GB2031330A

GB2031330A - Moulding pipes

Info

Publication number: GB2031330A
Application number: GB7930467A
Authority: GB
Original assignee: Ekstroem & Co H
Current assignee: Ekstroem & Co H
Priority date: 1978-09-06
Filing date: 1979-09-03
Publication date: 1980-04-23
Also published as: NO792871L; GB2031330B; DK361479A; JPS5546936A; SE413477B; FI792756A; NL7906548A; SE7809371L; DE2934360A1

Abstract

An expandable mandrel for use in the construction of pipes has a tubular sleeve (10) of rigid but flexible material, a mandrel contracting device in the form of a rubber sleeve (12) surrounding the tubular sleeve (10) and an expanding device (14) inside the tubular sleeve (10). The tubular sleeve (10) has a longitudinal slot (11) and it assumes its undeformed shape when the mandrel is in the expanded state, whereby the mandrel will have a true cylindrical shape. <IMAGE>

Description

SPECIFICATION Expandable mandrel for the use in the construction of pipes The present invention relates to an expandable mandrel for use in the construction of pipes, consisting of a tubular sleeve, a mandrel contracting device surrounding said sleeve, and a mandrel expanding device disposed in the tubular sleeve.

In the manufacture of pipes of for instance reinforced plastics or plastic, bituminous and glass fibre layers, or inner rubber layers and outer layers of thermosetting resin, use is made of a mandrel, on which the different layers are built up. The problem then isto provide an elongated mandrel (for instance 12 m) which is cheap to manufacture but nonetheless gives a sufficiently accurate cylindrical shape for the construction of pipes of such lengths.

It is previously known to design expandable mandrels from a tubular sleeve in which mandrel expanding device is disposed, the tubular sleeve further being surrounded by a means conferring a contracting effect on the tubular sleeve and thus the mandrel. A mandrel of this type is disclosed in for instance German Auslegeschrift DAS 1,128,797. The mandrel of this German Auslegeschrift is however not only complicated, but also little accurate in its cylindrical shape, since the end portions of the layer or layers of the outer tubular sleeve overlap and, what is more, this outer tubular sleeve has been manufactured with a diameter greater than the inner diameter of the pipe to be produced by means of the mandrel.

Hence, the object of the present invention is to provide a mandrel of simpler construction which permits giving a more accurate cylindrical shape to the inner side of the pipe to be manufactured with the aid of the mandrel.

To this end, the tubular sleeve consists of a rigid but flexible material and is manufactured in such a size that, when the mandrel has been expanded to the inner shape of the pipe to be constructed, it assumes its undeformed size which it was given when manufactured, and the tubular sleeve is provided with a longitudinal slot which permits contraction of the tubular sleeve to enable extraction of the mandrel from the finished pipe.

A tubular sleeve for such a mandrel may be manufactured by first producing a plastic sleeve of reinforced plastics on a steel mandrel. Once this plastic sleeve has been cured, a portion of a width of e.g. 30 mm is removed by sawing along the plastic sleeve. A pre-manufactured rubber sleeve is then forced onto and passed over the slotted plastic sleeve, the rubber sleeve having in its relaxed state a smaller diameter than the plastic sleeve (if this sleeve when manufactured is given an outer diameter of 500 mm, the rubber sleeve in the undeformed state may be of a diameter of for instance 480 mm) whereby it will compress the plastic sleeve, so as to close its slot.After mounting the plastic sleeve with the applied rubber sleeve on a support having a mandrel expanding device in the form of an outer or several successive outer inflatable curing bags, it is possible by means of the expanding device to expand the slotted plastic sleeve to its initial manufactured diameter (i.e. 500 mm in the above example). The outer rubber sleeve which serves as a mandrel contracting device will then, of course, be subjected to tensile stress. On the thus expanded mandrel it is possible to build up tubular products, such as pipes of reinforced plastics. Once the manufacture of the tubular products is finished, the pressure on the curing bag or bags is relieved, whereupon the rubber sleeve will compress the slotted plastic sleeve, until the slot is closed. The mandrel may then readily be extracted from the tubular product.

The invention will be described in greater detail hereinbelow with reference to the accompanying drawings, in which:~ Fig. 1 is a schematic longitudinal section of an expandable mandrel according to the invention.

Fig. 2 is a section along line Il-Il in Fig. 1, and Fig. 3 shows a portion of a section of another embodiment.

In the drawings, the mandrel according to the invention, shown by way of example, comprises a plastic sleeve 10 made of reinforced thermosetting resin, for instance polyester. A longitudinal portion of the plastic sleeve has then been sawn off to form a longitudinal slot 11. A rubber sleeve 12 has been forced onto and passed over the plastic sleeve. In the undeformed state, the rubber sleeve 12 must be of a smaller inner diameter than the plastic sleeve so as to be able to compress the plastic sleeve and close the slot 11.

The plastic sleeve and the rubber sleeve are arranged on a mandrel support 13 on which a number of successive expandable rubber bags 14 are mounted. The rubber bags are retained by hose clips 15. The rubber bags can be fed with compressed air by separate compressed-air inlets 16 opening into a distribution chamber 17 inside each rubber bag. The rubber bags support the slotted plastic sleeve and permit expanding it to its initial outer manufactured diameter, being for instance 500 mm. When relieving the pressure in the expandable rubber bags, the outer rubber sleeve 12 will compress the slotted plastic sleeve, such that the slot 1 1 is closed. The mandrel support 13 also has end flanges 18 which serve as end moulds when the tubular product is being constructed on the expandable mandrel.These end flanges may be removed and are attached to the mandrel support in a manner which is not shown here in greater detail. The end flanges 18 are provided with retaining rings 19 which, in the shown embodiment, are disposed outside the plastic sleeve 10 and the rubber sleeve 12 but which may also be disposed inside the sleeves.

The rings 19 serve to centre the mandrel which must often be rotated as the tubular product is built up.

A great advantage of the mandrel construction of the invention is that by slotting a plastic sleeve it has become possible to provide an expandable mandrel having a true cylindrical shape, which would not be the case if the plastic sleeve were manufactured with a greater diameter than the inner diameter of the tubular product to be manufactured, and if a longitudinal saw cut had then been made in the plastic sleeve to permit compressing the plastic sleeve such that the portions of the sleeve on either side of the saw cut overlap in the manner shown in German Auslegeschrift 1,128,797.

As is shown in Fig. 3, it is also possible in a mandrel according to the invention to have a certain degree of overlapping but in this case the slotted plastic sleeve must first be manufactured in the abovedefined manner by removing a portion of a width of e.g. 30 mm, an overlapping tongue 20 being thereafter attached on one side of the slot. In order that the true cylindrical shape of the plastic sleeve should not be impaired, the overlapping tongue must however be very thin and be inlaid on one side of the slot, the plastic sleeve 10 and the tongue 20 being bevelled on the opposite side of the slot 11, such that the tongue will follow the cylindrical external side of the slotted plastic sleeve.

Claims

1. Expandable mandrel for use in the construction of pipes, consisting of a tubular sleeve, a mandrel contracting device surrounding said sleeve and a mandrel expanding device disposed in the tubular sleeve, wherein the tubular sleeve consists of a rigid but flexible material and is manufactured in such a size that, when the mandrel is expanded to the inner shape of the pipe to be constructed, it assumes its undeformed size which it was given when manufactured, and wherein the tubular sleeve is provided with a longitudinal slot which permits contraction of the tubular sleeve to enable extraction of the mandrel from the finished pipe.

2. Mandrel as claimed in claim 1 for the construction of cylindrical pipes, wherein the mandrel, when the tubular sleeve is in the undeformed state, is of the same diameter as the interior of the pipe to be constructed.

3. Mandrel as claimed in claim 1 or 2, wherein the tubular sleeve has an overlapping tongue applied to the sleeve after the formation of the longitudinal slot, and inlaid therein so as to follow the shape of the plastic sleeve, when it assumes its undeformed state.

4. An expandable mandrel for use in the construction of pipes substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.