Tube Bending Process

Abstract

A method of forming a bend in a normally rigid tube of a material which becomes plastic at an elevated temperature, in which the material of the tube portion to be bent is rendered plastic by heating means, supporting means being provided to ensure that the tube remains substantially dimensionally stable in cross-section during the bending, the supporting means consisting of intermediate zones kept under substantially normal temperature conditions, which conditions are realized by screening said zones by means of aluminium strips from heat supplied by said heating means.

Description

This invention relates to a method of forming a bend in a normally rigid tube of a material which becomes plastic at an elevated temperature, in which the material of the tube portion to be bent is rendered plastic by raising its temperature, supporting means being provided to ensure that the tube remains substantially dimensionally stable in cross-section during the bending, and subsequently bending forces are applied to the tube portion to be bent to subject the plasticised material to plastic deformation and form a bend, and then the bend is set by cooling.

Conventionally, sand or other granular material, such as barley, or a helical spring narrowly fitting within the tube portion concerned, are selected as supporting means within a tube section to be bent. Both the introduction and, after the bend has been made, the removal of these supporting means require substantial work, which disadvantage is all the more cogent according as the tube to be bent has a larger diameter. Indeed, for tubes having a very large diameter, say 400 - 600 mm and over, these known supporting means are actually no longer acceptable, while there are no other methods of bending such large tubes.

It is an object of the present invention to overcome the above disadvantages, and to provide a method which renders the bending of tubes of average diameter more economical, and furthermore renders the bending of tubes of larger tubes possible at all.

According to the invention, there is provided a method of forming a bend in a normally rigid tube of a material which becomes plastic at an elevated temperature, in which the material of the tube portion to be bent is rendered plastic by raising its temperature, supporting means being provided to ensure that the tube remains substantially dimensionally stable in cross-section during the bending, and subsequently bending forces are applied to the tube section to be bent, to subject the plasticised material to plastic deformation and form a bend, and then setting the bend by cooling, characterized in that the supporting means are applied by raising the temperature of the tube material exclusively in peripheral zones substantially transverse to the longitudinal axis of the tube, such zones being separated by intermediate peripheral zones kept under substantially normal temperature conditions.

The alternating plastic and rigid peripheral zones provide a flexible tube portion, which, as it were, forms its own stiffening ribs.

The tube may be heated by all kinds of known heating means, while the portions to be maintained under normal conditions can be insulated from the heat applied in various manners. Thus, electric heating strips may be used, which are applied around the tube portion to be bent in spaced relationship.

In a preferred embodiment, in particular for plastics tubes, infrared irradiation is used for the heating thereof, while aluminium strips reflecting the infrared radiation is applied around the peripheral zones to be kept cool.

A number of embodiments of the invention will be described, by way of example, with reference to the accompanying drawings. In said drawings,

FIG. 1 is a perspective view of apparatus for carrying out the method according to the invention;

FIG. 2 is a side view of the showing of FIG. 1, but with differently applied screening means for the peripheral zones to be kept cool;

FIG. 3 is a plan view of a specific form of aluminium foil to be used;

FIG. 4 is a plan view of still form of aluminium foil;

FIG. 5 is a cross-sectional view on the line V -- V of FIG. 4;

FIG. 6 illustrates a special form and a special arrangement of aluminium foil;

FIG. 7 illustrates still another form and arrangement of aluminium foil.

FIG. 1 shows stands 1 on which rests a tube 2 of a material which becomes plastic upon heating and in which a bend must be formed. The tube 2 is spirally wound with aluminium strips 3, arranged in spaced relationship. The aluminium foil has a reflective outer surface. Disposed about the tube 2 are a plurality of infrared radiators 4, which direct their rays substantially radially to the tube. After a relatively short heating period, the infrared radiators are removed, and the tube can be subjected to bending forces, whereafter the aluminium wrappings are removed. FIG. 2 shows a tube 2 to be bent, wound with strips 3 of aluminium foil, the windings being spaced separate rings.

FIG. 3 shows a carrier foil 5 with aluminium strips 13 applied thereto. The foil 5 is of infrared transmitting material, so that in providing the tube 2 with aluminium strips, it is sufficient to wrap the foil 5 together with the strips 13 around the tube, the longer sides 6 of foil 5 being secured together, for example, by means of an adhesive. The presence of foil 5 permeable to infrared radiation often has the advantage of preventing the tendency of the outer surface of the heated circumferential zones to become scarred.

FIG. 4 shows a different arrangement of aluminium strips to be applied in the method according to the invention. An aluminium foil 15 has aluminium strips 23 to be separated along score lines 16. The strips 23 are provided with a self-adhesive layer 17, covered with a cover strip 18. On the opposite side, the strips 23 are also provided with a self-adhesive layer 19, which is covered with a cover strip 20.

The foil 15 is used as follows. After cover strip 18 has been peeled off, the foil 15 is affixed to the tube to be bent by the exposed adhesive surfaces 17. Next the cover strip 20 is removed, whereafter the foil 15 is wrapped around the tube 2. The strips 23 are of such length that they just fit the circumference of the tube with some overlap. The exposed adhesive surface 19 are now affixed to the aluminium foil edge already lying on the tube. whereafter foil 15 is pulled off. Owing to the presence of score lines 16, strips 23 remain on the tube.

It will be clear that, instead of the rows of adhesive surfaces 17 and 18 on the same side of foil 15, such adhesive surfaces may be provided on the front and back of the foil, while in addition there are many other possibilities within the scope of the invention.

FIG. 6 illustrates how, by means of a shape different from the strips of constant width, the width of the portion to be heated can be adapted to the elongation of the plasticised portions lenthwise of the tube, which occurs in practice during the bending operation. In the situation shown in FIG. 6, the neutral line 25 will remain in the heart of tube 2, as will also be the case when strips of constant width are applied.

In the method according to the invention, however, it is possible to position the neutral line at will.

FIG. 7 shows an aluminium foil 43 of such shape that at the inside bend the tube will be heated over shorter lengths, whereas at the outer bend longer lengths of the tube are heated. It will be clear that when, after the heating, bending forces are applied to the tube, the neutral line 35 will be positioned adjacent to the inside bend, which has remained almost entirely cool.

Claims

We claim:

1. A method of forming a bend in a normally rigid tube constructed of a material which becomes plastic at an elevated temperature comprising simultaneously heating to an elevated temperature a plurality of axially spaced apart circumferential band portions of the tube while maintaining the intermediate band portions which separate the heated band portions at substantially normal temperature; bending the tube over the desired angle; and cooling the tube.

2. A method as in claim 1 wherein heating is effected by directing heat radiation onto the tube while isolating the intermediate band portions of the tube from the radiation by applying strips of radiation-reflective material around the tube.

3. A method as in claim 1 wherein the band portions to be heated and the intermediate band portions have, viewed in the circumferential direction, a tapered form.

4. A method as in claim 1 wherein heating is effected by applying electric heating strips around the tube in axially spaced relationship.

5. A method as in claim 1 wherein the tube is heated along only a portion of its length, said portion consisting of said band portions and said intermediate band portions, whereby at least one end portion of said tube remains unheated and retains its initial shape.

6. A method as in claim 5 wherein the tube is constructed of plastic material and wherein heating is effected by directing infra-red radiation onto that portion of the tube which is to be bent, the intermediate band portions being protected from heating by applying to the circumference of the tube strips of infra-red reflective material.