The present invention relates to a pipe of thermoplastic material having a wall in which a plurality of channels is present which extend essentially in the longitudinal direction of the pipe. The present invention also relates to a method and a device for extruding such a pipe.

Such a pipe, as well as a method and device for extruding it have been disclosed in US 4 323 339. The most important advantage of a pipe having a large number of elongated channels in its wall is that the rat ⁱ o between, on the one hand, the weight of the pipe, and consequently the quantity of plastic material required, and, on the other hand, the strength and rigidity of the pipe is more advantageous than in the case of "standard" pipes having a wall of completely solid design.

In the case of the method disclosed in US 4 323 339, a single flow of plastic material is extruded which then passes through an extrusion head. The extrusion head has a single flow-channel system which debouches at an annular e _j ection ope„ni-;n,rg,. Tinn aann adnnnnuuliαari c ^v _a ^α v ^v i ^.ι. ty of the extrusion hea _d , which cavity is situated upstream of the ejection opening when viewed in the flow direction, an annular member is present which divides up the single flow of plastic material into two subflows which pass, respectively, along the inside and the outside of the annular member. Near the tapering rear side of the annular member, these subflows come together again supported in the process by a narrowing of the annular space in the extrusion head. Attached to the rear side of the annular member are a large number of channel- forming members designed as cylindrical pins, which each form a channel in the flow of plastic material passing through the extrusion head.

Pipes having acceptable engineering properties and an acceptabel exterior quality can be manufactured from various thermoplastic materials such as polyvinylchloride using the

method and device disclosed in US 4 323 339.

It is found, however, that there are limitations, as a result of which it is not possible to manufacture satisfactorily the known pipe with the aid of the known method with a desired capacity. It is also found that not all plastic materials which are suitable in principle for pipes can be utilized for manufacturing in the known manner this type of pipe. In particular, previously used plastic material cannot be used successfully because the parts of the wall of the pipe situated next to a channel are thin and the quality of previously used plastic material does not guarantee that these thin wall parts then meet the requirements.

The object of the present invention is to eliminate the abovementioned disadvantages.

For this purpose, the present invention provides a pipe according to the preamble which is characterized in that the wall of the pipe comprises a plurality of wall layers of thermoplastic material, and in that there is present in at least one of the wall layers a plurality of channels extending in the longitudinal direction of the pipe which are each enclosed by the thermoplastic material of said wall layer.

In the case of the pipe according to the invention, a plastic material other than that used for the other wall layer (or layers) can be used to provide the wall layer in which the channels are present. As a result of suitable choice of the types of plastic material used, for example, the effect described in US 4 323 339 can be counteracted and relatively rigid and non-rigid plastic materials can advantageously be combined, or plastic material can be used which has already been used previously and has been made suitable for re-use. Incidentally, "other" plastic material is also understood as meaning material of the same sort but of another quality, for example plastic material already used previously of the same type is used for the wall layer in which the channels are formed. If an inadequate mutual adhesion is expected between the wall layers, the invention

provides for the provision of an adhesive layer of a plastic material having strong adhesive properties between two wall layers. _S uch an adhesive layer has, furthermore, the advantageous property that it has a high density and is therefore, for example, virtually impermeable to oxygen.

_Th e present invention also relates to a method for manufacturing the pipe according to the invention by means of coextrusion.

The invention relates furthermore to an extrusion head which can be connected to one or more extruders and is suitable for manufacturing the pipe according to the invention.

The invention will be explained further hereinafter _b y reference to the drawing, in which: Figure ₁ shows a diagrammatic cross section of an example of a pipe according to the invention,

Figure 2 shows a diagrammatic longitudinal section of a part of a known extrusion head for the coextrusion of a multilayer pipe, Figure 3 shows a diagrammatic longitudinal section corresponding to Figure 2 of an extrusion head according to the invention,

Figure 4 shows a diagrammatic partial section of the separator head according to Figure 3 along the line I ^V -IV, Figure ₅ shows a partial section of an exemplary embodiment of the pipe according to the invention, an ^d

Figure ₆ shows a partial section of another exemplary embodiment of the pipe according to the invention.

Figure ₁ shows a plastic pipe 1 manufactured in the manner according to the present invention in cross section. The pipe ₁ has a wall comprising three concentric wall layers, namely an outer layer 2, a central layer 3 and an inner layer ₄ . The outer layer 2 and the inner layer ⁴ are of solid design, while the central layer 3 contains a single row comprising a large number of channels 5, which channels ₅ extend in the longitudinal direction of the pipe 1.

The channels ₅ are shown here with a circular section, but they may also have another section ⁽ see, for

example, Figures 5 and 6) . Furthermore, the channels ₅ are in this case at an equal distance from the centre line of the pipe 1, but the channels 5 may also be distributed differently within the central layer 3. Each channel 5 is enclosed completely by the plastic material of the central layer 3 and, consequently, not by the outer layer 2 or the inner layer 4. However, the thickness, dimension "d" in Figure 1, of the plastic material of the central layer ₃ may be very small at the position of a channel 5. As a result of constructing the pipe according to the invention, for example as shown in Figure 1, it is possible to use for each layer a plastic material which is optimally suitable for said layer. Furthermore, plastic materials may also be used which it was hitherto not possible to utilize successfully for the manufacture of pipes having channels in the wall. Thus, for example, it is possible to manufacture the central layer 3 from previously used plastic material, for example recycled material from plastic bottles, and the outer layer 2 and the inner layer 4 of virgin plastic material. Polyolefins, in particular polyethene and polypropene, can also be advantageously used to manufacture the pipe according to the invention.

It has been found that the coextrusion principle, which is known per se, can advantageously be utilized to manufacture the pipe according to the present invention. Figure 2 shows a part of a known extrusion head which is used for the coextrusion of multilayer pipe, in this case a three- layer section. In the case of coextrusion it is known to design the centremost wall layer of the pipe solidly, but it is also known to use foam plastic material to form the centremost wall layer.

The known extrusion head 20 shown in Figure ₂ comprises three inflow openings (not shown) which can each be connected to a separate extruder and can receive a flow of extruded plastic material from it. Furthermore, the extrusion head 20 has three flow-channel systems 21, 22 and 23 which each connect to an associated inflow opening and which each terminate at an associated annular opening 24, 25

and 26, respectively.

The openings 24, 25 and 26 of the three flow-channel systems ₂₁ , ₂₂ and 23 are concentric and adjoin an annular compression space 27 in the extrusion head 20. Said space 27 is characterized in that the clear width for the plastic material, viewed in the direction of conveyance of the plastic material, continuously decreases. The resultant compression of the plastic material brings about the tight mutual adhesion of the layers of plastic material lying against one another. The compression space 27 is adjoined by an annular space 28 having an essentially constant section.

The present invention provides for the advantageous use of an extrusion head of the type such as the extrusion head ₂₀ for the manufacture of the pipe according to the invention. However, adaptation of the extrusion head 20 is, of course, required. Figures 3 and 4 show an exemplary embodiment of an extrusion head 30 according to the present invention.

The extrusion head 30 shown in Figures 3 and 4 comprises three inflow openings (not shown) which can each be connected to a separate extruder and can receive a flow of extruded plastic material from it. Furthermore, the extrusion head ₃ 0 has three flow-channel systems 31, 32 and ₃₃ which each connect to an associated inflow opening and which each terminate at an associated annular opening 34, 35 and 36, respectively.

The openings ₃ 4, 35 and 36 of the three flow-channel systems 31, 3 ₂ and 33 are concentric and adjoin an annular compression space 37 in the extrusion head 30. Said space 37 is characterized in that the clear width for the plastic material, viewed in the direction of conveyance of the plastic material, continuously decreases. The resultant compression of the plastic material brings about the tight mutual adhesion of the layers of plastic material lying against one another. The compression space 37 is adjoined downstream by an annular space 38.

According to the present invention, channel-forming means are provided in space 38 of the extrusion head 30 in

such a way that channels 39 extending in the longitudinal direction in the flow of plastic material C are formed in said flow of plastic material C enclosed by two annular flows of plastic material A, B. The channel-forming means comprise an annular member

40 which is arranged in the path of the annular flow of plastic material C in which the channels 39 have to be formed. The annular member 40 divides said flow c into two subflows Cl and C2 which pass along the inside and the outside, respectively, of the annular member 40.

The annular member 40 is rigidly fixed to the extrusion head 30 by a plurality of slender spokes 41, in this case to the outer wall of said space 38. The annular member 40 could, however, also be attached to the inner wall of the compression space 37 or to both walls.

The annular member 40 has, on its downstream side, at which point the two subflows Cl and C2 come together again, an elongated projection 42 extending downstream in the path of the respective flow of plastic material C for each channel 39 to be formed in the wall of the pipe. In this example, each projection 42 is an elongated and hollow cylindrical pin.

Using the extrusion head 30, it is possible to manufacture a pipe such as that shown in Figure 1, in which the central layer 3 is composed of a first type of plastic material and the inner layer 4 and the outer layer 2 of a second type. Examples of this are the central layer 3 of recycled plastic material and the inner layer 4 and outer layer 2 of virgin plastic material; the inner layer 4 and the outer layer 2 of polypropene and the central layer of an extrudable plastic material having more beneficial rigidity properties.

It is also possible for the central layer 3, on the one hand, and the inner layer 2 and the outer layer 4, on the other hand, to be composed of exactly the same plastic material so that the plastic composition of the pipe is identical to the pipe obtained according to US 4 323 339.

According to a subsequent aspect of the present

invention, an internal cooling of the extruded pipe downstream of the extrusion head is provided in combination with a generally standard external cooling of the pipe. The internal cooling of the innermost wall layer, in particular, of the pipe is advantageous because a shape-retaining skin layer of cooled plastic material is then quickly formed on the inside of the pipe. The shape-retaining skin layer prevents the shrinkage of the plastic material situated further inwards affecting the shape of the inside of the pipe. In particular, the internal cooling prevents linear depressions forming on the inside of the pipe at the point where the wall sections situated between the channels adjoin the innermost wall layer.

_A pipe having a very smooth wall quality both on the inside and the outside and a well-defined outside diameter and inside diameter is obtained by using internal and external cooling in combination.

Because the outermost wall layer and the innermost wall _l ayer of t _h e pipe are now, as it were, frozen, the cooling of the layer on the inside, which cooling is inevitably accompanied by shrinkage of the plastic material, could give rise to the formation of small shrinkage cavities, as can _b e observed in the case of pipes having a solid wall. In the case of solid walls, said shrinkage cavities are very disadvantageous _b ecause, with their often pointed shape, they can form the start of cracks in the wall of the pipe.

_A s a result of the presence of the channels in the pipe according to the invention, the shrinkage occurring in the inside layer is absorbed, in particular, by a relative stretch and/or change in shape of the thin wall parts between the channels and no shrinkage cavities form in the plastic material itself.

The choice of the shape of the section of the channels in the wall of the pipe may also contribute to counteracting the formation of any shrinkage cavity. As a result, for example, of giving the channels not a circular section but a circular-segment-shaped section having small roundings, as indicated in Figure 5, the material volume at

the position where the wall parts situated between the channels adjoin the outside wall or inside wall, respectively, is much smaller and the formation of shrinkage cavities is counteracted. The triangular shape of the channels shown in Figure 6 also results in the advantageous effect described above.

In a variant of the extrusion head according to the invention which is not shown, provision is made for the attachment members which support the annular member not to extend, as shown in Figure 3, through the outermost wall layer of the pipe, but to join the annular member to a partition wall in the extrusion head which separates the flow channel for the flow of plastic material in which the channels are formed from a flow channel situated inside it or outside it. In Figure 3, the spoke 41 could then be joined, for example, to the wall section 50 and/or wall section 5 ₁ . This has the advantage that the spokes of the annular member then extend only through the wall layer of the pipe in which the channels are formed and no flow seams form in the outermost or innermost wall layer, respectively, of the pipe. s is known, such flow seams form a weakening of the pipe.