WO1992008594A1

WO1992008594A1 - A method and an apparatus for the manufacture of moulded components on thermally expandable cores

Info

Publication number: WO1992008594A1
Application number: PCT/GB1991/002009
Authority: WO
Inventors: Matthew Readman
Original assignee: MANCHESTER INSTITUTE OF SCIENCE, University of; MANCHESTER INSTITUTE OF SCIENCE and TECHNOLOGY, University of
Current assignee: MANCHESTER INSTITUTE OF SCIENCE, University of; MANCHESTER INSTITUTE OF SCIENCE and TECHNOLOGY, University of
Priority date: 1990-11-14
Filing date: 1991-11-14
Publication date: 1992-05-29
Anticipated expiration: 1993-05-14
Also published as: GB2266263A; GB9024789D0; AU8866491A; GB2266263B; GB9309736D0

Abstract

A method and apparatus for the manufacture of moulded components, in which a component forming curable material (2) is first deposited on the surface of a support (1). The support (1) is formed from a material which exhibits viscoelastic properties and has a coefficient of thermal expansion greater than that of the curable material. Heating means (8), e.g. a resistive electrical heater, are provided to heat the support (1) whilst it is restrained against displacement relative to the curable material. The restraint is then removed, the support cooled, and the cured component (2) removed. Because the curable material has a smaller coefficient of thermal expansion than the support (1), the thermal expansion of the support is restrained as it is heated. Thus, when subsequently cooled it contracts to a size that is less than its original size and therefore the cured component (2) can be readily removed.

Description

A method and an apparatus for the manufacture of moulded components on thermally expandable cores.

The present invention relates to the manufacture of moulded components, for example tubes formed from composite materials.

Composite^'tubes are well known in the art. Such tubes are made from a matrix of polyester resin, epoxy resin or the like and reinforcement such as woven or unwoven fibre. To construct the tube the reinforcement is wrapped around a mandrel and impregnated with resin in a liquid state. This can be done by hand or mechanically. For example, the mandrel may be rotated as re^'sin impregnated fibres are applied. The resin or matrix is then allowed to set or cure and after curing the mandrel is removed leaving the composite tube. In order to facilitate removal of the composite tube from the mandrel a release agent is applied to the mandrel prior to the wrapping of the matrix and reinforcement. During curing of the matrix the composite tube will shrink onto the mandrel. Shrinkage and physical or chemical adhesion makes the removal of the mandrel very difficult, and as a result the mandrel and/or the tube can be damaged.

It is an object of the present invention to obviate or mitigate the aforesaid problems.

According to the present invention, there is provided a method for moulding a component, wherein component-forming curable material is deposited on a surface of a support formed ^•from a material exhibiting viscoelastic properties, the curable material when cured and the support having different coefficients of thermal expansion, the curable material is cured on the support, the support is heated whilst restrained against displacement relative to the cured material, and the support is cooled whilst not restrained against relative displacement between the support surface and the cured material.

The term "curable material" is used herein to mean any material which can be deposited in liquid form and subsequently hardens to form a dimensionally stable structure. Examples are resin, plaster, concrete, plastics and rubber based materials.

The term "material exhibiting viscoelastic properties" is used herein to mean any material in which compressive stresses induced by heating the material when it is constrained against expansion relax at least partially such that when the material is subsequently cooled it contracts to a size which is less than the size to which it was constrained during heating.

Preferably, the material from which the support is made has non-adhesive properties to prevent adhesion of the curable material to the support. Alternatively a release agent may be applied to the mandrel before the resin is applied.

The material exhibiting viscoelastic properties may be for example polypropylene.

A tube may be formed in accordance with the present invention by wrapping tube-forming material around a mandrel formed from the material exhibiting viscoelastic properties, curing the tube-forming material on the mandrel, heating the mandrel, subsequently cooling the mandrel, and removing the cured tube from the mandrel.

The tube-forming material has a smaller coefficient of thermal expansion than the mandrel, and thus thermal expansion of the mandrel is restrained as the mandrel is heated. When subsequently cooled, it contracts to a size that is less than its original size, and thus the cured tube can be readily removed.

Preferably the mandrel incorporates a heating means, e.g. a resistive electrical heater. Preferably, means are provided for expanding the mandrel after use to return it to its dimensions before use. For example, excess internal pressure may be applied to the mandrel when it is heated and enclosed in a rigid tube the internal diameter of which is slightly greater than the initial outside diameter of the mandrel. When the mandrel is subsequently cooled it will then contract to its initial outside diameter.

Preferably said mandrel has stepped end portions of enlarged diameter to provide for consistent stressing of said tube. Preferably one of said stepped end portions is removable to allow said mandrel to be removed.

According to a second aspect of the present invention, there is provided a moulded component manufactured in accordance with the method as hereinbefore defined.

According to a third aspect of the present invention, there is provided moulded component forming apparatus comprising means for moulding curable material on a support exhibiting viscoelastic properties, means for heating the support, and means for cooling the support to facilitate the removal of the component from the support.

Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a section through a mandrel used in accordance with the present invention to form a tube and a tube formed on the mandrel;

Figure 2 is a section through the mandrel and tube of Figure 1 after heating and cooling; and

Figure 3 is a section through an improved mandrel in accordance with the present invention.

Referring to Figure 1 there is shown a mandrel 1, with a tube wrapped thereon. The mandrel 1 is constructed from, for example, polypropylene tubing. This is a thermoplastic that is viscoelastic. The tube 2 is wound around the mandrel without a release agent being previously applied to the mandrel. The composite tube 2 is formed from a matrix of resin, for example, epoxy resin, and a reinforcement material such as fibre which may be woven or unwoven. The fibre may be glass, boron, polyamide or other fibres capable of reinforcing composite tubes. Once the composite tube 2 material has been wound on the mandrel 1, it is allowed to set or cure from its liquid state to the solid state. As it cures at room temperature it shrinks tighter onto the mandrel 1. The amount of shrinkage depends on the resin used.

To remove the composite tube 2, the mandrel 1 and composite tube 2 are heated, for example, in an oven at a temperature of around 70 - 80°C. This heating step may be initiated before the tube is cured to accelerate curing. Heating causes the mandrel 1 and the tube 2 to expand. The expansion to the composite tube 2 is very small and thus constrains the relatively larger expansion of the mandrel 1. Constraining expansion of the mandrel 1 causes circumferential compressive stresses to be set up in the mandrel 1. These compressive stresses are relaxed at the elevated temperature. The amount of stress relaxation depends upon the temperature and length of time the temperature is maintained. For example at 70^GC using a polypropylene mandrel 1 about fifty percent of the stresses set up are relaxed relatively quickly such that the force the mandrel was exerting on the composite tube 2 is relaxed. The mandrel 1 and the composite tube 2 are now left to cool to room temperature. Given the viscoelastic properties of the mandrel 1 the diameter of the mandrel 1 is reduced relative to the diameter of the composite tube 2. This occurs because the compressive stresses that were relaxed from the mandrel result in the mandrel 1 contracting to a smaller diameter than its initial diameter.

Compressive stress is only applied to the mandrel 1 where it is constrained. The composite tube 2 is shorter in length than the mandrel 1 and thus there is part of the mandrel 1 at either end that is not stress relaxed. Thus after cooling the relative dimensions of the mandrel 1 and tube 2 are as illustrated in Figure 2. A cut is made through the composite tube 2 and the mandrel 1 at one end so that the tube 2 can then be removed from the mandrel 1 at that end. Given the contraction of the mandrel 1 relative to the tube 2, this is easily achieved. Of course, a release agent could be applied to the mandrel, but this is not generally necessary.

Figure 3 shows another example of a mandrel 1. The mandrel 1 again is formed from a viscoelastic material. It has an integrated heater 3 in the form of a resistive heating wire and glass fibres are laid unidirectionally along the axis of the tube to give it axial stiffness. The integrated heater 3 is wound circumferentially within the thickness of the mandrel 1. The ends of the mandrel 1 are sealed to allow it to be pressurized from the inside. Both ends of the mandrel 1 are stepped so as to have a greater outside diameter than that of the remainder of the mandrel 1 on which the composite tube (not shown) is wound. This ensures that the mandrel 1 can be used for making sure the tube 2 will always be stressed by the same amount and no inconsistencies in diameter will occur. One end 4 of the mandrel 1 is made detachable to enable the tube 2 to be removed without cutting the mandrel 1. After removal, the ends of the tube can be trimmed to remove parts of the tube formed on the stepped ends.

The diameter of the mandrel 1 can then be returned to its original size by heating the mandrel 1 through the integrated heater 3 and pressurizing the interior of the mandrel 1 via a pressure connector 5. This is done whilst the mandrel 1 is placed within another tube (not shown) of slightly larger internal diameter than the initial mandrel diameter. When the mandrel 1 is cooled it will thus contract to the initial diameter required to make the next composite tube 2.

Figure 3 shows one end 6 of the mandrel 1 having a protruding housing and electrical plug 7 which is used to supply electricity to the heater 3. A temperature sensor 8 is also provided within the mandrel 1 to gauge when the appropriate temperature has been reached during the heating processes.

It will be appreciated that the present invention can be applied to the production of tubes of material other than fibre reinforced resins. For example, tubes can be manufactured from plaster of Paris which may be reinforced with glass fibres.

Claims

t.CLAIMS

1. A method for moulding a component, wherein component- forming curable material is deposited on a surface of a support formed from a material exhibiting viscoelastic properties, ^•the curable material when cured and the support having different coefficients of thermal expansion, the curable material is cured on the support, the support is heated whilst restrained against displacement relative to the cured material, and the support is cooled whilst not restrained against relative displacement between the support surface and the cured material.

2. A method according to claim 1, wherein the material from which the support is made has non-adhesive properties thereby preventing adhesion of the curable material to the support.

3. A method according to claim 1, wherein a release agent is applied to the support before the curable material is applied.

4. A method according to any preceding claim, wherein the curable material is a material based on resin, plaster, concrete, plastics or rubber.

5. A method according to any preceding claim, wherein the material exhibiting viscoelastic properties is polypropylene.

6. A method according to any preceding claim, wherein the support incorporates a heating means.

7. A method according to claim 6, wherein the heating means comprises a resistive electrical heater.

8. A method according to any preceding claim, wherein the support is expanded after use to return it to its dimensions before use.

9. A method according to any preceding claim, wherein the moulded component is a tube, said tube being formed by wrapping tube-forming curable material around the support, said support being in the form of a mandrel formed from the material exhibiting viscoelastic properties, curing the tube- forming material on the mandrel, heating the mandrel, subsequently cooling the mandrel, and removing the cured tube from the mandrel.

10. A method according to claim 9 when dependent on claim 8, wherein the mandrel is expanded by the application of excess internal pressure to the support when it is heated and enclosed in a rigid tube the internal diameter of which is slightly greater than the initial outside diameter of the support such that when the support is subsequently cooled it will contract to its initial outside diameter.

11. A method according to claim 9 or 10, wherein the mandrel has stepped end portions of enlarged diameter to provide for consistent stressing of said tube.

12. A method as claimed in claim 11, wherein one of said stepped end portions is removable to allow said support to be removed.

13. Apparatus for forming a moulded component, comprising means for moulding curable material on a support exhibiting viscoelastic properties, means for heating the support, and means for cooling the support to facilitate the removal of the component from the support.

14. A method for moulding a component, substantially as hereinbefore described with reference to the accompanying drawings.

15. A moulded component manufactured in accordance with the method as hereinbefore defined.

16. Apparatus for forming a moulded component substantially as hereinbefore described with reference to the accompanying drawings.