GB2073648A

GB2073648A - Vacuum bagging

Info

Publication number: GB2073648A
Application number: GB8109317A
Authority: GB
Original assignee: Aerovac Systems Keighley Ltd
Current assignee: Cytec Process Materials Keighley Ltd
Priority date: 1980-04-11
Filing date: 1981-03-25
Publication date: 1981-10-21
Also published as: GB2073648B

Abstract

A method and apparatus is disclosed for forming a vacuum bag for moulding composite articles which comprises a base 10, triangular section ribbing 12 fixed to the base and about its perimeter, a stretchable membrane 14 of elastomeric material such as silicone rubber held within a framework 16 and able to form a seal with the ribbing 12. The framework 16, which may be a T-section aluminium frame, may be fastened to the base 10 by means of clips thereby forming a seal between the ribbing 12 and the stretchable member 14 allowing a vacuum to be drawn, e.g. by means of vacuum points 18 between the membrane, the ribbing and the base. Once the vacuum is drawn and the membrane conforms closely to the article to be moulded placed on the base 10, the whole is transferred to an autoclave curing. <IMAGE>

Description

SPECIFICATION Vacuum bagging This invention relates to a method and apparatus useful in the production of carbon fibre composite materials, glass reinforced Plastics, laminates and the like by the vacuum bagging system.

Vacuum bagging is a term used to describe a method of producing pressure on the various components in a carbon fibre composite material, glass reinforced plastics material, metal/metal bonds and other composite materials while these are cured, usually by means of heating in an autoclave. The method may also be used in the production of various parts useful in the aircraft industry by bonding laminates and honeycomb sections.

Previously the normal method which has been used involves "laying up" the component together with layers of release sheets, bleeder cloths, air bleed ropes, rubber compression sheets and the like, on a metal base plate, covering the whole with an impervious sheet of flexible plastics material. The latter has to be securely sealed to the base plate generally by means of a special putty and adhesive tapes to allow a vacuum to be drawn inside the bag. The bag is draped over the contours of the component and slack material has to be secured by adhesive tapes to prevent drainage by air movement when the lay-up is in the autoclave. To ensure a good lay-up and an efficient vacuum seal, the process needs careful workmanship and takes considerable time to operate.Further, after curing in the autoclave, the vacuum bag must be removed from the special putty, which entails considerable cleaning up of the base plate and the bag itself often must be replaced.

The invention seeks to provide a method and apparatus capable of achieving the same end result but without needing putty in a very much quicker time and with greater reliability.

According to the present invention there is provided an apparatus for forming a vacuum bag which comprises a base, ribbing fixed to the base about its perimeter, stretchable membrane held within a framework and able to form a seal with the said ribbing, means for fastening the framework to the base thereby forming a seal between the ribbing and the stretchable membrane, and means for forming a vacuum between the membrane the ribbing and the base.

In one embodiment, the base may be a rigid base plate, e.g. a metal plate, but in another embodiment the base may itself be a stretchable membrane held within a framework.

The invention further provides a method of moulding which comprises placing an item to be moulded on a base having ribbing fixed thereto about its perimeter, locating a stretchable membrane held in a framework over the item on the base, fixing the framework to the base thereby forming a seal between the stretchable membrane and the ribbing, drawing a vacuum between the membrane, the ribbing and the base plate, and curing the article.

Preferably, curing is carried out by heating the bagged lay-up, for example in an autoclave.

Consequently, the material from which the various components are made are of necessity capable of standing curing temperatures without degradation. The preferred material for the membrane is an elastomeric silicone rubber sheet having high elongation and tear-strength properties and being capable of continued service at 4000 F. Similarly the ribbing which forms the seal with the membrane is also of a silicone rubber material.

The membrane is attached to a framework, for example an aluminium T-section framework, so that the membrane may be handled easily. This is especially important when the apparatus used is capable of taking relatively large lay-ups. The membrane, within its frame, can easily be placed in its correct position over the base and the membrane firmly attached to the base by clipping the frame to the base. The ribbing will normally be indented somewhat from the perimeter of the base so that it forms a seal with the membrane within the perimeter of the base and the framework carrying the membrane. The rigid framework has the further advantage that it secures the perimeter of the membrane while the vacuum is drawn thus dispensing with the need for separate adhesive tape and the like which was formally used in these circumstances.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which~ Figure 1 is a perspective view of a base of one embodiment of~the invention; Figure 2 is a perspective view of a membrane suitable for use with the base of Figure 1; Figure 3 is a perspective view of a second embodiment of the invention: and Figure 4 is a cross-section, on an enlarged scale, of the ribbing used.

Referring to Figures 1 and 2, the apparatus comprises an aluminium base plate 10 having a sealing rib 12 located about its perimeter, and spaced in somewhat from the perimeter. The rib 12, as can be seen from Figure 4, is basically triangular in cross-section and its upper apex has been rounded off. An article to be moulded is laid up within the perimeter of the ribbing 12 on the base plate 10. A membrane 14 of flexible silicone rubber having high elongation and tear-strength properties is attached to a T-section aluminium frame 1 6 by means of an appropriate RTV adhesive. The membrane 14 contains two vacuum points 18, to which a vacuum extraction line can be attached. In use, the article to be moulded is placed on the base plate 10 and the membrane 14 placed over the article.Generally, the framework 16 of the membrane is co-extensive with the dimensions of the base plate 10 and, when the membrane 14 is in position over the base plate 10, the framework 16 surrounds the rib 12, which is of sufficient height to contact the membrane 14.

The framework 16 is clipped at various points to the base 10 holding it securely fixed thereto. This has the effect of ensuring that the rib 12 contacts the membrane 14 completely around its periphery forming a sealing contact therewith. The centre of the membrane of course conforms to the shape of the laid-up composite placed on the base plate. A vacuum is then drawn between the base, the rib 12 and the membrane 14 by means of a vacuum extract pipe attached to either or both of the vacuum points 18 when the membrane 14 will conform exactly to the contours of the lay-up. The cross-sectional shape of the rib 12 ensures that as the vacuum is drawn, the membrane 14 conforms closely to the rib in sealing contact therewith, as shown most clearly in Figure 4.On removing the vacuum extract pipes, the vacuum within the apparatus is maintained, and excellent vacuum integrity can be maintained for a long period of time. The apparatus is then transferred to an autoclave set at the correct temperature for curing the article for a time sufficient to effect cure. If desired, thermo-couples or the like may be inserted into the apparatus to ensure that the temperature in a vicinity of the composite is correct.

Referring now to Figure 3, the base 10 here is replaced by a second membrane held within a framework 20 co-extensive with the framework 16. The second membrane contains a rib 12 sealed to it just within the periphery of the framework 20 by means of a suitable RTV adhesive. The two frameworks 16 and 20:may be clamped to each other by means of a clamping strip 22 with the article to be moulded between the two membranes. When the two frameworks 16 and 20 are clamped together the sealing rib on the lower membrane contacts the membrane 14 in a sealing manner similar to that described with reference to Figures 1 and 2 above, and when the vacuum is drawn the membranes-mould themselves to the contours of the article and form a vacuum tight seal about the# rib 12. As before, the apparatus can be taken to an autoclave for heat curing.

As is normal, the composite to be moulded may be laid-up using layers of reinforced resin, release sheets, bleeder cloths, air bleed ropes, rubber compression sheets and the like as wiith conventional vacuum bag moulding.

If the article to be moulded is of unusual shape, the membrane may be made from an uncured silicone rubber moulded to the shape required and then fully cured before being inserted in the framework 14.

When the embodiment of the type shown in Figure 3 is used, it may often be desirable to provide stiffening in the form of cross members 24.

Vacuum monitoring connection points can be inserted into the membrane or the tool base as required. Generally however self-sealing connectors are used so that once the vacuum has been applied the vacuum pipes rnay-be removed without loss of vacuum within the apparatus. As before, thermo-couple leads may be led into the apparatus through the seal rib.

Using the above system, the use of conventional film and acrylic putty is eliminated and the membranes used have an extended life.

The location of the membrane within a framework enables the top frame to be clipped into position on the base very quickly compared with the skilled and laborious job of setting up the diaphragm with putty and adhesive tape under the previous method. Further, the base need not be cleaned after a moulding operation since putty is no longer used. Finally, the sealing of the membrane about the ribbing used in the apparatus of the invention is extremely good and vacuum integrity can be maintained at a higher level and for longer periods than hitherto possible.

Claims

1. An apparatus for forming a vacuum bag which comprises a base, ribbing fixed to the base about its perimeter, a stretchable membrane held within a framework able to form a seal with the said ribbing, means for fastening the framework to the base thereby forming a seal between the ribbing and the stretchable membrane, and means for forming a vacuum between the membrane, the ribbing and the base.

2. An apparatus according to claim 1 in which the base is a rigid plate.

3. An apparatus according to claim 2 in which the base is aluminium plate.

4. An apparatus according to claim 1 in which the base is a stretchable membrane held within a framework.

5. An apparatus according to claim 4 in which the ribbing is fixed to the stretchable membrane comprising the base about its perimeter.

6. An apparatus according to any of the preceding claims in which the ribbing is triangular in cross-section.

7. An apparatus according to any preceding claim in which the membrane is formed from an elastomeric silicone rubber sheet having high elongation and tear-strength properties.

8. An apparatus according to any preceding claim in which th-e means of fastening the framework to the base comprises clips.

9. An apparatus as claimed in any one of claims 1 to 7 in which the means for fastening the membrane to the base comprises a clamping strip.

10. An apparatus according to any preceding claim in which the means for forming a vacuum between the membrane, the ribbing and the base comprises one or more vacuum points set into the membrane.

1 An apparatus according to claim 10 in which the or each vacuum point comprises a selfsealing valve. -

12. An apparatus for forming a vacuum bag substantially as hereinbefore described with reference to-and as illustrated in Figures 1, 2 and 4, or Figures 3 and 4 of the accompanying drawings

13. A method of moulding which comprises placing an item to be moulded on a base having a ribbing fixed thereto about its perimeter, locating stretchable membrane held in a framework over the item on the base, fixing the framework to the base thereby forming a seal between the stretchable member and the ribbing, drawing a vacuum be#tween the membrane, the ribbing and the base plate, and curing the article.

14. A method according to claim 13 in which the curing is carried out by heating the bagged assembly in an autoclave.

15. A method according to either of claims 1 3 - or 14 in which the item to be moulded comprises a lay-up of carbon fibre composite materials, glass reinforced plastics materials, or laminates.

16. A method according to any of claims 13 to 15 in which the membrane is firmly attached to the base by clipping the frame to the base.

17. A method of moulding substantially as hereinbefore described with reference to and as illustrated in Figures 1, 2 and 4, or Figures 3 and 4 of the accompanying drawings.