In contrast to conventional pipes having thick walls with both a smooth inner and outer surface, so called"constructed"pipes have a smooth inner wall, while the outer wall is built using rigid rings which form visible flanges and steps.

Pipes constructed with a smooth inner wall, and a corrugated or ribbed outerwall have good material economics, but also have a greater total wall thickness, and thus less inner diameter.

In order to achieve a smooth inner pipe surface and avoid inner steps in such a pipe system, the inner diameter of the pipe sectopms of the system (bends, branch pipes, etc.) and pipes in each dimension class should be approximately the same size. Thus, pipe sections of constructed pipes will also have a comparatively large wall thickness.

For efficiency, it is desirable that pipe sections can be injection moulded; to accurately control the geometry of the parts. They should be formed in a casting mould comprising two halves which form the external shape of the cast, and retractable cores forming the bore inside of the pipe part.

Thick, large walls are not compatible with the injection moulding technique, as they give a low production rate and high material use. There is also an increased danger of product defects, (depressions and ovality) due to the long cooling time that is necessary in injection moulding.

Pipe sections for systems of constructed pipes can be injection moulded with ribs so that a sufficient total wall profile height is achieved, but without a large local wall thickness.

With this kind of rib design, it is necessary to omit a conventional sealing system such as a rubber ring in a groove near the sleeve nozzle, because no smooth tip end exists for sealing against. Instead, a packer sits between the two outermost profiles of the tip end of the pipe or the pipe section, so that the packer seals agains the sleeve wall almost only down by terminal edge of the sleeve end.

In some cases, it is necessary that the tip end is smooth both internally and externally, so that conventional sealing systems can also be used. The pipe section should be

injection moulded with a thin walled tip end which inevitably gives a larger inner diameter in the tip end than in the rest of the pipe system. Then inner steps are formed in the assembled pipe line.

It is thus an object with present invention to provide a pipe section having a tip end that is not thick, but has a smooth inner wall and outer wall, and that has a total wall profile height corresponding to a constructed pipe. It is also an object to provide a tool for the manufacture of such a pipe section.

The object of the invention is achieved with a pipe section and tool, respectively, according to the characterizing part of the independent claims. Further features are clear from the dependent claims.

According to the present invention, it is possible to injection mould pipe sections with a ribbed sleeve and a ribbed pipe part section, but with an outer (and inner) smooth tip end, with local wall thickness that is favourable for moulding and material economically.

Furthermore, according to present invention, it is made possible to injection mould pipe sections having a tip end constructed from two or more relatively thin concentrical pipes, between which extend ribs parallel to the axis of the pipe evenly distributed around and normal to the circumference of the pipe.

The tip end is preferably terminated in a bevel so that it is easier to press into a sleeve having a sealing ring in a groove near the opening of the sleeve. The cavities in the wall of the tip end (between the ribs) will be invisible when viewing into the tip end. In the transition between the rib pipe section and the smooth tip end, the cavity is covered by a mainly plate shaped wall perpendicular to the axis through the tip end.

In the following, the invention is described in more a detail by means of an example of an embodiment and with reference to enclosed drawings, where Fig. 1 shows a partly sectioned picture of a pipe section manufactured according to present invention, Fig. 2 shows the pipe section form Fig. 1, seen from the tip end, and Fig. 3 shows the detail of a casting tool having casting cores for the tip end and sleeve end. partly sectioned.

In Fig. 1 and 2 is shown a pipe section 1 formed as a bend. The pipe section 1 has a first end, or tip end 2, and a second end, or sleeve end 3. The pipe end 1 is in a per se known manner, provided with ribs 4 extending peripherally of the pipe section 1.

Furthermore, the sleeve end 3 is formed in a per se known manner with a sleeve end 5 which is provided with a first, outer recess 6 and a second, inner recess 7. The inner recess 7 is arranged for receiving a sealing ring 8, having an inner extending lip 9. The outer recess 6 is arranged for receiving a so called click ring 10, also previously known.

The click ring 10 has a first internal flange 11 which fits in the outer recess 6, and a second internal flange 12 which is arranged to bias the sealing ring 8. It is also possible that other alternative sealing means be incorporated into sleeve end 3.

The tip end 2 comprises an inner wall 13 and an outer wall 14. Between inner wall 13 and outer wall 14 is a distance means, in the form of ribs 15 arranged parallel to the axis of tip end 2. Preferably, inner wall 13, the ribs 15 and the outer wall 14 terminates in a bevel 16, to ease introduction of the tip end 2 intp a corresponding sleeve end 3.

In order for the sealing ring 8 not to be dislodged or cut to pieces during assembly of the tip end 2 into a sleeve end 3, the bevel 16 is preferably formed without sharp edges in places that can be in contact with the sealing ring 8 during the joining operation.

Manufacture of the pipe section 1 is by injection moulding using a moulding cast with two cores. This will be described in the following with reference to Fig. 3.

Fig. 3 shows a section of the casting tool 19, with a first casting core 17 for the tip end 2 and a second casting core 18 for the sleeve end 3. The separation plane between the casting cores 17,18 is situated so that both cores easily can be drawn out of the pipe section after the casting is completed.

The first casting core 17 comprises an inner element 20 having an outer shape corresponding to the desired shape of the tip end 2 of the pipe section 1. Furthermore lips 21 are situated parallel to the axis of the pipe section 1.

During manufacture, the casting cores 17,18 will be arranged inside the casting tool 19 and the cavities thus formed will be filled with molten plastic. When the plastic has hardened the casting tool 19 is opened, and the cores 17,18 are drawn out in directions parallel with their respective axis.

The fact that the tip end 2 is made hollow with axial ribs 15 will in most cases give a lower ring rigidty compared with a corresponding tip end with circular ribs vertical to

the axis of the tip end. However, functionally, the only requirement is that in the area where sleeve end 3 and tip end 2 overlap, the resulting ring rigidty shall be at least as rigid as the ring rigidty of the pipe itself. Also the part of the pipe section 1 that is free, should at least have this rigidty. The tip end 2 can thus be made weaker than the rest of the pipe section, if the sleeve end 3 encircling it is so rigid that the sum of the ring rigidties for the sleeve end 3 and the tip end 2 is sufficiently large.

It is also possible that the pipe section be integrated with some kind of bendable mantle upon the outer pipe 14 of the tip end 2 to provide a slight elongation (not shown).

This mantle can in an additional process step after the injection moulding be heat shaped, so that it covers the end of the ribs 15 of the tip end 2, and thus protects the sealing ring in the sleeve end 3 into which the tip end 2 is located.

The present invention will also be very useful in connection with the manufacture of pipe sections which can be used as trannsitions between pipe systems having a large difference in wall thickness. An example could be transition from concrete pipes to plastic pipes, as the plastic pipes usually have thinner walls than the concrete pipes.