1. Field of the Invention
This invention relates to a process and an apparatus for making a blow molded product, such as a hollow molding for an automobile.
2. Description of the Prior Art
Extrusion or injection molding has been a typical method for making a molding as a decorative or protective part of an automobile, but blow molding has also come to be employed for that purpose.
FIG. 8 shows a molding made by a known blow molding process. It is a blow molded product 8 comprising a main body 81 having a hollow interior 82. The hollow interior 82 provides a shock absorbing function and also contributes to reducing the weight of the molding 8 as a whole. That is why the molding has come to be made by blow molding.
FIGS. 9 and 10 show a known mold 9 which is used for making the blow molded product 8. The mold 9 comprises a core plate 91 and a cavity block 92 which define a cavity therebetween when they are put together. The cavity is so shaped as to define the shape of the blow molded product 8. The core plate 91 has a gas blowing device 93 and a gas blowing needle 931 which face the cavity. The cavity block 92 has a cavity surface 920 provided with a plurality of air vent holes 924, and is also provided with an air passage 925 and an air vent pipe 926 to which the air vent holes 924 are connected by the air passage 925. The core plate 91 and the cavity block 92 are supported on retainer plates 911 and 921, respectively.
The blow molded product 8 is formed from a parison 80 of a softened synthetic resin which is extruded into the space between the core plate 91 and the cavity block 92 which have been separated from each other. After the parison 80 has been extruded, the mold is closed. The blowing needle 931 is pierced through the wall of the parison 80 into its hollow interior 801, and air is blown through the needle 931 into the hollow interior 801 to hold the outer surface of the parison 80 against the surface of the cavity. The parison 80 which has been expanded to the desired shape is cooled to yield the blow molded product 8 as shown in FIG. 8.
When air has been blown into the parison 80, the air remaining in the cavity between the parison 80 and the cavity surface 920 is allowed to flow out through the air vent holes 924, the air passage 925 and the air vent pipe 926, whereby the parison 80 is brought into intimate contact with the cavity surface 920, as shown in FIG. 10.
The cavity surface 920 is usually embossed to form an embossed or grained surface on the blow molded product 8, as shown in FIG. 10. The grained surface makes the blow molded product 8 look as if it were of leather, or gives it an ornamental feature. It has also been usual to coat the blow molded product with a paint to improve its appearance.
The manufacture of blow molded products is described in, for example, Japanese Patent Application Laid-Open No. 206241/1984 and Japanese Utility Model Publication No. 22123/1988.
There has been growing a demand for a blow molded product having a highly lustrous surface. There is known a process which employs a mold having a mirror finish on its cavity surface for making a blow molded product having a highly lustrous surface. The mold is preheated to a high temperature to heat and melt the surface of a parison, and is cooled to cool it. The heating and cooling of the parison, however, result in a longer molding time and therefore a lower degree of productivity. The apparatus which are required for heating and cooling the mold, and for controlling its temperature, necessarily add to the cost of manufacture. The mirror finish on the cavity surface calls for a great deal of labor and time.
The mirror finish causes intimate contact between the softened surface of the parison and the cavity surface when the parison has been expanded. This makes it difficult to vent air from between the parison and the cavity surface. If the cavity surface is embossed, as shown at 920 in FIG. 10, it may be relatively easy to vent such air, but it is often the case that the direct contact of the softened surface of the parison with the embossed cavity surface disables such air to escape.
The incomplete removal of air results in the formation of concavities, or other defects in the surface of a blow molded product. This is the case with a product having an embossed surface, too, though the embossed surface may make any such defect less noticeable.
The direct contact of the parison with the cavity surface of the mold presents a number of problems. Any defect that the cavity surface may have is reproduced in the surface of a blow molded product. The softened material in the surface of the parison protrudes into the air vent holes, as shown at 924 in FIG. 9, and forms small projections on the surface of a blow molded product.
The known process as described above increases the manufacturing cost due to the process of heating and cooling the mold for a highly lustrous surface, and causes defects in the surface of a blow molded product resulted from incomplete removal of the air remained between the parison and the cavity surface. Those defects in the surface might occur when reproducing an embossed surface of the blow molded product.
It has, therefore, been difficult to make a blow molded product having a highly lustrous surface presenting a good and pleasing appearance at a low cost.