Firstly, the invention relates to a plastic tube with a smooth inner surface and a solid outer surface com¬ prising cross-ribs.

Such a tube is known from German Patent Speci- fication 2,709,395. In said patent specification such a tube is described together with a mo 'Iding of such a tube. The tube described therein has a smo <th inn r surface and is provided with solid ribs on the ? ter si' face over its entire length. Such a known tube which is provided with ribs over its entire outer surface has the disadvantage that connecting it to other similar sorts of pipes is relatively difficult. The usual widening, in normal smooth tubes, of one end in a manner such that the unwidened end of another tube fits therein has in practice hitherto not yielded satisfactory results in the case of tubes with solid ribs because the ribs are deformed during the widen¬ ing operation which leads to uneven, non-round shapes which may be accompanied by a lessening of the mechanical strength.

The object of the present invention is to provide a tube provided with ribs with a smooth inner surface which is directly suitable for use in all known tube con¬ nection met ods. According to the invention such a tube is charac¬ terized in that at least one end of said tube has a smooth outer surface of predetermined length, said end having a wall thickness which is greater, equal to or smaller than the smallest wall thickness of the tube between the solid ribs.

In particular, the wall thickness is greater than the smallest wall thickness of the tube between the

solid ribs .

By making at least one end of a pipe according to the invent on smooth and distr buting at least part of the volume of the solid ribs normally present there equally over the wall thickness of the tube, a tube end is moulded, on the one hand, which is suitable for all normal joining methods while, on the other hand, said tube end may satisfy, in a simple manner, the required tolerances of roundness and strength. In particular, in a tube according to the inven¬ tion, the volume of plastic per length unit of the ribbed section amounts to 0.5 to 1.4 times the volume per length unit of the smooth section; in many cases, however, this ratio may be 1.0. The transition section between the ribbed sec¬ tion of a tube and the smooth section of a tube is gener¬ ally not sharp, but the transition section will have an incline; said incline is generally between 0 and 90°.

In the case of many types of tube connection it is usual to widen one end of the tube in a manner such that an unwidened end of another tube may be suitably received therein; as a rule, use is made of sealing means which are appropriate for this purpose. The tube accor¬ ding to the invention may then, at its smooth end, be widened in a manner such that an end comprising solid ribs of another tube may be received; the widening may also be produced in a manner such that an unwidened smooth end of another tube may be received. In the latter case, the tube according to the nvention will have a smooth outer surface at both ends; one of the two ends will then be widened in a manner such that a smooth end of another tube may be received.

The invention also relates to a rib-shaped device for use in an installation for moulding a tube com- prising ribs according to the invention as described above, which device comprises at least two series of half- moulds; each half-mould having a half-tube-shaped moulding cavity, and the half-moulds being moveable along tracks, which

tracks run parallel to each other over a working path in a manner such that, over said working path, the half-moulds complement each other, during moulding, to an annular mould around a concentricall arranged smooth mandrel. The device according to the invention being hara terized in that at least one pair of all pairs of half-moulds is present, at least a section of which has a smooth inner surface, whilst the half-moulds in the remaining pairs have a ribbed inner surface. For such a device according to the invention it is, furthermore, the case that, in particular, at least one of all pairs of half-moulds is present, at least a section of which has the smooth profile which is not parallel to the surface of the above mentioned smooth mandrel; said sect on has, at least partially, an incline of between 0 and 90° with respect to the central axis of the device.

The sum of the length of all half moulds with a smooth inner surface which is parallel to that of the smooth mandrel is essentially equal to the desired length of the surface of a tube with a smooth outer surface.

In particular, in the device according to the invention, the volume ratio per length unit of the moulding cavity of the half moulds with a ribbed inner surface around the mandrel to the moulding cavity of the half moulds with a smooth inner surface around the mandrel has a value in the region of 0.5 - 1.4; in many cases, said volume ratio is essentially equal to 1.0.

The invention also relates to an installation comprising an extruder and a rib moulding device as des¬ cribed above.

Finally, the invention relates to a method for producing a tube made from plastic with a smooth inner surface and an outer s urface comprising sol i d cross-ribs using in an installation according to the invention as has been described above which is character zed in that the extrusion speed and the speed of displacement of the half-moulds of the invention are adjusted in relation to

the desired ratio of the olume of plastic per unit length of a tube sect on ith ribs and a tube sec ion th a smooth outer surface and, similarly, the desired ratio of the displacement speed of the half-moulds is adjusted to a value in the region of approximately 0.5 to 1.4 t mes the extrusion speed.

If the ratio differs from 1.0, a constant speed difference is maintained over a period of time which is essentially equal to and coinc des with the time that is necessary to permit formation, around a smooth mandrel, of the number of moulds having a smooth inner surface, which is required for the corresponding smooth sect on of a tube.

The invention will now be describ ed with reference to the dra ng, in which: Figure 1 shows a tube according to the invention in cross-section, with a smooth outer surface at one end; Figure 2 shows two tubes according to the inven¬ tion connected together;

Figure 3 shows a tube provided with solid ribs, a central sect on having a smooth outer surface;

Figure 4 shows an installation for moulding a tube provided with ribs;

Figure 5 shows in plan view a section through a number of half-moulds of a device according to the iπven- tion.

Figure 1 shows a tube 1 with a smooth inner wall 2 and solid ribs 3 on the outer side. One end of the tube has a smooth outer side 4 while the trans tion section between the smooth section 4 and the section where the ribs 3 are located is indicated by 6.

Figure 2 shows two ribbed tubes connected together, 1 and 1', the smooth section 4 is widened in order to receive the section of the tube 1' provided with ribs; 5 shows that a packing material may be located between the ribs.

Figure 3 shows a section of a continuously moulded ribbed tube in wh ch a sect on of the length has a smooth outer surface 4. In order to obtain tubes with a smooth

outer surface at both ends the section 4 in the centre may be sawn through; if a smooth outer surface is required at only one end, sawing through may take place at one of the ends of said smooth section 4. F igure 4 shows diagremmatically a device for the continuous moulding of rib tubes comprising a rib moulding device 10 and extruder 13.

The rib moulding device comprises two endless belts 11 and 12 on which half-moulds 11' and 12' are mounted. The half-moulds lock together around the smooth end of an extrus on die ring 20 during moulding of an annular hollow form.

The tube 22 delivered by le extrusion head 14 at the extrusion die ring 20 has a internal diameter which is smaller than the external iameter of the smooth mandrel 21. The half-moulds 11* and 12' deform the still plastic material, for example polyvinyl chloride, of the tube 22 and give the outer surface of the tube its profile. The moulds provided with a ribbed inner surface give a ribbed tube section 19. The moulds with a smooth inner surface lead to a smooth tube section 18. In all cases, the inner wall of the moulded tube is smooth.

The extruder 13 s driven by a drive means 15; the rib moulding device is, as shown diagremmatically, driven by drive means 16. The two drive means are coupled via a control unit 17 which regulates extrusion speed and transport speed of the half-moulds in accordance with a desired ratio of the volume of plastic per length unit of a ribbed tube section and a smooth tube section. The device sketched in Figure 4 shows a single extrusion head to which all necessary plastic is supplied in one delivery.

Of course, with the aid of co-extrusion, the required plastic may be supplied in several streams to a special extrusion head connected to the rib moulding device, in which case the tube is constructed in stages. Finally, Figure 5 shows three cross-sections of moulds in plan view; mould 30 is a normal mould with

r I D S i i .

Mould 32 is a -.ould with a ribbed section 33 and a smooth section 35; the transition section 34 has an incline. Mould 36 has a smooth inner surface 37.

The nvention will now be further illustrated with an example. A tube with an internal diameter of 180 mm with a wall thickness between the ribs of 2.0 mm is moulded in an installation according to the invention. The height of the ribs is 7.8 mm, the width of the ribs is 2.55 mm and the pitch of the ribs is 16.9 mm. The surface of the cross-sect on of the tube is, on average.

1785 mm*

The smooth end should have a wall thickness of 3.7 mm which corresponds with a cross-section of 2134 mm . This may be achieved by changing the ratio of the speeds of extrusion and transport of the half-moulds from 1:1 to 1:0.85 when the half-moulds with smooth inner surface pass through the zone in which the mouthpiece of the extruder supplies a tube.

As soon as the last mould with smooth inner surface has been completed, the speed ratio may be returned to 1:1.

Throughout the description and following claims the term section means to indicate a length of tube forming a part of the total length of a tube.