GB2182603A

GB2182603A - Transversely expanding and calibrating an extruded hollow product

Info

Publication number: GB2182603A
Application number: GB08626100A
Authority: GB
Inventors: David John Hill; Keith Andrew Wilson
Original assignee: Victaulic Co PLC
Current assignee: Victaulic Co PLC
Priority date: 1985-11-06
Filing date: 1986-10-31
Publication date: 1987-05-20
Anticipated expiration: 2006-10-31
Also published as: GB2182603B; DE3636960A1; DE3636960C2; GB8527300D0; GB8626100D0

Abstract

Plasticated thermoplastics or rubber material is extruded through an annular die (2) to form an elongate hollow product (7) which is then subjected to a pressure difference across its side walls whilst still soft and thereby transversely expanded, prior to being calibrated and cooled in its expanded configuration during passage through a perforated sleeve (6), within a vacuum tank (8). The pressure difference may be effected by internal pressure admitted through a die passage (4) and/or vacuum applied externally within an enclosure between the die and the perforated sleeve. The transverse expansion produces transverse orientation in the product wall, and increased strength arising from incorporated reinforcement e.g. glass fibres, or from the material being a self reinforcing polymer, such as a liquid crystal polymer. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the extrusion of products This invention relates to the extrusion of products.

More particularly, the invention relates to the extrusion of products in a hollow form, which may be retained as such as pipes ortubes or may be slit long itudinallytoformchannelsorstripshaped products.

Such products may be formed ofthermoplastics materials or rubber, including synthetic rubber, wherein the material from which the product to be extruded is melted or plasticated in a screw mixing cylinder or ram pressure cylinder and forced through an annular die of the required sectional con#figuration ofthe hollow form of the product. The basically crude shape so extruded must be calibrated and cooled to give a formed extrudate of the required dimensions.

Commonly, with pipes and tubes for example, this has hitherto been done by means of either pressure calibration or vacuum calibration. Typically, with pressure calibration the pipe, after extrusion from the mandrel, is immediately passed through a cool ingforming boxthroughwhich coldwatercirculates.

Calibration and heattransferareaided bytheapplication of an internal pressure within the pipe which, in practice, is maintained by sealing the advancing pipe end or by use of a sliding internal bung within the advancing pipe connected to the mandrel by a cable.

Similarly, typical vacuum calibration is by passing the pipe direct from the forming mandrel into a perforated sizing sleeve or set of sizing rings or plates around which a vacuum is applied. The bore ofthe pipe remains open to atmospheric pressure thus providing, across the pipe wall, a pressure difference during cooling.

It has been proposed to reinforce and strengthen such extruded products by means of reinforcing fibres, of glass for example, mixed with the product material.

In products produced by such extrusion method the fibre reinforcement will, due to the polymerflow path in the die lying along the length ofthe pipe, have a high proportion of fibres aligned along the length of pipe or hose. In a pipeline, for example, under pressure from within, or being subjected to high soil loading characteristics or factors from without, the high stresses developed in the pipe will most sig nificantly be in the circumferential or hooped direction.

Fibres aligned parallel to this direction Le. down the length of the pipe or hose contribute nothing to the circumferential strength of pipe or hose and all the hoop stresses are carried by the plastics or rubber matrix.

It is an object of the present invention to overcome, or at least substantially reduce, the above mentioned difficulty.

In accordance with one aspect of the present invention there is provided a method of producing an extruded product comprising the steps of melting or plasticating the material of the product and forcing it through an annular die to form an elongate hollow product, wherein the hollow product, after passing through the die, is subjected to a pressure difference across its side walls whilst soft and thereby expanding the hollow product transverse to the axis of extrusion and calibrating and cooling the product in its expanded configuration.

The expanded product may be calibrated and cooled by means of a vacuum sizing sleeve for vacuum calibration or by a former box for pressure calibration.

In accordancewithanotheraspectofthepresent invention there is provided apparatus for producing an extruded product comprising means for melting or plasticating the material of the product; an an nulardiethroughwhichthematerial is forced, to form an elongate hollow product; means for applying a pressure difference across the sidewalls ofthe productaftertheforming die to expand the product transverse to the axis of extrusion; and meansfor calibrating and cooling the product in its expanded configuration.

The material ofthe product itself may, for example, bethermoplastics material or natural or synthetic rubber. The material from which the product is formed may be reinforced with fibrous material, such as glass fibre, or be a self-reinforcing polymer such as a liquid crystal polymer.

It has been foundthatthe lateral expansion ofthe hollow product as it emerges from the mandrel and die provides, in practice, a considerable fibre reinforcement orientation in the hoop direction of the productthereby increasing the hoop or lateral strength of the product.

In orderthatthe invention may be more readily understood, one embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure lisa diagrammatic sectional elevation of apparatusforforming a tube or pipe according to the invention; Figure2 is a variation on the apparatus on Figure 1; Figure 3 is a further variation on the apparatus of Figure 1.

Figure 4 is an alternative arrangement of part ofthe apparatus of Figure 2; and Figure 5shows a vacuum control for use with the apparatus of Figures 2 or4.

Referring now to Figure 1, it will be seen that a soft tube of plastics material 1 (e.g. medium density polyethylene) is extruded from a die body 2 about an internal mandrel 3. Air is passed through a passage 4 in the mandrel so as to provide within the tube after extrusion an internal air pressure P. To maintain the air pressure, the tube end may, in a manner not shown, be crimped over, or an internal plug may be provided within the tube connected to the mandrel in the previously mentioned manner.After leaving the mandrel and diethestillsoftpipeexpands in a "bubble" 7 due to the# pressure difference across its sidewall, passesthrough a sizing sleeve 5 and then entersa perforated sleeve 6 in a vacuum spray tank 8 where the expanded dimensions of the pipe are retained whilst it is cooled. It is to be noted that the external diameter of the tube increases from Dl to D2 whilst the wall thickness may changefrom W1 to W2 in the formed pipe.It has been found thatthe shape ofthe melt"bubble"7forexpandedportionofsofttube can be controlled by varying the distance between thedie3andthevacuumspraytank8.

As previously stated, it has been found that this technique increases the orientation of fibre within the plastics in a hoop direction. There are, of course, a number of factors influencing the degree of fibre orientation in the hoop direction, these including the ratio of D1 to D2 and the ratio of W1 to W2 as well as the ratio ofthe meltvelocityinthe die and the formed pipe velocity as it is withdrawn, the shape of the expansion "bubble" of the still soft pipe, and the die swell ofthe polymer melt.

Whilst the invention described in relation to Figure 1 has proved successful, in some circumstances the use of an above atmospheric pressure within the pipe might possibly cause difficulties in association with a vacuum calibration technique, such as poor stability ofthe melt "bubble".

This potential slight problem is overcome with the embodiment of Figure 2.

With regard to Figure 2, it is to be seen that, in addition to the apparatus illustrated in Figure 1, an additional vacuum chamber 29 is fitted around the space between the die 11 and mandrel 12, and the end of the vacuum spray tank 20. In this instance, the bore of the formed pipe is left open to the atmosphere and the additional vacuum applied to the still soft pipe as it emerges from the mandrel acts in place ofthe air pressure within the pipe to create the expansion "bubble" 22.

The plate 14, separated from the die 1 1 and the mandrel 12 by a p.t.f.e. ring 13, is fitted with a rubber seal 16, allowing movement between the end plate and sizing sleeve 19. In this way the vacuum tank20 may be withdrawn allowing access to the die outlet to facilitate ease of starting extrusion, or it may be moved during running to allow adjustment ofthe bubble size in addition to varying the vacuum level.

The additional vacuum chamber is completed by perspextube 17. Vacuum and manometer inlets 15 and 21 are provided.

Figure 4shows an alternative embodiment of rubber seal 30 attached to the end plate 14. This arrangement allows greater capability of sealing under slight misalignment of the extrusion die 11, end plate and the additional vacuum chamber. In each case, the rubber seal enables an effective vacuum to be maintained within the additional vacuum chamber, whilst enabling adjustment in its effective overall internal length.

In Figure3,theadditionalvacuumchamberdis- posed between the die body and the vacuum spray tank is replaced by a forming block 10 manufactured from an appropriate material such as perforated polytetrachlorethylene or ceramic material. As can be seen,the forming block has an internal configuration 11 corresponding to the desired outside configuration of the expansion "bubble" 7 of the pipe on leaving the mandrel.Theforming block maybe drilled from solid material or constructed from a porous material so that, in either event, a vacuum can be applied to the outside periphery thereof so asto applyto the stili warm, justformed pipe, an app ropriatevacuumtodrawitagainsttheinternal shaped surface 11 of the block 10.With such an arrangementthe internal profile ofthe block can beso shaped as to give optimum orientation of the fibre reinforcement in the plastics material of the pipe for both longitudinal and hoop strength.

It is to be notedthat in this embodiment alsothe bore of the pipe is open to atmospheric pressure, although, alternatively, it can be closed and under positive pressure.

Figure 5 shows a simple vacuum control circuit which may be used in combination with the additi onalvacuumchambertogivefineadjustmentofthe bubble size. It comprises a vacuum pump 31 connected via line 32 (having a bleed valve 33) to additional vacuum chamber29.Amanometer34isalsoconnec- ted,via an isolation valve 35, to chamber 29.

The process of controlling fibre orientation hereinabove described in relation to pipe and tube may be applied to other profiles that are not of circular cross section as in pipes and hoses. Thus, oval or re ctangularsectionscan be similarly treated. Again, the possibility of producing open sections with controlled fibre orientation by slitting and opening profilescan be provided. Bywayofexamplesemicircular and U channels, or even flat sheet, can be produced.

It isto be noted that standard extrusion dies can be utilised. The method is applicable to a wide range of pipe extrusion materials.

Claims

1. Apparatus for producing an extruded product comprising meansformelting or plasticating the material ofthe product; an annulardiethrough which the material is forced, to form an elongate hollow product; means for applying a pressure difference across the side walls of the product afterthe forming dieto expand the producttransversetothe axis of extrusion; and means for calibrating and cooling the product in its expanded configuration.

2. Apparatus as claimed in claim 1 wherein the meansforapplying a pressure difference across the side walls of the product includes means for increas- ing the pressure within the product after it leaves the die.

3. Apparatus as claimed in claim 1 or 2 wherein the means for applying a pressure difference across the side walls of the product includes a vacuum chamberthrough which the product is arranged to pass after it leaves the die.

4. Apparatus as claimed in claim 3wherein an additional vacuum chamber is provided surrounding the product exit from the die.

5. A method of producing an extruded product comprising the steps of melting or plasticating the material ofthe product and forcing it th rough an an nulardietoform an elongate hollow product, wherein the hollow product, after passing through the die, is subjected to a pressure difference across its sidewalls whilst soft and thereby expanding the hollow product transverse to the axis of extrusion and calibrating and cooling the product in its expanded configuration.

6. A method as claimed in claim 5 including increasing the pressurewithinthe productto expand it.

7. A method as claimed in claims 5 or 6 including applying a vacuum around the product to expand it.

8. A method as claimed in claim 7 including applying a vacuum around the product as it leaves the die to expand it.

9. A method as claimed in any one of claims 5 to 8 wherein the expanded product is calibrated and cooled by means of a vacuum sizing sleeve or by means of a pressure former box.

10. A method as claimed in anyone of claims 5to 9 wherein the product is a thermoplastics material or natural or synthetic rubber reinforced with fibrous material.

11. A method as claimed in anyone of claims 5to 9 wherein the product is a self-reinforcing polymer.

12. Apparatusforproducing an extruded product substantially as shown and as hereinbefore described with reference to the accompanying drawings.

13. A method of producing an extruded product substantially as hereinbefore described with reference to the accompanying drawings.