Plastic and nonmetallic article shaping or treating: processes – With step of cleaning – polishing – or preconditioning...
Reexamination Certificate
1998-12-08
2001-09-04
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of cleaning, polishing, or preconditioning...
C264S040100, C264S328100, C425S135000, C425S557000, C425S558000, C425S583000
Reexamination Certificate
active
06284167
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for injection molding using a preplasticizing injection molding machine having a plasticizing device in which the molding resin material is plasticized. More particularly, the invention relates to a method and apparatus for injection molding using a preplasticization type molding machine having an injection device into which plasticized molding resin material is introduced. More particularly still, the invention relates to a method and apparatus for purging leaked resin from an injection device which injects melted resin from its injection cylinder into a metal-mold using an injection plunger.
BACKGROUND OF THE INVENTION
Injection molding machines include the in-line screw type and the preplasticizing type. The preplasticizing type of molding machine has a separate injection device and plasticizing device; the injection device being provided with an injection plunger in the injection cylinder. The preplasticizing type molding machine has a somewhat more complex structure compared to an in-line screw type molding machine, but has advantages such as more accurate metering and injection of plastic resin material, the capability of high injection pressures, as well as more accurate temperature control. Such molding machines are therefore used for injection molding of moldable resin materials in relatively severe molding conditions, such as with engineering plastics.
FIG. 8
is a partial sectional plan view illustrating an exemplary screw type preplasticizing injection molding machine. The plasticizing device includes a plasticizing chamber
12
, in which the screw
11
plasticizes the plastic resin material. An injection chamber
13
, from which a plunger
9
injects the molten resin material into the cavity of a metal mold (not illustrated), is provided on the screw preplasticizing injection unit. The tips of the plasticizing chamber
12
and injection chamber
13
are connected by a connection path
14
. The plasticizing device also includes a plasticizing screw
11
which fits into the plasticizing chamber
12
, a plasticizing motor
4
which rotationally drives the screw
11
and a shut-off hydraulic cylinder
5
which causes the screw
11
to advance over a short stroke in the axial direction so as to close the connection path
14
. Although it is not shown, a hopper which supplies resin material to the base edge portion of the screw
11
is provided on the base edge side of a plasticizing cylinder
16
which forms the plasticizing chamber
12
. A band heater is typically wound around the perimeter of the plasticizing cylinder
16
in order to heat the material.
The injection device further includes an injection plunger
9
which is inserted into an injection cylinder
15
, which forms the injection chamber
13
. A injection hydraulic piston
6
causes the plunger to advance and retract. A nozzle
17
is provided on the front of the injection chamber
13
through which molten resin is injected into a metal mold cavity (not shown).
As is shown in exaggerated size in
FIG. 9
, a step
9
c,
having a diameter difference of approximately 0.2 mm, is formed behind the head potion of the plunger, i.e., the piston head
9
a
and the plunger shaft
9
b
in order to ensure that when the plunger
9
moves during the injection process at high speed and with a high pressure inside the injection chamber, it will not bite into the inner wall of the injection cylinder
15
. Due to the large pressure differential between the front and back of the piston head
9
a
which occurs during injection, molten resin
30
may pass through the gap between the piston head
9
a
and the injection cylinder
15
and leak out behind. In that event, leaked resin
30
a
collects in a thin film around the plunger shaft
9
b.
The leaked resin
30
a
is pushed back by the step
9
c
when the plunger
9
retracts during the metering step; it is then intercepted by a scraper
23
provided at the rear of the injection cylinder
15
, which causes the leaked resin to separate from the plunger shaft
9
b
and fall down through a purge hole
20
.
However, in preplasticizing injection molding machines in which injection is performed by means of such a plunger
9
, the leaked resin
30
a
tends to adhere around the perimeter of the plunger shaft
9
b
making it difficult to purge through the purge hole
20
. Because the small amount of resin material leaked behind the plunger head
9
a
in the injection step is intercepted by the unpurged resin film, the retraction resistance of the plunger
9
increases during subsequent normal metering steps toward the metering stroke completed position.
More particularly, when molding with engineering plastic resin materials, leaked resin
30
a
tends to harden and it is therefore frequently difficult to purge it through the purge hole
20
. Moreover, in order to impart characteristics such as strength, heat-resistance, non-flammability, chemical resistance, etc. to this type of resin, polymer alloys, in which resins of differing physical characteristics are blended with the base resin and resin material to which a halogen base is added, are frequently used. Because the applicable temperature range at which stable melting occurs is quite narrow, the molten resin can easily degrade and its tendency to carbonize and solidify accelerates when it is being moved within the injection cylinder. Also, because some of these resin materials can solidify to high hardness, if they adhere to the plunger shaft area
9
b
they are not easily removed. Due to differences in molding conditions, if the time during which the leaked resin moves in the injection cylinder is extremely long, or if the temperature in the injection cylinder itself fluctuates greatly, hardening of the leaked resin will proceed, causing a phenomenon similar to that known as a “short stroke injection,” in which less than 5% of the molding machine standard stroke distance is used during the metering stroke.
When the plunger retraction resistance increases, not only is there a change in the back pressure experienced during the metering operation, but also a lengthening of metering time. As a consequence, the measured resin density and temperature differ, such that the actual metered resin quantity and its viscosity differ and injected amounts change, adversely effecting the molded part quality. In the worst case, retraction of the plunger during a metering operation may become impossible, or the capability to perform plasticizing for metering is lost. Some of the unpurged resin can also become mixed into the injected molten resin.
The present applicant, in JPN Patent Notices Hei 6-91695 and Hei 6-142245 (Jpn 2,654,897), proposed a method for forcibly purging molten resin in one stroke by forcibly moving the injection plunger over its entire movement stroke in the middle of the molding cycle.
The above noted applications were for purge methods used to change new and old resin materials when changing materials (e.g., color changes) wherein the molding cycle was interrupted and the injection plunger, caused to move a full stroke during that interval. When this purge operation was performed during the molding operation, particularly when the measuring stroke was small, the molding conditions changed temporarily in mid-operation, subtly affecting the injection, plasticization, and metering operations immediately following the purge.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a method and apparatus for purging leaked resin in the injection cylinder of a preplasticizing injection molding machine in which the distance moved by the injection plunger to purge leaked resin is a minimum motion distance, while at the same time, periodically performing the normal injection cycle. In other words, the purge operation is linked to the normal metering step, so that the purging operation can be carried out without interrupting the injection molding cycle, and no adverse effects will be imparted to the plasticizing, metering, or injection ope
Devinsky Paul
Heitbrink Jill L.
McDermott & Will & Emery
Sodick Co. Ltd.
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