Method for the non-resin fluid-assisted injection molding of a r

Details Method for the non-resin fluid-assisted injection molding of a r Method for the non-resin fluid-assisted injection molding of a r

1996-10-03

1998-03-24

Timm, Catherine

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

425130, B29C 4500, B29C 4517

Patent

active

057309264

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a non-resin fluid-assisted injection molding method. More particularly, the present invention is concerned with a method for the non-resin fluid-assisted injection molding of a resin to produce a shaped resin article having an improved surface finish, wherein an excess filling of a mold cavity with a specific amount of a molten resin is effected to form a molten resin mass having opposite, first and second surfaces respectively facing opposite inner walls of the mold cavity, and a pressurized non-resin fluid is subsequently introduced into the mold cavity on a side of the first surface of the molten resin mass to thereby press the second surface of the molten resin mass against the cavity inner wall remote from the side of the introduction of the non-resin fluid. By the non-resin fluid-assisted injection molding method of the present invention, the occurrence of a sink mark on a front surface of a shaped resin article can be prevented without causing the problem of the occurrence of burrs (flashes) thereon. The non-resin fluid-assisted injection molding method of the present invention is especially effective for producing a shaped resin article which has, on a front surface thereof, a locally protruded portion, at which the shaped resin article has an increased thickness, such as a rib, a boss or the like.
2. Discussion of Related Art
It is widely known that when a shaped resin article which has a relatively large thickness or has a locally protruded portion on a back surface thereof is produced by injection molding a molten resin, a dent, i.e., the so-called "sink mark," is likely to occur on a front surface of the shaped resin article in accordance with the volume shrinkage of the molten resin during the cooling thereof. For preventing the occurrence of a sink mark on a front surface of a shaped resin article, a method has conventionally been known, wherein when an injection pressure is applied, an increased level of pressure is used, and, after the injection, the application of pressure is continued while cooling the injected molten resin (this method is hereinafter referred to as "resin pressure method".). In this resin pressure method, the applied injection pressure is generally in the range from 500 to 2,000 kgf/cm.sup.2 in terms of the maximum resin pressure in the mold cavity.
However, as described in Japanese Patent Application Laid-Open Specification No. 50-75247, the above-mentioned resin pressure method has problems. For example, it is difficult to appropriately apply pressures in two stages so as to achieve the effects aimed at by the resin pressure method, so that the molding operation tends to become cumbersome and time-consuming. Furthermore, in the resin pressure method, the shaped resin articles produced are likely to have burrs formed along a parting line of the mold. Therefore, the removal of the burrs is necessary, thereby inevitably causing a problem such that additional work on steps are necessary in a molding operation. On the other hand, when a relatively low pressure is employed in the resin pressure method in order to prevent the occurrence of burrs, sink marks are likely to occur at a portion of a shaped resin article, which is positioned away from a gate in a mold cavity, i.e., a portion on which a satisfactory resin pressure cannot be obtained.
For solving the problems of the resin pressure method, the above-mentioned Japanese Patent Application Laid-Open Specification No. 50-75247 proposes a gas-assisted injection molding method in which a mold is used which comprises a fixed mold half and a movable mold half with its inner wall having a core protruding therefrom toward the inner wall of the fixed mold half, so that a concave mold cavity defined by the fixed mold half and the movable mold half having the core portion is provided. The core has a hole extending in the direction of the longitudinal axis thereof, and the hole has a poppet valve disposed at the top thereof. In this method, afte

REFERENCES:
patent: 4948547 (1990-08-01), Hendry
patent: 5344596 (1994-09-01), Hendry
patent: 5439365 (1995-08-01), Hendry
patent: 5643527 (1997-07-01), Carroll

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