1. Field of the Invention
The present invention relates to a process for producing a molded article and in particular to a process for producing molded articles formed of a synthetic resin which have less dimensional variation following manufacturing and less shrinkage after molding during use.
2. Background Information
Synthetic resin materials, such as, for example, polyvinyl chloride (PVC), are frequently used for molding elongated articles, such as side moldings, for use on automobiles.
Such elongated articles formed of a synthetic resin material are often produced by injection molding techniques and then annealed (heat treated) as will be described hereafter. The finished products are then mounted upon automobiles.
However, many such elongated molded articles experience so-called shrinkage in use. Accordingly, the spacing provided between adjacent automobile moldings, or between the molding and the edge of an automobile body, may be undesirably widened during use on automobiles. In some instances shrinkage will reduce the length of elongated moldings to a length that is so short that they do not meet a given minimum size prior to mounting upon an automobile or perhaps are so changed in their dimensions that they become impossible to mount upon the automobile.
Consequently, appearance qualities are lowered and obstacles develop in the desired mounting of moldings. Therefore, requirements for higher dimensional accuracy of elongated products molded from synthetic resin materials has recently become more severe.
Synthetic resin materials such as PVC and rubber-containing polypropylene generally have a shrinkage factor of 10/1000 to 20/1000. In usual molding processes, the synthetic materials do not reach an equilibrium shrinkage in the mold. The molded article is removed from the mold while it only partially shrunk. Accordingly, shrinkage occurs after molding so that the article is shortened in dimension and there exists increased dimensional variation among products.
On the other hand, annealing treatments of products performed prior to shipping from the factory or after users get them, has generally been known as an approach to suppress shrinkage after molding. The term "annealing treatment" includes techniques of heating molded articles at, for example, 40.degree. to 100.degree. for 10 to 150 minutes and then to gradually cool it.
This annealing treatment can suppress shrinkage after molding, but has little effect in suppressing other dimensional variations among products. Therefore, such annealing treatments are not suitable for elongated molded articles for which dimensional accuracy is required.
The present invention overcomes the above mentioned problems.
It is an object of the present invention to provide a process for producing molded products which have less dimensional variation and less shrinkage after molding in use.