Note: Descriptions are shown in the official language in which they were submitted.
CA 02350991 2001-06-18
METHOD AND APPARATUS FOR INJECTION MOLDING
BACKGROUND
The present invention relates to a method and apparatus for injection
molding, and in particular, to a method and apparatus for molding injection
molded parts.
Conventionally, a variety of methods have been utilized for injection
molding in various fields. Of these methods, a molding metal mold of a
runnerless
(hot runner) system has been widely used.
There are a variety of molds in such a hot runner system. For example,
there is a valve gate system in which it is possible to move a needle pin
forward
and backward relative to a gate so as to open and close the gate mechanically.
In a valve gate system, it is possible to prevent a gate portion from causing
problems such as gate stringiness and to enlarge the gate diameter so that the
flow
of a melted resin into a mold is smooth. Generally, the opening and closing
control of the gate in the valve gate system is carried out such that an air
cylinder
or a hydraulic cylinder type actuator is included in the metal mold, and such
an
actuator is controlled in synchronism with a molding process. This
synchronization is realized generally by a process of opening a valve in
response
to a mold clamping signal, starting a timer, and closing a gate when the timer
counts the lapse of the predetermined time.
In another example, a valve is opened in response to a mold clamping
completion signal from an injection molding machine, and a gate is closed in
response to a completion signal of a holding pressure.
A conventional molding process uses two platens, a movable platen and a
stationary platen. A hydraulic cylinder applies a certain force to push a
movable
platen against a stationary platen. Molding members within or attached to the
platens form a molding cavity. The force is maintained on the stationary
platen or
die plate while a molten material is injected into the molding cavity. The
molten
material is injected into the cavity with an extrusion screw until the
pressure inside
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the cavity reaches a predetermined molding pressure. After injecting the
molten
material, the molten material is allowed to cool and solidify, the force is
then
released, and the plates are separated and the process begins anew.
Some injection molding machines use a mold with only one cavity, thereby
allowing for the production of one molded object per cycle. Total cycle time
is the
sum of the fill time and the cool down time. The cool down time is generally
substantially longer than the fill time. For example, a typical fill time is
about 5
seconds, whereas a typical cooling time is about 30 seconds, for a total of
about 35
seconds for the production of one molded article.
To reduce process time per molded article, some injection molding
machines utilize molds with a plurality of cavities for forming a plurality of
molded articles. The molten material fills into each of the cavities
simultaneously.
While this may double the fill time, for example, to about 8 seconds, the
cooling
time remains fixed at about 30 seconds. The total time of this process is
about 38
seconds for the production of two molded articles. Thus, using multiple
cavities
increases the efficiency almost two-fold.
A problem with the multiple cavity method, however, is that the mold
clamping force must also be doubled since the article molding area is doubled.
As
a result, a larger injection molding machine must be used to apply the extra
force
needed to hold the molding platens together. A larger injection molding
machine
costs more, takes up more floor space, and requires more power. Therefore,
using
multiple cavity molds with the conventional method sacrifices cost for greater
time efficiency.
BRIEF SUMMARY
Taking the foregoing conventional problems into consideration, an object
of the present invention is to provide an injection molding method that does
not
require increasing the mold clamping force for molding articles in a multiple
cavity mold, thereby providing the simultaneous benefits of both time and cost
savings in the same process.
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In one aspect, the invention is a method for sequentially injecting a molten
material comprising clamping a stationary platen and a movable platen at a
clamping force to define at least two cavities, opening a first valve gate to
inject a
molten material into a first cavity, closing the first valve gate when the
pressure
inside the first cavity reaches a set-point pressure, opening a second valve
gate to
inject the molten material into a second cavity, and closing the second valve
gate
when pressure inside the second cavity reaches a set-point pressure.
In a second aspect, the invention is an injection molding apparatus
comprising a mold having at least two mold cavities, a molten material inlet
system in communication with said at least two mold cavities, at least two
valve
gates in said molten material inlet, wherein each of said at least two valve
gates
are associated with one of said mold cavities; and a controller adapted to
sequentially open and close said valve gates.
In a third aspect, the invention is a controller for use with a injection
1 S molding device having a mold with at least two cavities, the controller
comprises
means for opening a first valve gate associated with a first mold cavity to
initiate a
flow of molten material into the first mold cavity, means for closing the
first valve
gate when the pressure inside the first mold cavity reaches a set-point
pressure,
means for opening a second valve gate associated with a second cavity to
initiate a
flow of molten material into a second mold cavity, and means for closing the
second valve gate when pressure inside the second mold cavity reaches a set-
point
pressure.
Other objects, features and advantages of the present invention will become
apparent from the following detailed description. It should be understood,
however, that the detailed description and the specific examples, while
indicating
embodiments of the invention, are given by way of illustration only, the
invention
being defined only by the claims following this detailed description.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
The following drawings form part of the present specification and are
included to further demonstrate certain aspects of the present invention. The
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invention may be better understood by reference to one or more of these
drawings
in combination with the detailed description of specific embodiments presented
herein:
FIG. 1 is a schematic view of an injection molding apparatus suitable for
S the method of injection molding a molten material, provided by the present
invention.
FIG. 2 is a schematic view of a part of the injection molding apparatus
showing a state immediately after clamping the mold, and a state in which the
introduction of the molten material is initiated, in the method of injection
molding,
provided by the present invention.
FIG. 3 is a flowchart illustration of the sequential injection molding process
of the present invention.
FIG. 4 schematically shows a change in injection velocity with time for a
conventional injection molding method and a sequential injection molding
method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, the injection molding apparatus suitable for use in the method of
injection-molding a thermoplastic or thermoset resin, provided by the present
invention, will be outlined below with reference to FIG. 1. The injection
molding
apparatus includes an injection cylinder 12 having a resin-feeding screw 10
inside,
a stationary platen 40, a movable platen 44, an inlet 26, valve gates 50, tie
bars 34,
a clamping hydraulic cylinder 30 and a hydraulic piston 32. The movable
platen 44 is actuated with the hydraulic piston 32 in the hydraulic cylinder
30 to
move in parallel on the tie bars 34.
A mold is formed by a stationary mold member 36 and a movable mold
member 46. The stationary mold member 36 is attached to the stationary
platen 40, and the movable mold member 46 is attached to the movable platen
44.
The platens 40, 44, the tie bars 34, and the cylinder 30 and piston 32 define
a
clamping system for applying a clamping pressure to the mold members 36, 46.
The movable platen 44 is moved towards the stationary platen 40 until the
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movable mold member 46 is engaged with the stationary mold member 36, and the
mold is clamped to form cavities 22, 24. This clamped position is illustrated
in
FIG. 2.
After the mold has been clamped, the clamping force is controlled with the
clamping hydraulic cylinder 30. The clamping force may also be controlled by
toggle or an electric machine. The molten material flows into the cavities 22,
24
via an inlet 26. The valve gates 50, 52 face cavities 22, 24 and, in a
preferred
embodiment, there is a valve gate associated with each cavity. The valve gates
50,
52 open and close the inlet 26 to the cavities 22, 24. Suitable valve gates
50, 52
are any valves known in the injection molding art. Particularly preferred
valves
include valve gates. After the molten material cools and hardens, the clamping
force is released and the movable platen 44 is moved away from the stationary
platen 40, in order to release the molded product.
For the exemplary two-cavity mold shown in FIG. 2, the sequential
1 S injection molding method begins with clamping the mold with at a mold
clamping
force. The controller 60 then closes valve gate 52 and opens valve gate 50.
Molten material fills cavity 22. A pressure transducer P2 is associated with
valve
gate 50, and another pressure transducer P1 is associated with the cavity 22
itself.
Similarly, a pressure transducer P4 is associated with valve gate 52 and
another
pressure transducer P3 is associated with cavity 24. In an alternate
embodiment
the pressure transducers P1', P3' associated with the cavity are located near
an
opposite wall from the inlet. Pressure transducers P1', P3' are useful to
monitor
the pressure inside the cavity near the end of the fill. Lt will be understood
that it
is not necessary to have a pressure transducer at both the valve gate and the
cavity.
One of ordinary skill in the art will understand that it is only necessary to
have a
pressure transducer associated with either each valve gate, each cavity, or a
combination of both. The pressure transducers 66 transmit their respective
readings to the controller 60, which uses either or both pressure readings to
determine when to close valve gate 50 and open valve gate 52. The controller
closes valve gate 50 and opens valve gate 52 when the pressure inside cavity
22
reaches a set-point pressure. Molten material then fills into cavity 24, and
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pressure transducers P4, P3 associated with valve gate 52 and cavity 24,
respectfully, transmit information to the controller. When the pressure inside
cavity 24 reaches the set-point pressure, the controller closes valve gate 52
and the
molten material is allowed to cool and solidify.
S FIG. 3 is a flowchart illustration of the sequential injection molding
process
of the present invention. It will be understood that each step of the
flowchart
illustration can be implemented by computer program instructions or can be
done
manually. These computer program instructions may be loaded onto a computer
or other programmable data processing apparatus to produce a machine, such
that
the instructions which execute on the computer or other programmable data
processing apparatus create means for implementing the functions specified in
the
flowchart step. These computer program instructions may also be stored in a
computer-readable memory that can direct a computer or other programmable data
processing apparatus to function in a particular manner, such that the
instructions
stored in the computer-readable memory produce an article of manufacture
including instruction means which implement the function specified in the
flowchart step. The computer program instructions may also be loaded onto a
computer or other programmable data processing apparatus to cause a series of
operational steps to be performed on the computer or other programmable data
processing apparatus to cause a series of operational steps to be performed on
the
computer or other programmable apparatus to produce a computer implemented
process such that the instructions which execute on the computer or other
programmable apparatus provide steps for implementing the functions specified
in
the flowchart step.
It will be understood that each step of the flowchart illustration can be
implemented by special purpose hardware-based computer systems which perform
the specified functions or steps, or combinations of special purpose hardware
and
computer instructions, or can be done manually.
An injection molding machine utilizing a sequential injection molding
process has a plurality of cavities formed by the movable mold member 46 and
the
stationary mold member 36. For an injection molding machine with m cavities,
CA 02350991 2001-06-18
where n equals 1 to m, the process begins with step 100 by closing the clamp
with
a mold clamping force calculated by the equation:
mold clamping force = (molding pressure)/(surface area of cavity n)
The molding pressure is predetermined and is calculated based upon the type of
S molding material and the desired characteristics of the molded article. In
step 110,
a first valve gate is opened which faces a first cavity. 'The first cavity is
then
injected with molten material using a resin-feeding screw at a predetermined
injection velocity in step 120. The injection velocity may be changed or may
be
kept constant as the cavity becomes filled with molten material. The time it
takes
to fill the cavity to the set-point pressure depends on the size of the cavity
and the
injection velocity. In a preferred embodiment, the injection velocity is
varied and
it takes about one second to about ten seconds to fill the cavity to the set-
point
pressure. In step 130, the pressure inside the cavity is measured and compared
to
the set-point molding pressure. The process goes back to step 120 if the
pressure
1 S inside the cavity is less than the set-point molding pressure. The first
valve gate is
closed once the pressure inside the cavity reaches the set-point molding
pressure in
step 140. The process goes back to step 110 and repeats for n cavities. After
all of
the cavities are full at the set-point pressure, the molten material inside
the cavities
is allowed to cool and solidify in step 150. The cooling process takes about
20 seconds to about 40 seconds, depending upon the size of the molded article
and
the type and temperature of the molded material. After cooling, the mold
clamping force is released and the clamp is opened in step 160. The sequential
injection molding process ends with step 170, when the molded articles are
ejected
from the molding cavities.
The mold clamping force required is reduced significantly in a sequential
injection molding process for a multiple cavity mold. 'This is because the
area to
be pressurized does not increase when there are multiple cavities. For a mold
with
multiple cavities, the area to be pressurized remains constant and equals the
area
of one cavity since each cavity in the mold is pressurized and closed
sequentially.
Therefore, the mold clamping force required in a two-cavity mold is reduced to
almost half by using the sequential injection molding method compared to a
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conventional method. The mold clamping force required in a three-cavity mold
the force required is reduced by over fifty percent compared to the force
required
in the conventional method. This significant reduction in mold clamping force
allows for a reduction in the press size, which in turn allows for dramatic
cost
savings in terms of production cost per molded article.
FIG. 4 shows how the injection velocity varies during the step of filling a
cavity for a standard injection molding process compared to a sequential
injection
molding process in a two-cavity mold. The injection velocity is controlled by
the
machine set-point of the resin-feeding screw 10. In a standard injection
molding
process the cavities are filled with molten material simultaneously and in the
sequential method the cavities are filled sequentially. Both processes may be
carried out with more than two cavities. The sequential molding process,
however, has at least two cavities.
In a standard injection molding process, the injection pressure is set at the
set-point molding pressure. The injection velocity of the molten material is
at a
filling flow rate when the valve gate is first opened. As illustrated in FIG.
4, the
injection velocity is kept at filling flow rate until the cavities are almost
full. The
injection velocity is then gradually tapered down from the filling flow rate
so that
the molten material can fill up the entire cavity. Once the pressure inside
the
cavity reaches the set-point molding pressure, the injection velocity is
brought
down to zero. Decreasing the injection velocity ensures that the molten
material is
uniform inside the cavities, thereby yielding a higher quality molded article.
In the sequential injection molding process, the injection pressure is set at
the set-point molding pressure. The injection velocity of the molten material
is at
a filling flow rate when a valve gate is opened. The injection velocity is
kept at
the filling flow rate until a cavity is almost full and then gradually tapered
down
until the pressure inside a cavity reaches the set-point molding pressure. The
difference in the sequential method compared to the standard method, is that
the
injection velocity is increased again to the filling flow rate when the second
valve
gate is opened. This adds approximately 0.5 seconds to about 4 seconds to the
total fill-time for the process. In a preferred embodiment, the ramp up of the
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injection velocity to the filling flow rate is rapid so that the total process
time does
not increase significantly.
It is contemplated that numerous modifications may be made to the
injection molding method and apparatus of the present invention without
departing
from the spirit and scope of the invention as defined in the claims. For
example,
while the exemplary embodiment shown in the drawings has two mold cavities,
those skilled in the art will appreciate that the same sequential steps can be
used to
control the flow of molten material into molds having more than two cavities.
In
addition, for molds having more than two cavities, there may be a valve gate
associated with each cavity, with each valve gate opened and closed
sequentially.
Alternately, for molds having more than two cavities, there may be fewer valve
gates than cavities, as long as there are at least two cavities. In this
embodiment,
at least one of the valve gates would control the inlet to at least two
cavities.
Accordingly, while the present invention has been described herein in relation
to
1 S several embodiments, the foregoing disclosure is not intended or to be
construed
to limit the present invention or otherwise to exclude any such other
embodiments,
arrangements, variations, or modifications and equivalent arrangements.
Rather,
the present invention is limited only by the claims appended hereto and the
equivalents thereof.
