U.S. patent application number 13/476045 was filed with the patent office on 2012-11-22 for apparatus and method for injection molding at low constant pressure.
Invention is credited to Gene Michael Altonen, Ralph Edwin Neufarth, Gary Francis Schiller.
Application Number | 20120294963 13/476045 |
Document ID | / |
Family ID | 46149755 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120294963 |
Kind Code |
A1 |
Altonen; Gene Michael ; et
al. |
November 22, 2012 |
Apparatus and Method for Injection Molding at Low Constant
Pressure
Abstract
A low constant pressure injection molding machine forms molded
parts by injecting molten thermoplastic material into a mold cavity
at low substantially constant pressures of 6,000 psi and less. As a
result, the low constant pressure injection molding machine
includes a mold formed of easily machineable material that is less
costly and faster to manufacture than typical injection molds.
Inventors: |
Altonen; Gene Michael; (West
Chester, OH) ; Neufarth; Ralph Edwin; (Liberty
Township, OH) ; Schiller; Gary Francis; (US) |
Family ID: |
46149755 |
Appl. No.: |
13/476045 |
Filed: |
May 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61488564 |
May 20, 2011 |
|
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Current U.S.
Class: |
425/149 |
Current CPC
Class: |
B29C 2945/76859
20130101; B29K 2905/02 20130101; B29C 45/77 20130101; B29K
2995/0013 20130101; B29C 2945/76498 20130101 |
Class at
Publication: |
425/149 |
International
Class: |
B29C 45/03 20060101
B29C045/03; B29C 45/26 20060101 B29C045/26; B29C 45/77 20060101
B29C045/77 |
Claims
1. An injection molding apparatus comprising: a melt holder for
pressurizing molten plastic prior to injection into a mold having a
plurality of mold cavities; a sensor in dynamic communication with
the melt holder for sensing a characteristic of the molten plastic;
and a controller in communication with the sensor, the controller
receiving a signal from the sensor, the signal being indicative of
a melt pressure of the molten plastic entering at least one mold
cavity in the plurality of mold cavities, the controller further
being in communication with an injection element, the injection
element applying a force to the molten plastic to advance the
molten plastic from the melt holder into the mold, wherein the
controller controls the injection element to maintain a
substantially constant melt pressure entering the at least one mold
cavity of less than 6000 psi, and the mold has an average thermal
conductivity of more than 30 BTU/HR FT .degree. F. and the mold has
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%.
2. The injection molding apparatus of claim 1, wherein the at least
one mold cavity in the plurality of mold cavities is a thin-walled
mold cavity having an L/T>100.
3. The injection molding apparatus of claim 1, further comprising
an artificially balanced molten plastic feed system.
4. The injection molding apparatus of claim 1, wherein sensor
generates an electrical signal.
5. The injection molding apparatus of claim 1, wherein the sensor
generates a mechanical signal.
6. The injection molding apparatus of claim 1, wherein the sensor
generates a hydraulic signal.
7. The injection molding apparatus of claim 1, wherein the sensor
generates a pneumatic signal.
8. The injection molding apparatus of claim 1, further comprising a
mold frame and a mold base, wherein at least one of the mold frame
and the mold base is made from a material having a surface hardness
of greater than 165 BHN and less than 260 BHN.
9. The injection molding apparatus of claim 1, further comprising a
hot runner feed system constructed of a material having a thermal
conductivity that is substantially equal to the average thermal
conductivity of the mold.
10. The injection molding apparatus of claim 9, wherein the hot
runner feed system is directly connected to a gate that is fluidly
connected with the at least one mold cavity.
11. The injection molding apparatus of claim 1, wherein the mold
has a sinker EDM machining index of greater than 200%.
12. The injection molding apparatus of claim 11, wherein the mold
has a sinker EDM machining index of between 200% and 1000%.
13. The injection molding apparatus of claim 1, wherein the mold
comprises at least four mold cavities.
14. An injection molding apparatus comprising: a mold having at
least one mold cavity, the mold having an average thermal
conductivity more than 30 BTU/HR FT .degree. F., and a sinker EDM
machining index of greater than 200%; a melt holder for
pressurizing molten plastic prior to injection into the mold; a
sensor in communication with the melt holder for sensing a
characteristic of the molten plastic; and a controller in
communication with the pressure sensor, the controller receiving a
signal from the sensor, the signal being indicative of a melt
pressure of the molten plastic entering the mold cavity, the
controller further being in communication with an injection
element, the injection element applying a force to the molten
plastic to advance the molten plastic from the melt holder into the
mold, wherein the controller controls the injection element to
maintain a melt pressure of the molten plastic entering the mold
cavity of less than 6000 psi.
15. The injection molding apparatus of claim 14 wherein the
controller controls the injection element to maintain a
substantially constant melt pressure.
16. An injection molding apparatus comprising: a mold having at
least one mold cavity, the mold having an average thermal
conductivity more than 30 BTU/HR FT .degree. F., and the mold
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%; a melt holder for
pressurizing molten plastic prior to injection into the mold; a
sensor in communication with the melt holder for sensing a
characteristic of the molten plastic indicative of a melt pressure
of the molten plastic in a nozzle of the melt holder, just prior to
entering the mold; and a controller in communication with the
sensor, the controller receiving a signal from the pressure sensor,
the controller further being in communication with an injection
element, the injection element applying a force to the molten
plastic to advance the molten plastic from the melt holder into the
mold, wherein the controller controls the injection element to
maintain a melt pressure of less than 6000 psi.
17. The injection molding apparatus of claim 16, wherein the
controller controls the injection element to maintain a
substantially constant melt pressure.
18. An injection molding apparatus comprising: a mold having at
least one mold cavity, the mold having an average thermal
conductivity more than 30 BTU/HR FT .degree. F., and at least one
of a thin-walled mold cavity (e.g., L/T>100), at least four mold
cavities, and an artificially balanced molten plastic feed system;
a melt holder for pressurizing molten plastic prior to injection
into the mold; a sensor in communication with the melt holder for
sensing a characteristic of the molten plastic indicative of a melt
pressure of the molten plastic in a nozzle of the melt holder just
before entering the mold; and a controller in communication with
the sensor, the controller receiving a signal from the sensor, the
controller further being in communication with an injection
element, the injection element applying a force to the molten
plastic to advance the molten plastic from the melt holder into the
mold, wherein the controller controls the injection element to
maintain a melt pressure of less than 6000 psi.
19. The injection molding apparatus of claim 18, wherein the
controller controls the injection element to maintain a
substantially constant melt pressure.
20. An injection molding apparatus comprising: a mold having at
least one mold cavity, the mold being formed from a material having
a surface hardness less than 30 Rc and the mold having an average
thermal conductivity of more than 30 BTU/HR FT .degree. F.; a melt
holder for pressurizing molten plastic prior to injecting the
molten plastic into the mold; a guided ejection system; and a
controller that controls an injection element in the melt holder so
that the injection element advances molten plastic into the at
least one mold cavity at a melt pressure of less than 6000 psi.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/488,564, filed May 20, 2011.
TECHNICAL FIELD
[0002] The present invention relates to apparatuses and methods for
injection molding and, more particularly, to apparatuses and
methods for producing injection molded parts at low constant
pressure.
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. A molded part may have one or more gates. It
is common for large parts to have two, three, or more gates to
reduce the flow distance the polymer must travel to fill the molded
part. The one or multiple gates per cavity may be located anywhere
on the part geometry, and possess any cross-section shape such as
being essentially circular or be shaped with an aspect ratio of 1.1
or greater. 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 cause 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 an
injection element of 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] While it may be desirous to reduce the wall thickness of
injected molded parts to reduce the plastic content, and thus cost,
of the final part; reducing wall thickness using a conventional
injection molding process can be an expensive and a non-trivial
task, particularly when designing for wall thicknesses less than
15, 10, 5, 3, or 1.0 millimeter. As a liquid plastic resin is
introduced into an injection mold in a conventional injection
molding process, the material adjacent to the walls of the cavity
immediately begins to "freeze," or solidify and cure. As the
material flows through the mold, a boundary layer of material is
formed against the sides of the mold. As the mold continues to
fill, the boundary layer continues to thicken, eventually closing
off the path of material flow and preventing additional material
from flowing into the mold. The plastic resin freezing on the walls
of the mold is exacerbated when the molds are cooled, a technique
used to reduce the cycle time of each part and increase machine
throughput.
[0006] There may also be a desire to design a part and the
corresponding mold such that the liquid plastic resin flows from
areas having the thickest wall thickness towards areas having the
thinnest wall thickness. Increasing thickness in certain regions of
the mold can ensure that sufficient material flows into areas where
strength and thickness is needed. This "thick-to-thin" flow path
requirement can make for inefficient use of plastic and result in
higher part cost for injection molded part manufacturers because
additional material must be molded into parts at locations where
the material is unnecessary.
[0007] One method to decrease the wall thickness of a part is to
increase the pressure of the liquid plastic resin as it is
introduced into the mold. By increasing the pressure, the molding
machine can continue to force liquid material into the mold before
the flow path has closed off. Increasing the pressure, however, has
both cost and performance downsides. As the pressure required to
mold the component increases, the molding equipment must be strong
enough to withstand the additional pressure, which generally
equates to being more expensive. A manufacturer may have to
purchase new equipment to accommodate these increased pressures.
Thus, a decrease in the wall thickness of a given part can result
in significant capital expenses to accomplish the manufacturing via
conventional injection molding techniques.
[0008] Additionally, when the liquid plastic material flows into
the injection mold and rapidly freezes, the polymer chains retain
the high levels of stress that were present when the polymer was in
liquid form. The frozen polymer molecules retain higher levels of
flow induced orientation when molecular orientation is locked in
the part, resulting in a frozen-in stressed state. These
"molded-in" stresses can lead to parts that warp or sink following
molding, have reduced mechanical properties, and have reduced
resistance to chemical exposure. The reduced mechanical properties
are particularly important to control and/or minimize for injection
molded parts such as thinwall tubs, living hinge parts, and
closures.
[0009] In an effort to avoid some of the drawbacks mentioned above,
many conventional injection molding operations use shear-thinning
plastic material to improve flow of the plastic material into the
mold cavity. As the shear-thinning plastic material is injected
into the mold cavity, shear forces generated between the plastic
material and the mold cavity walls tend to reduce viscosity of the
plastic material, thereby allowing the plastic material to flow
more freely and easily into the mold cavity. As a result, it is
possible to fill thinwall parts fast enough to avoid the material
freezing off before the mold is completely filled.
[0010] Reduction in viscosity is directly related to the magnitude
of shear forces generated between the plastic material and the feed
system, and between the plastic material and the mold cavity wall.
Thus, manufacturers of these shear-thinning materials and operators
of injection molding systems have been driving injection molding
pressures higher in an effort to increase shear, thus reducing
viscosity. Typically, injection molding systems inject the plastic
material in to the mold cavity at melt pressures of 15,000 psi or
more. Manufacturers of shear-thinning plastic material teach
injection molding operators to inject the plastic material into the
mold cavities above a minimum melt pressure. For example,
polypropylene resin is typically processed at pressures greater
than 6,000 psi (the recommended range from the polypropylene resin
manufacturers, is typically from greater than 6,000 psi to about
15,000 psi. Resin manufacturers recommend not to exceed the top end
of the range. Press manufacturers and processing engineers
typically recommend processing shear thinning polymers at the top
end of the range, or significantly higher, to achieve maximum
potential shear thinning, which is typically greater than 15,000
psi, to extract maximum thinning and better flow properties from
the plastic material. Shear thinning thermoplastic polymers
generally are processed in the range of over 6,000 psi to about
30,000 psi.
[0011] 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, typically greater than 30 Rc, and more typically 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. These high hardness materials are also better able to
resist wear from the repeated contact between molding surfaces and
polymer flow.
[0012] 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 cycles per year or more.
Industrial quality production molds must be designed to withstand
at least 500,000 cycles per year, preferably more than 1,000,000
cycles per year, more preferably more than 5,000,000 cycles per
year, and even more preferably more than 10,000,000 cycles per
year. These machines have multi cavity molds and complex cooling
systems to increase production rates. The high hardness materials
are more capable of withstanding the repeated high pressure
clamping 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
through from the molten plastic material through the high hardness
material.
[0013] 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. In some class 101, more than 1 or 2 million cycles per
year may be run, 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.
[0014] Another drawback to using high hardness materials for the
molds is that high hardness materials, such as tool steels,
generally are 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] 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:
[0016] FIG. 1 illustrates a schematic view of an injection molding
machine constructed according to the disclosure;
[0017] FIG. 2 illustrates one embodiment of a thin-walled part
formed in the injection molding machine of FIG. 1;
[0018] FIG. 3 is a cavity pressure vs. time graph for the injection
molding machine of FIG. 1;
[0019] FIG. 4 is a cross-sectional view of one embodiment of a mold
of the injection molding machine of FIG. 1;
[0020] FIG. 5 is a perspective view of a feed system;
[0021] FIGS. 6A and 6B are top and front views of a naturally
balanced feed system;
[0022] FIGS. 7A and 7B are top and front views of another naturally
balanced feed system;
[0023] FIG. 8 is a top view of an artificially balanced feed system
that may be used in the injection molding machine of FIG. 1;
and
[0024] FIGS. 9A and 9B are top views of non-balanced feed systems
that may be used in the injection molding machine of FIG. 1.
DETAILED DESCRIPTION
[0025] 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.
[0026] 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.
[0027] 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.
[0028] Melt holder, as used herein, refers to the portion of an
injection molding machine that contains molten plastic in fluid
communication with the machine nozzle. The melt holder is heated,
such that a polymer may be prepared and held at a desired
temperature. The melt holder is connected to a power source, for
example a hydraulic cylinder or electric servo motor, that is in
communication with a central control unit, and can be controlled to
advance a diaphragm to force molten plastic through the machine
nozzle. The molten material then flows through the runner system in
to the mold cavity. The melt holder may by cylindrical in cross
section, or have alternative cross sections that will permit a
diaphragm to force polymer under pressures that can range from as
low as 100 psi to pressures 40,000 psi or higher through the
machine nozzle. The diaphragm may optionally be integrally
connected to a reciprocating screw with flights designed to
plasticize polymer material prior to injection.
[0029] 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.
[0030] The reciprocating screw 22 forces the molten thermoplastic
material 24, toward a nozzle 26 to form a shot comprising
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.
[0031] The mold may comprise a single mold cavity or a plurality of
mold cavities. The plurality of mold cavities may comprise similar
cavities or dissimilar cavities which will yield dissimilar parts.
The mold may also comprises grouped family of dissimilar
cavities.
[0032] Once the shot comprising 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.
[0033] 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. There
may be intermediary operative units in the communications path
between the sensor, the controller, and the screw control.
[0034] 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.
The sensor 52 need not be in direct contact with the injected fluid
and may alternatively be in dynamic communication with the fluid
and able to sense the pressure of the fluid and/or other fluid
characteristics. 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 disposed at a
location which is 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. In one aspect the controller 50 may maintain the
pressure according to the input from sensor 52.
[0035] A sensor may be located near the end of fill in the mold
cavity. This sensor may provide an indication of when the mold
front is approaching the end of fill in the cavity. The sensor may
sense pressure, temperature, optically, or other means of
identifying the presence of the polymer. When pressure is measured
by the sensor, this measure can be used to communicate with the
central control unit to provide a target "packing pressure" for the
molded component. The signal generated by the sensor can be used to
control the molding process, such that variations in material
viscosity, mold temperatures, melt temperatures, and other
variations influencing filling rate, can be adjusted for by the
central control unit. These adjustments can be made immediately
during the molding cycle, or corrections can be made in subsequent
cycles. Furthermore, several readings can be averaged over a number
of cycles then used to make adjustments to the molding process by
the central control unit. In this way, the current injection cycle
can be corrected based on measurements occurring during one or more
cycles at an earlier point in time. In one embodiment, sensor
readings can be averaged over many cycles so as to achieve process
consistency.
[0036] 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.
[0037] 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). In some injection molding
industries, thin-walled parts may be defined as parts having an
L/T>200, or an L/T>250. 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.
[0038] Thin-walled parts present certain obstacles in injection
molding. 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). For mold
cavities having a more complicated geometry, the L/T ratio may be
calculated by integrating the T dimension over the length of the
mold cavity 32 from a gate 102 to the end of the mold cavity 32,
and determining the longest length of flow from the gate 102 to the
end of the mold cavity 32. The L/T ratio can then be determined by
dividing the longest length of flow by the average part
thickness.
[0039] 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 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 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.
[0040] Traditional injection molding machines use 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.
[0041] 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, polypropylene, polyethylene, thermoplastic
elastomers, polyester, 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.
[0042] Exemplary thermoplastic resins together with their
recommended operating pressure ranges are provided in the following
chart:
TABLE-US-00001 Injection Pressure Material Brand Material Full Name
Range (PSI) Company Name PP Polypropylene 10000-15000 RTP RTP 100
series Imagineering Polypropylene Plastics Nylon 10000-18000 RTP
RTP 200 series Imagineering Nylon Plastics ABS Acrylonitrile
8000-20000 Marplex Astalac ABS Butadiene Styrene PET Polyester
5800-14500 Asia AIE PET 401F International Acetal Co- 7000-17000
API Kolon Kocetal polymer PC Polycarbonate 10000-15000 RTP RTP 300
series Imagineering Polycarbonate Plastics PS Polystyrene
10000-15000 RTP RTP 400 series Imagineering Plastics SAN Styrene
10000-15000 RTP RTP 500 series Acrylonitrile Imagineering Plastics
PE LDPE & 10000-15000 RTP RTP 700 Series HDPE Imagineering
Plastics TPE Thermoplastic 10000-15000 RTP RTP 1500 Elastomer
Imagineering series Plastics PVDF Polyvinylidene 10000-15000 RTP
RTP 3300 Fluoride Imagineering series Plastics PTI Poly-
10000-15000 RTP RTP 4700 trimethylene Imagineering series
Terephthalate Plastics PBT Polybutylene 10000-15000 RTP RTP 1000
Terephthalate Imagineering series Plastics PLA Polylactic Acid
8000-15000 RTP RTP 2099 Imagineering series Plastics
[0043] 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.
[0044] 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.
[0045] 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 form
crystalline structures that have better mechanical and optical
properties than conventionally molded parts. 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.
[0046] 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-00002 TABLE 1 Machining Technology Milling Drilling Sinker
EDM- Sinker EDM- Spindle Spindle Wire EDM Graphite 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% 0:05:58 248% 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: ~20%
2 graphite electrode wear: ~15% 3 Cu electrode wear: ~15% 4 Cu
electrode wear: ~3%
[0047] 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.
[0048] 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 Aleris). 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.
Additionally, non-naturally balanced feed systems are also possible
for use in the constant low pressure injection molding machines
described herein.
[0049] One example of a multi-cavity mold 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 may feed multiple
mold cavities 32. In many high capacity injection molding machines,
the runners are heated to enhance flowability of the molten
thermoplastic material. Because viscosity of the molten
thermoplastic material is very sensitive to shear and pressure
variations at high pressures (e.g., above 10,000 psi), conventional
feed manifolds are naturally balanced to maintain uniform
viscosity. Naturally balanced feed manifolds are manifolds in which
molten thermoplastic material travels an equal distance from the
sprue to any mold cavity. Moreover, the cross-sectional shapes of
each flow channel are identical, the number and type of turns are
identical, and the temperatures of each flow channel are identical.
Naturally balanced feed manifolds allow the mold cavities to be
filled simultaneously so that each molded part has identical
processing conditions and material properties. Naturally balanced
feed manifolds are expensive to manufacture and limit mold designs
somewhat.
[0050] FIG. 5 illustrates an example of a naturally balanced feed
manifold 60. The naturally balanced feed manifold 60 includes a
first flow path 70 from the sprue 62 to a first junction 72 where
the first flow path 70 splits into second and third flow paths 74,
76, the second flow path terminating at a second gate 78a and the
third flow path 76 terminating at a third gate 78b each gate
serving an individual mold cavity (not shown in FIG. 5). Molten
thermoplastic material flowing from the sprue 62 to either the
second gate 78a or the third gate 78b travels the same distance,
experiences the same temperatures, and is subjected to the same
cross-sectional flow areas. As a result, each mold cavity is filled
simultaneously with molten thermoplastic material having identical
physical properties.
[0051] FIGS. 6A and 6B illustrate the naturally balanced manifold
60 schematically. The naturally balanced manifold 60 of FIGS. 6A
and 6B is a multi-tier manifold. Each flow path 74, 76 has
identical characteristics at identical locations along the flow
path. For example, after the junction 72, each flow path narrows at
the same distance. Moreover, each flow path serves an identical
number of mold cavities 32. Naturally balanced flow manifolds 60
are critical to high pressure injection molding machines to
maintain identical plastic flow properties and to ensure uniform
parts.
[0052] FIGS. 7A and 7B illustrate another naturally balanced
manifold 60. The naturally balanced manifold 60 of FIGS. 7A and 7B
is a single tier manifold.
[0053] By contrast, FIGS. 8, 9A, and 9B illustrate non-naturally
balanced manifolds with FIG. 8 illustrating an artificially
balanced manifold and FIGS. 9A and 9B illustrating non-balanced
manifolds.
[0054] The low constant pressure injection molding machine
disclosed herein allows artificially balanced manifolds, and even
unbalanced manifolds, to be used because thermoplastic materials
injected at low constant pressure are not as sensitive to pressure
differences or shear differences due to flow channel characteristic
differences. In other words, the thermoplastic materials injected
at low constant pressure retain nearly identical material and flow
properties regardless of differences in flow channel length,
cross-sectional area, or temperature. As a result, mold cavities
may be filed sequentially instead of simultaneously.
[0055] The artificially balanced manifold 160 of FIG. 8 includes a
sprue 62, a first flow channel 174, and a second flow channel 176.
The first flow channel 174 terminates at a first gate 178a and the
second flow channel 176 terminates at a second gate 178b. The first
flow channel 174 is shorter than the second flow channel 178 in
this embodiment. The artificially balanced manifold 160 varies some
other parameter of the flow channel (e.g., cross-sectional area or
temperature) so that the material flowing through the manifold 160
provides balanced flow to each cavity, similar to a naturally
balanced manifold. In other words, thermoplastic material flowing
through the first flow channel 174 will have about equal melt
pressure to thermoplastic material flowing through the second flow
channel 176. Because artificially balanced, or unbalanced, feed
manifolds can include flow channels of different lengths, an
artificially balanced, or unbalanced, feed manifold can make much
more efficient use of space. Moreover, the feed channels and
corresponding heater band channels can be machined more
efficiently. Furthermore, naturally balanced feed manifolds are
limited to molds having distinct, even numbers of mold cavities
(e.g., 2, 4, 8, 16, 32, etc.). Artificially balanced, and
unbalanced, feed manifolds may be designed to deliver molten
thermoplastic material to any number of mold cavities.
[0056] The artificially balanced feed manifold 160 may also be
constructed of a material having high thermal conductivity to
enhance heat transfer to the molten thermoplastic material in hot
runners, thus enhancing flow of the thermoplastic material. More
specifically, the artificially balanced feed manifold 160 may be
constructed of the same material as the mold to further reduce
material costs and enhance heat transfer within the entire
system.
[0057] FIGS. 9A and 9B illustrate non-balanced manifolds 260. The
non-balanced manifolds 260 may include an odd number of mold
cavities 232, and/or flow channels having different cross-sectional
shapes, different number and type of turns, and/or the different
temperatures. Moreover, the non-balanced manifolds 260 may feed
mold cavities having different sizes, and or shapes, as illustrated
in FIG. 9B.
Drilling and Milling Machineability Index Test Methods
[0058] 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.
[0059] 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., stock 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.
[0060] The test milling or drilling machine was a Hass VF-3
Machining Center.
[0061] Drilling Conditions
TABLE-US-00003 TABLE 2 Spot Drill 120 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
[0062] Milling Conditions
TABLE-US-00004 TABLE 3 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
[0063] For all tests "flood blast" cooling was used. The coolant
was Koolrite 2290.
[0064] EDM Machineability Index Test Methods
[0065] 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.
[0066] 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.
[0067] Wire EDM
TABLE-US-00005 TABLE 4 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%
[0068] Sinker EDM--Graphite
TABLE-US-00006 TABLE 5 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
[0069] Sinker EDM--Copper
TABLE-US-00007 TABLE 6 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
[0070] 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 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.
[0071] 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.
[0072] 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. 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.
[0073] 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.
[0074] 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