Static molds – Including coating or adherent layer
Patent
1997-07-30
1999-02-02
Davis, Robert
Static molds
Including coating or adherent layer
249115, 264338, 425812, B29C 3340
Patent
active
058660259
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a mold for molding a synthetic resin and a molding method using the same. More particularly, the present invention relates to a heat insulating layer-coated mold for injection molding, blow molding, vacuum forming, compression molding, etc. which is endurable to molding of tens of thousands time, and a molding method using the same.
BACKGROUND ART
In injecting a thermoplastic resin into a cavity of a mold, improvement of reproducibility in imparting the mold surface profile on the molded article and improvement of gloss of the molded article can usually be achieved to some extent by making a proper selection of molding conditions, for example, by increasing the resin temperature, the mold temperature or the injection pressure.
Of these factors, a mold temperature is the most influential. The higher the mold temperature, the better the appearance of the resulting molded article. However, an increase in mold temperature makes a time required for cooling the plasticized resin longer, leading to a reduction in molding efficiency. Therefore, it has been demanded to develop a molding technique for achieving improved mold surface reproducibility without increasing the mold temperature or, even if the mold temperature is increased, without requiring an extension of a cooling time. A method comprising introducing a heat transfer medium and a coolant alternatively into the respective holes of a mold to make a heating and cooling cycle has been adopted. However, this method involves high consumption of heat and extended heating and cooling times, making the molding cycle time longer.
Many reports have been made to date on a method for improving mold surface reproducibility comprising coating a cavity wall of a mold with a substance having a small thermal conductivity, i.e., a heat insulating layer. For example, WO 93/06980 suggests use of polyimide as an insulating layer, and JP-A-54-142266 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the use of an epoxy resin as an insulating layer.
The merit of injection molding and the like resides in that an article of complicated shape can be obtained through a single shot. Therefore, the molds used in these molding methods usually have a complicatedly shaped cavity. On the other hand, the heat insulating layer of a heat insulating layer-coated mold has a thickness of about 0.1 to 0.2 mm for general injection molding but, under some molding conditions, the heat-insulating layer for use even in injection molding should have a thickness of about 0.2 to 0.5 mm. For blow molding, the insulating layer should have a thickness of not less than 0.3 mm and, in some cases, 0.4 mm or more. It has been demanded to uniformly and economically provide such a thick heat insulating layer on a cavity wall of complicated shape.
Uniform formation of a thick heat insulating layer on a cavity wall having a complicated shape has conventionally been carried out by repeating formation of a thin coating layer by spray coating or brush coating followed by hardening by, for example, heating several times or, in some cases, several tens of times until a desired thickness is reached. If a cavity wall of complicated shape is coated thick at a time, the coating sags during application, making it difficult to provide a coating layer of uniform thickness on the cavity wall of complicated shape. A quantitative sag (Q) is generally considered to be proportional to the thickness (t) of a coating film immediately after being applied, being represented by the following equation: coating; g represents an acceleration of gravity; and .eta. represents a viscosity of the coating; and t represents a thickness of a coating film.
In order to apply a coating to a uniform thickness while minimizing a sag amount (Q), it is necessary to reduce the amount of the coating applied at a time to reduce the coating thickness (t). In other words, the steps of applying a coating thinly and solidifying by heating, crosslinking, etc. ha
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Kataoka Hiroshi
Mawatachi Masanori
Asahi Kasei Kogyo Kabushiki Kaisha
Davis Robert
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