Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: METHOD FOR INJECTION MOLDING AND INJECTION MOLDING
MACHINE THEREFOR
I. Background of the Invention
1. Field of the invention
The present invention relates generally to methods and machines for injection
molding.
More specifically, it relates to a method for injection molding and injection
molding
machines therefor, the former and the latter being based on the continuous
rotation,
without rectilinear reciprocating displacement, of the plasticizing screw.
2. Description of the state of the art
As it is known in the field of injection molding processes and machines, the
synthetic
resinous material is usually supplied in a particulate feedstock. The latter
is heated,
masticated and plasticated into a moldable consistency and, then, is forced
into one or
more molding cavities. As it is also known, when dealing with hygroscopic
materials in
particulate form, it is required to pass those materials through a separate
pre-drying
apparatus, before entering into an injection molding machinf:. In the pre-
drying apparatus
the particulate material is exposed to a dry heat, for a predetermined period
of time, at a
temperature below the material melting temperature. This pre-drying operation
is
necessary, since water, present in the particulate material, vaporizes during
the
plasticization phase, forming pockets of volatilized substances.
The above situation leads to defects, such as, for example, polymer
degradation and
diminished physical material properties of the molded product. Moreover,
surface
defects, such as loss of gloss arid voids, sometimes occur in 'the molded
product. These
defects can not only alter the commercial aspect of the products, but also,
directly affect
the physical properties of the molded products.
Attempts have been made in the past to overcome, or, at least, alleviate the
above
essential drawbacks. Thus, for example, United States Patent No. 6,187,229,
granted Feb.
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13, 2001, to Kiyoto et al. for a "Process for Molding Information Recording
Disks"
discloses a molding process, wherein plasticization, injection and filing of
synthetic
resinous material which are carried out in separate cylinders. A synthetic
resinous
material, such as polycarbonate, is supplied, with out pre-drying, to an
injection molding
machine, for a first time molding. While synthetic resinous material is
masticated and
plasticated by the machine, certain components are heated. Thus, they become
volatile
and can be exhausted through a vent port.
A main disadvantage characterizes the forgoing process. This is due to the
fact that the
synthetic resinous material is extruded into an injection cylinder and then
transferred,
under high pressure, to a mold, while the plasticizing screw remains
stationary, allowing
a pooling of melted material. When the plasticizing screw resumes rotating,
the pool of
melted material is conveyed, at a relatively high speed, along the
plasticizing screw. A
surge causing vent bleed is formed. Vent bleed will cause a solidification of
the
plasticated material in a vent opening and, thereby, partially or wholly, will
block the
latter. When this occurs, the subsequent release of volatilized material from
the vent
opening is severely inhibited, if not actually prevented.
United States Patent No. 6,234,659 granted May 22, 2001, to Takashima et al.,
for a
"Surge Suppresser for Vented Injection Molding Machine Screw" describes a
machine
operating without pre-drying. The plasticizing machine includes a two-stage
plasticizing
screw, rotatably and reciprocably mounted within a barrel. A surge suppresser,
provided
between the first and second stages, comprises an integral protrusion of the
plasticizing
screw root and a deep channel section, the latter following the above integral
protrusion.
The surge suppresser prevents the surge of molten material into the vent area,
upon the
resumption of the plasticizing screw rotation, thereby allowing faster
recovery and
preventing bleeding of plasticized material from the vent opening. The main
disadvantage
of the above surge suppresser used with a molding machine plasticizing screw
resides in
the following: this design requires a support for the increased length of the
two-stage
plasticizing screw, in order to obtain an appropriate seal in the second stage
of the
plasticizing screw. Furthermore, the reciprocating nature of the plasticizing
screw within
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the barrel limits the range of shot size. Also, the surge suppressor must be
adapted to the
viscosity of the melted material.
II. Summary of the Invention
Accordingly, there is a need for an improved method and apparatus.
It is a first objective of the present invention to develop, based on a new
method, a simple
to operate, reliable, with a long service life, injection molding machine.
It is a second objective of the present invention to elaborate a method and
design an
apparatus that allows the use of non pre-dried synthetic resinous material.
It is a third objective of the present invention to obtain a wide range of
shot sizes, while
preventing vent bleed.
Broadly stating, the injection molding machine, according to the present
invention,
comprises the following main assemblies:
- a plasticization assembly adapted to receive, masticate and plasticate a pre-
established quantity of synthetic resinous material, the plasticization
assembly
being flanked by
- a pair of injection assemblies communicating with the plasticization
assembly, so
that while one of the pair of injection assemblies communicates with the
plasticization assembly to receive the synthetic resinous material, already
masticated and plasticated, the other one of the pair of injection assemblies
forcibly expels said synthetic resinous material, already masticated and
plasticated, into a molding assembly;
- an interconnecting assembly for providing
- a communication between the plasticization assembly and each one of the
pair of injection assemblies, alternatively, and
- a communication between one of the pair of injection assemblies, which is
not in communication with the plasticization assembly, and the molding
assembly;
- a common base incorporating a lower stationary base and an upper moving
base,
the latter being used
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- to support and
- alternatively translate the plasticization assembly and the pair of
injection
assemblies on the lower stationary base; and
- a translating assembly for alternatively translating the upper moving base
with
respect to the lower stationary base and the molding assembly.
In one aspect of this invention, the plasticization assembly comprises a
cylindrical-
housing subassembly incorporating a two stage plasticizing screw that rotates
continuously about its longitudinal axis of symmetry, without any rectilinear
displacement; thus, the pre-established quantity of synthetic resinous
material is
transformed and advanced. The two stage plasticizing screw is constituted by a
thread
having a constant external diameter, commensurate to cooperate with an
internal diameter
of the cylindrical-housing subassembly, and a variable root diameter. In order
to
accomplish a two stage plasticizing operation, the variable root diameter
basically
increases from a minimum to a maximum along a first stage, starting from one
extremity,
proximate to a drive end to an end of this stage. Along a second stage the
root diameter
basically replicates the first stage by increasing from the minimum to the
maximum
diameter. The plasticizing assembly further comprises band-heaters, mounted
around the
cylindrical-housing subassembly, respectively its external surface; a hopper
connected to
a feed port of the cylindrical-housing subassembly, wherein synthetic resinous
material is
discharged; an inlet for introducing an inert gas into the cylindrical-housing
subassembly
communicating with the feed port; an exhaust vent devise connecting the
interior of the
cylindrical-housing subassembly with the environment; and an electrical motor-
mechanical transmission unit being coupled to the two stage plasticizing
screw.
In another aspect of the present invention, each of the injection assemblies
comprises a
stepped body that incorporates an injection cylinder, coaxial with an
actuating cylinder.
The injection cylinder has a diameter relatively smaller than the diameter of
the actuating
cylinder, and is usually provided with heating elements. A plunger is located
within the
injection cylinder, while a piston is located in the actuating cylinder. A rod
conveniently
interconnects the plunger with the piston, and under the pressure of a fluid,
the piston is
displaced towards the injection cylinder.
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In yet another aspect of this invention, the interconnecting assembly
comprises a
manifold, having, centrally, a two position - four port valve. The manifold is
connected,
at its middle, to:
a) the plasticization assembly and, at its extremities, to
b) each one of the pair of injection assemblies; the two position - four port
valve
having
- a first port connected to the cylindrical-housing subassembly;
- a second port connected to one of the pair of inj action cylinders;
- a third port connected to another one of the pair of injection cylinders;
and
- a fourth port connected, via a nozzle, to a molding assembly, the manifold
being heated by conventional heating elements.
The method for injection molding, using the above described machine, comprises
- masticating and plasticizing a pre-established quantity of the synthetic
resinous
material, generally provided with hygroscopic properties, in the
plasticization
assembly;
- expelling the synthetic resinous material, already masticated and
plasticated, from the
plasticization assembly, into one of the pair of injection assemblies, while
- forcibly expelling the synthetic resinous material, already masticated and
plasticated,
from the other one of the pair of injection assemblies into a molding
assembly,
whereby
a) masticating and plasticizing a pre-established quantity of the synthetic
resinous material; generally provided with hygroscopic properties, in the
plasticization assembly;
b) expelling the synthetic resinous material, already masticated and
plasticated,
from the plasticization assembly, into one of the pair of injection
assemblies,
while
c) forcibly expelling the synthetic resinous material, already masticated and
plasticated, from the other one of the pair of injection assemblies into a
molding assembly,
whereby forgoing steps b and c are alternatively performed by each one of the
pair of
injection assemblies, so that the method for injection molding is continuous.
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III. Brief description of the Drawings
Although the characteristic features of the invention will be particularly
pointed out in the
claims, the invention itself and the manner in which it may be made and used
may be
better understood by referring to the following description and accompanying
drawings.
Like reference numerals refer to like parts throughout the several views of
the drawings
in which:
Figure 1 is a perspective view of the injection molding machine according to
the
presentinvention;
Figure 2 is a vertical cross-section along the plasticization assembly;
Figure 3 is an enlarged view of Detail A of Fig. 2;
Figure 4 is a horizontal crass-section taken along the paasticization,
injection and
interconnecting assemblies; and
Figure 5 is an enlarged view of Detail B of Fig. 4.
IV. Description of the Preferred Embodiment
FIGS. 1 - 5 illustrate an injection molding machine 100 which incorporates the
following
main assemblies:
- a plasticization assembly 200 adapted to receive, masticate and plasticate a
pre-
established quantity of synthetic resinous material, plasticization assembly
200
being flanked by
- a pair of injection assemblies 300 communicating with plasticization
assembly
200, so that while one of the pair of injection assemblies 300 communicates
with
plasticization assembly 200 to receive the synthetic resinous material,
already
masticated and plasticated, the other one of the pair of injection assemblies
300
forcibly expels the synthetic resinous material, already masticated and
plasticated,
into a molding assembly (not shown);
- an interconnecting assembly 400 for providing
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- a communication between plasticization assembly 200 and each one of the
pair of injection assemblies 300, alternatively, and
- a communication between one of the pair of injection assemblies 300, which
is not in communication with plasticization assembly 200, and the molding
assembly;
- a common base 500 incorporating a lower stationary base 510 and an upper
moving base 520, the latter being used
- to support and
- alternatively translate plasticization assembly 200 and the pair of
injection
assemblies 300 on lower stationary base 510; and
- a translating assembly 600 for alternatively translating upper moving base
520
with respect to lower stationary base 510 and a the molding assembly.
Referring now in detail, said plasticization assembly 200 comprises a
cylindrical-housing
subassembly 205 /constituted of several components/, wherein a two stage
plasticizing
screw 210 rotates about its longitudinal axis of symmetry, without rectilinear
displacement. Thus, the pre-established quantity of synthetic resinous
material is
transformed and advanced. The two stage plasticizing screw 210 is constituted
by a
thread 215 having a constant external diameter 220, commensurate to cooperate
with an
internal diameter 225 of cylindrical-housing subassembly 205, and a variable
root
diameter 230. In order to accomplish a two stage plasticizing operation, the
variable root
diameter 230 basically increases from a minimum to a maximum along a first
stage,
starting from one extremity, proximate to a drive end to an end of this stage.
Along a
second stage the root diameter 230 basically replicates the first stage by
increasing from
the minimum to the maximum diameter. Plasticizing assembly 200 further
comprises: a)
band-heaters 235, mounted around cylindrical-housing subassembly 205,
respectively its
external surface; b) a hopper 240, wherein synthetic resinous material is
discharged; and
c) a feed port 250 of cylindrical-housing subassembly 205. An inlet 255 for
introducing
an inert gas into cylindrical-housing subassembly 205 communicates with feed
port 250.
An exhaust vent device 260 connects the interior of cylindrical-housing
subassembly 205
with the environment. An electrical motor- mechanical transmission unit 265 is
coupled
to two stage plasticizing screw 210.
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Each of injection assemblies 300 comprises a stepped body subassembly 305 that
incorporates an inj ection cylinder 310, coaxial with an actuating cylinder
315. Inj ection
cylinder 310 has a diameter relatively smaller than the diameter of actuating
cylinder 315
and is usually provided with heating elements 313. A plunger 320 is located
within
injection cylinder 310, while a piston 325 is located in actLrating cylinder
315. A rod 330
conveniently interconnects plunger 320 with piston 325. Under the pressure of
a fluid,
piston 325 is displaced towards injection cylinder 310.
Interconnecting assembly 400, comprising a manifold 405., wherein, centrally,
a two
position - four port valve 410 is incorporated. Manifold 405 is connected, at
its middle,
to:
a) plasticization assembly 200 and, at its extremities, to
b) each one of the pair of injection assemblies 300.
Two position - four port valve 410 has
- a first port 415 connectable to cylindrical-housing subassembly 205;
- a second port 420 connectable to one of the pair of injection cylinders 310;
- a third port 425 connectable to another one of the pair of injection
cylinders 310;
and
- a fourth port 430 connectable via a nozzle 435 to the molding assembly.
Manifold 405 is also generally heated by conventional external elements (not
shown).
Two position - port valve 410 is operated as follows:
in a first position, first port 415 communicates with second port 420, so that
the synthetic
resinous material, already masticated and plasticated, is extruded from
cylindrical
housing 205 of plasticization assembly 200, into one of the pair of injection
assemblies
300, via manifold 405; simultaneously, when third port 42S communicates with
forth port
430, the synthetic resinous material is forcible expelled from one of the pair
of injection
assemblies 300 via manifold 405 and through nozzle 435 to the molding
assembly;
in a second position, first port 415 communicates with third port 425, so that
the
synthetic resinous material, already masticated and plasticated, is extruded
from
cylindrical housing 205 of plasticization assembly 200, into another one of
the pair of
injection assemblies 300, via manifold 405; simultaneously, when second port
420
communicates with forth port 430, the synthetic resinous material is forcible
expelled
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from one of the pair of injection assemblies 300 via manifold 405 and through
nozzle 435
to the molding assembly(not shown).
The method for injection molding, according to the present invention, is
adapted for use
with the foregoing injection molding machine.
The method for injection molding comprises
- masticating and plasticizing a pre-established quantity of the synthetic
resinous
material, generally provided with hygroscopic properties, in the
plasticization
assembly 200, namely in cylindrical-housing subassembly 205;
- expelling the synthetic resinous material, already masticated and
plasticated, from the
cylindrical-housing subassembly 205, into one of the pair of injection
assemblies 300,
namely into one injection cylinder 310, while
- forcibly expelling the synthetic resinous material, already masticated and
plasticated,
from the other one of the pair of injection cylinders 310 into the molding
assembly,
whereby
a) masticating and plasticizing a pre-established quantity of the synthetic
resinous material, generally provided with hygroscopic properties, in the
cylindrical-housing subassembly 205;
b) expelling the synthetic resinous material, already masticated and
plasticated,
from the plasticization cylinder, into one of said pair of injection cylinders
310, while
c) forcibly expelling the synthetic resinous material, already masticated and
plasticated, from the other one of the pair of injection cylinders 310 into
the
molding assembly, whereby
forgoing steps b and c are alternatively performed by each one of the pair of
injection
assemblies 300, so that the method for injection molding is continuous.
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