1. Field of the Invention
The present invention relates to an apparatus for preparing a heat-set plastic hollow vessel. More particularly, the present invention relates to an apparatus for preparing a heat-set plastic hollow vessel, which is biaxially molecularly oriented by drawing, in one mold at a high efficiency.
2. Description of the Related Art
A biaxially draw-blow-formed vessel of a thermoplastic polyester such as polyethylene terephthalate (PET) has a high transparency and an excellent surface gloss and also has high impact strength, rigidity and gas barrier property required for a bottle. Accordingly, vessels of this type are utilized for bottling various liquids.
However, polyester vessels are defective in that the heat resistance is poor, and when hot-filled, thermal deformation or change of the volume by shrinkage occurs. Accordingly, various proposals have been made for heat-setting a biaxially draw-blow-formed vessel after the forming operation.
The heat-setting method is roughly divided into a so-called one-mold method in which the draw-blow forming and heat-setting are carried out in one common mold and a so-called two-mold method in which the draw-blow forming and heat-setting are carried out in different molds.
According to the one-mold method, as disclosed in Japanese Patent Publication No. 6216/84, the draw-blow forming and heat setting are simultaneously carried out in a blow-forming mold, and a relatively long residence time in the mold is necessary for effecting the heat-setting after the draw-blowing operation and the cooling for the withdrawal of a formed hollow vessel and therefore, the manufacturing speed is still low. As the one-mold method, Japanese Patent Application Laid-Open Specification No. 95666/79 proposes a process in which the temperature of a hollow mold is maintained at a highest level within the range where a final hollow formed body can be withdrawn without deformation without any substantial cooling, for example, at 100.degree. C., and high-temperature high-pressure air is blown in a polyester preform and simultaneously, the preform is biaxially drawn. According to this process, the temperature of the mold need not be elevated or dropped, but the heat-setting is effected by heat transferred from the high temperature gas and a relatively long time is necessary for completion of the heat-setting because of the presence of the heat transfer interface. Moreover, the attained resistance to thermal shrinkage is still insufficient.
As the two-mold method, a process in which a formed body obtained by the draw-blow forming is taken out from a blow-forming mold and is then held in a heat-setting mold to effect the heat-setting, as disclosed in Japanese Patent Publication No. 56606/85, and a process in which the draw-blow forming and the heat treatment are simultaneously carried out in a primary mold and the formed body is taken out from the primary mold and is blow-formed again in a secondary mold without cooling, as disclosed in Japanese Patent Application Laid-Open Specification No. 53326/86 are known.
However, the two-mold method is defective in that two molds are necessary for the forming and heat-setting operations, the apparatus cost is increased and the number of the manufacturing steps is increased. Therefore, further improvements are desired.