The present invention relates to an assembly for extruding circular, cylindrical plastic piping, comprising an extruder with a feed screw for transporting and softening the mass, and an annular nozzle with a central mandrel^ which constitutes a circular, cylindrical, coaxial extension of the feed screw.

In the hitherto known extruders for producing plastic piping, the mandrel is secured centrally by means of spokes fixed in the outer mold section of the annular nozzle. The spokes thus divide the softened plastic, and even though the plastic flows together after having passed the spokes, axial flow lines will arise which can constitute a weakening of the pipe wall. This is a particulax drawback if the pipe is to be subjected to radially-directed pressure, for instance, an internal water pressure. Bursting of the pipes most often occurs along such weakened portions.

The purpose of this invention is to produce by extrusion a pipe having external, helical reinforcing ribs and a smooth interior. One has never before been able to produce such pipes by direct extrusion. Such pipes have previously been made in the form of corrugated pipes , a thin-walled pipe being extruded which thereafter is given a corrugated config ^¬ uration downstream of the extruder by means of pressure or

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vacuum. The corrugations will then be of the same configur both on the inside and outside of the pipe. Such pipes hav been used as so-called electrician's piping and, for example as drainage pipes. Such pipes, however, are poorly suited f transporting a medium, e.g., as waste pipes, because the resistance of the pipe is. much too great. It has hitherto n been possible to produce pipes with relatively thick walls immediately in extruders. Where there has been a need for p having external ribs and a smooth interior, it has previousl been necessary to extrude a corrugated pipe while at the sam time producing a smooth-walled pipe in a second extruder. T two pipes are then formed and welded together in a common to complex with the corrugated pipe lying outside the smooth pi Another method is to machine the outer surface of the pipe w cutting tools. Both of these methods are naturally costly a are suited only for certain types of plastics.

A massive pipe having external ribs and a smooth interior, however, confers great advantages in such applicat as, for instance, ground water drainage pipes (sewer pipes) other pipes which must withstand internal pressure. Because the pipe is provided with rein orcing ribs, one can obtain a large savings of plastics material and thus manufacture a le expensive product.

This is obtained in accordance with the invention by the assembly mentioned in the introduction to the specifi¬ cation, which is characterized in that the stationary outer mold section of the annular nozzle is provided with one or more parallel, helical grooves,_with the direction of the thread being in the direction of rotation of the screw and mandrel. It is a further feature of the invention, that the outside surface of the rotating mandrel is roughened or pro¬ vided with axial grooves or ribs over at least a portion of i length. By means of this feature, the mandrel or extension w more easily carry the plastic mass along and bring it into rotation. In order to obtain a satisfactorily smooth interio surface on the pipe, it is a feature of the invention that th outer portion of the mandrel is smooth.

A further feature of the invention is that the oute

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mold section is provided with lubricating means for lubrication of the sliding surface for the plastic. This is naturally needed primarily when the outer mold section has a threaded groove and the plastic is supposed to rotate during its passage through the nozzle. The lubrication can be effected in several differ¬ ent ways, as may readily be imagined; for instance, a portion of the nozzle's outer mold might be made of a porous material and the lubricant pressed through this material. The outer mold might also be made of massive metal, with lubrication bores being provided over all or a portion of the outer mold section.

A second embodiment of the invention in which the outer mold section is of a straight, circular, cylindrical con¬ figuration is characterized in that the rotating mandrel, is provided with helical grooves on its outside surface and that lubrication of the sliding surface is provided on the extension. With this modification, one can produce pipes having a smooth exterior surface and a ribbed interior. Such ^' pipes can be utilized in connection with a coupling system for pipes having external helical ribs, short lengths of a pipe with an internal ^' thread serving as a threaded collar for connecting the pipes. Such pipes could also be used, for instance, as vertical trunk pipes for the sewer lines in houses. Owing to the internal threaded rib, the water will flow along the pipe wall to a greater extent and there will be better airing of the pipes, and noise will be reduced.

The invention will be elucidated further hereinbelow with reference to the accompanying schematic illustrations, where ___

Figure 1 shows a cross section through an extruder according to the invention for the production of circular, cylindrical pipes with external ribs, and

Figure 2 shows an extruder in accordance with the invention for the production of pipes with internal ribs.

The extruder, as is conventional, has a feed screw 1 with a surrounding cylindrical housing 2. The annular nozzle of the extruder consists of a stationary outer section 3 ^* and a mandrel 4. The mandrel 4 is a direct extension of the feed screw 1 and has in Figure 1 the same diameter as the stem 5 of

the feed screw. In Figure 1, the outer mold section 3 in the annular nozzle has grooves 6 disposed along a helical line, or in other words, a threaded groove. The mold section 3 can be provided in the conventional manner (although this is not show in Figure 1) with a heating device at its first portion (to th left on the drawing1. At the middle portion 7, it can be pro¬ vided with a lubricant under pressure for lubricating the slid surface between the stationary mold section and the plastic. The last portion can be provided with water cooling 8.

The plastic 9 is transported and softened owing to heat and pressure in the feed screw 1,2. The softened plastic is given a rotary movement by the rotating mandrel 4 , and by suitably adapting the rate of feed and rotation, the plastic mass will be given a helical rotation such that it will be screwed out through the threaded outer mold section, producing a plastic pipe 10 with external ribs 11 and a smooth interior 1

Figure 2 shows a modification of the assembly in Figure 1. In this assembly, too, there is an extruder with a feed screw 1 and cylinder 2. Here, however, the outer mold section 3' does not have a threaded groove, but has a smooth mold surface 13. Conversely, the extension 4' is provided with a threaded groove 14. The plastic mass will in this case move rectilinearly out through the annular nozzle while the mandrel 4 ¹ is screwed through the mass, producing a pipe 15 with inter¬ nal ribs 16.