GB2171167A

GB2171167A - Manufacture of thermoplastic pipe

Info

Publication number: GB2171167A
Application number: GB08602270A
Authority: GB
Inventors: Eric Bridgstock
Original assignee: Fusion Plastics Ltd
Current assignee: Fusion Plastics Ltd
Priority date: 1985-01-31
Filing date: 1986-01-30
Publication date: 1986-08-20
Also published as: GB8602270D0; GB8502439D0

Abstract

In the manufacture of thermoplastic pipe, e.g. for gas and water supply lines, a pipe extrusion (P) is produced by an extruder (1), and a layer of the extruded material is removed from the outer surface of the pipe by a scraping unit (5) before the pipe is cut off into lengths. The finished pipe lengths do not require further scraping before making welded pipe joints. <IMAGE>

Description

SPECIFICATION Manufacture of thermoplastic pipe This invention is concerned with the manufacture of pipes of thermoplastic material, such as polyethylene.

Extruded thermoplastic pipe is used in the construction of gas and water supply systems and welded pipe joints are frequently used for connecting pipe lengths or connecting branch pipes to a main pipe. For making the welded joints so-called electro-fusion fittings may be employed. An electrofusion fitting generally comprises a body of thermoplastic material capable of fusing with the material of the pipe to which it is to be joined, and a winding of electrical resistance heating wire incorporated in the fitting adjacent the surface thereof which is to contact and become welded to the pipe. Heat generated by an electric current passing through the winding softens the material of the fitting and the material at the outer surface of the pipe so that they fuse together and, upon cooling a secure welded joint is obtained.Electro-fusion fittings may take different forms according to the type of joint which is to be completed. For example, for joining lengths of pipe end-to-end the fitting can be a sleeve into the opposite ends of which the pipe ends to be joined are fitted. Alternatively, for making branch connections to a main pipeline, the fitting may consist of a generally semi-cylindrical member which is clamped against the pipe during the welding process.

Hitherto, when making pipe joints using electrofusion techniques it has been the normal practice to remove by scraping an outer surface layer of the pipe in the vicinity of the required joint as a preliminary step before positioning the fitting onto the pipe. The scraping of the pipe immediately before making a joint has been considered essential to obtain a good strong weld between the pipe and fitting, but it has also constituted a serious drawback of electro-fusion jointing methods since it must be performed in the field at the site of the joint being made and it is difficult to ensure that the pipes will be prepared properly to ensure good quality welds.

It has been the generally accepted view that the need to scrape the pipe is due to an 'oxidised' surface layer which inhibits the fusion process, so that scraping the pipe just before making a joint is unavoidable if a good secure weld is to be obtained.

It is with great surprise that we have now found by experiment that the time at which the surface layer of the fitting is removed is unimportant and does not affect the weld strength ultimately achieved. Thus, the layer removal operation can be performed by machining befoe the pipe is delivered to the installation site, and most conveniently in the factory when the pipe is manufactured so that no preparatory scraping or machining operations have to be carried out in the field at the jointing site.

The present invention therefore resides a pipe comprising an extrusion of thermoplastic material having had a layer of the extruded material removed from the entire outer surface thereof.

Also provided by the invention is a method of manufacturing thermoplastic pipe, comprising the steps of extruding thermoplastic material to form the pipe, and removing a layer of the extruded material from the outer surface of the pipe over its entire length.

Most conveniently the surface layer is removed by continuously machining the extruded pipe before it is cut off into lengths. If preferred, however, the cut off lengths of extruded pipe could be machined by feeding them through a suitable device.

It has been found that a weld obtained when the surface layer of the pipe has been removed at the time of manufacture has equal strength to welds obtained when the pipe is only scraped immediately prior to making the joint, and the interval between manufacture and making a joint with the pipe does not affect the weld strength. Preparing the pipe in the factory is more convenient since it can be carried out under controlled conditions so that the strength of the welds ultimately obtained is not dependent upon the proper scraping of the pipe at the installation site, and uncertainly about weld quality is removed.

By removing the entire outer surface layer of the pipe it is ensured that the pipe is properly prepared whatever the position of a joint eventually required along its length.

The success of the invention implies that the fusion inhibiting surface layer is not formed by oxidation, but is instead an effect produced as a result of the high temperatures and pressures involved in the extrusion process.

It is thought that this effect is probably a molecular orientation resulting in a surface energy which is insufficient to promote fusion. When the inhibiting surface layer, which is probably only a few molecules thick, is removed the new surface layer includes unpaired molecular sites and has the energy needed to initiate fusion. Therefore, to achieve satisfactory weld strength it is only necessark to remove a surface layer from the pipe and this can be done at any convenient time after is has been extruded. An especially convenient method of removing the surface layer is by feeding the pipe longitudinally through a device having either stationary scraping blades or scraping blades which are rotated around the pipe. Alternatively the layer may be pared away with a peeling action, e.g. using a cutting tool having a blade which is rotated around the pipe.

In accordance with the invention there is provided also an apparatus for manufacturing thermoplastic pipe comprising an extruderfor producing an extruded pipe, a cutting device for cutting the extruded pipe into lengths, and means located between the extruder and the cutting device for removing a layer of material from the entire outer surface of the pipe as the pipe advances to the cutting device from the extruder.

Illustrated schematically in the accompanying drawing is an extrusion plant suitable for manufacturing pipe according the invention. The screw extruder 1 extrudes the pipe Pthrough an extrusion die 2, after which the pipe passes through a cooler 3 to harden the thermoplastic material and then to a haul-off device 4. The continuously advancing pipe then enters a scraping unit 5 provided in accordance with the invention and adapted to scrape the pipe over its entire circumference so that a surface layer is removed from the outer surface of the pipe. The scraper 5 could include an iris-type blade which is adjustable to the size of the pipe, or alternatively may comprise a set of blades arranged to scrape respective portions of the pipe periphery, for example two pairs of blades spaced apart in the direction of pipe travel and the blades of each pair being diametrically opposite with respect to the pipe. After scraping and possibly marking, if required, the pipe is delivered to a saw 6 which cuts it off at the required lengths.

Other scraper constructions besides those mentioned above are also possible, such as a device having one or more blades which are rotated around the pipe as it is passed through the device. In place of the scraping unit 5 there could be used a paring device for cutting or peeling away the pipe surface.

Claims

1. A pipe comprising an extrusion of thermoplastic material having had a layer of the extruded material removed from the entire outer surface thereof.

2. A pipe according to claim 1 and substantially as herein described.

3. A method of manufacturing thermoplastic pipe, comprising the steps of extruding thermoplastic material to form the pipe, and removing a layer of the extruded material from the outer surface of the pipe over its entire length.

4. A method according to claim 3, wherein the outer layer is removed from the extruded pipe before the pipe is cut off into pipe lengths.

5. A method according the claim 3 or 4, wherein the outer layer is removed by scraping or paring the layer from the extruded pipe.

6. A method according to claim 5, wherein the pipe is scraped by feeding it longitudinally through a scraping device having stationary scraper blade means.

7. A method according to claim 5 wherein the pipe is fed longitudinally through a scraping device or paring device having blade means rotated about the pipe.

8. A method of manufacturing thermoplastic pipe according the claim 3 and substantially as herein described.

9. An apparatus for manufacturing thermoplastic pipe comprising an extruder for producing an extruded pipe, a cutting device for cutting the extruded pipe into lengths, and means located between the extruder and the cutting device for removing a layer of material from the entire outer surface of the pipe as the pipe advances to the cutting device from the extruder.

10. An apparatus according to claim 9, wherein said means for removing the outer surface layer comprises a device having blade means for acting on the pipe to remove the layer with a scraping or paring action.

11. An apparatus for manufacturing thermoplastic pipe, substantially as herein described with reference to the accompanying drawing.