Plastic and nonmetallic article shaping or treating: processes – With step of making mold or mold shaping – per se
Patent
1995-06-28
1997-08-05
Heitbrink, Jill L.
Plastic and nonmetallic article shaping or treating: processes
With step of making mold or mold shaping, per se
2491141, 249134, 264327, 26432812, 26432816, 264338, 425542, B29C 4537, B29C 3340, B29C 4526
Patent
active
056539320
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an injection molding method for manufacturing a product having the external appearance free from defects such as sink marks or weld lines. The present invention is also directed to a mold assembly suitable for manufacturing such a product.
According to a method of injection molding a thermoplastic resin, the product which even has a complicated profile can be easily manufactured with high productivity. In this regard, the injection molding process has been adopted in various field, e.g. for manufacturing industrial parts or living necessaries.
A molten resin to be injection molded is forcibly poured through a gate into a molding cavity defined in a mold assembly. The poured resin passes through a predetermined channel to fill the cavity. The resin is diverged to a plurality of flows and then joined together during the fluidization in the cavity. When a plurality of the resin flows are incompletely joined together at the confluent point in the cavity, the incomplete joining causes the formation of weld lines. Especially in the case of manufacturing a product having a complicated configuration, a plurality of gates are opened to the molding cavity. Consequently, weld lines are formed on the surface of the product with high frequency.
The molten resin poured in the molding cavity is cooled and solidified in the state to fill the cavity. The resin reduces its volume due to phase transformation or thermal shrinkage during the cooling and solidification. The volumetric shrinkage is not uniformly promoted in the resin body, but is likely to concentrate at a certain part. The uneven shrinkage causes the formation of defects such as sink marks on the surface of an obtained product.
The resin especially at the thick wall part, where cooling and solidification would be delayed due to mass effect, is held in a highly fluid state, although the resin at the surroundings has been already solidified. Consequently, a stress originated in the shrinkage of the resin at the surroundings is accumulated in the resin at the thick wall part, so that sink marks are concentratedly formed on both the surfaces of the thick wall part.
Such a thick wall part is effective in an obtained product, e.g. as a reinforcing rib or a boss for fixture maldistributed on the periphery or interior of the product, or an outer or inner frame. However, surface defects such as sink marks or weld lines formed at the thick wall part or its neighborhood remarkably reduces the commercial value of the product. In general, the sink marks becomes deeper as the thickness of the rib, the larger diameter of the boss or the higher temperature of the molten resin.
For instance, when a molten resin 20 is poured into a cavity 13 between a stationary mold 11 and a movable mold 12 as shown in FIG. 1, the fountain flow 21 from which the molten resin 20 continuously gushes out is formed at the top end of the molten resin 20 passing into the cavity 13. The fountain flow 21, after being sent to the top end of the molten resin 20, changes along the direction toward the inner surfaces of the molds 11, 12 and then arrives at parts near the inner surfaces, as shown as a resin flow unit 22 in FIG. 1. The resin 20 is rapidly cooled and solidified by thermal diffusion through the molds 11, 12, to form a skin layer 23 in contact with the inner surface of each mold 11, 12. The cooling and solidification of the resin 20 is interpreted from the movement of the resin flow unit 22 as such that the resin 20 is stretched longer near the inner surface of each mold 11, 12 and formed to the skin layer 23 orientated due to the stretching.
When the molding cavity 13 in the mold assembly is filled with the molten resin 20, the molten resin 20 stops its fluidization. The interior of the molten resin 20 in the stationary state is cooled through the molds 11, 12, so that the skin layer 23 grows up to a thick solidified layer along the direction from the inner surface of each mold 11, 12 to the interior of the resin 20. At the same tim
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Aida Hiroshi
Hosoyama Takamitsu
Yoshizaki Haruhiko
Heitbrink Jill L.
Nippon Steel Chemical Co. Ltd.
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