injection molds require
extensive machining time and expensive machining equipment to form,
and expensive and time consuming post-machining steps to relieve
stresses and optimize material hardness. Milling and/or forming
cooling channels within these complex molds adds even more time and
costs to the manufacture of typical high throughput injection
molds.
[0009] There is a tradeoff between machining complexity and cooling
efficiency in traditional, high hardness molds. Ideally, cooling
channels should be machined as close to the mold cavity surfaces as
possible. Additionally, conformal cooling is desirable and most
effective. However, machining conformal cooling channels close to
molding surfaces is difficult, time consuming, and expensive.
Generally, machining cooling channels within about 5 mm of the mold
surfaces is considered to be the practical limit. This practical
limit reduces cooling efficiency due to material between the
cooling fluid and the hot plastic having low thermal conductivity.
Conventional machining techniques, along with conventional mold
materials (i.e., high hardness and low thermal conductivity) place
a lower limit on cycle time and cooling efficiency for a given
mold.
[0010] Furthermore, locating cooling lines close to the mold
surfaces requires precise machining of the cooling lines in the
molds. Because the molds are supported by mold support plates when
placed in a clamping device of the injection molding machine, fluid
seals must be located where the cooling lines transition from the
mold support plate to the mold (because the fluid circulating
systems (e.g., pumps) must be located outside of the molds). These
fluid seals may fail, causing cooling fluid to escape. As a result,
parts may be incompletely cooled, which produces an inferior part,
or the plastic in the mold may be contaminated with cooling fluid,
which is also undesirable.
[0011] Still further, practical limitations on machining cooling
channels results in unequal cooling within the mold. As a result,
temperature gradients are produced within the mold cavity. Often
the temperature of the surface of a mold cavity can vary by ten
degrees Celsius or more. This wide variation in temperature within
the mold can lead to imperfections in the molded parts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The embodiments set forth in the drawings are illustrative
and exemplary in nature and not intended to limit the subject
matter defined by the claims. The following detailed description of
the illustrative embodiments can be understood when read in
conjunction with the following drawings, where like structure is
indicated with like reference numerals and in which:
[0013] FIG. 1 illustrates a schematic view of an injection molding
machine constructed according to the disclosure;
[0014] FIG. 2 illustrates one embodiment of a thin-walled part
formed in the injection molding machine of FIG. 1;
[0015] FIG. 3 is a cavity pressure vs. time graph for a mold cavity
in a mold of the injection molding machine of FIG. 1;
[0016] FIG. 4 is a cross-sectional view of one embodiment of a mold
assembly of the injection molding machine of FIG. 1;
[0017] FIGS. 5A-5E illustrate different views of various mold
assemblies having a plurality of cooling lines machined in a mold
support plate;
[0018] FIG. 6 illustrates a cross-sectional view of a mold assembly
having a plurality of cooling lines machined in a mold support
plate that extend into a mold side;
[0019] FIG. 7 illustrates a close-up sectional view of a cooling
line including a baffle;
[0020] FIG. 8 illustrates a perspective cross-sectional view of a
mold assembly including a plurality of cooling lines machined along
at least two different axes;
[0021] FIG. 9 illustrates a perspective cross-sectional view of a
mold assembly having a plurality of terminal cooling lines and a
plurality of through bore cooling lines machined along at least two
different machining axes;
[0022] FIG. 10 illustrates a perspective partially transparent view
of a mold assembly having a plurality of cooling lines, at least
one of the cooling lines being formed by two terminal cooling lines
that join one another at terminal ends to form a non-terminal
cooling line, each terminal cooling line being machined along a
different machining axis;
[0023] FIG. 11 illustrates a perspective view of a mold assembly
having an actively cooled dynamic part;
[0024] FIG. 12 illustrates a perspective view of a mold assembly
having at least one cooling line that includes non-linear,
non-coaxial, or non-planar cooling channel;
[0025] FIG. 13 illustrates one embodiment of a cube mold that
incorporates a mold having a simplified cooling system;
[0026] FIG. 14 illustrates one embodiment of a vapor compression
evaporative cooling system;
[0027] FIG. 15 illustrates another embodiment of an evaporative
cooling system;
[0028] FIG. 16A illustrates an embodiment of an evaporative cooling
system contained, at least partially, within a mold support
plate;
[0029] FIG. 16B illustrates an alternate embodiment of an
evaporative cooling system contained, at least partially, within a
mold support plate; and
[0030] FIG. 17 illustrates one embodiment of an external
evaporative cooling system.
DETAILED DESCRIPTION
[0031] Embodiments of the present invention generally relate to
systems, machines, products, and methods of producing products by
injection molding and more specifically to systems, products, and
methods of producing products by low constant pressure injection
molding.
[0032] The term "low pressure" as used herein with respect to melt
pressure of a thermoplastic material, means melt pressures in a
vicinity of a nozzle of an injection molding machine of 6000 psi
and lower.
[0033] The term "substantially constant pressure" as used herein
with respect to a melt pressure of a thermoplastic material, means
that deviations from a baseline melt pressure do not produce
meaningful changes in physical properties of the thermoplastic
material. For example, "substantially constant pressure` includes,
but is not limited to, pressure variations for which viscosity of
the melted thermoplastic material do not meaningfully change. The
term "substantially constant" in this respect includes deviations
of approximately 30% from a baseline melt pressure. For example,
the term "a substantially constant pressure of approximately 4600
psi" includes pressure fluctuations within the range of about 6000
psi (30% above 4600 psi) to about 3200 psi (30% below 4600 psi). A
melt pressure is considered substantially constant as long as the
melt pressure fluctuates no more than 30% from the recited
pressure.
[0034] Referring to the figures in detail, FIG. 1 illustrates an
exemplary low constant pressure injection molding apparatus 10 for
producing thin-walled parts in high volumes (e.g., a class 101 or
102 injection mold, or an "ultra high productivity mold"). The
injection molding apparatus 10 generally includes an injection
system 12 and a clamping system 14. A thermoplastic material may be
introduced to the injection system 12 in the form of thermoplastic
pellets 16. The thermoplastic pellets 16 may be placed into a
hopper 18, which feeds the thermoplastic pellets 16 into a heated
barrel 20 of the injection system 12. The thermoplastic pellets 16,
after being fed into the heated barrel 20, may be driven to the end
of the heated barrel 20 by a reciprocating screw 22. The heating of
the heated barrel 20 and the compression of the thermoplastic
pellets 16 by the reciprocating screw 22 causes the thermoplastic
pellets 16 to melt, forming a molten thermoplastic material 24. The
molten thermoplastic material is typically processed at a
temperature of about 130.degree. C. to about 410.degree. C.
[0035] The reciprocating screw 22 forces the molten thermoplastic
material 24, toward a nozzle 26 to form a shot of thermoplastic
material, which will be injected into a mold cavity 32 of a mold
28. The molten thermoplastic material 24 may be injected through a
gate 30, which directs the flow of the molten thermoplastic
material 24 to the mold cavity 32. The mold cavity 32 is formed
between first and second mold parts 25, 27 of the mold 28 and the
first and second mold parts 25, 27 are held together under pressure
by a press or clamping unit 34. The press or clamping unit 34
applies a clamping force in the range of approximately 1000 psi to
approximately 6000 psi during the molding process to hold the first
and second mold parts 25, 27 together while the molten
thermoplastic material 24 is injected into the mold cavity 32. To
support these clamping forces, the clamping system 14 may include a
mold frame and a mold base, the mold frame and the mold base being
formed from a material having a surface hardness of more than about
165 BHN and preferably less than 260 BHN, although materials having
surface hardness BHN values of greater than 260 may be used as long
as the material is easily machineable, as discussed further
below.
[0036] Once the shot of molten thermoplastic material 24 is
injected into the mold cavity 32, the reciprocating screw 22 stops
traveling forward. The molten thermoplastic material 24 takes the
form of the mold cavity 32 and the molten thermoplastic material 24
cools inside the mold 28 until the thermoplastic material 24
solidifies. Once the thermoplastic material 24 has solidified, the
press 34 releases the first and second mold parts 25, 27, the first
and second mold parts 25, 27 are separated from one another, and
the finished part may be ejected from the mold 28. The mold 28 may
include a plurality of mold cavities 32 to increase overall
production rates.
[0037] A controller 50 is communicatively connected with a sensor
52 and a screw control 36. The controller 50 may include a
microprocessor, a memory, and one or more communication links. The
controller 50 may be connected to the sensor 52 and the screw
control 36 via wired connections 54, 56, respectively. In other
embodiments, the controller 50 may be connected to the sensor 52
and screw control 56 via a wireless connection, a mechanical
connection, a hydraulic connection, a pneumatic connection, or any
other type of communication connection known to those having
ordinary skill in the art that will allow the controller 50 to
communicate with both the sensor 52 and the screw control 36.
[0038] In the embodiment of FIG. 1, the sensor 52 is a pressure
sensor that measures (directly or indirectly) melt pressure of the
molten thermoplastic material 24 in the nozzle 26. The sensor 52
generates an electrical signal that is transmitted to the
controller 50. The controller 50 then commands the screw control 36
to advance the screw 22 at a rate that maintains a substantially
constant melt pressure of the molten thermoplastic material 24 in
the nozzle 26. While the sensor 52 may directly measure the melt
pressure, the sensor 52 may measure other characteristics of the
molten thermoplastic material 24, such as temperature, viscosity,
flow rate, etc, that are indicative of melt pressure. Likewise, the
sensor 52 need not be located directly in the nozzle 26, but rather
the sensor 52 may be located at any location within the injection
system 12 or mold 28 that is fluidly connected with the nozzle 26.
If the sensor 52 is not located within the nozzle 26, appropriate
correction factors may be applied to the measured characteristic to
calculate the melt pressure in the nozzle 26. In yet other
embodiments, the sensor 52 need not be fluidly connected with the
nozzle. Rather, the sensor could measure clamping force generated
by the clamping system 14 at a mold parting line between the first
and second mold parts 25, 27.
[0039] Although an active, closed loop controller 50 is illustrated
in FIG. 1, other pressure regulating devices may be used instead of
the closed loop controller 50. For example, a pressure regulating
valve (not shown) or a pressure relief valve (not shown) may
replace the controller 50 to regulate the melt pressure of the
molten thermoplastic material 24. More specifically, the pressure
regulating valve and pressure relief valve can prevent
overpressurization of the mold 28. Another alternative mechanism
for preventing overpressurization of the mold 28 is an alarm that
is activated when an overpressurization condition is detected.
[0040] Turning now to FIG. 2, an example molded part 100 is
illustrated. The molded part 100 is a thin-walled part. Molded
parts are generally considered to be thin-walled when a length of a
flow channel L divided by a thickness of the flow channel T is
greater than 100 (i.e., L/T>100). The low constant pressure
injection molding systems and molds having simplified cooling that
are described herein become increasingly advantageous for molding
parts as L/T ratios increase, particularly for parts having
L/T>200, or L/T>250 because the molten thermoplastic material
includes a continuous flow front that advances through the mold
cavity, which fills the mold cavity with thermoplastic material
more consistently than high variable pressure injection molding
systems. The length of the flow channel L is measured from a gate
102 to a flow channel end 104. Thin-walled parts are especially
prevalent in the consumer products industry and healthcare or
medical supplies industry.
[0041] Thin-walled parts present certain obstacles in injection
molding. For example, the thinness of the flow channel tends to
cool the molten thermoplastic material before the material reaches
the flow channel end 104. When this happens, the thermoplastic
material freezes off and no longer flows, which results in an
incomplete part. To overcome this problem, traditional injection
molding machines inject the molten thermoplastic material into the
mold at very high pressures, typically greater than 15,000 psi, so
that the molten thermoplastic material rapidly fills the mold
cavity before having a chance to cool and freeze off. This is one
reason that manufacturers of the thermoplastic materials teach
injecting at very high pressures. Another reason traditional
injection molding machines inject molten plastic into the mold at
high pressures is the increased shear, which increases flow
characteristics, as discussed above. These very high injection
pressures require the use of very hard materials to form the mold
28 and the feed system.
[0042] Traditional injection molding machines use molds made of
tool steels or other hard materials to make the mold. While these
tool steels are robust enough to withstand the very high injection
pressures, tool steels are relatively poor thermal conductors. As a
result, very complex cooling systems are machined into the molds to
enhance cooling times when the mold cavity is filled, which reduces
cycle times and increases productivity of the mold. However, these
very complex cooling systems add great time and expense to the mold
making process.
[0043] The inventors have discovered that shear-thinning
thermoplastics (even minimally shear-thinning thermoplastics) may
be injected into the mold 28 at low, substantially constant,
pressure without any significant adverse affects. Examples of these
materials include but are not limited to polymers and copolymers
comprised of, polyolefins (e.g., polypropylene, polyethylene),
thermoplastic elastomers, polyesters (e.g. polyethelyne
terephthalate, polybutelene terephthalate), polystyrene,
polycarbonate, poly(acrylonitrile-butadiene-styrene), poly(latic
acid), polyhydroxyalkanoate, polyamides, polyacetals,
ethylene-alpha olefin rubbers, and styrene-butadiene-stryene block
copolymers. In fact, parts molded at low, substantially constant,
pressures exhibit some superior properties as compared to the same
part molded at a conventional high pressure. This discovery
directly contradicts conventional wisdom within the industry that
teaches higher injection pressures are better. Without being bound
by theory, it is believed that injecting the molten thermoplastic
material into the mold 28 at low, substantially constant, pressures
creates a continuous flow front of thermoplastic material that
advances through the mold from a gate to a farthest part of the
mold cavity. By maintaining a low level of shear, the thermoplastic
material remains liquid and flowable at much lower temperatures and
pressures than is otherwise believed to be possible in conventional
high pressure injection molding systems.
[0044] Turning now to FIG. 3, a typical pressure-time curve for a
conventional high pressure injection molding process is illustrated
by the dashed line 200. By contrast, a pressure-time curve for the
disclosed low constant pressure injection molding machine is
illustrated by the solid line 210.
[0045] In the conventional case, melt pressure is rapidly increased
to well over 15,000 psi and then held at a relatively high
pressure, more than 15,000 psi, for a first period of time 220. The
first period of time 220 is the fill time in which molten plastic
material flows into the mold cavity. Thereafter, the melt pressure
is decreased and held at a lower, but still relatively high
pressure, 10,000 psi or more, for a second period of time 230. The
second period of time 230 is a packing time in which the melt
pressure is maintained to ensure that all gaps in the mold cavity
are back filled. The mold cavity in a conventional high pressure
injection molding system is filled from the end of the flow channel
back to towards the gate. As a result, plastic in various stages of
solidification are packed upon one another, which may cause
inconsistencies in the finished product, as discussed above.
Moreover, the conventional packing of plastic in various stages of
solidification results in some non-ideal material properties, for
example, molded-in stresses, sink, and non-optimal optical
properties.
[0046] The constant low pressure injection molding system, on the
other hand, injects the molten plastic material into the mold
cavity at a substantially constant low pressure for a single time
period 240. The injection pressure is less than 6,000 psi. By using
a substantially constant low pressure, the molten thermoplastic
material maintains a continuous melt front that advances through
the flow channel from the gate towards the end of the flow channel.
Thus, the plastic material remains relatively uniform at any point
along the flow channel, which results in a more uniform and
consistent finished product. By filling the mold with a relatively
uniform plastic material, the finished molded parts may form
crystalline structures that have better mechanical and/or better
optical properties than conventionally molded parts. Amorphous
polymers may also form structures having superior mechanical and/or
optical properties. Moreover, the skin layers of parts molded at
low constant pressures exhibit different characteristics than skin
layers of conventionally molded parts. As a result, the skin layers
of parts molded under low constant pressure can have better optical
properties than skin layers of conventionally molded parts.
[0047] By maintaining a substantially constant and low (e.g., less
than 6000 psi) melt pressure within the nozzle, more machineable
materials may be used to form the mold 28. For example, the mold 28
illustrated in FIG. 1 may be formed of a material having a milling
machining index of greater than 100%, a drilling machining index of
greater than 100%, a wire EDM machining index of greater than 100%,
a graphite sinker EDM machining index of greater than 200%, or a
copper sinker EDM machining index of greater than 150%. The
machining indexes are based upon milling, drilling, wire EDM, and
sinker EDM tests of various materials. The test methods for
determining the machining indices are explained in more detail
below. Examples of machining indexes for a sample of materials is
compiled below in Table 1.
TABLE-US-00001 TABLE 1 Machining Technology Milling Drilling
Spindle Spindle Wire EDM Sinker EDM-Graphite Sinker EDM-Copper Load
Index % Load Index % time Index % time Index % time Index %
Material 1117* 0.72 100% 0.32 100% 9:34 100% 0:14:48 100% 0:24:00
100% 6061 Al 0.50 144% 0.20 160% 4:46 201% 0:05:58 243% 0:15:36
154% 7075 Al 0.55 131% 0.24 133% 4:48 199% 0:05:20 278% 0:12:27
193% Alcoa QC-10 Al 0.56 129% 0.24 133% 4:47 200% 0:05:11 286%
0:12:21 194% 4140 0.92 78% 0.37 86% 9:28 101% 0:09:36 154% 0:19:20
124% 420 SS 1.36 53% 0.39 82% 8:30 113% 0:10:12 145% 0:23:20 103%
A2 0.97 74% 0.45 71% 8:52 108% 0:08:00 185% 0:20:12 119% S7 1.20
60% 0.43 74% 9:03 106% 0:12:53 115% 0:20:58 114% P20 1.10 65% 0.38
84% 9:26 101% 0:11:47 126% 0:20:30 117% PX5 1.12 64% 0.37 86% 9:22
102% 0:12:37 117% 0:23:18 103% Moldmax HH 0.80 90% 0.36 89% 6:00
159% 6:59:35 4% 1 0:43:38 55% 3 Ampcoloy 944 0.62 116% 0.32 100%
6:53 139% 3:13:41 8% 2 0:30:21 79% 4 *1117 is the benchmark
material for this test. Published data references 1212 carbon steel
as the benchmark material. 1212 was not readily available. Of the
published data, 1117 was the closest in composition and machining
index percentage (91%). 1 Significant graphite electrode wear:
.sup.~20% 2 graphite electrode wear: .sup.~15% 3 Cu electrode wear:
.sup.~15% 4 Cu electrode wear: .sup.~3%
[0048] Using easily machineable materials to form the mold 28
results in greatly decreased manufacturing time and thus, a
decrease in manufacturing costs. Moreover, these machineable
materials generally have better thermal conductivity than tool
steels, which increases cooling efficiency and decreases the need
for complex cooling systems.
[0049] When forming the mold 28 of these easily machineable
materials, it is also advantageous to select easily machineable
materials having good thermal conductivity properties. Materials
having thermal conductivities of more than 30 BTU/HR FT .degree. F.
are particularly advantageous. For example easily machineable
materials having good thermal conductivities include, but are not
limited to, Alcoa QC-10, Alcan Duramold 500, and Hokotol (available
from Alerts). Materials with good thermal conductivity more
efficiently transmit heat from the thermoplastic material out of
the mold. As a result, more simple cooling systems may be used.
[0050] One example of a multi-cavity mold assembly 28 is
illustrated in FIG. 4. Multi-cavity molds generally include a feed
manifold 60 that directs molten thermoplastic material from the
nozzle 26 to the individual mold cavities 32. The feed manifold 60
includes a sprue 62, which directs the molten thermoplastic
material into one or more runners or feed channels 64. Each runner
64 may feed multiple mold cavities 32. High productivity molds may
include four or more mold cavities 32, sometimes as many as three
hundred and eighty four mold cavities 32, and often also may
include heated runners 64. Some embodiments of constant low
pressure injecting molding machines may include non-naturally
balanced feed systems, such as artificially balanced feed systems,
or non-balanced feed systems.
[0051] Drilling and Milling Machineability Index Test Methods
[0052] The drilling and milling machineability indices listed above
in Table 1 were determined by testing the representative materials
in carefully controlled test methods, which are described
below.
[0053] The machineability index for each material was determined by
measuring the spindle load needed to drill or mill a piece of the
material with all other machine conditions (e.g., machine table
feed rate, spindle rpm, etc.) being held constant between the
various materials. Spindle load is reported as a ratio of the
measured spindle load to the maximum spindle torque load of 75
ft-lb at 1400 rpm for the drilling or milling device. The index
percentage was calculated as a ratio between the spindle load for
1117 steel to the spindle load for the test material.
[0054] The test milling or drilling machine was a Haas VF-3
Machining Center.
TABLE-US-00002 TABLE 2 Drilling Conditions Spot Drill 118 degree
0.5'' diameter, drilled to 0.0693'' depth Drill Bit 15/32''
diameter high speed steel uncoated jobber length bit Spindle Speed
1200 rpm Depth of Drill 0.5'' Drill Rate 3 in/min Other No chip
break routine used
TABLE-US-00003 TABLE 3 Milling Conditions Mill 0.5'' diameter 4
flute carbide flat bottom end mill, uncoated (SGS part # 36432
www.sgstool.com) Spindle Speed 1200 rpm Depth of Cut 0.5'' Stock
Feed Rate 20 in/min
[0055] For all tests "flood blast" cooling was used. The coolant
was Koolrite 2290.
[0056] EDM Machineability Index Test Methods
[0057] The graphite and copper sinker EDM machineability indices
listed above in Table 1 were determined by testing the
representative materials in a carefully controlled test method,
which is described below.
[0058] The EDM machineability index for the various materials were
determined by measuring the time to burn an area (specifics below)
into the various test metals. The machineability index percentage
was calculated as the ratio of the time to burn into 1117 steel to
time required to burn the same area into the other test
materials.
TABLE-US-00004 TABLE 4 Wire EDM Equipment Fanuc OB Wire 0.25 mm
diameter hard brass Cut 1'' thick .times. 1'' length (1 sq. '')
Parameters Used Fanuc on board artificial intelligence, override at
100%
TABLE-US-00005 TABLE 5 Sinker EDM - Graphite Equipment Ingersoll
Gantry 800 with Mitsubishi EX Controller Wire System 3R pre-mounted
25 mm diameter Poco EDM 3 graphite Cut 0.1'' Z axis plunge
Parameters Used Mitsubishi CNC controls with FAP EX Series
Technology
TABLE-US-00006 TABLE 6 Sinker EDM - Copper Equipment Ingersoll
Gantry 800 with Mitsubishi EX Controller Wire System 3R pre-mounted
25 mm diameter Tellurium Copper Cut 0.1'' Z axis plunge Parameters
Used Mitsubishi CNC controls with FAP EX Series Technology
[0059] The disclosed low constant pressure injection molding
machines advantageously employ molds constructed from easily
machineable materials. As a result, the disclosed low constant
pressure injection molds (and thus the disclosed low constant
pressure injection molding machines) are less expensive and faster
to produce. Additionally, the disclosed low constant pressure
injection molding machines are capable of employing more flexible
support structures and more adaptable delivery structures, such as
wider platen widths, increased tie bar spacing, elimination of tie
bars, lighter weight construction to facilitate faster movements,
and non-naturally balanced feed systems. Thus, the disclosed low
constant pressure injection molding machines may be modified to fit
delivery needs and are more easily customizable for particular
molded parts.
[0060] Moreover, the disclosed low constant pressure injection
molds (e.g., mold assemblies that include one or more mold sides
and one or more mold support plates) may include simplified cooling
systems relative to cooling systems found in conventional high
pressure injection molds. The simplified cooling systems are more
economical than conventional cooling systems because the simplified
cooling systems are more quickly and easily produced. Additionally,
the simplified cooling systems use less coolant, which further
reduces cooling costs during molding operations. In some cases, the
simplified cooling systems may be located solely in the mold
support plates, which allows the mold sides to be changed without
the need for changing the cooling system. In summary, the
simplified cooling systems of the disclosed low constant pressure
injection molding molds are more economical and more effective than
conventional complex cooling systems found in conventional high
pressure injection molds.
[0061] Generally speaking, a mold support plate physically supports
and reinforces a mold side. Two or more mold sides (or mold cores)
define a mold cavity. A mold support plate may support a mold side
by continuous contact along a length and width of a mold side.
Alternatively, a mold support plate may support a mold side by
intermittent or partial physical contact with the mold side. Such
intermittent or partial physical contact may be used for a variety
of reasons such as (i) to focus the load bearing contact on certain
locations (e.g., reinforced locations) of the mold side, (ii) to
direct the location of portions of the thermal exchange or heat
flow between specific parts of the mold side and the mold support
plate, or (iii) to accommodate the special needs for a given
apparatus. The mold support plate may remain in contact with the
mold side throughout the molding process, or the mold support plate
may completely separate from the mold side for certain periods of
time during the molding process. Moreover, the mold support plate
may be formed of two or more separate pieces that are fixed to one
another. The mold support plate may be made of a material with a
high thermal conductivity (e.g., 30 BTU/hr Ft .degree. F.). In some
embodiments, the mold support plate may be made of a material
having a higher thermal conductivity than the material of the mold
side, or vice versa. In yet other embodiments, the mold support
plate may have a thermal conductivity that is identical to the
thermal conductivity of the mold side. In one example, the mold
support plate may be made of CuBe, or least a portion of the mold
support plate may be made of CuBe, which contacts the mold side
that may be made of aluminum. While the mold support plates
illustrated in the drawings are generally formed from a single
piece of material, in other embodiments, the mold support plates
may be formed from multiple pieces of similar or different material
that are fixed to one another.
[0062] Cooling systems of all sorts may be categorized in a system
of cooling complexity levels, with cooling complexity level zero
representing the most simple cooling system and higher cooling
complexity levels representing progressively more complex cooling
systems. This system of cooling system categorization is discussed
below in more detail. However, conventional high productivity
consumer product injection molding machines (e.g., class 101 and
102 molding machines) employ complex cooling systems to reduce
cycle time and improve productivity. Generally speaking, high
productivity consumer product injection molding machines include
complex cooling systems (i.e., cooling systems having a level four
cooling system complexity level or higher). Level zero to level
three cooling complexity level systems generally do not produce
cooling capacity that is sufficient for conventional high
productivity injection molds, which include molds made of high
hardness, low thermal conductivity materials.
[0063] Advantageously, the disclosed low constant pressure
injection molds include cooling systems having cooling complexity
levels of three or less, preferably cooling complexity level three,
two, or one, which lowers production costs and increases efficiency
over conventional high pressure injection molding machines.
[0064] As used herein, a cooling complexity level zero mold
assembly is defined as a mold assembly that includes no active
cooling system. In other words, a cooling complexity level zero
mold assembly is only passively cooled through the conduction of
heat through the mold sides and through the mold support plates,
and eventually to the atmosphere surrounding the mold assembly.
Cooling complexity level zero mold assemblies typically have
relatively long cycle times (as it takes a significant amount of
time for the plastic within the mold to freeze because of the slow
cooling rate). As a result, high productivity consumer product mold
assemblies (e.g., mold assemblies used in class 101-102 molding
machines) do not use cooling complexity level zero mold
assemblies.
[0065] Turning now to FIGS. 5A-5E, different embodiments of a
cooling complexity level one mold assembly 328 (and/or different
embodiments of a mold support plate in the mold assembly) are
illustrated. The mold assembly 328 may include a mold 370 having a
first side 372 and a second side 374. The first side 372 and the
second side 374 form a mold cavity 376 therebetween. The first side
372 may be supported by a first mold support plate 378 and the
second side 374 may be supported by a second mold support plate
380. The first and second mold support plates 378, 380 may be
attached to a press (not shown), which actuates to move the first
and second sides 372, 374 during the molding process. One or more
cooling lines 382 may be formed in one or more of the mold support
plates 378, 380. Because the first and second sides 372, 374 are
made from a highly thermally conductive material, heat flows
through the first and second sides 372, 374 to the mold support
plates 378, 380 at a rate that is sufficient to cool plastic in the
mold cavity 376 in an acceptable amount of time.
[0066] The mold support plates 378, 380 may include posts or other
projections 381 that extend outward, away from the mold support
plate 378, 380, towards the mold 370. The cooling lines 382 may
extend into the projections 381. The mold 370 may include a
complementary feature so that the mold may fit around (FIG. 5B),
within (FIG. 5C), or upon (FIGS. 5D and 5E) the projections 381. In
this way, the cooling lines 382 may be located closer to the mold
cavity without extending the cooling lines 382 into the mold 370 or
into the first and second mold sides 372, 374. As a result, the
mold support plates 378, 380 may receive molds having a variety of
different mold cavity shapes. The molds may thus be formed without
cooling lines integrated into the first and/or second sides 372,
374, which reduces manufacturing costs of the molds 370.
[0067] Conventional high output consumer product injection mold
assemblies do not use cooling complexity level one mold assemblies
because such mold assemblies do not adequately cool plastic with in
a mold cavity formed by two high hardness, low thermal conductivity
materials. Cooling complexity level one mold assemblies are defined
as containing all active cooling lines 382 within the mold support
plates 378, 380, even if more than one machining axis is needed to
form the cooling lines 382. In the example of FIGS. 5A-5E, the mold
may be a stack mold, a cube mold, a shuttle mold, a helicopter
mold, a mold having rotating platens, or other multi-cavity molds
to increase productivity if desired.
[0068] Turning now to FIG. 6, a cooling complexity level two mold
assembly 328 is illustrated. The cooling complexity level two mold
assembly 328 is identical to the cooling complexity level one mold
assembly 328 of FIGS. 5A-5E, with the exception that the cooling
lines 382 in the embodiment of FIG. 6 extend through at least one
mold support plate 378, 380 and into at least one mold side 372,
374 (i.e., as opposed to the cooling lines 382 only extending
through the mold support plates 378, 380). The cooling lines 382
have terminal ends 384. However, each cooling line 382 is machined
along an axis that is parallel to a single machining axis. The
cooling lines 382 may include a baffle 386, as shown in more detail
in FIG. 7, to facilitate cooling fluid flow through the cooling
line 382. Cooling complexity level two mold assemblies have not
been used in high output consumer product injection molding
machines (i.e., class 101-102 injection molding machines) because
cooling complexity level two mold assemblies do not have enough
flexibility to machine cooling lines close to the mold surfaces of
the mold cavity and therefore, cooling complexity level two mold
assemblies do not provide adequate cooling for conventional high
output mold assemblies having high hardness, low thermal
conductivity molds.
[0069] Turning now to FIG. 8 an embodiment of a cooling complexity
level three mold assembly 328 is illustrated. A cooling complexity
level three mold assembly 328 is defined by cooling channels 382
having at least two different machining axes. At least one cooling
line 382 may include two different machining axes and a terminal
end. More particularly, the cooling line 382 may have a bend or
turn. For example, the cooling line 382 may include a first
machining axis that is substantially parallel to the
opening-closing stroke S of the mold assembly 328 and a second
machining axis that is angled with respect to the first machining
axis. Like cooling complexity level two mold assemblies, cooling
complexity level three mold assemblies have not been used in high
output consumer product injection molding machines (e.g., class
101-102 injection molding machines) because level three cooling
complexity does not have enough flexibility to machine cooling
lines close to the mold surfaces of the mold cavity and therefore,
cooling complexity level three mold assemblies do not provide
adequate cooling for conventional high output mold assemblies
having high hardness, low thermal conductivity molds.
[0070] Turning now to FIG. 9, a cooling complexity level four mold
assembly 328 is illustrated. The cooling complexity level four mold
assembly 328 includes a plurality of cooling lines 382, a first
cooling line 382a having a terminal end 384 and a second cooling
line 382b being a through-bore without a terminal end. The first
cooling line 382a extends from the mold support plate 378 into the
first mold side 372 and the second cooling line 382b extends
through the first mold side 372. A machining axis for the first
cooling line 382a is different from a machining axis for the second
cooling line 382b. In other words, the cooling lines 382 have at
least two different machining axes for formation. Cooling
complexity level four mold assemblies have been used in some high
output consumer product injection molding machines (e.g., class
101-102 injection molding machines) having mold assemblies with
very simple mold cavity geometries.
[0071] Turning now to FIG. 10, a cooling complexity level five mold
assembly 328 is illustrated. The cooling complexity level five mold
assembly 328 includes a first cooling line 382 that is a
through-bore having two different machining axes. As illustrated in
FIG. 10, the first cooling line 382 includes a first section 390
and a second section 392 that are angled with respect to one
another and meet at a junction or turn 394. Machining the first
cooling line 382 with two different axes that must meet at an
internal location in the mold part requires great precision and
thus more costly equipment, along with a greater manufacturing
time. However, cooling complexity level five mold assemblies 328
have been used in high output consumer product injection molding
machines (e.g., class 101-102 injection molding machines) because
cooling complexity level five mold assemblies allow for greater
customization in cooling line placement. Thus, cooling lines can be
placed closer to the mold cavity than in cooling complexity mold
assemblies of lesser complexity. As a result, the more complex
cooling complexity mold assembly can at least partially offset the
drawback of lower thermal conductivity found in conventional
injection molds made of high hardness, low thermal conductivity
materials.
[0072] Turning now to FIG. 11, a cooling complexity level six mold
assembly 328 is illustrated. The cooling complexity level six mold
assembly 328 is a cooling complexity level one to five mold
assembly that also includes at least one actively cooled dynamic
molding part 398. Forming cooling channels in a dynamic molding
part 398 requires great precision. Moreover, actively cooled
dynamic molding parts 398 require complicated flow mechanisms that
move with the dynamic molding part 398 during operation of the mold
assembly 328. Cooling complexity level six mold assemblies have
been used in high output consumer product injection molding
machines (e.g., class 101-102 injection molding machines).
[0073] Turning now to FIG. 12, a cooling complexity level seven
mold assembly 328 is illustrated. The cooling complexity level
seven mold assembly 328 is a cooling complexity level two through
six mold assembly that includes at least one conformal cooling
cavity 399. The conformal cooling cavity 399 at least partially
complements the contours of the mold cavity to provide maximum
active cooling. The conformal cooling cavity 399 is non-linear,
non-coaxial, and/or non-planar. Conformal cooling cavities 399
require complex machinery to form. Additionally, conformal cooling
cavities 399 take significant amounts of time to form. As a result,
cooling complexity level seven mold assemblies are very expensive
and are generally reserved for high output consumer product
injection molding machines that have very intricate part
geometries.
[0074] The simplified cooling systems described herein may be
incorporated into virtually any type of conventional injection
mold, such as an injection molding machine having a cube mold
assembly 428, as illustrated in FIG. 13.
[0075] Returning now to FIGS. 5A-5E, in some embodiments, a level
one cooling complexity level injection mold assembly may include an
evaporative cooling system. Evaporative cooling systems are more
efficient in removing heat than liquid based cooling systems. In
some examples, evaporative cooling systems may be 100 times more
efficient, or even 500 times more efficient, at removing heat than
liquid based cooling systems. Because the injection mold assemblies
described herein are made of materials having high thermal
conductivity, these mold assemblies may include more simplified
cooling systems (by moving cooling lines farther away from the mold
cavity) while increasing heat removal by including evaporative
cooling systems. Moving cooling lines away from the mold cavity
produces more uniform temperature distributions throughout the mold
sides. Advantageously, level one cooling complexity mold assemblies
having evaporative cooling systems do not need complex dynamic
seals between the mold support plates and the mold sides because
the cooling fluid lines do not extend into the mold sides. As a
result, the evaporative cooling systems in level one cooling
complexity level mold assemblies are more robust and less prone to
failure than evaporative cooling systems in prior art mold
assemblies that required the cooling fluid to extend into the mold
sides.
[0076] Generally speaking, evaporative cooling systems exploit a
phase change in a cooling fluid to extract more heat from the mold
assembly than a conventional all liquid cooling system could
extract. By using localized pressure differentials, the circulating
fluid alternates between a liquid and a gas phase. Transition from
a liquid to a gas phase is highly endothermic. When the liquid
cooling fluid passes through a region of elevated temperature (such
as the mold support plate, or an evaporator), the cooling fluid
absorbs heat from the mold support plate and changes phase to a
gas. The gas then passes to a region of lower temperature, such as
a condenser, where heat is transferred from the gas to the
environment. This heat transfer causes the gas to condense back
into a liquid that may be pumped back into the mold support plate
to absorb more heat and the cycle is repeated. Evaporative cooling
systems may be ten times more effective, 100 times, or even 500
times more effective at removing heat than conventional all liquid
cooling systems.
[0077] More specifically, a refrigeration-type vapor compression
cooling system 500 is illustrated in FIG. 14. The vapor compression
cooling system 500 includes a compressor 510 that increases
pressure of a cooling fluid within cooling lines 512a,b of a
cooling circuit 514, which causes the temperature of the cooling
fluid to rise in cooling line 512a according to the combined gas
law (i.e., p.sub.1V.sub.1/T.sub.1=p.sub.2V.sub.2/T.sub.2). After
being compressed, the elevated temperature cooling fluid enters a
heat exchanger or condenser 516. The elevated temperature cooling
fluid exchanges heat with the atmosphere (or other medium), and the
cooling fluid cools below its evaporation temperature, thus
condensing into liquid form. The liquid cooling fluid then moves
through cooling line 512b to an expansion valve 520, where the
volume of the cooling liquid is increased, causing the pressure of
the cooling liquid to decrease, which causes the cooling fluid to
at least partially evaporate, again according to the combined gas
law. Some of the cooling fluid may vaporize so that a combination
of liquid and gas cooling fluid moves through cooling line 512c to
an evaporator 522. In one embodiment, a mold support plate may
comprise the evaporator 522. The mold support plate may include one
or more cooling channels for moving cooling fluid through the mold
support plate to remove heat from the mold support plate. The
evaporator surfaces are relatively warm, compared to the cooling
fluid in the evaporator 522. Thus, heat is transferred from the
evaporator (e.g., the mold support plate) to the cooling fluid,
which results in vaporization of a majority of the remaining
cooling fluid.
[0078] After exchanging heat and vaporizing in the evaporator 522,
the cooling fluid moves through cooling line 512d to the compressor
510 and the process is repeated. The evaporator 522, the compressor
510, the condenser 516, and the expansion valve 520 are all fluidly
connected to one another by the cooling lines 512a-d. In some
embodiments, the entire cooling circuit 514 may be located within
or on the evaporator 522 or mold support plate. In other
embodiments, the mold support plate may comprise the evaporator 522
(having one or more cooling channels located within the mold
support plate), while one or more of the compressor 510, the
condenser 516, and the expansion valve 520 may be physically
separated from the mold support plate, while being fluidly
connected to the mold support plate via the cooling lines
512a-d.
[0079] FIG. 15 illustrates one embodiment of an evaporative cooling
system 600 that may be used in an injection molding machine. The
evaporative cooling system 600 includes the same elements as the
evaporative cooling system of FIG. 14, with respective elements
having reference numerals increased by 100. The evaporative cooling
system 600 includes a compressor 610, a condenser 616, an expansion
valve 620 and an injection mold 622, all fluidly connected by a
plurality of cooling lines 612a-d to form a closed loop cooling
circuit 614. In the embodiment of FIG. 15, the injection mold 622
itself, and more specifically a mold support plate of the injection
mold 622, forms the evaporator.
[0080] Cooling fluid flowing through the injection mold 622 removes
heat from the injection mold 622, thereby cooling molten plastic
within the injection mold 622. The increased cooling capacity of
the evaporative cooling system 600 reduces cycle time of the
injection mold 622 by removing heat more quickly than traditional
cooling systems that move cooling liquid only through cooling
channels.
[0081] Turning now to FIG. 16A, another example of an evaporative
cooling system 700, which is located in a mold support plate 478 of
an injection mold, is illustrated. The evaporative cooling system
700 includes a chamber 710 within the mold support plate 478. An
evaporative liquid 712, such as water, is disposed within the
chamber 710. A percolator tube 714 connects a reservoir portion 716
of the chamber 710 to a condensing portion 718 of the chamber 710.
The percolator tube 714 assists in moving water from the bottom of
the chamber 710 to the top of the chamber 710. A cooled condenser
720 may be located near the top of the chamber 710. As heat from
the mold side (which would be located to the right in FIG. 16A)
warms the mold support plate 478, liquid water in the reservoir
portion 716 evaporates and travels upwards through the chamber
towards the condensing portion 718. This evaporation process
removes heat from the mold support plate 478 and thus, from the
mold side adjacent the mold support plate 478. When the water vapor
reaches the condenser 720, heat again transfers from the water
vapor to the condenser 720, which causes the water vapor to
condense back into liquid form. This liquid water then runs down
the sides of the chamber 710 towards the reservoir portion 716.
Some of the liquid water will re-evaporate from the side walls of
the chamber and some of the liquid water will reach the reservoir
portion 716 before re-evaporating. To enhance evaporation from the
side walls of the chamber 710, a coating may be applied to the side
walls that enhances retention of liquid water against the side
walls by increasing surface tension between the liquid water and
the side walls. A controllable heat source 722 may optionally be
disposed within the mold support plate 478 (or be attached to the
mold support plate 478) to regulate the volume of water returned to
the condensing section 718 through the percolator tube 714.
[0082] FIG. 16B illustrates an alternate embodiment of an
evaporative cooling system 800. Elements of the evaporative cooling
system 800 like those of the evaporative cooling system 700 of FIG.
16A have reference numerals that are 100 greater than the elements
in FIG. 16A. The main difference in the evaporative cooling system
800 of FIG. 16B is that an additional collection portion 830 is
located within the chamber 810 vertically between the condensing
portion 818 and the reservoir portion 816. The collection portion
830 may facilitate collection and re-evaporation of liquid water
for larger (or longer) mold support plates.
[0083] While not illustrated, the evaporative (and vapor
compression) cooling systems 500, 600, 700, and 800 of FIGS. 14,
15, 16A, and 16B may include a vacuum system to lower relative
pressures of the cooling fluids. Lowering relative pressures of the
cooling fluids lowers the evaporation temperature for a given
cooling fluid (all other factors being equal). Conversely, the
evaporative (and vapor compression) cooling systems 500, 600, 700,
and 800 may include a pressurization system to increase relative
pressure of the cooling fluid. Raising relative pressure of the
cooling fluid raises the evaporation temperature of a given cooling
fluid (all other factors being equal). In this way, the evaporation
temperature may be tailored to the temperatures typically
experienced by a particular mold.
[0084] Evaporative cooling systems may use many different types of
cooling fluids, such as refrigerants (e.g., chlorofluorocarbons,
chlorofluoroolefins, hydrochlorofluorocarbons,
hydrochlorofluoroolefins, hydrofluorocarbons, hydrofluoroolefins,
hydrochlorocarbons, hydrochloroolefins, hydrocarbons, hydroolefins,
perfluorocarbons, perfluoroolefins, perchlorocarbons,
perchloroolefins, and halon/haloalkane, and blends thereof), water,
glycol, propylene glycol, alcohol, or mercury. Other refrigerants
having cooling capacities and/or physical or chemical properties
similar to the refrigerants listed above may also be used.
Similarly, other cooling fluids that undergo a phase change when
exposed to temperatures between about 0.degree. C. and about
200.degree. C. at pressures between 0 psi (i.e., complete vacuum)
and about 2000 psi, may also be used. In some cases a surfactant
may be added to the cooling fluid. Some evaporative cooling systems
may utilize a vacuum system to create differential pressure, while
other evaporative cooling systems may utilize compressors to create
differential pressure.
[0085] In yet other embodiments, the evaporative cooling system may
employ atmospheric liquid evaporation to remove heat. Because the
disclosed mold assemblies are made of highly thermally conductive
materials, in some cooling complexity level zero mold assemblies it
is possible to simply spray a cooling liquid on the outer surface
of the mold support plates or mold sides, which evaporates as the
liquid absorbs heat, thereby cooling the mold support plate or the
mold side. One type of liquid that may be advantageously employed
in these types of systems is distilled water. Distilled water will
completely evaporate without leaving any type of residue on the
mold support plate or mold side. In some embodiments, fins or
radiator structures may be used to increase surface area of the
mold support plate or mold side to further facilitate evaporation
and heat removal.
[0086] One example embodiment of an evaporative cooling system 900
that employs atmospheric liquid evaporation is illustrated in FIG.
17. The mold 910 may include a first mold side 925 and a second
mold side 927. A first mold support plate 978 and a second mold
support plate 980 may be located adjacent the first and second mold
sides 925, 927, respectively. A spray bar 911 may be positioned
near one of the mold support plates 978, 980, and/or near one of
the mold sides 925, 927. The spray bar 910 is fluidly connected to
a pump 912, which pumps liquid (for example water) to the spray bar
910 under pressure. The liquid is sprayed out of a nozzle 914 so
that the sprayed liquid coats an outer surface of one of the mold
support plates 978, 980 and the mold sides 925, 927. As the liquid
coats the outer surface, heat from the mold support plate 978, 980,
and/or the mold side 925, 927, causes the liquid to evaporate, thus
cooling the mold support plate 978, 980 and/or the mold side 927,
927. Liquid that does not evaporate may drip down and collect in a
liquid collection area or sump 940. The sump 940 is an area or
reservoir for collecting liquid that does not evaporate. A return
line 942 extends from the sump 940 to the pump 912 to channel water
from the sump 940 back to the spray bar 911. The pump 912 may also
be connected to a source of liquid 944 so that a supply of liquid
is always available to the spray bar 911, regardless of the level
of liquid in the sump 940.
[0087] In the embodiment illustrated in FIG. 17, evaporated liquid
simply vents to the atmosphere and fresh liquid is supplied through
the source of liquid 944 to make up for the lost evaporated liquid.
In other embodiments, the entire mold 910 may be located in a
closed environment and the evaporated liquid may be condensed and
returned to the sump 940.
[0088] The disclosed mold assemblies may include cooling systems
having cooling channels that are completely confined within a mold
support plate. As a result, the disclosed systems do not need any
dynamic seals (e.g., seals between moving parts) and the risk of
cooling fluid escaping to the atmosphere, or being released into
the environment is reduced.
[0089] As described above, the cooling systems disclosed herein,
for a level one cooling complexity mold, include cooling channels
only in one or more of the mold support plates. In other words,
there are no cooling channels in either the first mold side or the
second mold side. As a result, all seals in the cooling systems are
static in nature and very robust. Stated another way, there are no
seals between components that move relative to one another in the
disclosed cooling systems, which would require softer, dynamic
seals. Thus, the disclosed cooling systems may use dangerous,
hazardous, or expensive cooling fluids (sometimes referred to as
"exotic cooling fluids") as there is little chance of seal breach,
which would result in release of the cooling fluid. Some dangerous,
hazardous, or expensive cooling fluids may have superior heat
absorption properties when compared to traditional cooling fluids.
However, these dangerous, hazardous, or expensive cooling fluids
have not been previously used in cooling systems for injection
molds for fear of seal breach (especially a breach in dynamic seals
between moving parts), which would release these cooling fluids to
the atmosphere. Particularly useful dangerous, hazardous, or
expensive cooling fluids that may now be used in the disclosed
cooling systems (due to the very low risk of these cooling fluids
escaping to the atmosphere) include heating oil, hydraulic fluid,
glycols, cesium, mercury, potassium (which has a thermal
conductivity of approximately 42 W/mK at 25.degree. C.),
lead-bismuth eutectic, sodium potassium alloy, sodium potassium
cesium alloy, and lead-bismuth.
[0090] Desirable cooling fluids may have a thermal conductivity of
1 W/mK or more. More desirable cooling fluids may have a thermal
conductivity of between about 1 W/mk to about 42 W/mk. Some
desirable cooling liquids maintain a flowable viscosity (e.g., a
cpi of 100,000 or less) at temperatures between about 5.degree. C.
and about 100.degree. C. In addition to hazardous or dangerous
cooling fluids, non-hazardous cooling fluids that are relatively
expensive may also be used in the disclosed cooling systems. One
such expensive, but useful, cooling fluid is distilled water, which
advantageously dose not corrode internal components of the
disclosed cooling systems. Distilled water, however, has not been
used in conventional high productivity injection molding systems
because of the need to constantly replace distilled water lost
through seal breaches. These losses generally required a
distillation plant on site to produce make up distilled water,
which is cost prohibitive in the highly competitive consumer
products injection molding industry.
[0091] By eliminating dynamic seals from the disclosed cooling
complexity level one molds, a wider range of cooling fluids may be
used. As described above, some potentially harmful, hazardous, or
expensive cooling fluids may be used. Additionally, nanofluids may
be used as cooling fluids. Nanofluids include a carrier liquid,
such as water, having tiny nano-scale particles known as
nanoparticles dispersed throughout the carrier liquid.
Nanoparticles of solid materials (e.g., copper oxide, alumina,
titanium dioxide, carbon nanotubes, silica, or metals, including
copper, or silver nanorods) may be dispersed into the carrier
liquid, which enhances the heat transfer capabilities of the
resulting coolant compared to the carrier liquid alone. The
enhancement can be theoretically as high as 350%. In some examples,
nanofluids have been experimentally shown to have thermal
conductivities of 50%-100% greater than the thermal conductivity of
the carrier liquid alone. Nanofluids also exhibit a significant
increase in heat flux when compared to traditional cooling fluids.
In one example, a nanofluid may comprise ethylene glycol and copper
nanoparticles, which has a thermal conductivity of approximately
1.4 W/mK at 25.degree. C.
[0092] For example, silver nanorods of 55.+-.12 nm diameter and
12.8 .mu.m average length at 0.5 vol. % can increase thermal
conductivity of water by 68%, and 0.5 vol. % of silver nanorods
increased thermal conductivity of ethylene glycol based coolant by
98%. Alumina nanoparticles at 0.1% can increase the critical heat
flux of water by as much as 70%.
[0093] Because the seals in level one cooling complexity molds are
very robust in nature (because they are static seals), the seals
are much more tolerant of the nanoparticles in nanofluids, which
tend to degrade dynamic (softer) seals more quickly. As a result,
the disclosed cooling complexity level one molds may use nanofluids
to increase heat transfer rates, which results in more efficient
cooling. Desirable nanofluids may have thermal conductivities of 1
W/mK or more. Examples of nanoparticles that may be added to a
carrier fluid include copper oxide, alumina, titanium oxide, boron
nitride nanotubes, carbon nanotubes, carbon uranium nano rods, and
silver nano rods. Additionally, these nanofluids have a greater
heat capacity than traditional cooling fluids. As a result, fluid
circulation rates may be slowed to allow the nanofluid to remove
more heat per unit volume than traditional cooling fluids. Thus,
the overall cooling fluid volume needed for such a system may be
reduced, causing a corresponding reduction in overall cost and
complexity of the cooling system.
[0094] This increase in thermal conductivity and reduction in
overall fluid volume may allow some cooling complexity level one
molds that use nanofluids to employ a radiant heat type heat
exchanger to cool the nanofluid before circulation through the mold
support plate, as the nanofluid would linger in the heat exchanger
long enough to adequately cool the nanofluid.
[0095] In some level zero cooling complexity molds, the mold may be
cooled completely by convection/conduction of heat to the
atmosphere. Radiator fins may be formed on the mold support plates
or the mold sides to enhance convection of heat to the atmosphere.
Additionally, a gas moving device, such as a fan, may move
atmospheric gases over the molds and/or over the radiator fins to
further enhance heat dissipation though conduction.
[0096] Generally speaking, the low constant pressure injection
molding machines of the present disclosure include molds and/or
mold assemblies manufactured from materials having high thermal
conductivity, as discussed above. This high thermal conductivity
allows the disclosed low constant pressure injection molding
machines, molds, and mold assemblies to cool molded parts using
cooling complexity level three mold assemblies or lower for
virtually any part geometry. Preferably a cooling complexity level
two mold assembly will be used to cool a molded part. More
preferably a cooling complexity level one mold assembly will be
used to cool a molded part. For some part geometries, a cooling
complexity level zero mold assembly may even be used. The cooling
complexity level three or lower mold assemblies may be used even in
ultra high output consumer product injection molding machines
(e.g., class 101-102 injection molding machines) where more complex
cooling systems were needed for conventional injection molds made
from high hardness, low thermal conductivity materials. As a
result, the disclosed low constant pressure injection molds and
mold assemblies, and thus the injection molding machines, are less
costly to manufacture, while decreasing mold cycle times and
increasing mold productivity due at least in part to the
availability of less complex cooling systems.
[0097] An additional benefit of molds made from high thermal
conductivity materials is that a temperature profile for the mold
is more uniform during the injection molding process than in
conventional molds. In other words, there is less temperature
variation from point to point within the mold. As a result, parts
manufactured in molds with high thermal conductivity have less
internal stress (and a more uniform crystalline structure) than
parts manufactured in conventional molds. This lower internal
stress and more uniform crystallinity result in lower rates of part
warp. In conventional molds, the mold cavity is often designed to
offset part warp due to non-uniform temperature gradients, which
adds to the cost and complexity of conventional mold assemblies.
Finalizing a particular offset usually requires an iterative and
time consuming trial process. In high thermal conductivity molds,
the mold cavity need not be designed to offset warp because the
molded part does not experience a significant amount of warp, as
internal stresses are more uniform due to the more uniform cooling.
Thus, the iterative offset process used in the design of
conventional molds may be avoided, further reducing manufacturing
costs and time.
[0098] It is noted that the terms "substantially," "about," and
"approximately," unless otherwise specified, may be utilized herein
to represent the inherent degree of uncertainty that may be
attributed to any quantitative comparison, value, measurement, or
other representation. These terms are also utilized herein to
represent the degree by which a quantitative representation may
vary from a stated reference without resulting in a change in the
basic function of the subject matter at issue. Unless otherwise
defined herein, the terms "substantially," "about," and
"approximately" mean the quantitative comparison, value,
measurement, or other representation may fall within 20% of the
stated reference.
[0099] It should now be apparent that the various embodiments of
the products illustrated and described herein may be produced by a
low constant pressure injection molding process. While particular
reference has been made herein to products for containing consumer
goods or consumer goods products themselves, it should be apparent
that the low constant pressure injection molding method discussed
herein may be suitable for use in conjunction with products for use
in the consumer goods industry, the food service industry, the
transportation industry, the medical industry, the toy industry,
and the like. Moreover, one skilled in the art will recognize the
teachings disclosed herein may be used in the construction of stack
molds, multiple material molds including rotational and core back
molds, in combination with in-mold decoration, insert molding, in
mold assembly, and the like.
[0100] All documents cited in the Detailed Description of the
Invention are, in relevant part, incorporated herein by reference;
the citation of any document is not to be construed as an admission
that it is prior art with respect to the present invention. To the
extent that any meaning or definition of a term in this written
document conflicts with any meaning or definition of the term in a
document incorporated by reference, the meaning or definition
assigned to the term in this written document shall govern.
[0101] While particular embodiments have been illustrated and
described herein, it should be understood that various other
changes and modifications may be made without departing from the
spirit and scope of the claimed subject matter. Moreover, although
various aspects of the claimed subject matter have been described
herein, such aspects need not be utilized in combination. It is
therefore intended that the appended claims cover all such changes
and modifications that are within the scope of the claimed subject
matter.
* * * * *
References