The invention further relates to a method of providing a plastic pipe with a socket, in which method at least one end of a body portion of the plastic pipe is heated and expanded so as to form the socket.

The socket portion is typically the weakest portion of a pipe. To re- inforce the socket, Finnish Patent 94902 teaches a solution for generating in the socket an orientation which differs from the orientation of the rest of the pipe. However, such a forcefully orientated material is not always suitable for the inner surface of a socket. In some plastics, for example, the solvent glue may trigger stress-cracking in the surface, thus making a glue joint impossible.

Finnish Patent 945107, in turn, discloses a socket comprising an expanded pipe end and a sleeve adapted inside it. However, even such a solution does not often sufficiently improve the characteristics of the socket.

It is an object of the present invention to provide a plastic pipe and a method for avoiding at least some of the above drawbacks.

The plastic pipe of the invention is characterized in that the orienta- tion of at least one layer of the socket differs from the orientation of another layer, and the orientation of at least one layer differs from the orientation of the body portion.

The method of the invention is characterized in that the plastic pipe comprises at least two layers and that the end of the plastic pipe is expanded so as to provide the plastic pipe with a socket, the orientation of at least one layer of which socket differs from the orientation of another layer and the ori- entation of at least one layer differs from the orientation of the body portion.

An essential idea of the invention is that the pipe comprises a socket having at least two layers and that the orientation of at least one layer differs from the orientation of the body portion of the pipe. A further essential idea is that the orientation of at least one layer of the socket differs from the orientation of another layer of the socket. The idea of a preferred embodiment is that the socket is composed of a portion of the pipe which is formed broader and of a sleeve disposed therein. The pipe is expanded for example at a tem- perature which does not substantially change its orientation, and the orienta-

tion of the socket differs from the orientation of the pipe, or the pipe is ex- panded at a temperature which changes its orientation, and the orientation of the socket differs from that of the expanded portion of the pipe. It is the idea of another preferred embodiment that the socket is composed of the end of a pipe having at least two layers, the end being expanded so that a socket forms therein. For example the layers of the pipe are of a different material so that their orientation temperatures are different. Before the end of the pipe is ex- panded, said end is heated to a temperature which causes orientation in the material of at least one layer during the expansion but which substantially causes no orientation in the material of at least one layer during expansion.

Furthermore, a different orientation in different layers can be achieved for ex- ample by heating only one surface of the feedstock pipe. The surface being heated reaches such a high temperature that the tension brought about by the drawing relaxes immediately and the surface does not get oriented, but the inner, colder surface does get oriented.

A preferred construction is achieved when the material is oriented by drawing, but also contains staple fibres. Ideally, the socket comprises two layers which are reinforced by fibres and in which the opposite fibre directions are at an angle of about 50 degrees with respect to the axis and provide pres- sure resistance. The third layer is provided with an axial reinforcement which thus makes the orientation different as compared with the other layers. In a tension resistant pipe, said axial layer is needed only in the socket portion, not in the pipe. Finally, the entire construction is covered with a thermoplastic fi- bre-free inner and outer layer.

It is an advantage of the invention that the mechanical strength of the socket becomes as good as or even better than the rest of the pipe. By arranging different orientations to the socket layers, at least one layer, for ex- ample, is provided with an axial orientation which makes the socket extremely tension resistant, and at least one layer with a radial orientation which makes the socket pressure resistant. On the other hand, at least one layer may be oriented, whereby the mechanical and, possibly, also barrier characters, of the socket improve and at least one layer may be arranged substantially non- oriented, whereby its chemical characteristics are good.

The orientation of the pipe and/or socket can be achieved by differ- ent methods. Molecular orientation is concerned when it is achieved for exam- ple by drawing or shearing (for example a rotating mandrel in a tool). Different

grades of orientation may also be achieved by making some layer of the pipe an/or the socket from a fibre-reinforced thermoplastic. The present invention only relates to self-reinforcing thermoplastic pipes and/or thermoplastic pipes reinforced with staple fibres. Plastic pipes reinforced with continuous fibres are not covered by the invention since the pipe cannot be provided with a socket by expansion. Instead, staple fibres glide in matrix plastic and present no problems for expanding the socket.

In the present description, the term'socket'refers to that portion of the end of a pipe which is arranged broader than the rest of the pipe either by expanding the pipe or by arranging a sleeve on the outside of the pipe. Fur- ther, the term'a socket having at least two layers'implies that at least one layer of the socket is an integral part of the wall of the rest of the pipe and the sleeve may constitute the second layer, or both the pipe and the socket may have at least two layers such that the socket layers are integral with the corre- sponding pipe layers. In the latter case thus refers to an integral pipe socket having at least two layers.

The invention will be described in greater detail in the attached drawings, in which Figures 1a, 1b, and 1c are schematic sectional side views of the process steps of a method of the invention, Figure 2 is a schematic sectional side view of a second plastic pipe provided with a socket according to the invention, Figure 3 is a schematic side view in partial section of a third plastic pipe provided with a socket according to the invention.

Figure 1a shows a mould 1, inside which is arranged a three-layer plastic pipe 2. Heating means 3 for heating the end of the plastic pipe 2 are arranged in a socket support 1a in the mould 1. Inside the plastic pipe 2 is arranged an expansion mandrel 4. The expansion mandrel 4 is a device with which the plastic pipe 2 can be mechanically expanded. The expansion man- drel 4 may be for example of rubber, whereby it expands when air is pumped inside thereof and simultaneously expands the plastic pipe 2. The expansion mandrel 4 is used to expand that part of the plastic pipe 2 from which a socket 2e forms. The expansion mandrel 4 is used to expand the plastic pipe 2 so that it is compressed against the socket support 1 a of the mould 1 in the man- ner shown in Figure 1 b.

Layers 2a to 2c in the plastic pipe 2 are of different materials to

cause orientation therein at different temperatures. The inner layer 2a and the outer layer 2c can be of polyethene PE, for example, and the intermediate layer 2b of crosslinked polyethene PEX. The heating means 3 are used to heat the end of the plastic pipe 2 to a temperature above the crystallization point of polyethene PE, i. e. to 150°C, for example. In this case, during expan- sion, the inner layer 2a and the outer layer 2c, which are of polyethene, do not undergo orientation. However, the intermediate layer 2b undergoes a radial orientation, since at said temperature the crosslinked polyethene PEX under- goes orientation, since the network bonding produced by crosslinking pre- vents the material from relaxing.

Consequently, the plastic pipe 2 shown in Figure 1c is achieved, the body portion 2d of which has three layers and to which a three-layered integral socket 2e is connected, the inner layer 2a and outer layer 2c of which are non-oriented and the intermediate layer 2b is oriented. The oriented in- termediate layer 2b increases the mechanical strength of the socket 2e and also improves its barrier characteristics. The chemical characteristics of the inner layer 2a and the outer layer 2c are extremely good. Such layers endure for example solvent glues that are used in pipe joints without the socket 2e shrinking or its surface being cracked.

The inner layer 2a and the outer layer 2c can also be of modified polyvinyl chloride PVC, for example, in which case said layers are not ori- ented. In this case the oriented intermediate layer 2b may be of polyvinyl chlo- ride PVC, for example.

When a plastic pipe is provided with a socket in the manner shown in Figures 1 a to 1 c, the characteristics of the materials in the different layers may also be substantially the same, if desired, whereby for example the outer surfaces the feedstock pipes only are heated. In this case the outer surface is heated to such a high temperature that the tension brought about by the draw- ing relaxes immediately with no orientation for example in the outer layer 2c.

This being the case, the temperature in the inner layer (s) may be so low that orientation occurs therein.

Figure 2 shows a second plastic pipe 2 according to the invention.

In. the embodiment of Figure 2, the socket 2e is provided by a layer having a larger diameter than the body portion 2d of the pipe 2 and by a sleeve 5 dis- posed inside the socket. The socket 2e portion of the pipe 2 can be made larger for instance by heating the material of the pipe to a temperature at

which it does not get orientated during expansion, and by expanding the pipe after heating. In this case the sleeve 5 is oriented such that its orientation dif- fers from the orientation of the body portion 2d of the pipe and the orientation of the wall of the expanded portion of the pipe. In the sleeve 5, the direction of the orientation may be either radial or axial or it may be in both directions. A radial orientation increases the pressure resistance of the socket 2e and an axial orientation increases the tensile strength of the socket 2e.

Another way to make the socket 2e is to heat the plastic pipe to a temperature at which orientation is created in the wall of the plastic pipe dur- ing expansion and to expand the plastic pipe at said temperature. This being the case, the orientation in the wall of the plastic pipe is different in the body portion 2d and the socket portion 2e. The orientation of the sleeve 5 is se- lected such that it differs from the orientation of the wall of the plastic pipe which is an integral part of the body portion 2d of the socket 2e. The sleeve 5 may also be completely non-oriented and the orientation of the sleeve 5 may be the same as or different from the orientation of the body portion 2d.

The sleeve 5 may also be displaced outside the pipe 2. Conse- quently, the sleeve 5 may be a separately oriented sleeve or a conventionally injection moulded one. The sleeve 5 can be pushed inside the pipe 2 such that when being pushed inside, the sleeve 5 expands the end of the pipe 2.

On the other hand, the end of the pipe 2 may already have been expanded exactly up to the outer dimensions of the sleeve 5. Furthermore, the end of the pipe 2 may be expanded so that it is larger than the sleeve 5, and the end of the pipe 2 is allowed to shrink onto the sleeve 5 either in such a way that a potential pipe orientation is at least partly relaxed or in such a way that it is not relaxed. The relaxation of the orientation can be prevented for example by keeping the end of the pipe axially drawn at the shrinking stage.

Figure 3 shows a third plastic pipe 2 according to the invention. The outer surface of the sleeve 5 can be provided with grooves, projections, in- clined surfaces or the like, which keep the sleeve 5 and the pipe 2 immobile with respect to one another. The sleeve 5 is preferably made by injection moulding, whereby the dimensions of the sleeve 5 are accurate and which also enables difficult shapes to be made. The sleeve 5 can be made from such a rigid material, for example glass-fibre reinforced plastic, that the shrink- ing of the pipe 2 onto the sleeve 5 does not substantially change the size and shape of the inner part of the sleeve 5. A second pipe 6 is arranged inside the

sleeve 5. A wedge ring 7 can be arranged between the sleeve 5 and the sec- ond pipe 6. The wedge ring 7 comprises a wedge surface 7a, arranged against a wedge surface 5a of the sleeve 5. This way the wedge surfaces 5a and 7a prevent an axial motion between the second pipe 6 and the sleeve 5.

The axial motion is prevented because the wedge surfaces 5a and 7a direct a strong radial friction force to said points, whereby the wedge ring 7 is tightly pressed against the second pipe 6.

The inside of the sleeve 5 can be provided with a sealing groove 5b into which a sealing ring 8 can be placed to ensure the tightness of the joint between the sleeve 5 and the second pipe 6.

The sleeve 5 comprises a bevelled end 9 that facilitates pushing the sleeve 5 inside the pipe 2. The orientation degrees and/or orientation di- rections of the materials of the pipe 2 and the sleeve 5 do not have to be the same and they can be modified so as to be suitable for each particular appli- cation. The orientation can be polymer molecule orientation and also orienta- tion of potential fibre reinforcement. If the sleeve 5 is mainly subjected to ra- dial stresses and forces, the orientation is directed particularly in the periph- eral direction. If again the sleeve 5 is mainly subjected to axial forces, the main orientation of the sleeve 5 is axial. For example, if axial tensile stress is caused in the sleeve 5 when the wedge ring 7 rests against the wedge surface 5a of the sleeve 5, then the sleeve 5 is preferably oriented or fibre reinforced in the axial direction. The pipe 2 and the sleeve 5 can be made from the same polymer, but their filling agents and/or reinforcement agents and their contents may be different. The end of the pipe 2 is provided at the orifice of the pipe 2 with a flanged additional reinforcement 10 that increases the peripheral strength of the orifice in particular.

The drawings and the related description are only intended to illus- trate the inventive idea. The details of the invention may vary within the scope of the claims. Accordingly, the orientation of the different layers of the socket 2e may be different in such a way that the different layers have different orien- tation degrees and/or orientation directions. Furthermore, the socket 2e may comprise more than two or three layers. The sleeve 5 can also be disposed inside the socket 2e which comprises at least two layers and forms an integral part of the body portion 2d, that is, the sleeve 5 could be disposed inside the socket 2e of Figure 1 c, for example.