GB2300138A

GB2300138A - Seal arrangement in vacuum bagging moulding

Info

Publication number: GB2300138A
Application number: GB9508152A
Authority: GB
Inventors: Peter Butterworth
Original assignee: Fluorovac Ltd
Current assignee: Fluorovac Ltd
Priority date: 1995-04-21
Filing date: 1995-04-21
Publication date: 1996-10-30
Also published as: GB9508152D0

Abstract

An apparatus for forming composite articles comprises a stretchable membrane 12 adapted to be placed over an article 34 to be moulded on a platen 16, and carrying a seal 14 around its outer periphery characterised in that the underside of the outer periphery of the membrane 12 adjacent the seal 14 is provided with a vacuum path 32. There is a vacuum valve 39 and the provision of vacuum path 32 12 ensures that the vacuum drawn through this valve extends completely to the outer seal, holding the latter into sealing contact with platen 16. In order to initiate the seal, a framework member 38 is pressed around the periphery to hold the seal 14 in contact with platen 16 while the initial vacuum is drawn. Thereafter the framework 38 can be removed. The assembly may then be placed in an autoclave as before. The fact that no permanent manifold is in position allows the apparatus of the invention to be used with small autoclaves where there was insufficient clearance for an apparatus of the type having a permanent manifold. Morover, less clearance between the stack and the edge of the platen is required.

Description

VACUUM BAGGING This invention relates to an apparatus useful in the production of composite products by the vacuum bagging system and in particular relates to an improved vacuum bag and seal assembly.

Vacuum bagging is a term used to describe a method of producing pressure on various resin filled, carbon, glass, synthetic material and/or metal laminates (known as composites) while curing and consolidation takes place, usually by means of heating in a pressurized autoclave or oven or hydraulic press.

The method may be used in the production of various composite parts useful in the aircraft and motor industries, as well as for the production as articles such as printed circuit boards for the electronic industry.

EP-A-0175510 describes an apparatus of the above type which comprises a stretchable membrane or vacuum bag held within a rigid framework which is hollow and forms a vacuum manifold supplying vacuum to a seal affixed to the membrane and carried on the framework by one or more passages communicating with a cavity. The resilient seal defines the cavity within which a vacuum can be drawn when the framework and membrane are placed on a mould base.

EP-A-0641276 discloses a modification of this apparatus in which the outer seal is attached to the manifold as before but the inner seal is attached only to the membrane. This allows a number of membranes of different pre-shaped sizes to be provided with a single manifold. Nevertheless the manifold is always present during the curing process within an autoclave.

In a typical operation, for example curing a stack of printed circuit boards, the boards are laid up and covered with a breather fabric (which allows the vacuum to percolate round the stack) and then the vacuum bag membrane. The vacuum is drawn locking the manifold, and thus the stack, to a platen or mould base, and the assembly placed within an autoclave. The pressure within the autoclave (up to 300 psi normally) acts on the outside of the vacuum bag membrane, or hood, and crushes the breather fabric which causes the periphery of the membrane, between the stack and the manifold, to be stretched, and it is at this point that failure most often occurs. Moreover, the manifold assembly requires a clearance of a minimum of approximately 50mm from the edge of the stack to the edge of the platen, and thus the size of the stack must be at least lOOmm less than the size of the platen.In addition, the height of the manifold is such that there is often insufficient clearance in the top of cylindrical autoclaves for the above types of vacuum bagging assembly to be used.

The invention seeks to provide a vacuum bagging apparatus improved in the above respects.

According to the present invention there is provided an apparatus for forming composite articles which comprises a stretchable membrane adapted to be placed over an article to be moulded on a platen, the membrane carrying a seal around its outer periphery characterised in that the underside of the outer periphery of the membrane adjacent the seal is provided with a vacuum path.

In contrast to the apparatus as described in the above European patent publications, no permanent manifold is provided, nor is there a double seal arrangement defining a cavity therebetween.

The vacuum hood is provided with a vacuum valve as is conventional in the art, and the provision of the vacuum path below the outer periphery of the membrane ensures that the vacuum drawn through this valve extends completely to the outer seal, holding the latter into sealing contact with the platen.

In order to initiate the seal, a framework member may be provided which is pressed around the periphery of the hood, over the seal area, to hold the seal in contact with the platen while the initial vacuum is drawn. Thereafter the framework can be removed and the vacuum maintains the hood assembly in place. The platen/vacuum hood assembly may then be placed in an autoclave as before. The fact that no permanent manifold is in position allows the apparatus of the invention to be used with small autoclaves where there was insufficient clearance for an apparatus of the type having a permanent manifold.

Morover, less clearance between the stack and the edge of the platen is required. In addition, energy usage may be reduced as the need to heat up and cool down the mass of the manifold is eliminated. Since up to eighteen hood/manifold assemblies could be loaded into an autoclave, the saving here can be significant.

In a preferred form of the invention, the periphery of the hood membrane is formed with a fold of excess material (like a "wrinkle") to allow horizontal movement of the vertical walls of the hood in an autoclave and prevent undue stress on the peripheral edge adjacent the seal.

The hood membrane may be produced from silicone rubber as is known in the art, or from a fluoropolymer material where higher temperature resistance is required. The seal may be of the same general shape as that described in EP-A-0641276, being of generally square section and having an outwardly extending tapered curtain, as this has been found to be extremely effective in practice.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which Figure 1 is a diagrammatic representation of a manifold according to EP-A-0641276; and Figure 2 is a similar view to figure 1 illustrating an apparatus in accordance with the invention.

Referring to the drawings, and in particular figure 1, a vacuum bagging apparatus in accordance with EP-A-0641276 comprises a vacuum manifold 10, for example, of aluminium, to which is affixed a stretchable membrane or hood 12 and an outer seal 14 is bonded to the underside of the manifold 10. The seal 14 will in use squash against a mould base or platen 16 to provide good sealing contact. In the apparatus of figure 1 a second seal 18 is provided and the two seals between them define a vacuum cavity 20 which communicates with the manifold 10. The manifold 10 has a vacuum valve so that when a vacuum is pulled within the interior of the manifold 10 it evacuates the cavity 20 also pulling the seals 14,18 into close sealing contact with the platen 16. A separate vacuum valve (not shown) is also provided for the hood 12.

As illustrated in figure 1 the curvature and size of small autoclave diagrammatically shown at 20 may be such as to prevent the use of the apparatus of figure 1 since the size of the peripheral manifold is such as to interfere with the walls of the autoclave.

This problem is overcome iii accordance with the invention by the apparatus illustrated in figure 2, in which like numerals will be used for like parts. In this, the outer peripheral edge 22 of the hood 12 is provided with a seal 14 of the same general shape as that in figure 1. The seal 14 has an outwardly extending tapered curtain 24 and a small cut out section 26. The seal 14 may be formed integrally with the hood 12 but, more probably, will be bonded thereto at 28. The outer periphery 22 is formed with an excess of material in the form of a fold 30 to allow for stretching as hereinafter more fully described.

The under portions of the outer periphery 22 are formed with, either, a moulded-in pattern or a laminated open weave fabric of, for example, woven wire or woven glass fibre, which may be formed in place during the moulding of the hood 12. This constitutes a vacuum path 32.

The hood 12 encloses a stack 34 of articles to be moulded, for example, a stack of printed circuit boards. Around the stack 34 is placed a breather medium such as a breather fabric 36.

A separate, unattached, framework 38 is also provided for the purpose to be described more fully hereinafter.

In use, a stack of articles to be moulded 34, for example a stack of printed circuit boards, is placed on the platen 16 and covered with a collapsible breather fabric 36. The hood 12 is then placed over the stack as illustrated in figure 2.

The outer periphery 22 comes into contact with the platen 16 and the undersides 32 sit on the upper surface of the platen 16. In order to ensure good contact between the seal 14 (in particular the curtain 24) and the platen 16, the framework 38 (which extends around the entire periphery) is placed over the outer periphery 22 and manual pressure applied so as to firmly press the seal 14 against the platen 16. Vacuum is drawn at one point only, via a valve 39 in the hood 12 and is transmitted through the vacuum path 32 right to the seal 14 thus pulling the seal tight against the platen 16. At this point the frame 38 can be removed and stored for further use as necessary.

The vacuum bag/platen assembly may now be placed within an autoclave where it will be subjected to heat and pressure in order to laminate the printed circuit boards within the stack 34. Absence of a manifold 10 enables the assembly to placed in a smaller autoclave than with the apparatus of figure 1.

Once in the autoclave, pressure is applied and the breather medium 36 is crushed. This causes the vertical walls of the hood 12 to move to the right (as viewed in figure 2) closer to the stack 34. The excess material within the fold 30 allows this to happen without putting any excessive strain on the outer periphery 22, thus eliminating a common point of failure in previous vacuum bag arrangements.

A further advantage with the apparatus according to the invention is as follows. Many autoclaves in service have platens 16 which have bowed owing to poor design of support framework or overloading. The apparatus described above in relation to figure 1 requires a flat platen to be effective.

However, the apparatus in accordance with the present invention may be used with a bowed platen as follows. The framework 38 in plan view, is rectangular and resembles a picture frame. If two opposed sides of the frame are manufactured from a relatively rigid material, and the other opposed sides from a flexible material, a bowed platen 16 can be accommodated.

The stack is oriented so that the rigid sides lie along the unbowed dimension while the flexible sides overlie the bowing.

Manual pressure approximately half way along the flexible sides bends them until they conform to the concavity thus pressing the outer periphery 22 underneath them to the surface of the platen evenly. The vacuum is then drawn as before. An alternative construction is to make the whole framework 38 of relatively rigid material and line the underside with a resilient material such as soft rubber foam to take up uneveness in the platen.

The apparatus of the invention thus has a number of advantages over the earlier proposals. Firstly, the absence of a manifold 10 enables a given snack size to be cured in a smaller autoclave; secondly, a preferred embodiment of the invention eliminates a common point of failure by providing an excess of fabric to allow for horizontal movement within the autoclave; thirdly, the apparatus of the invention can be employed with platens which have some concavity; and, fourthly, the fact that the hood 12 is not attached in any way to a rigid manifold enables it to be rolled up for storage or transport in contrast to designs with an attached rigid manifold.

Claims

1. An apparatus for forming composite articles which comprises a stretchable membrane adapted to be placed over an article to be moulded on a platen, the membrane carrying a seal around its outer periphery characterised in that the underside of the outer periphery of the membrane adjacent the seal is provided with a vacuum path.

2. An apparatus as claimed in claim 1 in which the periphery of the hood membrane is formed with a fold of excess material to allow horizontal movement of the vertical walls of the hood in an autoclave and prevent undue stress on the peripheral edge adjacent the seal.

3. An apparatus as claimed in either of claims 1 to 2 in which the hood membrane is produced from silicone rubber or from a fluoropolymer material where higher temperature resistance is required.

4. An appartus as claimed in any of claims 1 to 3 in which the seal is of generally square section having an outwardly extending tapered curtain.

5. An apparatus as claimed in any of claims 1 to 4 in which the under portions of the outer periphery of the membrane are formed with a moulded-in pattern or a laminated open weave fabric of woven wire or woven glass fibre to constitute the vacuum path.

6. A method of moulding which comprises placing an article to be moulded on a platen, covering it with a breather fabric and then a stretchable membrane such that the outer periphery of the membrane, and the seal, come into contact with the platen, placing a removable framework which extends around the entire periphery over the outer periphery and seal, applying manual pressure so as to firmly press the seal against the platen, drawing vacuum thus pulling the seal tight against the platen, removing the frame and placing the assembly within an autoclave where it will be subjected to heat and pressure in order to complete the moulding.

7. An apparatus substantially as hereinbefore described with reference to and as illustrated the accompanying drawings.

8. A method of moulding substantially as hereinbefore described with reference to and as illustrated the accompanying drawings.