GB2132129A - Lined moulds - Google Patents

Abstract

A mould for the manufacture of elongate tubular articles, such as flagpoles, comprises a casing (1) provided by an extruded aluminium tube, and a lining consisting of a main part (3) of filled or reinforced polyester resin and an inner layer (4) of epoxide resin providing a smooth hard inner surface. The mould lining is rotationally cast. <IMAGE>

Description

SPECIFICATION Moulds for the manufacture of elongate tubular members This invention concerns the manufacture of elongate tubular members, in particular hollow poles, made from synthetic materials, and the invention relates especially to moulds for manufacturing such members.

The method generally used to manufacture hollow poles from fibre reinforced plastics (FRP) material involves the use of a rotating mould having its axis inclined at a small angle to the horizontal. The fibre reinforcement is introduced into the mould to line the inner surface, after which the plastics resin is introduced preferably by spraying onto the inside of the rotating mould.

Hitherto the moulds which have been used to make hollow poles by this method have been manufactured from steel. The steel moulds must be accurately machined to ensure a completely smooth interior surface, and they must be rotationally balanced. Consequently, the steel moulds tend to be extremely expensive. A separate mould is required for making poles of each different shape and whereas the moulds have a very long useful lifetime, in some cases the numbers of poles to be made with a mould make a steel mould uneconomic.

The present invention seeks to overcome the above drawback of steel moulds, and accordingly provides a mould for the manufacture of elongate tubular members made of synthetic material, comprising a rigid tubular casing, and a tubular core located in the casing, the core including a main part of filled or reinforced synthetic resin material, and an inner layer of hard synthetic resin material formed in situ within said main part and providing a smooth interior surface.

In a preferred construction the casing is a metallic tube, conveniently extruded aluminium, the core main part is polyester resin, or other relatively cheap plastics material, and the inner layer is epoxy resin.

The outer metallic tube does not need to be machined accurately, while the epoxy resin layer of the core ensures a smooth, hard and seamless inner surface. The mould of the invention may not be as durable as a steel mould, but it can be made at a fraction of the cost and it should be satisfactory for moulding several hundreds of articles. The same metallic tube may accommodate different cores received removable therein to enable different pole shapes to be moulded. It is possible for the mould to be provided with a thin metal lining, e.g. of bismuth alloy, to assist release of moulded articles from the mould. The mould of the invention is of particular benefit in the manufacture of articles, such as flag poles and aerial masts, for which production rates are expected to be relatively low compared with the possible maximum output of a mould.

Afull understanding of the invention will be had from the following description which is given by way of example with reference to the accompanying drawings in which: Figure 1 is an axial section through a mould embodying the invention; Figure2 is an axial section through an alternative mould in the course of manufacture; Figure 3 is a transverse section of the mould shown in Figure 2; Figure 4 is an axial section through another mould according to the invention; Figure 5is an enlarged scale view illustrating a detail of the forward end closure of the mould of Figure 4; Figure 6 is an enlarged scale view of the rear end of the mould of Figure 4; and Figure 7 is an axial section through a casting mould for producing inserts used in the mould of Figure 4.

The mould illustrated schematically in Figure 1 comprises a tubular metal casing 1 of aluminium which is readily available in the form of constant diameter seamless tube. The casing 1 is supported for rotation about its axis, which is inclined at a small angle to the horizontal, by sets of rollers 2. Received in the casing with a good fit is a core which consists of an outer layer 3 of relatively cheap plastics resin, such as polyester resin, reinforced with glass fibres, and a comparatively thin inner layer 4 of harder epoxide resin.

To make the mould the casing 1, fitted with end cover 5 and flange 6, is mounted on the rollers 2. The glass fibre reinforcement is introduced to line the inside of the casing which is then set in rotation.

Next the liquid polyester resin is sprayed onto the inside of the rotating casing in a series of coatings, each coating being allowed to harden before the next is applied, until the required shape and thickness of the polyester resin layer 3 is obtained.

Finally, the liquid epoxide resin is sprayed onto the inside of layer 3, possibly also as a series of coatings, and allowed to harden to complete the laminated core with a smooth, seamless inner moulding surface. By careful selection of the inclination angle of the mould casing, its speed of rotation and the amount of resin introduced, the inner surface of the completed mould can be made to an accurately controlled parabolic section.

The outer layer 3 of the core may alternatively be made of plastics foam, such as polyurethane foam.

In this case the layer 3 may be assembled in the casing by introducing premoulded rings of foam.

After completion of the laminated core it can be removed from the casing 1, and to assist core removal a releasing agent is preferably applied to the inner surface of the casing before the glass fibre is inserted. The casing may then be used to make further mould cores.

When the mould is to be used for manufacturing articles the core is replaced into the casing and it is then used in the same way as a conventional steel mould. As will be appreciated from the foregoing, the same casing 1 may serve for several moulds it being necessary only to provide respective cores with inner surfaces of the required shape, so that a substantial cost saving is possible.

In use a releasing agent is applied to the mould surface to facilitate removal of moulded articles.

Sprayed metal has the property of absorbing the releasing agent so that releasing operations may be less frequent. Metal also has the advantage of better resistance to wear than epoxy resin. Consequently, it may be of advantage for the mould to be provided with a coating of metal, e.g. bismuth alloy on its inner surface. A method of making such a mould will now be described with reference to Figures 2 and 3.

A model 10 having the shape of the articles to be manufactured in the finished mould is prepared. As shown this model has the form of a hollow moulded FRP pole. Alternatively, the model could be made of wood, or the like, as long as it has a density less than that of liquid resin, for the reason which will become clear. A releasing agent is first applied to the model 10 and it is then coated by spraying with bismuth alloy to form a continuous metal layer 11.

In the same way as described above in relation to Figure 1, a layer 12 of glass fibre reinforced polyester resin is formed on the inside of a seamless aluminium casing 13. The metal coated model 10 is then introduced into the casing and the space between it and layer 13 is filled with epoxide resin 14. Due to the lower density of the model it initially occupies an uppermost position, as seen in Figure 3, but when the casing is set in rotation it moves automatically to an axial position. Rotation continues until the epoxide resin has hardened, after which the model 10 is withdrawn leaving the metal coating as a liner on the finished mould core. In this way a mould with a smooth seamless metal surface is obtained at a fraction of the cost of a steel mould.This method has the additional advantage that the mould cavity can be of any shape, e.g. stepped and is not necessarily parabolic.

In the production of very long moulds, e.g. with a length of 7 metres or more, shrinkage of the resin material can be a problem. There is a tendency for the resin to crack and result in an unsatisfactory surface finish. This problem is avoided with the mould of Figure 4. The mould has a tubular casing 20 of extruded aluminium supported by spacers 21 in a main machine cylinder 22. The cylinder 22 isjournalled in bearings for rotation about its axis which is downwardly inclined to the horizontal towards its forward end, i.e. to the left as seen in Figure 1, at a small angle typically of about 3 . Welded to the rear end of the casing 20 is a flange 23 which is fastened to the cylinder 22 by bolts. Received in the forward end of the casing 20 with a sliding fit is an annular plug or piston 24 which is sealed to the casing by 0-rings 25.The piston has a stepped bore, as seen in Figure 5, and has an end cover plate 26 secured to it by bolts. The piston and cover plate are retained in the casing by a Circlip 27. Attached by bolts to the flange 23 at the rear end of the casing are three rings 28,29,30. The inner steel ring 28 defines an annular groove 31 and normally serves to retain the mould core in the casing, as will become clear. The second steel ring 29 has a stepped inner diameter and is used for withdrawing a moulded pole from the mould. The third and outer ring 30 is of nylon and its purpose is to retain the resin in the mould when making a pole. A cover 32 is provided to close the central opening of the ring 30 and conveniently takes the form of a sheet of clear film which is secured by adhesive tape.

In the manufacture of the mould, with the piston 24 and cover plate 26 removed, a series of preformed annular inserts 33-36 are introduced in correct order into the casing 20. The inserts are shaped so that their interior surfaces define a smoothly tapering bore with a diameter selected according to the dimensions of the poles to be produced in the finished mould. The inserts 33-36 are cast in moulds as shown in Figure 7. The mould has a casing 40 which is cut from the same length of extrusion as the casing 20, but to facilitate removal of the inserts it is slit longitudinally with a slot 3mm wide.A plurality of expandable clamping rings 41 are provided for holding the casing 40 closed and the casing is fitted also with top and bottom end plates 42, 43. Held centred in the casing between the end plates is a core 44, e.g. turned out of wood to the required internal shape of the insert. It will be understood that different cores of corresponding profile will be used for making each of the series of inserts 33-36. The core 44 is wrapped with woven glass fabric 45 and inserted into the mould casing, after which the mould parts are clamped together by tie bars extending between the end plates 42,43. The top end plate includes one or more holes 46 through which the casting material is poured into the mould cavity. The casting material is polyester resin heavily filled with hollow glass microspheres or other light weight filler.When the material has set hard the mould is opened and the clamps 41 released to allow the completed insert to be removed. Because the casting mould 40 is split the inserts 33-36 are an easy sliding fit in the casing 20.

The inserts are loaded into the casing 20 as mentioned above. The inserts become thinner towards the rear end of the casing and over a portion of the casing length at this end inserts may not be required depending on the casing diameter. Woven glass fabric 47 supported by an expanded metal cylinder 48 is positioned in the casing ahead of the inserts and the end of the expanded metal is turned outwardly into the groove 31. (Figure 6). The first insert 36 is then introduced and the glass fabric 47 is lapped over its rear edge and spot bonded to the inner surface of the insert by a quick setting resin.

The remaining inserts are then inserted in turn with the joins between them being lapped with strips 49 of woven glass fabric which are similarly spot bonded to the glass fabric 45 which lines the insides of the inserts. The piston 24 is replaced and an expanded metal cylinder 50 is positioned to bridge the join between insert 33 and the piston 24. The assembly is then completed by refitting the cover plate 26 and the retainer clip 27, and it is then mounted in the machine cylinder 22. The cylinder 22 and casing are rotated at a controlled speed and a first layer of epoxy resin is poured in through the open rear end of the casing 20. The resin coats the inserts 33-36 as well as the metal cylinders 48,50 and the glass fabric strips 49. The resin also flows between the joins between the inserts, and between the end insert 33 and the piston 24 so that it moulds partly to the inside of the casing 20. The resin of this first pour includes a flexible resin addition to give extra toughness and especially resistant to shrink age cracks during curing. The glass fabric 45 of the inserts is partially wetted out by the polyester resin when preparing the inserts, and it becomes com pletely wetted out by the epoxy resin so that there is a good key between the epoxy and polyester and a good mould strength is obtained, especially in the longitudinal direction.

When the first layer 51 of epoxy resin has cured to thumbprint hardness, a second and final coating 52 of hard epoxy resin about 1-3 mm thick is applied.

When the epoxy has been poured into the casing the open end is closed by the cover 32 to exclude dust.

The rotation of the casing is continued until the hard epoxy has cured fully to provide an inner mould surface 51 with good finish and hard wear resistance.

It will be noted that the piston 24 and cover plate 26 are free to move to take up any thermal shrinkage stresses during curing of the epoxy resin. Furthermore, the whole core is free to move in the casing, which is coated internally with a release agent to facilitate such movement, but the rear end is held by the integral lip produced by the groove 31. When the mould has been completed as described the retaining clip 27 is removed to enable full freedom of axial movement of the core end relative to the casing.

The finished mould may be used in conventional manner to produce elongate tubular members, such as flagpoles. A finished product 52 is removed from the mould by detaching the ring 30 and then withdrawing the product by means of the ring 29. If it is required to remove the core from the casing, e.g.

to allow the casing to be used with another core, after removing the ring 28 the core can be pushed out through the rear end of the casing.

Claims (17)

1. A mould for the manufacture of elongate tubular members made of synthetic material, com prising a rigid tubular casing, and a tubular core located in the casing, the core including a main part of filled or reinforced synthetic resin material, and an inner layer of hard synthetic resin material formed in situ within said main part and providing a smooth interior surface.

2. A mould according to claim 1, wherein the casing is a metal tube.

3. A mould according to claim 2, wherein the metal tube is an aluminium extrusion.

4. A mould according to claim 1,2 or 3, wherein the inner layer is an epoxide resin.

5. A mould according to claim 4, wherein the epoxide resin is applied to said main part of the core in two or more coatings.

6. A mould according to any one of claims 1 to 5, wherein the main core part comprises fibre reinforced polyester resin.

7. A mould according to any one of claims 1 to 6, wherein the main core part is formed in situ within the casing.

8. A mould according to any one of claims 1 to 6, wherein the main core part comprises a plurality of preformed annular elements.

9. A mould according to claim 8, wherein the annular elements are moulded from filled polyester.

10. A mould according to claim 8 or 9, wherein each element is made in a mould including a casing cut from the same tube as forms said tubular mould casing.

11. A mould according to claim 8,9 or 10 wherein the annular elements are inserted end to end in the tubular casing and have the joins therebetween lapped with fibre reinforcement before the inner layer is formed on the inside of said elements.

12. A mould according to any one of claims 8 to 11,wherein each of the elements is moulded around a layer of fibre reinforcing material.

13. A mould according to any one of the preceding claims, wherein the core is free to expand and contract axially in the casing.

14. A mould according to any one of the preceding claims, wherein the core is removable from the casing to allow the same casing to be used with different cores.

15. A mould according to any one of the preceding claims, wherein the core is provided with an inner surface coating of sprayed metal.

16. A mouldforthe manufacture of elongate tubular members, substantially as herein described with reference to Figure 1, Figures 2 and 3 or Figures 4 to 7 of the accompanying drawings.

17. An elongate tubular article of reinforced synthetic material having been manufactured in a mould as defined in any one of the preceding claims.