GB2104436A - Forming a connecting socket on a pipe of thermoplastic material - Google Patents

Abstract

A method of integrally forming a connecting socket, having an internal groove, on the end portion of a pipe of thermoplastic synthetic material is described wherein the end portion is heated to shaping temperature, is then deformed to increase its wall-thickness until the latter fills an annular gap in a heatable upsetting apparatus, and thereafter is shaped to form the connecting socket during axial displacement in a forming device, and is finally allowed to cool. The method is characterized in that the heating up and preliminary deformation of the end portion proceed from the zone of the free end-face of the end portion and by the use of a relatively narrow shaping- temperature range just below the crystallite- melting temperature of the crystalline or partially crystalline synthetic material, in that the heated-up and preliminary deformed end portion is removed from the apparatus for carrying out preliminary deformation, and in that during the forming of the connecting socket in the forming means, maintained at shaping temperature, the end portion is finally deformed to compensate for reductions in wall thickness that have occurred during expanding.

Description

SPECIFICATION Method of forming a connecting socket in a pipe of thermoplastic material This invention relates to a method of forming a connecting socket on the end portion of a pipe of thermoplastic material. In particular, the invention relates to a method of integrally forming a connecting socket, having an internal groove, on the end portion of a pipe of thermoplastic synthetic material, wherein the end portion is heated to shaping temperataure, is then upset to increase its wallthickness until the latter fills an annular gap in a heatable upsetting apparatus, and thereafter is shaped to form the connecting socket during axial displacement in a forming device, and is finally allowed to cool.

Many methods and apparatuses for carrying out such operations for the integral shaping of sockets are known in the prior art. In this context, reference may be made to German Patent Specification 1,257,413, in which is described an apparatus for providing hollow bodies of synthetic material with socket ends, an annular groove and thickened walls.

The ends of the pipes treated in this apparatus are preheated for shaping in, for example, what are called annular-gap furnaces, and thereafter their final shape is imparted to them in a shaping apparatus. When imparting the final shape, use can be made of an upsetting force whereby the reductions in wall thickness occurring during shaping are made good by pushing the heated material of the pipe wall back into position so as to achieve the original wall-thickness.

Furthermore, German Auslegeschrift 23 19398 discloses a method and apparatus for integrally forming a socket, having an annular groove, on an end portion of a pipe of thermoplastic synthetic material, wherein thickening is achieved by upsetting at a temperature above the softening temperature of the synthetic material. The upsetting of the end portion proceeds continuously from a zone, remote from the end-face of the end portion, towards this end-face. The upsetting process is continued until an annular gap, present between the mandrel and a die of the apparatus concerned, is filled by the softened synthetic material of the pipe wall and the thickened portion of the socket wall in this zone, this filling of the annular gap proceeding from the insertion opening towards the base of the annular gap.

In the known method, the upsetting operation is followed by the forming of the socket by pushing the upset end portion on to an expanding mandrel which is at a lower temperature than that used for shaping the synthetic material. In this way a certain degree of stability in the peripheral zones of the surface of the socket is already achieved during the process of forming the socket which stabilization is further increased by holding the inner wall of the die at a temperature below the shaping temperature of the synthetic material during the process of forming the socket.

The annular peripheral groove is then pressed in the socketed end portion, partly stabilized in this manner, expanding elements associated with the socket-forming apparatus being used for this purpose.

Thus, in the known method, the formation of the connecting socket from the thickened, upset end portion is achieved by axial displacement of the end portion on the expanding mandrel and by subsequently pressing out the annular peripheral groove by means of known expanding elements. In this system, it is important that the thickening operation as well as the shaping operation should be carried out by means of axial and radial displacements achieved in one and the same apparatus. The thickening of the wall must be completed before the expanding operation is begun.However, during the expanding operation itself - because of the need for pushing material on to the expanding mandrel work-hardening of the wall of the socket occurs, and the cross-section of the wall in the zone of the socket is further reduced when the annular groove is formed by pressing, and it has not been possible to eliminate this disadvantage by further upsetting.

Finally, German Offenlegungsschrift 28 05 518 discloses apparatus for forming seats for seals in socket connections for pipes of synthetic material, and this specification is concerned with the problem of establishing fit tolerances in the interior of the socket and in particular for that part of the socket that is to accommodate the seal. In addition to not dealing with the specific form of the shaping ring for forming the annular groove, this publication makes no particular reference to the problem of upsetting orto possible difficulties in expanding the cylindrical socket part.

By means of the methods and equipment known in the prior art, pipes made of synthetic materials having an amorphous structure, such as hard-quality PVC for example, can be readily processed, since these synthetic materials can be efficiently worked because of their property of softening progressively with rise in temperature in the range 90' to 140 C while heat is applied to a uniformly increasing extent. The material can be hot-shaped between the stated temperature limits, and the forces for achieving shaping are dependent upon the particular temperature of the pipe.

A comparable deformation of pipes made of crystalline or part-crystalline thermoplastic materials, such as polyethylene, polypropylene, etc., is not however possible by the known methods of processing. In contrast to the amorphous synthetic materials, these substances do not continue to soften during heating, so that distortions occur in the radial and axial directions. Pipe ends distorted in this way cannot, however, be used for further processing.

Although US-PS 3,899,565 discloses the heating and upsetting of the end portion of a pipe made of crystalline synthetic material, e.g. polyethylene, upsetting is carried out in one operation, and, because of the subsequent expansion of the sockets and the forming of the peripheral groove, the material of the upset wall of the pipe is weakened again in these zones, so that considerable reduction of the wall thickness occurs, particularly in the sensitive zone of the peripheral groove, as a result of the increase in circumference occurring during expansion, and this reduction in wall-thickness may easily lead to fracture in this zone of the pipe which is the one that is particularly subjected to stress.

It is in this connection that use is made of the present invention, which seeks to provide a method whereby pipes of thermoplastic synthetic materials of crystalline or partially cyrstalline structure can also be provided with a socket, comprising a peripheral groove, without the disadvantageous creation of the above-described special circumstances during the working of such materials.

According to the invention there is provided a method of integrally forming a connecting socket, having an internal groove, on the end portion of a pipe of thermoplastic synthetic material, wherein the end portion is heated to shaping temperature, is then upset to increase its wall-thickness until the latter fills an annular gap in a heatable upsetting apparatus, and thereafter is shaped to form the connecting socket during axial displacement in a dorming device, and is finally allowed to cool, said method being characterized in that the heating up and preliminary upsetting of the end portion proceed from the zone of the free end-face of the end portion and by the use of a relatively narrow shaping-temperature range just below the crystallite-melting temperature of the crystalline or partially crystalline synthetic material, in that the heated-up and preliminary upset end portion is removed from the apparatus for carrying out preliminary upsetting, and in that during the forming of the connecting socket in the forming means, maintained at shaping temperature, the end portion is finally upset to make good reductions in wall thickness that have occurred during expanding.

Thus, in a first step of the method, uniform thorough heating of the end portion is carried out until the melting temperature of the crystallites of the thermoplastic material is almost reached, during which step the end portion is subjected to preliminary upsetting by the application of a first axial upsetting force which continues until the annular space in the heating means is filled and, in a second step of the method that is taken immediately thereafter and while the working temperature is maintained, the preliminary upset end portion is brought to its final shape in a shaping means with the aid of a second axial upsetting force.

The method proceeds in two consecutive steps, namely the raising of the end of the pipe to shaping temperature accompanied by thickening of the wall by preliminary upsetting, and subsequent expanding of the socket, comprising the peripheral groove, by final upsetting. The end of the pipe is brought to shaping temperature in a three-sided annular-gap furnace known perse which is provided with an upsetting means. During heating to the shaping temperature, care has to be taken to break up, as far as possible, the crystallite constituent of the materials used, by the application of heat. By reducing the crystallite content of the materials to be processed, they are rendered softer and therefore easier to work.An advantage of this procedure is that, at each pipe temperature below the melting point of the crystallites, a constant crystallite content is set up in the material, and this content remains uniform when the temperature in question is maintained, i.e. it cannot be varied when the heating time is extended.

When the crystalline contents of these materials are converted into the amorphous or partially amorphous condition, an increase in volume, i.e. swelling of the material, occurs. Maximum swelling is achieved when the melting temperature of the crystallites is reached or exceeded. Problems encountered in the heating of pipes of crystalline or partially crystalline synthetic materials arise, for example, from differences in wall-thickness and from uneven breakdown of the crystallites in the cross-section of the pipe because of the way in which the pipe is heated. This results in uneven swellings, which are further increased by the release of internal stresses in the pipe.

Contrary to this, the present method, even when crystalline or partially crystalline thermoplastic materials are used, results in a pipe-end, which has been brought to shaping temperature, acquiring specific dimensions including that of the diameter provided by the final shaping. In the first step used in the method and by means of the breakdown of the crystallites caused by heating, the entire crosssection of the annular gap in the heating equipment is completely filled as a result of the swelling of the pipe wall that occurs and of the first upsetting force applied at the same time, so that a pipe-end of precise shape is obtained for the shaping operation.

Because of the swelling, due to heat, of the pipe wall in the cross-section of the annular gap of the furnace and because of the forward movement occurring during upsetting, the contact between the outer and inner walls of the pipe, on the one hand, and the surface of the furnace, on the other, is improved, so that as the result of the contacts on all faces, very even heating of the cross-section of the pipe wall takes place continuously until the heating-up process is completed. Thus, a very uniform arrangement of the residual crystallites in the cross-section of the pipe wall can be achieved, and this of considerable importance as regards uniform expansion of the end contour, comprising the socket and the peripheral groove, at the extremity of the pipe.

The stresses present in the end of the pipe are relieved to a large extent by the selection of the correct temperature for heating up to the vicinity of the crystallite melting temperature of the crystalline or partially crystalline material used and by the temperature-controlled swelling, combined with mechanical upsetting in the optimum temperature range, so that the shape of the heated, thickened, pipe end remains sufficiently stable after its removal from the heating means and until it is introduced into the device for carrying out the final shaping.

It has already been mentioned that, presently described method, the preliminary shaping of the material in the heating means and the final shaping in the socket-forming station are carried out at a temperature close to that at which the crystallites of the material melt. In the case of hard-quality polyethylene, the preferred temperature is 130 C for example. However, depending upon the types of material used, hard-quality polyethylene has a crys tallite melting point which varies between 124 and 133 C. When using the present method, it is impor tant that the working temperature be just below the crystallite-melting temperature of the material con cerned.

On the other hand, tin the case of polypropylene, the crystallite-melting temperature lies between 160' and 165"C, depending upon the type of material used. In the case of polybutene, this temperature is lower than in the case of polyethylene, i.e. below 124 C. It has also to be borne in mind when performing the present method that the temperature range for the hot-shaping of crystalline or partially crystalline materials is relatively narrow compared with that of amorphous materials such as hard quality PVC. Experience has shown that a possible tolerance range of aproximately 5 C is available here. This calls for maximum accuracy in the regulation of the heating means.A result of this limitation is that, when heating crystalline or partial ly crystalline materials, the furnace temperature should not be appreciably higher than the tempera ture required for shaping the pipe (overheating being possible in the case of hard-quality PVC, for example) since otherwise an excessively steep gra dient in the crystallite break-down occurs between the inner and outer peripheral surfaces of the pipe on the one hand, and the interior of the pipe wall, on the other. The outer and inner peripheral surfaces of the pipe wall, which are more or less in direct contact with the surfaces of the furnace, would become plastic too rapidly in this case, and this would lead to displacement of surface zones relatively to internal zones of the pipe wall and thus would result in loss of strength in the zone of the socket.

Thus, when carrying out the first step in the present method the temperature gradient during the heating-up phase must be carefully selected to bring it into line with the insertion of the pipe end and the force used for upsetting. In this connection, it should be borne in mind that the pipe end, introduced into the annular gap in the heating means at the above mentioned temperatues, first begins to swell at the end face under the effect of heat and, from the end face towards the inlet end, bears continuously against the core of the furnace and the outer sleeve.

The controlled force used in advancing the pipe during the preliminary upsetting process results in the annular gap in the heating means being filled with material of the pipe wall in the outward direction as heating upoof the pipe ends proceeds, and this filling-up phase results both in equalization of the longitudinal contraction occurring during heating and in prevention of uncontrolled swelling of the material of the pipe wall.On completion of the heating-up phase, the pipe end lies in the annular gap of the heating means while at a uniform temperature that is below the melting temperature of the crystallites, and the swelling, that has occur red during the heating-up phase, in combination with the upsetting force, will have had the effect of applying the inner and outer peripheral surfaces of the pipe end to the periphery of the core of the heating means in just the same way as to the inner periphery of the outer sleeve. The core as well as the outer sleeve of the heating means will, of course, be at the same temperature necessary for transmitting the optimum shaping heat to the pipe end.

Filling up of the annular gap in the heating means with the material of the wall of the pipe results in a specific increase in the initial wall thickness of the pipe, since the internal dimension of the annular gap in the heating means must be at least slightly greater than the cross-sectional area of the pipe in order to enable the end of the pipe to be easily inserted into the annular gap. In one example, involving the processing of a pipe having a nominal diameter of 100 mm, its initial wall-thickness was 10 mm. During the preliminary upsetting phase, this wall-thickness was increased to 12.5 mm because of the size of the annular gap in the heating means.However, preliminary upsetting in accordance with the invention not only resulted in this increase in wall-thickness, but also in the maintenance of the length of the heated zone of the pipe end, required in the socketforming phase, with increase in wall-thickness in the stated order of magnitude over the entire circumference and length of the pipe end.

The heated preliminarily upset pipe-end, which in this case was at a temperature of approximately 130"C, was then removed from the annular-gap furnace in such a way that neither the contour nor the thickening of the wall, so achieved, were disadvantageously affected. Immediately thereafter, the pipe end was introduced into an upsetting device such as that described, for example, in German Patent Specification 1,257,413. It is very important that this upsetting device should be heated to the same temperature as that of the heated pipe end so as to avoid chilling of the heated periphery of the pipe and therefore unrequired work-hardening of the outer zones of the wall of the pipe.

After the heated pipe end has been brought to its final shape by the heated socket-forming means in which the socket, comprising a peripheral groove, is expanded by applying the second upsetting force, the finally upset pipe end is removed by opening the tool and pulling the expanded socket off the mandrel. Before opening the tool, it is possible to initiate an indirect cooling operation by cooling the surface of the finally upset socket by means of a cooling medium passed into the heating ducts of the tool.

This cooling operation is advantageously continued until the cooled inner and outer skin in the heated zone of the socket is thick enough to enable the shape of the socket to remain stable during its withdrawal from the mandrel.

Immediately after withdrawal of the pipe end with its integrally formed socket, it is recommended that direct further cooling be carried out, e.g. by spraying a cooling medium on to the heated inner and outer peripheral surfaces of the pipe end. This prevents the heat from the internal zone of the wall of the pipe from warming up the cooled peripheral zone again and causing contraction of the heated end zone of the pipe.

Claims (2)

1. A method of integrally forming a connecting socket, having an internal groove, on the end portion of a pipe of thermoplastic synthetic material, wherein the end portion is heated to shaping temperature, is then upset to increase its wall-thickness until the latter fills an annular gap in a heatable upsetting apparatus, and thereafter is shaped to form the connecting socket during axial displacement in a forming device, and is finally allowed to cool, said method being characterized in that the heating up and preliminary upsetting of the end portion proceed from the zone of the free end-face of the end portion and by the use of a relatively narrow shaping-temperature range just below the crystallite-melting temperature of the crystalline or partially crystalline synthetic material, in that the heated-up and preliminary upset end portion is removed from the apparatus for carrying out preliminary upsetting, and in that during the forming of the connecting socket in the forming means, maintained at shaping temperature, the end portion is finally upset to make good reductions in wall thickness that have occurred during expanding.

2. A method as claimed in claim 1 substantially as hereinbefore described.