This invention relates to an apparatus for manufacturing ribbed pipes with a smooth inner sur- face from a mouldable material, said apparatus com¬ prising a core including a shaft, a conically widen¬ ing mandrel located downstream of the shaft seen from the direction of production of the apparatus, and a kernel of a substantially uniform diameter disposed after the mandrel, an extrusion sleeve surrounding the core and forming with the core a nozzle for the material, the extrusion sleeve having an end face located upstream of the kernel seen in the direction of production of the apparatus, and moulds encasing the extrusion sleeve and the core and being movable along an endless path, having grooves on the inner surface for forming ribs on the pipe, whereby an ini¬ tial zone of the kernel which is in the vicinity of the nozzle has means for heating this zone and an end zone of the kernel which is remote from the nozzle has means for cooling this zone.

The production of closed-wall ribbed pipes having prominent and narrow ribs and a smooth inner surface has proved unexpectedly difficult. Prominent and narrow ribs would be the most advantageous in view of the use of the pipe. In order for the deep and narrow grooves in the inner surface of the moulds to be filled, high pressures must be used which place severe requirements on the endurance of the appar- atus. These problems are aggravated when a stiff plastic, such as unsoftened PVC, is used.

Another important problem relates to the qual¬ ity of the inner surface of the pipe. The smoothness of the inner surface is impaired e.g. by the follow- ing factors: air bubbles generated in the forming

space, i melt tears resulting from a high shearing rate, irregularities and cold seams resulting from the laminar structure of the material, the adhesion of the inner surface to the kernel, and depressions and a±r bubbles in the material resulting from the uneven cooling of the mass.

The publication WO 87/04113 discloses an appar¬ atus wherewith it is possible to manufacture ribbed •pipes of a relatively high quality. In this known apparatus the kernel is divided into two zones where¬ of an initial zone located closest to the nozzle is heated and an end zone located after it is cooled.

The reference discloses that the diameter of the initial zone of the kernel increases slightly in the direction of movement of the material. By heating the pipe in the region of the kernel and simul¬ taneously forcing the material toward the moulds, the material is caused to fill the grooves in the moulds simultaneously as the inner surface of the pipe is made fully smooth. This effect can probably be ex¬ plained by the fact that after the extrusion pressure has filled the grooves in the moulds in the region of the conical mandrel, the material begins to cool by the action of the moulds and thereby also to shrink. Imperfections due to different cooling rates and shrinkage of the material are prevented by generating a considerable afterpressure in the grooves of the moulds, on account of which the material to be formed closely follows the surfaces of the moulds and the kernel. This afterpressure is independent of the ex¬ trusion pressure and can therefore be easily regu¬ lated by varying the heating effect.

However, in practice it has been found that the inner surface of pipes manufactured by this known apparatus is not always smooth but contains irregu-

larities.

The object of this invention is to provide apparatus for manufacturing ribbed pipes, wherewith plastic pipes having a fully smooth inner surface can be produced. The apparatus of the invention is char¬ acterized in that the surface of the initial zone of the kernel is cylindrical and that the outset of the end zone of the kernel has an end face conically widening in the direction of production of the appar- atus the smallest diameter of said end face substan¬ tially corresponding to the diameter of the initial zone.

The invention is based on the realization that the uneven inner surface of the pipe is a result from the fact that the plastic material adheres to the heated and widening initial zone of the kernel, whereby shearing stresses are generated in the inner wall of the pipe which will make the surface uneven and which may produce cracks. Therefore, in accord- ance with the invention the heated initial zone of the kernel is cylindrical and not widening, whereby the plastic material will not adhere thereto and whereby no shearing stresses are generated. The afterpressure which is essentially important from the point of view of the smoothness of the inner surface of the pipe is generated by means of a conically widening end face disposed at the outset of the end zone. Since the end face is part of the cooled end zone, the plastic material will not adhere to it, wherefore no stresses are generated on the inner sur¬ face of the pipe.

By a kernel is meant in this specification the part of the core which has a substantially uniform diameter and by means of which the inner surface of the pipe is calibrated. Therefore, it is evident that

its diameter cannot vary very much. In known devices, the kernel has a uniform diameter or the diameter diminishes slightly towards the end, in order for the cooling and thereby contracting pipe not to adhere to the kernel. Thus it is preferred in the present invention that the difference between the smallest and largest diameter of the end face is 0.1% to 0.5%, preferably about 0.25%, of the diameter of the ker¬ nel. The difference between the diameters is prefer- ably about 0.5 mm when the diameter of the kernel is 200 mm.

The end face shall be rather steep in order for the inner surface of the pipe not to have time to become too stiff prior to the smoothing thereof with the end face. The angle between the end face and the central line of the core is thereby e.g. 2° to 45°, preferably about 20°.

In devices for manufacturing plastic pipes, the largest diameter of the kernel has to be slightly greater than the desired inner diameter of the pipe to be produced, since the pipe shrinks upon cooling at the kernel and thereafter. The difference between the largest diameter of the kernel and the inner dia¬ meter of the produced pipe can be calculated by means of the formulae for heat expansion used in the art. In accordance with the invention, it has been unex¬ pectedly found that by providing a cold end face according to the invention in the kernel, it is pos¬ sible to make the largest diameter of the kernel — which is simultaneously the largest diameter of the end face — larger than it should be according to the formulae used in the art. In other words, when the pipe is widened by means of the cold end face accord¬ ing to the invention, it will shrink after the end face more than could be expected according to the

formulae. This results in that the inner surface of the pipe can be compressed more than expected, on account of which the inner surface of the produced pipe is smoother than the inner surface of pipes man- ufactured by prior apparatus.

One advantageous embodiment of the invention is explained more closely in the following with refer¬ ence to the accompanying drawing, wherein

Figure 1 shows a schematic view of the appar- atus for manufacturing a ribbed pipe and

Figure 2 shows a detail of the apparatus on a larger scale in a longitudinal section.

The apparatus disclosed in Figure 1 comprises two moulds 1 and 2 moving along an endless path, which meet one another at guide rails 3 to form a cylindrical form. An extrusion sleeve 4 being in com¬ munication with the press end 6 of an extruder 5 ex¬ tends into said form. Figure 1 also shows how pro¬ duced pipe 7 protrudes from one end of the form cons- tituted by the moulds.

Figure 2 presents more closely those parts of the apparatus which take part in the forming of the pipe. On the central line of the apparatus, there is a shaft 8 within the extrusion sleeve 4, said shaft having a constant diameter. Downstream of the shaft there is a conically widening mandrel 9 which is lo¬ cated, at least for the most part, within the ex¬ trusion sleeve and after which a kernel 10 having a substantially constant diameter is provided. The shaft 8, the mandrel 9 and the kernel 10 together form the core of the apparatus. The extrusion sleeve 4 has an end face located upstream of the kernel 10 seen in the direction of production of the device. In the embodiment shown, the end face is located at the mandrel.

The extrusion sleeve 4 and the mandrel 9 form between them an annular nozzle 11 wherethrough the material to be moulded, e.g. plastic material, is fed into the moulding space 12 between the moulds 1, 2 and the kernel 10. To produce a pipe having ribs on the outer surface thereof, the inner surface of the moulds has annular grooves 13 at a distance from one another, whereinto the plastic material is pressed to form ribs. The kernel 10 comprises two parts in succes¬ sion, i.e. an initial zone 14 and an end zone 15. The zones are closely connected, and the transition sur¬ face between them has been indicated by the reference numeral 16. The mandrel 9 and the initial zone 14 of the kernel have heating means 17 schematically shown in the drawing, e.g. for a liquid heating medium, and the end zone 15 of the kernel has ducts 19 for a cooling medium. The moulds 1, 2 are conventionally cooled. In accordance with the invention, the surface of the initial zone 14 of the kernel 10 is cylind¬ rical, i.e. it has a constant diameter D _x , and the outset of the end zone 15 of the kernel has an end face 20 conically widening in the direction of pro- duction of the apparatus. The smallest diameter D ₂ of the end face equals the diameter of the initial zone D _x , the largest diameter D ₃ of the end face being slightly greater than D ₂ . For clarity, the difference between the diameters D ₂ and D ₃ has been exaggerated in Figure 2. In practice, the difference between these diameters is 0.1% to 0.5%, preferably about 0.25%, of the diameter of the kernel. The angle bet¬ ween the end face 20 and the central line of the core is 2° to 45°, preferably about 20°. The diameter D ₄ of the rear end of the kernel may be the same as D ₃ or

slightly smaller. The length of the end zone of the kernel is about 50% of the entire length of the ker¬ nel and is 200 to 1000 mm.

According to a preferred embodiment of the invention, the largest diameter D ₃ of the end face is larger than the calculated largest permissible dia¬ meter of the core at pipe forming temperature. The diameter D ₄ of the rear end of the core substantially corresponds to said calculated diameter at pipe out- let temperature.

Under a high pressure, PVC material is a very erosive substance. It was found in tests carried out that if the outset of the end zone of the kernel was not well cooled, strong abrasion took place in the outset. In one embodiment of the invention, said out¬ set is very effectively cooled on a distance of about 50 mm by means of a separate cooling spiral, and coated with hard chromium, nickel or an equivalent coating material having a hard wear surface. The remaining material of the mandrel is preferably stainless steel.

The apparatus shown in the drawing operates in the following manner. Pressurized material to be moulded, such as plastic material, is fed through the nozzle 11 between the extrusion sleeve 4 and the mandrel 9 into the moulding space 12 so that it fills up the grooves 13 in the moulds as well as the space between the moulds and the core of the apparatus. The material pressed into the grooves 13 will form the ribs on the pipe, and the material remaining between the moulds and the core will form the wall of the pipe.

As the material meets the moulds 1, 2, it begins to cool, but since the initial zone 14 of the kernel is heated with means 18, the parts of the

material which are clcaest to the kernel remain in a plastic state.

After the material has passed the transition surface 16 between the zones 14, 15, it encounters the effectively cooled and relatively steeply widen¬ ing end face 20 which presses the material still slightly outward. This will increase the afterpres¬ sure or terminal pressure in the material and compact and smooth the inner surface of the pipe. The pipe will thereafter advance along the cylindrical or decreasing-diameter surface of the end zone 15 simul¬ taneously as a cooling effect is applied thereto through the moulds and the kernel.

■ In the embodiment presented above, the initial zone and end zone of the kernel are separate material units which are joined with one another along a plane 16 wherein the smallest diameter of the end face 20 lies. The end face 20 may alternatively be located at a short distance from the surface 16.