Blow molded product and production method therefor

Details Blow molded product and production method therefor Blow molded product and production method therefor

2003-01-23

2004-08-10

McDowell, Suzanne E. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Producing multilayer work or article

C264S521000, C264S904000

Reexamination Certificate

active

06773656

ABSTRACT:

TECHNICAL FIELD
The present invention relates to blow moldings of crystalline resin and to a method for producing them. More precisely, the invention relates to scratch-resistant blow moldings of crystalline resin having good surface gloss and high surface hardness, and relates to an efficient method for producing them.
BACKGROUND OF THE INVENTION
Since blow moldings of crystalline thermoplastic resin are much more lightweight than injection moldings thereof, and have good mechanical strength, they have many applications in various industrial fields. In particular, since blow moldings of polyolefin-type crystalline resin such as those of crystalline polypropylene are recyclable and since the load of their wastes to the environment is low, they cover a broad range of practical applications. For example, they are much used for automobile parts such as spoilers and other aerodynamic parts, side moldings, bumpers, instrument panels; and for housing facilities such as bath units, toilets, walls, ceilings, etc. The appearance of these articles is much influenced by the surface characteristics of blow moldings for them. Therefore, for obtaining articles having better surface appearance, employed is a method of overcoating the blow moldings produced for them to thereby improve the surface appearance of the thus-coated articles. However, moldings to be overcoated require surface treatment of sanding them, or require pre-treatment of primer coating or plasma processing for improving the adhesiveness of coating compositions thereto, and, in addition, the coating films must be dried. Therefore, the method of overcoating moldings is problematic in that such pre-treatment and post-treatment take much labor and a lot of time and therefore the productivity of the coated articles is low.
In that situation, some blow-molding techniques for obtaining blow moldings having good surface gloss and good appearance even though not post-treated for overcoating them have been developed. For example, in Japanese Patent Publication No. 40498/1990, proposed is a method for producing blow moldings having good surface gloss. The method comprises preparing a resin parison of such that its surface has a large number of fine recesses having a depth of from 2 to 100 microns, in the space of a mirror-finished blow mold heated up to a temperature not lower than the crystallization temperature of the resin, then blow-molding the parison therein, and thereafter cooling the mold to a temperature lower than the crystallization temperature of the resin. However, the method is still problematic in that it requires specific control in the step of parison formation and requires a prolonged molding cycle for producing large-size moldings, and therefore the productivity of the moldings is low.
Another method is proposed in Japanese Patent Laid-Open No. 77231/1992, which comprises preparing a crystalline resin parison in the space of a blow mold, clamping the mold, and injecting a pressure fluid into the parison so that the parison is blown and cooled while being tightly pressed against the inner wall of the mold, and is thereby formed into a blow-molded article. In the method, the blow mold used is so controlled that its temperature may fall between a temperature around which the crystallization rate of the crystalline resin is the highest and the melting point of the resin while the coolant fluid is injected into the resin parison prepared therein, and the coolant is circulated under pressure in the parison. In the method, the die line and the weld line in the surface of the blow moldings produced could be reduced in some degree probably to a negligible level. However, the method is still problematic in that the transferability of the mold cavity design onto the moldings produced and also the surface gloss of the moldings are not on a satisfactory level.
Still another method is proposed in Japanese Patent Laid-Open No. 138324/1998, which comprises preparing an extruded parison of polypropylene in a blow mold, clamping the mold with the parison therein being pre-blown, and finally blowing the parison in the mold. In the method, the mold is temporarily heated at a temperature not lower than 140° C. after it has been clamped and before it is opened to take the blow-molded article out of it. The method is superior to the case where the mold temperature is kept lower than 140° C. all the time for blow molding, as the blow moldings produced have improved surface gloss. However, the method is still problematic in that the surface hardness of the blow moldings produced is not so high and the blow moldings are often scratched.
The present invention is to provide scratch-resistant blow moldings of crystalline resin having good surface gloss and high surface hardness, and to provide a method for producing them.
DISCLOSURE OF THE INVENTION
To attain the object as above, we, the present inventors have assiduously studied and investigated the molding conditions for blow moldings of crystalline resin, and, as a result, have found that, when the temperature control of the inner surface of the blow mold used is suitably combined with the retention time of the parison in the mold within a specific temperature range, then the blow molding from the parison has good characteristics intrinsic to crystalline resin. On the basis of this finding, we have completed the present invention.
Specifically, the invention is summarized as follows:
(1) A blow molding of crystalline resin, of which the surface gloss is at least 80% measured at an incident angle of 60° according to JIS K7105, and of which the surface scratch resistance in terms of the pencil hardness measured in a pencil scratch test according to JIS K5400 is at least on the level of HB.
(2) The blow molding of above (1), which has a multi-layered structure of at least two layers, and of which the outermost layer is a crystalline resin layer.
(3) A method for producing blow moldings, which comprises preparing a crystalline resin melt parison in the space of a blow mold, clamping the mold, and introducing a pressure fluid into the parison to blow and tightly press the parison against the inner wall of the mold, thereby solidifying the parison along the wall; the method being characterized in that the temperature of the inner wall of the mold is so controlled that it falls between a temperature lower by 20° C. than the crystallization temperature of the crystalline resin and a temperature below the temperature lower by 5° C. than the melting point of the crystalline resin, while the parison is prepared in the mold, and that, after the mold has been clamped to pinch the parison therein, a pressure fluid is introduced into the thus-pinched parison, then the mold with the parison therein is so heated that the temperature of its inner wall falls between a temperature lower by 10° C. than the melting point of the crystalline resin and a temperature below the melting point of the crystalline resin, the parison is kept in the mold at the controlled temperature falling between a temperature lower by 10° C. than the melting point of the crystalline resin and a temperature below the melting point of the crystalline resin for a satisfactory period of time enough for the thus blow-molded article of the parison to have a surface hardness of not lower than the level of HB in terms of the pencil hardness measured in a pencil scratch test, and finally the mold is cooled and the blow-molded article is released from the mold.
(4) A method for producing blow moldings, which comprises preparing a crystalline resin melt parison in the space of a blow mold, clamping the mold, and introducing a pressure fluid into the parison to blow and tightly press the parison against the inner wall of the mold, thereby solidifying the parison along the wall; the method being characterized in that the temperature of the inner wall of the mold is so controlled that it is lower by from 5 to 10° C. than the crystallization temperature of the crystalline resin, while the parison is prepared in the mold, and that, after the mold has

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