GB2301059A

GB2301059A - Resin transfer moulding with controlled heating of resin

Info

Publication number: GB2301059A
Application number: GB9610984A
Authority: GB
Inventors: Michael Sylvester Johnson; Victor Middleton; Christopher Douglas Rudd; Patrick James Blanchard; Alan Robert Harrison
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1995-05-24
Filing date: 1996-05-24
Publication date: 1996-11-27
Anticipated expiration: 2016-05-24
Also published as: GB9610984D0; GB2301059B

Abstract

In a resin transfer moulding, the resin, along a conveying line 42, is heated before it is introduced into the mould 22, 24 to a temperature which is below the temperature at which curing of the resin will begin. As injection proceeds, the temperature of the resin being injected is continuously varied through microwave heating unit 48 upstream of the mould 22, 24. The resin may be catalysed before being heated (see Figure 2) or the resin and the catalyst may be heated separately before being mixed together (see Figure 3). In the latter a plurality of catalyst chambers 126, 128, 130, 132 are connected to a mixing chamber 136. The resin cures inside the mould. The temperature is controlled by a control circuit using sensors 64, 66, 67, 72, 74/140, 144 etc.

Description

RESIN TRANSFER MOULDING This invention relates to resin transfer moulding and in particular to an apparatus for resin transfer moulding as well as to a method of resin transfer moulding.

In resin transfer moulding (RTM) a fibrous reinforcement is placed in a mould cavity and the mould is closed. Liquid resin is then injected into the mould cavity so that it flows through the reinforcement to wet the reinforcement thoroughly. The resin is then allowed to cure and after cure the moulded product is ejected from the mould.

Curing of the resin is highly temperature dependent, and takes place at temperatures only slightly above ambient temperatures. For example one typical polyester resin system consists of a base resin sold under the designation CV 6345 by Cray Valley Total Chemie and 2% of the catalyst Perkadox 16 sold by AKZO. At room temperature, this system would last for about 4 hours before curing, whereas at 60"C it would cure in about 6 minutes. In using this system, it would be the intention to raise the mould temperature to 600C to cure the resin, ie by about 40"C from a typical ambient temperature of 200C. In another example, the same base resin is used with 1% of TBPEH (sold by AKZO) as catalyst. At room temperature this system would last for about 10 hours before curing, whereas at 700C it would cure in about 16 minutes.Since the ambient temperature may vary between about OOC and 300C from day to day, and during each day, changes in ambient conditions can significantly affect the curing time of the resin and it is therefore necessary to ensure that corrections are made to the moulding conditions to compensate for changes in ambient conditions which would otherwise make the process inconsistent from one cycle to the next.

It is possible to produce quite complex moulded products using this process. The reinforcement can be formed to provide necessary strength in areas where strength is required, because the reinforcement can be "designed" independently of the moulding process. However for RTM to become a viable process for use in mass production of articles, it is desirable to minimise the cycle time, i.e.

the time from ejection of one product from the mould to ejection of the following product from the same mould.

According to the present invention, there is provided apparatus for resin transfer moulding, the apparatus comprising a mould, means for heating the mould, a reservoir for containing liquid resin, a pipe through which resin can flow in a continuous stream from the reservoir to the mould, microwave heating means arranged to heat the resin flowing along the pipe, immediately before the resin enters the mould, and means for controlling the heating means to heat the resin to varying temperatures during the course of each injection cycle.

The invention also provides a method of resin transfer moulding in which liquid resin flows in a continuous stream into a mould and is heated by microwave radiation to a temperature above the resin storage temperature immediately before the resin enters the mould, and the temperature to which the resin is heated varies during the course of each injection cycle.

The resin may be mixed with a catalyst either before or after the resin passes through the microwave heating means.

If the heating takes place when the resin is uncatalysed, the temperature gradient across the cross section of the flowing body of resin is not critical. However if the resin has had catalyst mixed with it, it is important to ensure that the energy in the microwave cavity is properly distributed across the flowing resin, to prevent premature cure of the resin at the edges of the cavity. When the resin is mixed with catalyst prior to heating it is preferred to adopt the features described in our European Patent Application 0 618 057, and in particular to use a Two20 cavity.

There are two time periods to be considered in respect of the process after resin begins to enter the mould. Firstly there is the impregnation time, i.e. the time required for the resin to flow from the resin inlet to all parts of the mould and throughout the reinforcement whilst ensuring thorough wetting-out of the reinforcement. To reduce this impregnation time the viscosity of the resin should be as low as possible. One method of achieving this is to heat the resin, but not to heat it so far that cure takes place before the resin has flowed to all parts of the mould.

Secondly it is necessary to consider the time from entry of the resin into the mould to completion of resin cure. For the thermoset resins used in resin transfer moulding, curing takes place by raising the temperature of the resin. The higher the temperature is raised the faster the cure.

In order to reduce the cure time as far as possible, it is necessary to heat the mould. In order to reduce the injection time the resin is heated before it is introduced into the mould. The heating of both parts of the apparatus must however be done under fully controlled conditions to avoid premature cure of the resin and to prevent waste of resin between cycles.

The course of injection can be monitored either by time, or by monitoring the quantity of resin injected to the mould, or by monitoring the temperature within the mould.

The controlling means preferably comprises electronic storage means to store a mathematical function representing the intended temperature variation over the course of injection, a temperature sensor to monitor the actual resin temperature leaving the microwave heating means and a processor which receives temperature information from the sensor and adjusts the power of the microwave heating means to ensure that the temperature profile during injection follows the stored temperature map. The stored temperature map may be a map of temperature against time or a map of temperature against quantity of resin injected.

A proportional integral derivative controller can be used to adjust the power of the microwave heating.

A proximity sensor can be provided in the injection line, between the heating means and the mould, to detect resin arrival, and another proximity sensor or sensors can be provided at a mould vent to detect filling of the mould.

The sensors can be capacitive proximity sensors.

In addition, or alternatively, to the temperature control steps set out above, the cycle time can be speeded up by heating the resin immediately before it is introduced into the mould, to reduce the resin viscosity, and by varying the catalyst composition in the resin stream during the course of resin introduction to the mould.

The catalyst (sometimes also called an initiator) may be mixed with the resin, either before or after heating of the resin. In a preferred embodiment, there are a plurality of catalyst reservoirs which can be connected sequentially to a mixing chamber in the rein feed line, with the mixing chamber being located between the resin heater and the mould.

Where catalyst is mixed with resin in the feed line, the catalyst may itself be heated before it is mixed with the resin, so that catalyst and resin are at the same temperature when they are mixed.

Alternatively, two or more resin reservoirs in which the resin is mixed with different catalyst compositions can be maintained, and resin can be drawn from one or other of the reservoirs during the course of resin introduction.

In another alternative, there may be a single catalyst reservoir, and provision may be made for varying the catalyst concentration during the course of resin introduction.

The variation in the catalyst composition may be a variation in strength/concentration, or there may be catalysts of different chemical compositions or different reactivity.

The invention therefore also provides apparatus for resin transfer moulding comprising a resin feed line through which resin and catalyst can be introduced into a mould, a resin heater associated with the feed line for heating resin passing through the line, means for feeding catalysed resin along the feed line and means for varying the catalyst composition in the catalysed resin.

A plurality of reservoirs, each for containing a different resin/catalyst mixture, can be connected to the feed line by means of valves which allow any one reservoir, or a combination of reservoirs, to be in communication with the line at one time. Alternatively, a resin reservoir can be provided at one end of the feed line, with a plurality of catalyst reservoirs being connected to the line by means of valves which allow any one, or a combination of the catalyst reservoirs, to be in communication with the line at one time. A mixer is then provided in the line to mix catalyst and resin between the ends of the line.

The resin heater can be located upstream of the mixer, and can be a microwave heater with a rectangular cavity operating in the TE,02 mode.

Where catalyst is mixed with resin in the feed line, there may be an additional heater to heat the catalyst before it is mixed with the resin, so that catalyst and resin are at the same temperature when they are mixed.

Where it is desired to vary the concentration of the catalyst during resin injection, the catalyst may be continuously diluted with a suitable solvent (eg styrene) from a reservoir connected to the catalyst feed line through a mixing valve.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view illustrating a conventional resin transfer moulding process; Figure 2 schematically illustrates a first embodiment of resin transfer moulding apparatus in accordance with the invention; Figure 3 schematically illustrates a second embodiment of resin transfer moulding apparatus in accordance with the invention; and Figures 4 and 5 are graphs showing resin temperature against time for, respectively, a constant resin injection temperature and a variable resin injection temperature in accordance with the invention.

Figure 1 illustrates a process where a product 30 is made by resin transfer moulding. A reinforcing mat is first made into a preform and is then placed into a mould into which resin is introduced to produce the product. Figure 1 shows on the left hand side the preparation of the preform and on the right hand side the moulding process itself. As a first stage in the preparation of the preform, a mat 10 is heated in a heater 12 to soften a thermoplastic binding agent so that the mat can be shaped. The mat may be made up from a single type of fibrous reinforcement (e.g. glass fibres, carbon fibres, Kevlar [Registered Trade Mark]) coated with a binding agent and these fibres can be combined in any suitable way in accordance with the demand which will eventually be made on the final product.

The mat is then shaped in a tool 14 which, when closed, cools the mat to solidify the binding agent thus retaining the desired shape. The mat is trimmed at 20 to form a preform 18 which is then placed in the bottom half 22 of a mould. The mould has a top half 24 with an inlet 26 for resin, the flow of which is controlled by a valve 28. Once the preform is in position the mould halves 22, 24 are closed and resin is injected through the inlet 26 to fill the mould cavity and to wet-out the preform 18 thoroughly.

Once the mould has been filled with resin, it is left for sufficient length of time for the resin to cure, and the mould is then opened so that the product 30 can be ejected.

A final stage in the process is the removal of flash 32 from the edges of the product 30. The present invention is particularly concerned with the stage of injecting the resin which stage is indicated by the reference numeral 34 in Figure 1.

Figure 2 shows the resin handling equipment by means of which the resin is introduced into the mould 22, 24.

Resin mixed with catalyst is stored in a reservoir 36 beneath an air space 38. The reservoir 36 is suspended from a load cell 70 and is replenished through a resin inlet 40.

When resin is required at the mould 22, 24 it is fed in the direction of the mould along a flow pipe 42. To replenish the reservoir 36, the valve 44 is closed, the valve 46 is opened and air is pumped out of the space 38 to draw resin into the reservoir through the inlet 40. To pump resin to the mould 22,24, the valve 46 is closed, the valve 44 is opened and the space 38 is pressurised to force resin along the pipe 42. Monitoring the reading on the load cell 70 during injection gives a reading of resin mass flow rate into the mould 22, 24.

The catalysed resin is maintained in the reservoir 36 at a temperature significantly below the catalyst activation temperature so that a reservoir of catalysed resin can be maintained in a usable condition for an extended period of time.

When resin is fed to the mould 22, 24 it flows along the pipe 42 to a microwave heating unit 48 where the temperature of the resin is raised immediately before the resin flows into the mould cavity. The microwave heating unit 48 comprises a magnetron 50 in which the microwaves are generated, a waveguide 52 along which the waves pass and an applicator 54 in which the waves act on the resin to raise the resin temperature. The magnetron 50 is controlled by a control unit 56. The waveguide includes a water load 58 in a branch of the waveguide 52, and a circulator 60 allows forward transmission of microwaves along the waveguide.

Tuning screws 62 in the waveguide can be screwed in or out to alter the shape of the electromagnetic field within the waveguide. The waveguide 52 and the tuning screws 62 are set up so that microwaves produced by the magnetron 50 are directed entirely to the applicator 54. Microwave energy will be absorbed by the resin in the pipe 42, but some of the microwave energy will not be absorbed and will be reflected. The circulator 60 will direct the reflected energy into the water load 58 through which water at a known flow rate passes. By monitoring the temperature rise of the water in the water load 58, and by knowing the quantity of microwave energy produced by the magnetron 50 it is possible to obtain a result relating to the quantity of energy or power absorbed by the resin. Further details of the microwave applicator, and of the way in which the applicator uses a cylindrical cavity and is operated is operated in the Two20 mode to align the temperature profile across the flowing stream of resin with the flow rate profile are to be found in our European Patent Specification 0 618 057.

In order to be able to control the system, thermocouples 64 monitor the water temperature and thermocouples 66 and 67 monitor the resin temperature. An injection line capacitive proximity sensor 72 is provided to detect resin arrival at the mould, and vent capacitive proximity sensors 74 are provided at the mould vents.

The signals from the thermocouples and from the sensors are directed to a programmable logic controller (PLC) 76 which acts as a switching unit. The PLC 76 is interfaced with a personal computer (PC) 68 which contains a digital to analog converter to control the magnetron 50 automatically.

Detection of resin arrival by the sensor 72 causes the PLC 76 to send a digital signal to the PC 68 which signals the control unit 56 to switch the magnetron 50 ON via a digital channel on the digital/analog converter. A proportionalintegral-derivative (PID) controller, based on feedback of the resin temperature derived from the thermocouple 67 and/or based on mass of resin injected as derived from the load cell 70, adjusts the microwave power through the analog portion of the digital/analog converter.

The vent sensors 74 are also linked to the PLC 76. When these sensors sense resin arrival at the vents, the signal which they send to the PLC results in the magnetron being switched off.

In use, the resin injection stage is performed as follows.

After the mould has been closed and the mould closing clamps have been applied, the resin reservoir 36 is pressurised, the valves 44 and 45 are opened and resin injection begins.

The magnetron 50 is switched ON automatically when resin reaches the sensor 72, to start microwave heating of the resin. The heated resin temperature is monitored by the downstream thermocouple 67 which acts in a feedback loop, thereby maintaining a predetermined resin temperature. This set point temperature may be constant or may be set to follow a predetermined temperature profile over the course of one resin injection cycle. The temperature profile may vary with time or with mass of resin injected. Set point temperatures corresponding to Okg of resin injected and corresponding to the maximum amount of resin required to fill the mould are determined at the start of injection.

The set point temperature between these points is varied according to a mathematical function. The PID controller adjusts the microwave power in order to maintain the set point temperature. Alternatively, a similar function based on time required to fill the mould may be used to profile the resin set point temperature. The magnetron 50 is switched off after resin is detected at the mould vents, and the injection valve 45 is then closed. The mould is opened after a predetermined dwell time to allow the resin to cure to the stage where the product can be removed from the mould and handled.

The embodiment shown in Figure 3 shares many features with the embodiment of Figure 2, but will be described separately. In Figure 3, a mould 110 has upper and lower mould halves 112 and 114 with a mould cavity 116 enclosed by the two mould halves. The mould is fed with resin through a resin feed line 118, which includes a pump 120. Resin is stored in a reservoir 122 and is pumped from that reservoir by the pump 120 through the feed line 118 into the mould cavity 116, where a reinforcement preform has been placed before the mould was closed. The resin is pumped into the mould cavity 116 to wet out the preform and to fill the cavity. When the cavity is full, resin introduction is stopped and the resin is allowed to set. Setting of the resin will be assisted by a catalyst which will have been mixed with the resin before the resin is introduced into the mould.

In order to reduce the viscosity of the resin, and therefore to enable it to flow freely into the mould cavity and through the preform, the resin is heated in a microwave heater 124 which is located in the feed line 118, upstream of the mould 110. Because this heater 124 is heating uncatalysed resin, there is no risk of the resin setting in the feed line 118 and it is not necessary to ensure that the temperature gradient across the flowing stream of resin is aligned with the rate of flow profile across the flowing stream. The heating can therefore be done in a different cavity from that required when heating catalysed resin, preferably in a rectangular cavity using a To102 mode.

In order to vary the catalyst composition in the resin, catalyst at different concentration levels is stored in four separate catalyst reservoirs 126, 128, 130 and 132. The catalyst is pumped from one or more of these reservoirs by a catalyst pump 134 to a mixing chamber 136 in the feed line 118. The catalyst can be heated by a heater 137 before entering the mixer 136. The heater 136 can bring the temperature of the catalyst up to the same temperature as that of the resin leaving the resin heater 124, so that a temperature equilibrium is established prior to mixing.

The arrangement shown is all controlled by a computer 138, and the operation of the system will now be described.

A number of temperature sensing thermocouples 140 are built into the mould and are connected by signal lines 142 to the computer, to report on in-mould temperatures. A thermocouple 144 senses the temperature of the resin entering the mould and will report this also to the computer 138.

Depending upon the thermal history of the resin at different points within the mould, the catalyst type(s) which is (are) mixed with the resin will be varied to produce the desired sequence of cure. For example, in order to minimise the overall cycle time, it may be desirable to force the resin to cure at all points simultaneously, irrespective of the mould fill time. The information necessary to control the catalyst dosing sequence will be determined from the thermal history inside the mould. For example, in the case of a resin shot which has been injected at a constant (elevated) temperature, it will be necessary to vary the resin chemistry (and therefore kinetics) during the shot in order to promote simultaneous cure across the mould. This is necessary to compensate for the different "age" of the resin.

If catalysts of different reactivity are used, the catalyst peroxides will decompose at different temperatures to spread the energy release more evenly over the temperature range.

Normally, the computer 138 will be used to produce predetermined temperature profiles, both with the resin entering the mould and at the different temperature sensing points 140 within the mould. Because external conditions may vary (for example the ambient temperature is likely to vary), the heating and catalyst admixture may vary from cycle to cycle. In accordance with the temperature signals received by the computer 138, a control module 146 controlling the microwave heater 124 will be operated at a required level. Furthermore, valves 148 controlling catalyst flow into the pump 134 will themselves be controlled by the computer 138 to dispense the required amount of the required catalyst at the required time, to the mixing chamber 136 where mixing takes place with the resin.

In addition to thermocouples to monitor the thermal history of the process, pressure transducers can also be incorporated in the mould walls to provide additional information about the progress of each cycle.

It will therefore be seen that the catalyst type is adjusted or varied on-line, during the course of resin introduction to the mould.

Since, in the embodiment shown in Figure 3, the microwave heater 124 is heating the resin in the non-reactive state, a rectangular cavity operating in the TE102 mode can be used.

In an alternative embodiment however, the microwave cavity could be located downstream of the mixer 136, thus necessitating a cavity tuned to a different mode, as described in EP 0 618 057.

Figures 4 and 5 are temperature versus time profiles. In Figure 5, which represents the prior art where the incoming resin is unheated, the total cycle time 78 is over 700 seconds, and the injection time 80 is about 220 seconds. In this prior art apparatus, the mould is held at a desired, elevated temperature before any resin is introduced. When the cold resin is introduced, the part of the mould around the resin inlet experiences a "thermal quench", ie the temperature of the mould is caused to fall because of the influence of the incoming cold resin. As the contact time between the resin and the mould increases, so the resin heats up. When the resin reaches a certain temperature level as a result of being heated by the hotter mould walls, an exothermic reaction takes place producing temperature peaks 82.As the reaction continues, the exothermic effect reduces so the temperature decays.

Considering Figure 4, the traces 82a and 82b are produced from temperature sensors located, respectively, close to the resin inlet and close to the mould edge. It will be seen that there is a sharp temperature drop at the resin inlet as cold resin is first introduced. When injection is complete, at the end of period 80, the temperature starts to rise and continues to rise until it reaches an exothermic peaks at 84a. The trace 82b which relates to a temperature sensor located remote from the resin inlet shows that by the time the resir. reaches the mould edge, it has been heated by contact with the mould and consequently the exothermic peak 84b takes place relatively early in the cycle.As a result, and as is shown clearly by Figure 4, curing takes place much earlier at the remote part of the tool than it does close to the inlet and since the product cannot be ejected until curing is complete, curing takes a considerable period of time.

However in contrast when the resin is pre-heated and the temperature of the injected resin varies during injection, a situation can be produced as shown in Figure 5, where there is very little temperature drop (thermal shock) when the resin enters the mould. The exothermic peaks 84a and 84b are very close together and both peaks 84a and 84b are occurring after a considerably shorter time than in Figure 4. The overall cycle time is then dramatically reduced, both as compared with the prior art shown in Figure 4 and as compared with the cycle time achievable by the invention described in EP 0 618 057.

This results from heating of the resin at the start of injection, in order to reduce the resin viscosity which allows it to flow quickly into the mould, followed by an upward ramping of the resin temperature as injection progresses so that the last resin to enter the mould is at a higher temperature than the initially injected resin.

This results in the resin at the edge of the mould and at the injection gate curing at practically the same time.

Different temperature profiles will be required for different mould sizes and shapes, and the optimum temperature profile can be determined by empirical means.

It is a particular advantage of microwave heating that it can be switched on or off over very short periods. Resin transfer moulding is an intermittent process, ie once a mould has been filled with resin, the resin flow has to stop and wait until the product has been ejected before the flow can be restarted to refill the mould in the next cycle. In order to avoid waste of resin, it is important that the resin does not degrade whilst it is stationary in the feed pipe, and the use of microwave heating which can be switched on only whilst the resin is flowing and which imposes no residual thermal load on the resin can be very effective.

In practice the microwave circuit may be switched off a short time period before the end of injection, so that when injection comes to an end, the feed pipe 42 is completely filled with resin which has not been subjected to any heating at all.

The apparatus described can provide very accurate control over the resin in the mould. In many cases (but not all) it may be desirable to achieve cure of the resin at the same moment in time throughout the mould. This is likely to result in a short cycle time, which is the objective of the invention.

Claims

1. A method of resin transfer moulding in which liquid resin flows in a continuous stream into a mould and is heated by microwave heating to a temperature above the resin storage temperature, immediately before the resin enters the mould, and the temperature to which the resin is heated varies during the course of injection.

2. A method as claimed in Claim 1, wherein the course of injection is monitored by time.

3. A method as claimed in Claim 1, wherein the course of injection is monitored by monitoring the quantity of resin injected to the mould.

4. A method as claimed in any preceding claim, wherein the temperature inside the mould is monitored and the temperature to which the resin is heated is varied in accordance with the temperature inside the mould.

5. A method as claimed in any preceding claim, wherein a catalyst introduced into the resin has its composition varied during the course of resin introduction.

6. A method as claimed in Claim 5, wherein a plurality of catalyst reservoirs are connected to a mixing chamber in the resin feed line, and catalyst can be introduced into the mixing chamber for mixing with the resin from more than one of the reservoirs, in any sequence or in any combination, during the course of resin introduction to the mould.

7. A method as claimed in Claim 5, wherein the concentration of the catalyst is altered during the course of resin introduction to the mould.

8. A method as claimed in Claim 5, wherein a plurality of resin reservoirs are provided in each of which the resin is mixed with different catalyst compositions, and resin is drawn from more than one of the reservoirs, in any sequence or in any combination, during the course of resin introduction to the mould.

9. A method as claimed in Claim 6 or Claim 7, wherein the catalyst is mixed with the resin before heating of the resin.

10. A method as claimed in Claim 6 or Claim 7, wherein the catalyst is mixed with the resin after heating of the resin.

11. A method as claimed in Claim 10, wherein the catalyst is heated before mixing with the resin.

12. A method as claimed in any one of Claims 5 to 11, wherein the variation in the catalyst composition is a variation in strength/concentration.

13. A method as claimed in any one of Claims 5 to 12, wherein catalysts of different chemical compositions are used.

14. Apparatus for resin transfer moulding, the apparatus comprising a mould, means for heating the mould, a reservoir for containing liquid resin mixed with catalyst at a temperature below the catalyst activation temperature, a pipe through which resin can flow in a continuous stream from the reservoir to the mould, microwave heating means arranged to heat the resin flowing along the pipe, immediately before the resin enters the mould, and means for controlling the heating means to heat the resin to varying temperatures during the course of injection.

15. Apparatus as claimed in Claim 14, wherein the controlling means comprises electronic storage means to store a map of the intended temperature variation over the course of injection, a temperature sensor to monitor the actual resin temperature leaving the microwave heating means and a processor which receives temperature information from the sensor and adjusts the power of the microwave heating means to ensure that the temperature profile during injection follows the stored temperature map.

16. Apparatus as claimed in Claim 15, wherein the stored temperature map is a map of temperature against time.

17. Apparatus as claimed in Claim 15, wherein the stored temperature map is a map of temperature against quantity of resin injected.

18. Apparatus as claimed in any one of Claims 15 to 17, wherein a proportional integral derivative controller is used to adjust the power of the microwave heating.

19. Apparatus as claimed in any one of Claims 14 to 18, wherein a proximity sensor is provided in the injection line, between the heating means and the mould, to detect resin arrival.

20. Apparatus as claimed in any one of Claims 14 to 19, wherein a proximity sensor is provided at a mould vent to detect filling of the mould.

21. Apparatus as claimed in Claim 19 or Claim 20, wherein the sensors are capacitive proximity sensors.

22. Apparatus as claimed in any one of Claims 14 to 21, including a resin feed line through which resin and catalyst can be introduced into a mould, means for feeding catalysed resin along the feed line and means for varying the catalyst composition in the catalysed resin.

23. Apparatus as claimed in Claim 22, wherein a plurality of reservoirs, each for containing a different resin/catalyst mixture, are connected to the feed line by means of valves which allow any one reservoir, or a combination of reservoirs, to be in communication with the line at one time.

24. Apparatus as claimed in Claim 22, wherein a resin reservoir is provided at one end of the feed line, a plurality of catalyst reservoirs are connected to the line by means of valves which allow any one, or a combination of the catalyst reservoirs, to be in communication with the line at one time, and a mixer is provided in the line to mix catalyst and resin between the ends of the line.

25. Apparatus as claimed in Claim 24, wherein the resin heater is located upstream of the mixer, and is a microwave heater with a rectangular cavity operating in the TE102 mode.

26. Apparatus as claimed in Claim 23 or Claim 24, wherein a catalyst heater is provided upstream of the mixer.

27. Apparatus as claimed in any one of Claims 24 to 26, wherein means are provided for varying the concentration of the catalyst as it flows towards the mixer.

28. A method of resin transfer moulding using a settable resin, wherein the resin is heated immediately before it is introduced into the mould, to reduce the resin viscosity, and wherein the catalyst composition in the resin stream is varied during the course of resin introduction to the mould.

29. A method as claimed in Claim 28, wherein the preheating of the resin before introduction into the mould is carried out using a microwave heater.

30. A method as claimed in Claim 28 or Claim 29, wherein a plurality of catalyst reservoirs are connected to a mixing chamber in the resin feed line, and catalyst can be introduced into the mixing chamber for mixing with the resin from more than one of the reservoirs, in any sequence or in any combination, during the course of resin introduction to the mould.

31. A method as claimed in Claim 28 or Claim 29, wherein the concentration of the catalyst is altered during the course of resin introduction to the mould.

32. A method as claimed in Claim 28 or Claim 29, wherein a plurality of resin reservoirs are provided in each of which the resin is mixed with different catalyst compositions, and resin is drawn from more than one of the reservoirs, in any sequence or in any combination, during the course of resin introduction to the mould.

33. A method as claimed in Claim 30 or Claim 31, wherein the catalyst is mixed with the resin before heating of the resin.

34. A method as claimed in Claim 30 or Claim 31, wherein the catalyst is mixed with the resin after heating of the resin.

35. A method as claimed in Claim 34, wherein the catalyst is heated before mixing with the resin.

36. A method as claimed in any one of Claims 28 to 35, wherein the variation in the catalyst composition is a variation in strength/concentration.

37. A method as claimed in any one of Claims 28 to 36, wherein catalysts of different chemical compositions are used.

38. Apparatus for resin transfer moulding comprising a resin feed line through which resin and catalyst can be introduced into a mould, a resin heater associated with the feed line for heating resin passing through the line, means for feeding catalysed resin along the feed line and means for varying the catalyst composition in the catalysed resin.

39. Apparatus as claimed in Claim 38, wherein a plurality of reservoirs, each for containing a different resin/catalyst mixture, are connected to the feed line by means of valves which allow any one reservoir, or a combination of reservoirs, to be in communication with the line at one time.

40. Apparatus as claimed in Claim 38, wherein a resin reservoir is provided at one end of the feed line, a plurality of catalyst reservoirs are connected to the line by means of valves which allow any one, or a combination of the catalyst reservoirs, to be in communication with the line at one time, and a mixer is provided in the line to mix catalyst and resin between the ends of the line.

41. Apparatus as claimed in Claim 40, wherein the resin heater is located upstream of the mixer, and is a microwave heater with a rectangular cavity operating in the TE102 mode.

42. Apparatus as claimed in Claim 39 or Claim 40, wherein a catalyst heater is provided upstream of the mixer.

43. Apparatus as claimed in any one of Claims 40 to 42, wherein means are provided for varying the concentration of the catalyst as it flows towards the mixer.

44. Apparatus for resin transfer moulding substantially as herein described with reference to the accompanying drawings.

45. A method of resin transfer moulding substantially as herein described with reference to the accompanying drawings.