GB2186833A

GB2186833A - Pultrusion method

Info

Publication number: GB2186833A
Application number: GB08604245A
Authority: GB
Inventors: Stewart David Erskine Shaw
Original assignee: FIBERFORCE Ltd
Current assignee: FIBERFORCE Ltd
Priority date: 1986-02-20
Filing date: 1986-02-20
Publication date: 1987-08-26
Also published as: GB8604245D0

Abstract

The method comprises pultruding a resin impregnated bundle of fibres (10), for example optical fibres or copper wires, and then enclosing the pultruded bundle within a coating prior to heating (24) the coated bundle (22) to polymerise the resin. The coating is then stripped off leaving a smooth surface to the profile. The coating is preferably applied by continuously extruding thermoplastic material in a tube around the bundle, but may be applied as a wrapped and optionally heat shrunk plastics film. The bundle may be preheated (18) to a temperature below the resin polymerisation temperature, prior to extrusion of the tube from an extrusion head (20). <IMAGE>

Description

SPECIFICATION Method of pultrusion The invention relates to a method of pultrusion and to a pultruded profile.

One method of producing continuous profiles or rods using fibres and resin is commonly called pultrusion since it is a form of extrusion process where the fibres are pulled.

In the pultrusion process fibres, which are usually glass, are first impregnated with a resin mixture containing a suitable catalyst and curing agent, are pulled through suitably sized bushings, and finally pass to a heating curing section where the resin polymerises to form a solid material.

The presently known types of heated curing section can be divided up essentially into two classes, open surface and enclosed surface. In the former, the resin-coated fibres are left exposed to the atmosphere as they pass through a heated oven; if necessary a number of guide bushes can be used to maintain the integrity of the fibres as they pass through.

This process produces rods having a relatively rough outer surface, but it has the advantage that it can be run at fairly high speeds since the oven can be made as long as is necessary, for example several metres long, to ensure that the coated fibres take long enough to pass through it for the resin to polymerise.

The second glass curing system, known as the enclosed surface system, is more common. Here the coated fibres are passed through a highly polished heated tool or die so that the outside surface of the fibres is in contact with the die as they pass through. It is essential for this die to have a good surface finish to prevent particles of resin adhering to its surface. Typically, the dies are made from steel which has been precision ground, polished, and has a hard chrome plated surface designed to prevent wear by the rather abrasive fibres.

Dies generally come in one of two forms and are made either from a single piece of metal or from two or more pieces of metal held together. When the die is made from the single piece of metal it is generally produced by drilling and fine honing a hole from one end to the other. Dies of this form can typi cally be made up to about a metre in length, but if a longer die is needed or if it is necessary to have a small hole, less than about 6mm, the manufacture of a single-piece die becomes difficult, and it is then necessary to make the die in two pieces. A two-piece die, for example for a circular rod, is generally made so that each half of the die has a semicircular cavity in it. Unfortunately, it is difficult to clamp two halves of the die together sufficiently accurately to get a perfect match.Consequently, two join lines inevitably appear on the outer surface of the pultruded profile. Dies of this form can be made typically up to a maximum length of about two metres.

The speed of pultrusion is governed by a number of factors of which the most important are the time that the resin takes to polymerise, and the length and temperature of the heating system. Present typical production rates using two-part dies are between about 1 and 2 metres per minutes, and it is difficult to increase this speed substantially because of the limitations on the length of dies which can easily be manufactured.

It is an object of the present invention to provide a method of pultrusion which will, if necessary, produce profiles which have a smooth outer surface while at the same time being capable of running at higher speeds than prior art systems producing similar profiles.

According to a first aspect of the invention, a method of producing a pultruded profile comprises impregnating a bundle of fibres with resin, coating the impregnated bundle, and heating the coated bundle to polymerise the resin. Preferably the bundle is coated with a thermoplastic material; but it may also be coated by wrapping a plastics film around it.

The coating is preferably as thin as possible.

The coated bundle may be pre-heated preferably to a temperature just below the polymerisation temperature of the resin. The preheating may be by means of an oven, or by microwave or radiofrequency energy; a combination may also be used.

If the process is run with the bundle vertical, there is an advantage in that there is no sagging of the thermoplastic coating which would therefore remain uniform. Also, the bundle is constantly under tension without any need for support.

Finally, the coating may be removed to leave a smooth-surfaced profile useful, for example, as the strength members for fibre optic cable assemblies.

The invention may be carried into practice in various ways and some specific pultrusion methods will now be described, by way of example, with reference to the drawings, in which Figure 1 shows the general arrangement of a pultrusion system; Figure 2 is a sectional view of a first type of extrusion head; Figure 3 is a sectional view of a second type of extrusion head; and Figure 4 is a part-sectional view of the curing section.

Figure 1 shows the general arrangement of the process. A number of fibres 10, typically glass, carbon or aramid, are drawn from a creel 12 and pass through a resin impregnation system 14 of known type, shown here as an open resin bath through which the fibres are guided. The exact type of resin in the bath will vary according to the application, but will typically be epoxy. On leaving the bath, the fibres are passed through a squeeze-out bushing 16 which removes excess resin. They then pass through a preheating system 18 which preheats them to a first temperature below the polymerisation temperature of the resin. The exact form of the preheating system 18 may vary; for example, radiant heat, contact with a hot surface, microwave, radiofrequency, and so on.

After leaving the preheating system 18 the fibres then pass to an extrusion head 20, further details of which will be described below.

As the fibres pass through the extrusion head they are surrounded by a thermoplastic tube of a material with a reasonably high melting point, typically nylon. The output 22 of the extrusion head therefore consists of the wet impregnated fibres encased within a thermoplastic tube.

These encased fibres then pass to a heating system 24 which polymerises and cures the thermosetting resin. The heating system is of the open surface type, and the heat source may be a conventional oven, or microwave or radiofrequency energy. A combination of systems may also be used.

After curing, the encased rod passes through a cooling section 26 either at ambient temperature or having forced cooling by air or water. The rod then passes to a continuous pulling mechanism 28 comprising a series of cooled rubber rollers or two opposed belts.

The pulling speed is synchronised with the extrusion rate of thermoplastic covering.

The coated rod is then coiled onto- a drum 30, or, should a number of short rods be required, is cut to length.

Finally, if the thermoplastic coating is not required it may be removed by means of a high pressure water jet, leaving the fibre and resin rod with a smooth surface. Alternatively, the coated rods may be used with the thermoplastic coating in place.

Figure 2 shows the extrusion head 20 in more detail, the head comprising a screw 32 rotating in a barrel 34 which is surrounded by a heater 36. The barrel opens into a chamber 38 having an exit hole 40 and an inlet hole 42. Extending from the inlet hole 42 inwardly of the chamber is a cylindrical inlet flange 44.

In use, thermoplastic material in the barrel 34 is melted by the heater 36 and forced under pressure into the chamber 38 by the rotating screw 32. The impregnated fibres 46 enter the chamber 38 via the hole 42 and the inlet flange 44, which are both of a suitable size to be a close fit around the fibres, and come into contact with the resin 48 in a chamber. The diameter of the hole 40 is somewhat larger than the diameter of the resin bundle 46 and the pressure in the chamber 38 causes the thermoplastic material to extrude, preferably generally vertically, through the exit hole and to form a coating around the fibres 46. The thickness of the thermoplastic coating depends both upon the size of the hole 40 and the speed of rotation of the screw 32.By suitably controlling the speed of rotation of the screw the coated fibres 22 can he arranged to have a diameter equal to that of the exit hole 40. The lengths of the inlet flange 44 is so chosen that the length of fibre in contact with the thermoplastic material 48 in the chamber is optimized.

An alternative form of extrusion head is shown in Figure 3. This is similar to the extrusion head of Figure 2 except that the inlet flange 44 is replaced by a longer inlet tube 50 which extends from the inlet hole 42 right through the chamber and out of the exit hole 40. The tube has a bevelled end 52. The operation of the extrusion head of Figure 3 is rather different from that of Figure 2. The thermoplastic material is extruded through the annular space between the exit hole 40 and the inlet tube 50 and itself forms a tube 54 before it is brought into contact with the impregnated fibres 46. The rate of extrusion of the thermoplastic tube 54 is controlled so that it is fractionally less than the speed of the fibres 46, thus ensuring that the extruded tube is drawn into close contact with the fibres.

The temperature of the thermoplastic tube 54 when it comes into contact with the fibres 46 will depend, amongst other things, on the length of the tube 50 protruding from the exit hole 40; the exact amount by which it protrudes can be adjusted as necessary.

The curing section 24 is shown in more detail in Figure 4 and comprises an enclosed chamber 56 having an inlet hole 58 and an exit hole 60 (which may be vertically aligned) between which the coated fibres 22 pass.

The chamber 56 is heated by heaters 62 and is pressurized, by means of air or some other fluid, via an inlet port 64. The internal pressure is maintained by means of seals 66 and 68 which surround the coated fibres at the inlet and outlet holes respectively. In use, the fluid (preferably liquid) is heated to a temperature sufficient to polymerise the resin, typically 130"C. The pressure may be produced by the head of liquid.

Instead of the bundle being coated with a thermoplastic material it may be coated by wrapping a plastics film around it. This may then be heated to shrink it into contact with the bundle.

Some of the fibres may be copper wires and/or optical fibres.

Claims

1. A method of producing a pultruded profile comprising impregnating a bundle of fibres with resin, coating the impregnated bundle, and heating the coated bundle to polymerise the resin.

2. A method as claimed in Claim 1 in which the bundle is coated with a thermoplastic ma terial.

3. A method as claimed in Claim 2 in which the bundle is coated by being received within a continuously-extruded tube of a thermoplastic material.

4. A method as claimed in Claim 3 in which the tube is formed prior to being brought into contact with the bundle.

5. A method as claimed in Claim 3 or Claim 4 in which the extruded tube, when formed, is larger but slower moving than the bundle, and is subsequently drawn into contact with the bundle.

6. A method as claimed in any one of Claims 2 to 5 in which the thermoplastic tube is extruded generally vertically.

7. A method as claimed in Claim 1 in which the bundle is coated by wrapping a plastics film around it.

8. A method as claimed in Claim 7 in which the plastics film is subsequently heated to shrink it into contact with the bundle.

9. A method as claimed in any one of the preceding claims in which the coated bundle is heated to polymerise the resin under pressure.

10. A method as claimed in Claim 9 in which the coated bundle is heated under pressure by passing it through a heated liquid.

11. A method as claimed in Claim 10 in which the pressure is produced by the head of liquid above the coated bundle.

12. A method as claimed in any one of the preceding claims in which the impregnated bundle is preheated to a temperature below the polymerisation temperature of the resin, before being coated.

13. A method as claimed in Claim 12 in which the preheated temperature of the bundle is just below the polymerisation temperature of the resin.

14. A method as claimed in any one of the preceding claims in which the coated bundle is supported generally vertically.

15. A method as claimed in any one of the preceding claims which inciudes the final step of removing the coating.

16. A method as claimed in Claim 15 in which the coating is removed by means of a jet of water.

17. A pultruded profile comprising a bundle of fibres impregnated with a polymerised resin, the bundle being coated with a thermoplastic material.

18. A pultruded profile as claimed in Claim 17 in which at least some of the fibres are copper wires.

19. A pultruded profile as claimed in Claim 17 or Claim 18 in which at least some of the fibres are optical fibres.

20. A method of producing a pultruded profile substantially as specifically described herein with reference to Figures 1, 2 and 4 or to Figures 1, 3 and 4 or to Figures 1 and 4 alone.

21. A pultruded profile substantially as specifically described herein.