BACKGROUND OF THE INVENTION

In the production of articles made from carbon fibre/resin mixtures having high fibre contents and requiring cure at temperatures of up to 400°C it is essential to have an autoclave whose temperature can be controlled very accurately. The temperatures involved are higher than can conveniently be reached with steam heated autoclaves and the use of live steam at high pressure requires elaborate and expensive safety equipment. Electrically heated autoclaves are more commonly employed for curing articles of this kind, but are expensive to install and operate and suffer from the additional problem that if a heating element fails while the autoclave is in the middle of a cure cycle the element cannot be replaced and the whole batch of material being treated may be lost.

In the case of high value materials such as carbon fibre this is a serious problem.

SUMMARY OF THE INVENTION Accordingly it is an obj ect of the invention to provide an autoclave suitable for the treatment of fibre reinforced plastics materials requiring to be cured at high temperatures with accurate temperature control .

Broadly stated the invention provides an autoclave suitable for the heat treatment of fibre reinforced plastics materials comprising a gas -tight tank having a closed gas flow loop defined by a heat exchange pipe that is exposed to the interior of the tank and has a gas inlet and outlet accessed from outside the tank, a gas burner connected to the inlet of the gas flow loop so that burnt g as theref rom f lows through the loop, an impe l ler conne c ted to th e out le t from the ga s flow lo op and operable to suck burnt gas through the loop and a fan inside the furnace for circulating gas within the furnace to exchange heat with said heat exchange pipe. It has been found that temperature control of + 1°C can be obtained in an autoclave of the above kind . The autoclave is of simple and inexpens ive construction and does not require elaborate safety precautions because the heating unit can be a well establ ished natural gas or propane burner and the flue gas leaves the impeller at a comparatively low temperature and not under pressure . Moreover the or each gas burner is mounted outside the autoclave where in the event of failure it can be removed and replaced in a matter of minutes so that loss of a charge o f compon ent s i s no longer an in ev it ab l e consequence of burner failure.

The tank may be generally bell- shaped, is mounted with its axis horizontally and is closed at its mouth with an access door or port , the or each heat exchange pipe taking a sinuous path and entering and leaving the tank through the end wall thereof . Pre ferably there are at least two heat exchange loops mounted to opposed sides of

the tank, each heat exchange loop making two passes along the tank and back and arranged so that a horizontal diameter between the two passes of the opposed loops is unobstructed for a substantial distance from the tank mouth whereby articles having substantially the full diameter of the tank may be inserted. An additional heat exchange loop may be provided in the floor. Preferably each heat exchange pipe enters and leaves the tank through a flanged sleeve and has flanges that are fastened to the flanges of the inlet and outlet end sleeves whereby said heat exchange pipes are removably retained in the autoclave.

BRIEF DESCRIPTION OF DRAWINGS . An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal vertical section of a gas fired autoclave having three heating circuits;

Figure 2 is a view of the interior of an autoclave similar to Figure 1 but having only two heating circuits. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In Figure 1 an autoclave ^' tank structure 10 that may typically be about 12 feet (3.658 metres) in length and diameter about 5 feet (1.524 metres) has a loading door 12. Vacuum lines 14 are provided for connection to the mould side of a mould former (not shown) having a flexible diaphragm thereover and connectable through valve 18 to vacuum and through valve 20 to air for purposes that will be described hereinafter. Pressure lines 16 are also provided with a control valve 22 cross-linked to the door 12 so that the interior of the autoclave cannot be pressurised with the door open.

The interior of the autoclave tank is provided with exposed heater tubes 24 running the length of the tank in serpentine manner as shown and divided into side sets 24b,

24c and an optional bottom set 24a, each set defining a closed gas flow loop that enters and leaves through the

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end wall 28. The tubes 24 are of bore typically 2-2.5 inches (5.080-6.350 cm) and are fabricated of mild steel or stainless steel or any other material resistant to high temperatures, eg. up to 600°C. The tubes 24 may be coated on their exterior surfaces with sprayed on aluminium metal or other coating material that promotes increased efficiency of surface radiation.

The tubes 24 are removably mounted in the autoclave tank by means of flanged sleeves 30 and welded-on flanges 32. The entry side of each tube 24 is provided with a gas fired heater 34 including a gas burner, a regulator valve ^' to sense gas flow and maintain combustion, and an ignition system. The flue outlet from the burner is directly connected by a sleeve to the inlet to the respective tube 24 in such a manner that the heater 34 can be replaced rapidly in the event of boiler failure. The discharge end of the tube 24 is provided with an impeller 36 by which a negative pressure is produced towards said discharge end of the tube 24 to maintain a flow of flue gas therethrough. Spent flue gas from the impeller may be vented through an ordinary flue. The number of independent heater circuits constituted by heater 34, flow tube 24 and impeller 36 may be varied depending upon the capacity of the intended heater 34, the size of the autoclave tank and operational convenience but normally there will be at least two independent circuits. The provision of two loops aids heat distribution, and in the embodiment shown the tube loops 24b, 24c are configured so that a plate can be inserted horizontally to the maximum diameter of the tank 12. The burner power should, of course, be correlated with the bore of the tubes 24 and the length to the first elbow therein so that an unacceptable heat build up at the first elbow does not take place. Where the length of the autoclave is sufficient, usually more than 30-40 feet (9.144-12.801 metres) the tubes 24 may be straight and may simply exit via an elbow through the tank wall 10. A motor 38 mounted

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on the tank end wall is operatively connected to a radial flow impeller 40 within the tank 10 to produce circulatory gas flow with the autoclave tank.

The tank 10 may be insulated with mineral wool and covered with a metal sheath. Thermocouples 42 through the tank wall 10 responsive to the gas temperature are connected to a control unit 44 which is operatively connected to the several heaters 34 to turn them on or off to maintain a preset temperature in the autoclave. The control unit 44 may also be operatively connected to motor 38 so that the said motor 38 and fan 40 always run when the heaters 34 are operational. A time delay relay in the ignition system of heater 34 responds to signals from the control unit 44 so that the heater 34 is switched on only after the control thermocouple has indicated a temperature below the rated temperature for a predetermined time period, thereby preventing short duration ON-OFF cycles of the heater 34.

For the hot moulding of fibre reinforced plastics components, a mould form with laid up resin, bleeder cloth and pressure plate is enclosed in a vacuum bag introduced into the autoclave and the vacuum bag connected to lines 14. Vacuum is applied and heat is applied to the furnace to typically about 250°F (121.11°C)at a preset heat up rate of 1-5°F (-17.22° - -15.00°C) per minute. A first dwell period of 30-60 minutes is required for resin gelation and when the resin flow characteristics are correct pressure of from 6-7 bar is applied from the interior of the vessel using the line 16 so as to form the moulding and at an appropriate time after the pressure has stabilised the vacuum is switched off and the temperature in the furnace is raised through a second heat-up ramp to the cure temperature, typically 350°C which is maintained for an appropriate time. After cure is complete the autoclave is cooled either naturally or by air cooling, cool air being blown by another impeller through the tubes 24 or simply by running fan 36 with heater 34 off to draw

cool air into the tubes 34.

It will be appreciated that various modifications may be made to the described embodiment without departing from the invention , the scope of which is defined in the appended claims.