MOULDING METHOD USING A PREPREG

Abstract: A method of moulding an article from prepreg including laying a sheet of prepreg on a mould to conform with the mould includes also treating the resin of the prepreg to increase its viscosity to a predetermined value which, when the sheet is cured by the simultaneous application of pressure and heat results in a reduction in voids within the laminate and pits on the moulded surface as compared with a conventional prepreg moulding technique, curing the resin and removing the cured or partially cured article from the mould. The treatment of the resin may be achieved by applying heat to the resin of the prepreg amending its formulation or any other appropriate method.

Description: MOULDING METHOD USING A PREPREG The present invention concerns a method of moulding an article from a reinforced resin in the form of a prepreg. Prepreg materials are produced by impregnating a fibrous reinforcement material with a resin to produce a material which can be subsequently processed by the simultaneous application of pressure and heat curing the resin in the prepreg to give a solid laminate which is suitable for purposes where high structural performance or some other specific properties are required. The fibrous reinforcement material is usually in the form of a sheet but may be presented in a narrow strip or tow. In this specification the term "sheet" includes tows and other reinforcement structures. According to the present invention there is provided a method of moulding an article from prepreg including laying a sheet of prepreg on a mould to conform with the mould, treating the resin of the prepreg to increase its viscosity to a predetermined value which, when the sheet is cured by the simultaneous application of pressure and heat results in a reduction in voids within the laminate and pits on the moulded surface as compared with a conventional prepreg moulding technique, curing the resin and removing the cured or partially cured article from the mould. Preferably the method includes sequentially laying a plurality of sheets of prepreg on the mould and on each other, and treating each sheet to increase the viscosity to said predetermined value prior to laying the next sheet. Preferably said treatment of each sheet is carried out after it has been laid. Preferably the resin of a conventional prepreg formulation is amended such that the said increase in viscosity of the prepreg can be achieved. Alternatively the resin of a prepreg which cures at temperatures of less than 80°C is impregnated into the fibrous material and its viscosity is increased to said predetermined value by the application of ambient or mildly elevated temperature thereto. In a further alternative the resin of a prepreg which cures at temperatures of less than 80°C has its formulation amended such that the said increase in viscosity can be achieved. Preferably the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by applied cure initiation circumstance, for example ultra-violet light or electron beam. Alternatively the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by the addition to the resin formulation of encapsulants/absorbents which release the active ingredients after a specified time period or time/temperature cycle. Further alternatively the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by the addition to the resin formulation of encapsulants/ absorbents which are activated by radiant heat or infra-red or electron beams or mechanical shock or a change in the pressure applied to the sheets of prepreg or some other physical or chemical releasing/initiating circumstances. An embodiment of the invention will now be described by way of example only. The resins used in prepreg materials are usually thermosetting and formulated to give a level of viscosity which produces a suitable degree of tack and drape in the prepreg material. This implies that the resin is a semi-solid, as a liquid resin would give a wet sticky prepreg, whilst a solid resin would give a dry, stiff prepreg. Applications for both 'wet' and 'dry' prepreg exist, but most prepregs are required to be sufficiently drapeable and tacky to form to the shape of complex moulds and stick to the surface of the mould while further plies of prepreg are laid in place, without any resin transferring from the prepreg to the gloves or tools being used by the person laminating. The range of resin viscosity within which the correct prepreg handleability is achieved is therefore relatively small, and constrains the resin formulator when developing new prepreg resin systems. At different stages of the process of prepreg manufacture and use there are different levels of resin viscosity that are most desirable for specific purposes. Furthermore, different impregnation processes and different types of fibrous reinforcement format also demand different resin viscosities during the impregnation process. For thick fabrics a very thin resin is required in order to achieve a reasonable degree of impregnation of the fabric by the resin. Particularly in this case, the resin may be formulated so that the resin reacts to a limited extent prior to its use, so that the viscosity increases and therefore gives the correct degree of tack and drape in the final prepreg state. The prepreg may be heated during the impregnation process, or stored after manufacture in a warm environment for a set time to achieve the correct degree of further reaction, and hence the correct level of tack and drape. Such technology is well known and used routinely in the advanced composites industry, as is the incorporation of a dwell in the cure cycle to allow the resin viscosity to increase before, say, applying full pressure, or increasing the temperature to complete the cure cycle. Articles produced from prepregs of the type described in the preceding eight paragraphs which are referred to throughout this specification as "conventional prepregs" have not been completely satisfactory in that the laminates incorporate voids within their structure and often the moulded surfaces are pitted. The present invention is based on the discovery that the viscosity of the resin at the start of the final curing process when vacuum/pressure is first applied to the mould and the prepreg layers thereon should be at or above a predetermined value which is considerably greater than that which is most desirable for optimum prepreg tack and drape properties. The actual viscosity level required varies from prepreg to prepreg and moulding situation to moulding situation. It is significantly higher than the viscosity required for the optimum combination of tack, drape and outlife parameters during a normal lay up operation especially where low temperature cure prepregs are used, for example LTM (Registered Trade Mark, Advanced Composite Components Ltd). In a typical moulding procedure a mould is prepared in the normal way and the first layer of prepreg is laid on the mould to conform to the shape of the mould. The resin formulation of the prepreg has been modified, for example by the addition of materials thereto, such that once the sheet of prepreg has been laid the additives can be activated with the result that the process resulting in the increase in the viscosity of the resin therein is started and over time the viscosity is increased to the said predetermined value when the process, known as the lay up B-staging process, is effectively finished. The viscosity will then remain at or close to this value until the final curing process is initiated. A further sheet of prepreg is then laid on the original treated sheet on the mould and when the further sheet is finally in position the viscosity of the resin therein is increased by the same method to the predetermined value. Subsequent sheets are then added in a similar manner with a viscosity increasing step applied after the sheet has been laid until the material on the mould achieves the desired thickness. After leaving the laid up sheets for a period of time sufficient for the required viscosity increase to occur the moulding process is then completed following normal techniques, for example by covering the mould with an impervious membrane, applying vacuum/pressure across the membrane and elevating the temperature of the assembly until curing is achieved. Thereafter the cured article can be removed from the mould and, in accordance with the invention, the formation of voids within the moulded material will have been reduced or eliminated as will the surface pitting on the mould side of the laminate. This desirable objective will also be influenced by the fibre/resin ratio, the degree of impregnation of the fibre in the prepreg, the distribution of the resin in the prepreg, the temperature the pressure and the time employed during the moulding process and the design of the auxiliary consumable pack, as well as, in some cases, other parameters. Increasing the viscosity as soon as possible after laying each sheet has the additional advantage of ensuring that the prepreg remains in place on the mould surface around, for example sharp corners or similar features. As indicated above, to achieve an increased viscosity in the resin content of the prepreg prior to the initiation of the cure cycle, modifications are made to the resin formulation. One method of achieving the desired formulation changes is to incorporate resin ingredients and curing agents which are activated by ultra-violet light, electron beam or some other easily controlled and applied cure initiation circumstances during the lay-up procedure. Another technique is to incorporate curing agents which are automatically activated or released from encapsulants/absorbents after a specified time period or time/temperature cycle. Other methods, such as using direct radiant heating or infra-red radiation could be adopted and more energetic forms of radiation such as electron beams may be used. Mechanical shock or the application of vacuum, pressure or ultra-sound either alone or in combination with other methods may be possible initiation methods to promote the increase in viscosity to the predetermined level. In a modified arrangement a low temperature curing prepreg, that is a prepreg which cures at less than 80°C, can be employed. With such a resin it is possible to increase the viscosity to the desired level by simply leaving the laid- up prepreg sheet at ambient temperature. As this is likely to be a time consuming process it is considered preferable to modify the resin content of the low temperature curing prepreg such that the increase of viscosity to the predetermined level prior to initiation of the final cure is speedily achieved. In a modification of the embodiments described above, the moulded article could be formed from a single layer of prepreg. In a further modification any suitable curing technique, for example autoclaving may be employed.

Claims: CLAIMS 1. A method of moulding an article from prepreg including laying a sheet of prepreg on a mould to conform with the mould, characterised in that the method includes treating the resin of the prepreg to increase its viscosity to a predetermined value which, when the sheet is cured by the simultaneous application of pressure and heat results in a reduction in voids within the laminate and pits on the moulded surface as compared with a conventional prepreg moulding technique, curing the resin and removing the cured or partially cured article from the mould. 2. A method of moulding an article as claimed in claim 1, characterised in that a plurality of sheets of prepreg are sequentially laid on the mould and on each other, and each sheet is treated to increase the viscosity to said predetermined value prior to laying the next sheet. 3. A method moulding an article as claimed in claim 2, characterised in that said treatment of each sheet is carried out after it has been laid. 4. A method of moulding an article as claimed in any of claims 1 to 3, characterised in that the resin of a conventional prepreg formulation is amended to achieve said increase in viscosity of the prepreg. 5. A method of moulding an article as claimed in any of claims 1 to 3, characterised in that the resin of a prepreg which cures at temperatures of less than 80°C is impregnated into the fibrous material and its viscosity is increased to said predetermined value by the application of ambient or mildly elevated temperature thereto. 6. A method of moulding an article as claimed in any of claims 1 to 3, characterised in that the resin of a prepreg which cures at temperatures of less than 80°C has its formulation amended such that the said increase in viscosity can be achieved. 7. A method of moulding an article as claimed in any of claims 1 to 3, characterised in that the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by applied cure initiation circumstance. 8. A method of moulding an article as claimed in claim 7, characterised in that the cure initiation circumstances are obtained by the application of ultraviolet light or electron beam. 9. A method of moulding an article as claimed in any one of claims 1 to 3, characterised in that the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by the addition to the resin formulation of encapsulants/absorbents which release the active ingredients after a specified time period or time/temperature cycle. 10. A method of moulding an article as claimed in any of claims 1 to 3, characterised in that the resin ingredients and curing agents in the prepreg are modified such that the increase in viscosity to said predetermined value is obtained by the addition to the resin formulation of encapsulants/absorbents which are activated by radiant heat or infra-red or electron beams or mechanical shock or a change in the pressure applied to the sheets of prepreg or some other physical or chemical releasing/initiating circumstances.

Inventors: SLOMAN ROGER MARK, SLOMAN, ROGER, MARK
Applicants: ADVANCED COMPOSITES GROUP LTD, SLOMAN ROGER MARK, ADVANCED COMPOSITES GROUP LTD., SLOMAN, ROGER, MARK
Application year: 1999
Main classification: B29B15
IPC codes: B29B11, B29B13, B29B15, B29C43, B29C70, C08J5, C08K9