Plastic article or earthenware shaping or treating: apparatus – Female mold and charger to supply fluent stock under... – With accumulator – trap chamber – or serially arranged valves...
Reexamination Certificate
1998-11-12
2002-10-08
Silbaugh, Jan H. (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Female mold and charger to supply fluent stock under...
With accumulator, trap chamber, or serially arranged valves...
C425S561000, C425S564000
Reexamination Certificate
active
06461142
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an injection device used for molding a resin.
BACKGROUND ART
A known injection device used for molding a resin comprises a cylinder having an injection nozzle at an end thereof, a plunger sliding in the axial direction in a position opposite to the injection nozzle within the cylinder, and a driving device for advancing and retracting the plunger. The cylinder has a port for supplying a molding material between the sliding position of the plunger and the injection nozzle, and the molding material supplied into the cylinder is compressed by advancement of the plunger, so that the molding material is injected into a cavity of molds connected to the injection nozzle. In such an injection device, a screw for melting the molding material is provided outside the cylinder in the case of a pre-plasticizing type, and is provided integrally with a leading end of the plunger within the cylinder in the case of an in-line type. The plunger of such an injection device of the pre-plasticizing type may be provided with a larger diameter portion in contact with an inner circumferential wall surface of the cylinder for isolating inner and outer regions of the cylinder from each other and a smaller diameter portion in a leading end thereof.
The above injection device of a conventional injection molding machine has presented such issues as described below.
When the molds are opened for removing a product, a molding material is forced out of a leading end of the injection nozzle, causing a so-called drooling condition. Therefore, if the pressure (back pressure) of the molding material in the side of a plunger head is too high, while weighing comes to be unstable, or if the back pressure is too low, a suck-back operation has been conducted for moving the plunger slightly further backward from a position of the plunger after it has been pushed back by the back pressure to an extent sufficient to prevent the drooling.
In order to prevent the molding material from leaking out of the cylinder through a clearance (gap) between the cylinder and the plunger at the time the molding material is supplied into the cylinder, the molding material is injected into the mold cavity and the like, the length of a portion of the plunger in contact with the cylinder has been made longer. In addition, the clearance between the plunger and the cylinder has been reduced, that is, narrowed to such a minimum extent to allow the plunger to slide.
In the cylinder, a check valve (ring valve) has been employed between a fluid holding part with the injection nozzle and the supply port for preventing a molten liquid of molding material held in the fluid holding part from flowing back to the supply port. The check valve is brought into tight contact with a valve seat of the plunger by the back pressure of the molten liquid of molding material, when the backward movement of the plunger is discontinued after weighing, so that back flow of the molten liquid to the supply port is prevented.
On the other hand, in an attempt to prevent back flow of the molding material from the mold cavity to an inner region of the cylinder until completion of a pressure-holding and compression process after injection of the molding material to the mold cavity, a pressure control has been conducted by applying a force to the plunger in the advancing direction of the plunger until completion of the pressure-holding and compression process after injection of the molding material to the mold cavity. Therefore, a force of the molding material flowing back from the mold cavity into the cylinder is counterbalanced with the force applied to the plunger.
Such an injection device according to the prior art, however, has the below-mentioned issues.
During the suck-back operation, because the inner region of the cylinder is reduced in pressure, the air is brought into the cylinder from outside the cylinder through a trailing end of the plunger, and the air is entrapped in a product as a void (bubble), causing a defect in the product. Although P
2
>P
0
at the time of weighing of the molding material, P
2
<P
0
after completion of the weighing where a pressure of molding material from the plunger head to a guide is P
1
, a pressure of the molding material (back pressure) in the side of a leading end of the plunger head is P
2
, and a pressure outside the cylinder is P
0
(atmospheric pressure). Entrainment of the air from outside the cylinder is caused at P
1
<P
0
due to such reason as insufficient closing of the check-valve.
Even if the clearance between the plunger and the cylinder is narrowed, the molding material is adhered to the wall surfaces of the plunger and the cylinder in the sliding zone of the plunger, and it is unavoidable that the molding material enters between the plunger and the cylinder. Moreover, once the molding material enters between the plunger and the cylinder, it does not readily flow between them, because the clearance is very narrow, and retention of the molding material in the clearance is caused. The molding material retained therein is reduced in quality and carbonized due to a frictional heat caused by sliding movement of the plunger. Then, the carbide is entrained in the molding material in the cylinder, and contaminates a molded product, causing a defect in the product.
Shown in
FIG. 8A
is an injection device of pre-plasticizing type comprising a plunger
2
with a larger diameter portion
21
in contact with an inner circumferential wall surface of a cylinder
1
for isolating inner and outer regions of the cylinder from each other, and a smaller diameter portion
22
in a leading end thereof. A molten liquid of molding material fed from a port
12
for supplying the molding material in the cylinder
1
is guided toward an injection nozzle (lower side of the figure, not shown) by a stepped surface
23
in a boundary between the larger diameter portion
21
and the smaller diameter portion
22
. However, a part of the molten liquid of molding material supplied from the molding material supply port
12
is divided into two streams flowing in the right and left directions about the smaller diameter portion
22
of the plunger
2
as shown in
FIG. 8B
, and these streams collide against each other in a position backside in relation to the supply port
12
. This collision causes a stagnating area
9
at that position. Then, the molding material in the stagnating area
9
is reduced in quality and carbonized by heat, and the carbide is removed from the wall surface of the smaller diameter portion
22
, and contaminates a product, causing a defect in the product.
The check valve is not brought into sufficiently tight contact with the valve seat of the plunger, that is, it is not completely closed, even when the backward movement of the plunger is discontinued after completion of weighing, and is completely closed only after injection is completed. Therefore, a significant and unstable quantity of molten liquid of the molding material flows back before the valve is completely closed. As a result, a present quantity of the molding material to be supplied and an actual quantity of molding material supplied are unequal to each other, because the actual quantity supplied has been varied. Thus, it has been difficult to assure a fixed quantity of molding material to be supplied into the mold cavity.
In the pressure control described above, the position of the plunger in the cylinder should be stable. However, as the pressure of the molding material in the mold cavity during the pressure-holding and compression process and the force applied to the plunger in the advancing direction of the plunger are balanced with each other, the balance between them is actually varied in a subtle manner. Therefore, the position of the plunger is varied, so that it has been difficult to consistently maintain the forces in perfectly balanced condition in order to provide a constant holding pressure. Accordingly, the molding material is forced in and out of the molding cavity during the pressure-holdin
Higashida Takaaki
Kadoriku Shinji
Murata Tadao
Yutani Hiroshi
Luk Emmanuel S.
Matsushita Electric - Industrial Co., Ltd.
Silbaugh Jan H.
Wenderoth , Lind & Ponack, L.L.P.
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