Method of producing biaxially stretched polyester bottles

Abstract

In effecting the stretching by using a stretch rod in a method of producing a biaxially stretched polyester bottle, the air is blown into the preform at such a flow rate that the preform does not come in contact with the stretch rod but the end thereof and does not come in contact with the surfaces of the mold, either. There can be produced a biaxially stretched polyester bottle having an average thickness of about 0.05 to about 0.2 mm or having a further decreased thickness using a resin in decreased amounts and having a decreased weight without developing burst or whitening caused by over-stretching, without forming ring-like thick portion yet maintaining a good yield and efficiency.

Description

TECHNICAL FIELD[0001]The present invention relates to a method of producing biaxially stretched polyester bottles. More specifically, the invention relates to a method of producing biaxially stretched polyester bottles, which is capable of forming thin polyester bottles preventing burst and suppressing the formation of ring-like thick portions maintaining good productivity.BACKGROUND ART[0002]Biaxially stretched polyester bottles have heretofore been known being obtained by biaxially stretch-blow-forming the preforms made of a polyester resin such as a polyethylene terephthalate, and have been widely used as containers for containing a variety of kinds of beverages, foods, liquid detergents and the like owing to their excellent transparency, surface luster, shock resistance and gas barrier property.[0003]Biaxially stretched polyester bottles used for containing beverages, usually, have an average thickness of about 0.25 to 0.30 mm. From the standpoint of lowering the cost and decrea...

AI Technical Summary

Benefits of technology

[0010]It is therefore an object of the present invention to provide a method of producing a biaxially stretched polyester bottle having an average thickness of about 0.05 to about 0.2 mm like the one as obtained by the above prior art proposed by the present inventors or having a further decreased thickness, using a resin in decreased amounts and having a decreased weight, without whitening caused by over-stretching, without developing burst during the blow-forming or without forming a ring-like thick portion in the finally formed article, yet maintaining good yield and efficiency.

[0013]In effecting the stretching by using the stretch rod in the method of producing a biaxially stretched polyester bottle, an important feature resides in that the air is blown into the preform at such a flow rate that the preform does not come in contact with stretch rod but the end thereof and does not come in contact with the surfaces of the mold. This makes it possible to form a thin biaxially stretched polyester bottle without causing the burst or without forming a ring-like thick portion and maintaining a good yield.

[0015]According to the production method of the present invention as shown in FIG. 2(B), on the other hand, the air has been blown to such an extent that the preform does not come in contact with the stretch rod (except an end thereof) and does not come in contact with the surfaces of the mold, either, at the time of stretching by using the stretch rod. Therefore, there is no drop in the temperature of the preform which, therefore, can be entirely stretched uniformly and sufficiently in the longitudinal direction (axial direction) by using the stretch rod as shown in FIG. 2(C) in a state where the preform can be easily stretched since it has been heated.

[0016]According to the present invention, even after the stretching by using the stretch rod shown in FIG. 2(C) is discontinued, the stretching is continued without varying the rate of blowing the air to effect the stretching in substantially the circumferential direction, too, making it possible to stretch the preform up to the size of the finally formed article shown in FIG. 2(D). In order to improve the productivity, however, it is desired that the air is blown as a pre-blow when the stretching is to be effected, the pre-blow is discontinued simultaneously with the end of stretching by using the stretch rod and, thereafter, the stretching is effected relying upon a main blow by blowing the air at a flow rate greater than that of the pre-blow. According to this method, the preform that is sufficiently stretched in the longitudinal direction by using the stretch rod is further stretched efficiently in the circumferential direction relying upon the main blow, making it possible to produce a biaxially stretched polyester bottle having a reduced thickness maintaining a good productivity by shortening the stretching time.

Problems solved by technology

However, it becomes difficult to decrease the weight when the containers have capacities which are as relatively small as 2000 mL or less.

If it is attempted to further increase the longitudinal stretching ratio, there occur such problems as whitening due to over-stretching, exfoliation of the layers (delamination) and rupture (burst), etc.

So, there comes a problem of molding difficulty.

However, when a bottle having such a large change in the thickness is to be stretch-blow-formed, problems arouse in that the thickness must have been varied in advance when the preform is to be injection-formed and difficulty is in involved in setting the conditions for stretch-blow-forming the preform.

However, when the stretch-blow forming is to be effected relying on the production methods described in the above prior arts, there often occurs the burst in the initial stage of blow-forming or a ring-like thick portion is formed if the burst does not occur leaving room for improving the yield of the products.

Besides, as shown in FIG. 1(C), when the preform is forcibly stretched in a state where it can be little stretched since its temperature has dropped upon coming in contact with the surface of the mold on the way of being stretched, there occurs the burst or a parting line is formed, which, then, becomes a cause of burst.

Further, even when there does not occur the burst, there occurs a problem in that a portion where the temperature has dropped remains as a ring-like thick portion.

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