Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to...
Patent
1999-01-20
2000-11-14
Kuhns, Allan R.
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
26432817, B29C 4500
Patent
active
061465771
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for facilitating filling of a resin into a mold cavity in injection molding of a thermoplastic resin so as to transfer the mold surface state precisely onto a molded article surface.
BACKGROUND ART
In injection molding of a thermoplastic resin, the resin is generally heated and melted at a temperature to obtain sufficient flowability of the resin for filling into a mold cavity. The flowability of the molten resin affects not only ease of filling into a mold cavity but also sufficiency of pressure transmission to the resin filled in the cavity. Therefore, it also affects dimensional precision and external appearance of molded articles, and transfer of fine information of the mold surface as required for molded articles such as optical disks. Thus, the molten resin flowability is an important factor for the moldability of a resin. An index of the flowability of the molten resin is the melt viscosity of a resin.
Thermoplastic resins have a high melt viscosity and is inferior in flowability as a molding material. This tends to cause poor appearance such as irregular gloss and weld-line of the molded article, poor transfer of fine pattern of a mold surface such as pits of an optical disk, and incomplete filling of a resin into a thin article portion in the mold, disadvantageously.
Conventionally, there are the following three methods for modifying resins to improve the flowability thereof. The first method is lowering of the molecular weight of a resin, for example, by lowering the average molecular weight or by broadening the molecular weight distribution to increase the content of low molecular components. This method is disadvantageous in that the impact strength and chemical resistance are deteriorated although the flowability is improved. The second method is introduction of a comonomer into the molecule. This method is disadvantageous in that the rigidity of the molded article is deteriorated at high temperatures. The third method is addition of a plasticizer such as a low molecular weight oily substance, e.g., a mineral oil, and a higher fatty acid ester. This method is disadvantageous in that the rigidity upon application of heat is deteriorated by the plasticizer, or the plasticizer adheres onto a mold during the molding operation to stain the mold.
With regard to molding conditions for increasing the flowability, it is effective to elevate a resin temperature or a mold temperature. However, higher temperatures of a resin cause thermal decomposition of the resin itself or of additives to thereby tend to deteriorate the strength of molded articles or to cause undesired foreign matter generation, mold staining, and resin discoloration due to the deterioration of the resin. Further, higher temperatures of a mold retard the cooling of a resin in a mold to lengthen the molding cycle time, disadvantageously.
On the other hand, it is known that when carbon dioxide is absorbed by a resin, it serves as a plasticizer to lower the glass transition temperature of the resin, as disclosed in many documents such as J. Appl. Polym. Sci., Vol.30, p. 2633 (1985). This phenomenon has not been widely utilized in resin molding. One of a few application examples thereof is a method disclosed in JP-A-5-318541 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") in which a gas such as carbon dioxide and nitrogen is dissolved into a thermoplastic resin, and the resin is filled into a cavity while the gas in the cavity is removed, to thereby improve the resin flowability and to produce molded articles without deterioration in strength and external appearance. In this method, however, the amount of the gas dissolved in the resin is as small as 0.18% by weight at the maximum if carbon dioxide is used as the gas. Such a small amount is insufficient to achieve the desired improvement of flowability. Since the cavity is kept at the atmospheric pressure or a reduced pressure in this method, the external appearance of the molded artic
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Kataoka Hiroshi
Matsuura Yoshimasa
Yamaki Hiroshi
Asahi Kasei Kogyo Kabushiki Kaisha
Kuhns Allan R.
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