This invention relates to an apparatus for the production of ribbed pipes having a smooth inner face, of a plastic material, said apparatus comprising a core which is formed by a spindle, a conically enlarging mandrel positioned after said spindle in the production direction of the apparatus, and a kernel which has a substantially constant diameter and which is positioned after said mandrel; an extrusion sleeve which surrounds said spindle so as to form a nozzle for the material together with the core? and chill moulds which surround said extrusion sleeve and said core ans which are transportable along an endless path and the inner surface of which is provided with grooves for the formation of ribs on a pipe; whereby an end zone of the kernel spaced form the nozzle is provided with means for the cooling of said zone. The invention is also concerned with a method for the production of ribbed pipes.

Various apparatuses and methods have been suggested for the production of solid-wall ribbed pipes; none of these, however, is in industrial use on account of some major disadvantages thereof. GB Patent Specification 1 431 796 discloses an extrusion moulding method which is characterized by the step of creating a rather low moulding pressure mainly by means of the extruder. DE Patent Specifications 2 362 444 and 1 233 128 and FR Patent Specification 7 315 485 dis¬ close injection moulding methods which are character¬ ized by a small closed moulding space wherein there prevails a high pressure. From FI Patent Specification 60 825 and DE Patent Specification 2 709 395, such press moulding methods are known which are character¬ ized by an open moulding space and by the step of

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creating a pressure by means of the extruder on one hand and by means of the traction force of a conically enlarging part of the core, i.e. the mandrel, and the chill moulds on the other hand. A similar straight- kernel structure is also known from the manufacturing technique of double-walled corrugated pipes, see e.g. CA Patent Specification 1 172 813.

A structure corresponding to the apparatus disclosed at the beginning of this specification is known from EP Patent Specification 0 005 104 (US Patent Specification 4 365 948) , wherein Figure 6 illustrates an apparatus operated by the extrusion moulding method and by means of which solid-wall plastic pipes having a smooth inner face and a corrugated outer face can be manufactured. This figure shows a kernel the cooling means of which are positioned only at the terminal end of the kernel, while the initial portion of the kernel is uncooled. The kernel has a constant diameter. A major disadvantage of the manufacturing methods mentioned above is that it is very difficult to produce by means thereof pipes having high narrow ribs which would be the most advantageous ones in view of the use of the pipe. In order that the deep narrow grooves on the inner surface of the chill moulds were filled up, extremely high pressures have to be applied, which requires high durability from the apparatus. These problems are further aggravated when a stiff type of plastic, such as a PVC without any softener, is used. Another major problem is the surface finish of the inner face of the pipe. The smoothness of the inner face is impaired by e.g. the following factors: air bubbles are formed in the moulding space; the high shearing speed causes cracksin the melt; irregularities and cold seams are formed due to the layer structure of

the material; the inner face may adhere to the kernel; and an uneven cooling of the substance gives rise to recesses and air bubbles within the material. For these reasons, it has not previously been possible to manufacture high-quality ribbed pipes industrially by the.methodsmentioned above.

The object of the present invention is to provide an apparatus and a method for the production of ribbed pipes, by means of which the pipes can be provided with complete ribs and a smooth inner face.

The apparatus according to the invention is character¬ ized in that means are provided in an initial zone of the kernel between said mandrel and said end zone of the kernel for the heating of said initial zone, and that the diameter of the initial zone of the kernel increases slightly in the direction of motion of the material.

In the apparatus according to the invention, the initial zone of the kernel is heated and its diameter increases slightly towards the end zone. When the pipes are heated still within the area of the kernel and the plastic mass or material is simultaneously forced towards the chill moulds, the material unexpectedly fills all the grooves of the moulds reliably, and the inner face of the pipe becomes completely smooth. The advantageous influence of the structure according to the invention might be due to the fact that after the grooves of the chill moulds have been filled by the action of the extrusion pressure within the area of the conical mandrel, the moulds begin to chill the material, which begins to contract. Any imperfections caused by the different cooling rates and the contraction of the material are prevented by creating a notable terminal pressure in the grooves of the chill moulds, as a result of which the material to be moulded

follows accurately the surfaces of the chill moulds and the kernel. This terminal pressure is independent of the extrusion pressure, and it is therefore easily adjustable by changing the heating effect. The length of the initial zone of the kernel, too, affects the terminal pressure.

The heating means can be positioned in one part of the initial zone only, but it is of advantage in view of the operation that the heating means of the initial zone extend over the whole length of the zone. For the same reason, it is desirable that the diameter of the initial zone increases essentially evenly.

The kernel in the present description means that part of the core which has an essentially constant diameter and by means of which the inner face of the pipe is calibrated. Therefore it is evident that the diameter cannot vary to any greater degree. Conse¬ quently, the kernel of known apparatuses has a constant diameter or the diameter decreases to some extent towards the terminal end so that a cooling, contracting pipe would not adhere thereto. For this reason, the diameter of the initial zone of the kernel according to the present invention increases only slightly in the production direction so that the diameter of the kernel at the end of the initial zone is 0,2 to 2,0 per cent, preferably about 1,0 per cent, larger than at the beginning of the initial zone.

In order that the initial zone of the kernel would have the desired effect on the moulding of the pipe, it must be sufficiently long. According to the invention the length of the initial zone in the axial direction of the apparatus is 10 to 100 per cent, preferably about 50 per cent, of the outer diameter of the pipe. An unusually high friction exists between the material and the kernel due to the terminal pressure.

Any disadvantageous effects resulting therefrom can be eliminated by providing the kernel with a suitable plastic repellent coating, e.g. by a boric treatment, by the use of a lubricant, or by providing part of the kernel with a low spiral-shaped groove or grooves which prevent the material from sticking fast to the surface of the kernel.

The invention is also concerned with a method for the production of ribbed pipes having a smooth inner face, of a plastic material, wherein a material which is in a plasticized state is extruded into a ring-shaped moulding space which is defined between the end face of an extrusion sleeve, a core of the appara¬ tus, said core comprising a spindle, a conically enlarging mandrel and a kernel having a substantially constant diameter, and chill moulds which surround said extrusion sleeve and the core and move in the production direction of the apparatus and the inner surface of which is provided with grooves for the formation of the ribs; the material which has been moulded into a pipe being cooled by the chill moulds and an end zone of the kernel.

The method according to the invention is characterized in-that the inner face of the material being in the shape of a pipe is heated from the end zone, and that the diameter of the inner face of the pipe is increased slightly in the area of the initial zone.

One preferred embodiment of the invention will be described more closely below with reference to the attached drawing, wherein

Figure 1 is a general view of an apparatus for the production of ribbed pipes, and

Figure 2 is an enlarged longitudinal section of one detail of the apparatus.

The apparatus shown in Figure 1 comprises two chill moulds 1 and 2 which move along endless paths and which meet each other within the area of guide rails 3 to form a cylindrical mould. An extrusion sleeve 4 connected to an extrusion head 6 of an extruder 5 extends into said mould. It is also shown in Figure 1 how a finished pipe 7 protrudes from the other end of the mould formed by the chill moulds .

Figure 2 is a more detailed view of those parts of the apparatus which take part in the moulding of the pipe= A spindle 8 is positioned on the central line of the apparatus partially within the extrusion sleeve 4, which spindle 8 is straight, i.e. it has a constant diameter. A conically enlarging mandrel 9 is positioned after the spindle so that it is wholly outside the extrusion sleeve, and a kernel 10 having an essentially constant diameter is positioned after the mandrel. The spindle 8, the mandrel 9 and the kernel 10 together form the core of the apparatus. The extrusion sleeve 4 and the spindle 8 define therebetween a ring nozzle 11 wherefrom the material to be moulded, e.g. a plastic substance, is fed into a moulding space 12 defined between the extrusion sleeve 4 , the chill moulds 1 , 2, the spindle 8 and the mandrel 9. In order to obtain a pipe having a ribbed outer face the innersurface of the chill moulds is provided with mutually spaced ring-shaped grooves 13 into which the plastic material is forced for the formation of the ribs. The kernel 10 is formed by two successive parts: an initial zone 14 and an end zone 15. These zones are separated from each other by means of a peripheral groove 16 out of which a lubricant, e.g. air, can be fed on the surface of the end zone 15 of the kernel. Cooling means 17 (shown schematically in the drawing)

are provided within the end zone 15 for e.g. a liquid coolant. The cooling means enable the surface of the end zone 15 and, consequently, the inner face of the pipe moulded of the material to be cooled so that the pipe 7 keeps its shape when removed from the manu¬ facturing apparatus.

The diameter of the end zone 15 is slightly reduced from the groove 16 onwards so that the diameter D.. is about 1 per cent larger than the diameter D„. So this tapering is very gentle, although it has been exaggerated in the drawing for the sake of clarity.

According to the invention the initial zone 14 of the kernel, which zone is positioned between the mandrel 9 and the groove 16, enlarges slightly towards the groove 16 and is provided with heating means.

The diameter of the initial zone 14 increases evenly from the diameter D_, which is the diameter of the initial zone adjacent the mandrel 9, to the dia¬ meter D,, which is the diameter of the initial zone at the groove 16, so that the diameter D ₄ is about 1 to 2 per cent larger than the diameter D ₃ - Although this enlargement is very small it has proved to be of great importance in practice. The initial zone is further provided with heating means extending over the whole length of the zone, which means are shown schematically in the drawing and indicated therein by the reference numeral 18.

The length of the initial zone is 10 to 100 per cent, preferably about 50 per cent, of the outer dia- meter of the pipe, and the length of the end zone 15 is 50 to 200 per cent of the outer diameter of the pipe.

The chill moulds 1 , 2 are provided with cooling means (not shown) for the cooling of the outer face of the pipe. The apparatus shown in the drawing operates in the following way. A pressurized material to be moulded.

such as a plastic substance, is fed from the nozzle 11 between the extrusion sleeve 4 and the spindle 8 into the moulding space 12, in which it is forced outwards by the action of the conically enlarging mandrel 9 so that it fills the grooves 13 of the chill moulds and the space defined between the chill moulds and the core of the apparatus. The material which has been forced into the grooves 13 forms the ribs of the pipe, and the material which remains between the chill moulds and the core forms the wall of the pipe.

When the material makes contact with the chill moulds 1, 2, it begins to cool, while those portions of the material which are positioned closest to the kernel are maintained in a plasticized state since the initial zone 14 of the kernel is heated by said means 18. By virtue of the heating of the material and the enlarging diameter of the initial zone 14, the material fills properly all the grooves 13 and becomes very homoge¬ neous in the area of the initial zone, on account of which the inner face of the pipe, too, becomes very smooth.

After the pipe has passed by the groove 16, it is began to be cooled also on the inside by means of the cooling means 17, whereby the material is stiffened to such an extent that it keeps its shape when it is removed from the apparatus. The pipe is contracted during the cooling step, and the end zone of the kernel tapers slightly towards the end thereof in order to prevent the pipe from adhering to the kernel. For the same reason, a lubricant is fed out of the groove 16 on the surface of the end zone.

As distinct from the above-described, the heating means 18 may extend merely over a part of the length of the initial zone and the groove 16 can, of course, be left out if there is no need to feed a

lubricant on the surface of the kernel. It is further to be noted that the inclination of the surface of the initial zone 14 has been exaggerated to some extent in the drawing for the sake of clarity.