Mold for synthetic resin molding

Details Mold for synthetic resin molding Mold for synthetic resin molding

1993-05-28

1994-11-08

Nguyen, Khanh

Plastic article or earthenware shaping or treating: apparatus

Female mold and means to shape parison directly by internal...

With heating or cooling means

249111, 2491141, 249115, 249134, 264337, 425552, B28B 736, B29C 3356, B29C 4526

Patent

active

053622260

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a mold for synthetic resins. More particularly, it relates to an injection mold or a blow mold which withstands tens of thousands of molding cycles.


BACKGROUND OF THE INVENTION

In injection molding of a thermoplastic resin, good reproduction of the molding surface of a mold on the molded articles and satisfactory surface gloss of molded articles can generally be achieved to some extent by increasing the resin temperature or injection pressure.
The greatest factor affecting the surface conditions of resin molded articles is the temperature of a mold. The higher the mold temperature, the better. However, as the mold temperature increases, the mold-cooling time required for cooling and solidification of the plasticized resin must be so extended, resulting in a reduction in molding efficiency.
It has therefore been demanded to establish a molding technique for assuring satisfactory molding surface reproducibility without increasing a mold temperature or without lengthening the cooling time even when a mold temperature is increased. A method of mold heating and cooling by alternately introducing a heating medium and a cooling medium through the respective holes of the mold is currently employed. However, this method involves large consumption of heat and requires a long mold-cooling time.
A method of coating the cavity wall with a substance having a small heat conductivity to improve molding surface reproducibility is disclosed, e.g., in U.S. Pat. No. 3,544,518. Polyethylene terephthalate, polyphenylene sulfide, etc. are mentioned as examples of the substance having a small heat conductivity. Further, a method of providing a heat insulating layer near the molding surface is suggested in WO 89/10829. The publication teaches a method for cooling an injected resin in a mold in which the resin can be slowly cooled with a small difference in temperature between the outer surface and the central portion thereby to provide a strain-free molded article, such as a lens. The means disclosed for slow cooling is a mold structure comprising a mold body having formed thereon a heat insulating layer and, as the outermost layer, a metal layer, such as aluminum or nickel. The purpose of providing the heat insulating layer is to greatly reduce the cooling rate of the heated and injected resin. As the heat insulating layer there are mentioned a liquid crystal polymer plate and a plate of Vespel (molded polyimide, a trade name of E. I. du Pont de Nemours & Co., Inc.) each having a thickness of several millimeters.
Japanese Patent Application Laid-Open No. Sho-62-37107 discloses a mold with an air-permeable heat insulating layer on the surface thereof by which silver streaking, etc. can be prevented. The publication has a mention of polyimide with reference to the heat insulating layer but furnishes no further details.
U.S. Pat. No. 5,004,627 suggests coating a mold with a fluorinated polyimide for the purpose of improving parting properties in injection molding. The fluorinated polyimide used here functions as a parting agent. Fluorine-containing compounds generally exhibit markedly excellent slip properties or parting properties for the same or different materials and are widely employed as lubricants, releasants or parting agents.
In U.S. Pat. No. 5,004,627, the fluorinated polyimide is used as a parting agent taking advantage of these characteristics of fluorine-containing compounds. The fluorinated polyimide cannot be firmly adhered to a metallic mold.
It is difficult, in general, to adhere a polyimide a metal, whose coefficient of thermal expansion differs from that of the polyimide by one figure, with high adhesive strength endurable in a heating-cooling cycle test of several tens of thousands of times.
The most outstanding merit of injection molding resides in that a molded article of complicated shape can be obtained through one shot. It has been demanded to obtain a mirror-smooth injection molded article with good molding surface reproduction without extending the mold-

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