GB2057962A

GB2057962A - Stretch-blow moulding a two layer article

Info

Publication number: GB2057962A
Application number: GB8027804A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-08-31
Filing date: 1980-08-28
Publication date: 1981-04-08
Also published as: JPS5951889B2; CA1157611A; ES494669A0; AU540004B2; GB2057962B; DE3032663A1; MX155165A; BR8005546A; FR2464135A1; NL8004721A; ES8104044A1; AU6168180A; JPS5637125A; FR2464135B1

Abstract

A method for producing a hollow product, such as a bottle 4, by injection stretching and blow moulding involves injection moulding a two-layer parison 3 with a closed bottom, one layer 1 consisting of a readily stretchable, blow mouldable resin (e.g. polypropylene) and the other layer 2 consisting of a resin which is hard to stretch and blow mould (e.g. acrylic resin). The parison is adjusted to an appropriate temperature before stretching the two layers of the parison and at the same time blowing air into the parison, while the latter is mounted in a blow mould, to form the article by blow moulding. The method avoids the difficulty of rupture, and undue thickness variation in the layer of resin which is hard to stretch and blow mould. <IMAGE>

Description

SPECIFICATION Method of producing a hollow article THIS INVENTION relates to a method of producing a hollow article by forming a parison by injection moulding, stretching the parison and blowing the parison within a blow mould.

As methods of forming a hollow article of synthetic resin, such as a bottle or the like, there are known a method which comprises moulding a parison with a closed bottom by injecting a molten resin into a closed mould consisting of a core and a cavity mould, and blow moulding the parison after transferring it together with the core to a blow mould; and another method which comprises injection moulding a parison with a closed bottom, in a closed mould again comprising a core and a cavity mould, transferring the parison to a blow mould after separating it from the core, inserting into it a blow core equipped with a stretching rod, and blow moulding the parison while stretching it in the axial direction in the blow mould.

The former method is generally called injection blow moulding, and the latter is generally called injection stretching and blow moulding. Injection stretching and blow moulding can provide a moulded product having a thinner wall, a higher transparency and a greater strength than products of the former method.

Injection stretching and blow moulding, however, may not be applied favourably to every resin. Of stretchable, blow mouldable resins, some resins cannot be moulded readily. In particular vinyl chloride resins may be stretched and moulded with extreme ease into hollow moulded products having various shapes. However, sometimes difficulties arise in moulding them into hollow moulded products by means of injection stretching and blow moulding because a layer of the resin may break before it is sufficiently stretched or before it is sufficiently expanded to fit a blow mould, and because it is extremely difficult to mould these resins into hollow moulded products having a uniform wall thickness.

There are several possible reasons for this poor mouldability. One of these reasons stems from the problem of maintaining the appropriate thermal distortion temperature at the time of stretching and blow moulding, as evidenced by the facts that the resin can be subjected to injection blow moulding and that the distribution of wall thicknesses in a product produced by means of stretching and blow moulding is not uniform. Therefore, attempts have been made to find a procedure suitable for maintaining a thermal distortion temperature appropriate for stretching and blow moulding.As a result, it has been found that it is very difficult to maintain an optimum thermal distortion temperature for a parison with a closed bottom obtainable by injection moulding a resin such as, for example, a vinyl chloride resin, styrol resin, an acrylic resin, an acrylonitrile resin, an ABS resin, or an AS resin, as compared with a parison obtainable from a polyethylene terephthalate resin which is readily subjected to injection stretching and blow moulding. Thus, it is apparent that a further procedure is needed to maintain such a parison at a thermal distortion temperature suitable for stretching and blow moulding.

From the foregoing, the inventor considered a procedure comprising stretching and blow moulding by inserting a temperature-adjustable core into a parison with a closed bottom and allowing the parison to be maintained at an appropriate temperature by means of the core. The procedure was found to be suitable in the case of a resin extremely mouldable by means of stretching and blowing, such as polyethylene terephthalate or polypropylene, but relatively unsuitable to the stretching and blow moulding of a resin which is hard to mould by means of stretching and blowing, such as the above-mentioned vinyl chloride resin or the like.The reason for this is that, when the parison in which the core is inserted is stretched in the axial direction by means of the stretching rod at the centre of the core, the inner surface of the parison is brought into firm contact with the heated core by tensile forces and is partially welded, so that the parison cannot be fully expanded in the mould by blowing air thereinto.

It is an object of the present invention to provide a process for stretching and blow moulding a hollow article incorporating, in the wall thereof a layer of a resin that is hard to stretch and blow mould, and without using a heated core as a means for maintaining a parison at an appropriate temperature.

According to the invention there is provided a method of producing a hollow article which comprises forming, by injection moulding, a two-layer parison with a closed bottom, one layer consisting of a readily stretchable. blow mouldable resin, and the other layer consisting of a resin which is hard to stretch and blow mould, adjusting the temperature of the parison to a temperature suitable for stretching and blow moulding both layers, and stretching the layer of the resin which is hard to stretch and blow mould together with the readily stretchable, blow mouldable resin layer and blowing air into the parison while the latter is mounted in a blow mould, to blow mould the hollow article.

Readily stretchable, blow mouldable resins are generally less critical as to the temperature at which stretching and blow moulding is carried out than resins which are hard to stretch and blow mould. Thus it is desirable that, prior to stretching and blowing the parison, the parison should be adjusted to a temperature close to the optimum temperature for stretching and blowing the layer of resin which is hard to stretch and blow mould, and in order that this can be achieved, it is of course desirable that the optimum temperature for stretching and blow moulding the layer of resin which is hard to stretch and blow mould should be within the range of temperatures at which the layer of easily stretched and blow moulded resin can be stretched and blow moulded well.

However, it has been found that provided an appropriate choice of resins is made, good results may be obtained by adjusting the temperature of the parison, prior to stretching and blowing, to a temperature most suitable for stretching and blow moulding the resin which is easy to stretch and blow mould.

It may be advantageous to adjust the thermal distortion temperature of the resin which is hard to stretch and blow mould to a temperature in the thermal distortion temperature range of the readily stretchable blow mouldable resin.

An embodiment of the invention is described in more detail below, by way of an example, with reference to the accompanying drawings in which: Figure 1 is a longitudinal section view illustrating a parison with a closed bottom prior to stretching and blowing, and Figure 2 is a longitudinal sectional view illustrating a hollow moulded product prepared by stretching and blowing the parison of Fig! 1.

Referring to the drawings, to form the parison shown in Fig. 1, a polyethylene terephthalate resin is first injection moulded to form an inner layer 1 having a thickness of 2 mm. Outside the inner layer 1, a vinyl chloride resin is then injection moulded so as to form an outer layer 2 having a thickness of 3 mm. The two-layer parison 3, with a closed bottom, thus prepared is heated to a temperature appropriate for stretching and blow moulding both layers and is then transferred to a blow mould where the parison is subjected simultaneously to stretching in the axial direction and air blowing to form a two-layer hollow moulded product 4. The thermal distortion temperature may be preferably adjusted to a temperature most suitable for stretching and blow moulding the polyethylene terephthalate resin.The blow moulding may be carried out by stretching the outer layer 2 together with the inner layer 1 by means of a stretching rod. This procedure permits the maintenance of the thermal distortion temperature in the outer layer 2 by means of the inner layer 1 without causing a remarkable decrease in the temperature during the blow moulding. This procedure also can provide a hollow moulded product having no difference in stretching and expansion between the two layers after moulding because the outer layer 2 is expanded together with the inner layer 1 which is readily stretchable and blow mouldable and is uniformly expandable. Consequently, the hollow moulded products 4 thus produced have a high strength and a uniform thickness in their inner and outer layers.

Although the above-mentioned embodiment employs an inner layer 1 of polyethylene terephthalate resin, and an outer layer 2 of vinyl chloride resin, the vinyl chloride resin may instead be employed as the inner layer 1 and the polyethylene terephthalate resin employed as the outer layer 2. In this case, some irregularity in wall thickness may be caused in the inner layer as compared with the outer layer; however, the. degree of irregularity of the hollow moulded products may be considered negligible.

The thickness of the two different layers in a parison with a bottom may not necessarily be the same and, from an economic point of view, the cheaper resin layer is preferably made thicker than the more expensive resin layer.

The following table illustrates the thermal distortion temperatures of various resins which are hard to stretch and blow mould; the polyethylene terephthalate resin is used as the readily stretchable, blow mouldable resin: PET 75-85GC (inner layer) PVC 84 C (outer layer) Styrol 82 C (outer layer) Acryl 88tC (outer layer) ABS 82 C (outer layer) AS 85 C (outer layer) Acrylonitrile 74 C (outer layer)

Claims

1. A method of producing a hollow article which comprises forming, by injection moulding, a two-layer parison with a closed bottom, one layer consisting of a readily stretchable, blow mouldable resin, and the other layer consisting of a resin which is hard to stretch and blow mould, adjusting the temperature of parison to a temperature suitable for stretching and blow moulding both layers, and stretching the layer of the resin which is hard to stretch and blow mould together with the readily stretchable, blow mouldable resin layer and blowing air into the parison while the latter is mounted in a blow mould, to blow mould the hollow article.

2. A method according to claim 1, wherein the readily stretchable, blow-mouldable resin constitutes the inner layer of the two-layer parison and the resin which is hard to stretch and blow mould constitutes the outer layer thereof.

3. A method according to claim 1, wherein the readily stretchable, blow mouldable resin constitutes the outer layer of the two-layer parison and the resin which is hard to stretch and blow mould constitutes the inner layer thereof.

4. A method according to claim 1, 2 or 3, wherein the readily stretchable, blow mouldable resin is a polyethylene terephthalate resin or a polypropylene resin.

5. A method according to claim 1, 2 or 3, wherein the resin which is hard to stretch and blow mould is a vinyl chloride resin, a styrol resin, an acrylic resin, an acrylonitrile resin, an ABS resin or an AS resin.

6. A method of producing a hollow article, substantially as hereinbefore described with reference to the accompanying drawings.

7. A hollow article produced by the method of any of claims 1 to 6.

8. Any novel feature or combination of features described herein.