WO1991014564A1

WO1991014564A1 - An evacuation chamber

Info

Publication number: WO1991014564A1
Application number: PCT/GB1991/000421
Authority: WO
Inventors: Robert Samuel Wilson; Mark Anthony Braniff
Original assignee: Short Brothers Plc
Priority date: 1990-03-20
Filing date: 1991-03-19
Publication date: 1991-10-03
Also published as: GB2245215B; AU7460691A; IL97588A0; GB9006253D0; GB9105455D0; GB2245215A

Abstract

An evacuation chamber includes a base plate (56), a vacuum cover (72) located over the base plate, and an evacuation valve to enable the chamber to be evacuated. The valve includes a valve body (42) having a vent passage extending through the body from an inner vent hole (43) to an outer vent hole. The valve body (42) is located between opposed edge sealant strips (64, 66) which provide an airtight seal between the base plate and the vacuum cover at their periphery. The inner vent hole (43) is covered with wire mesh (54) to prevent blockage of the vent passage, and a valve coupling (40) communicates with the outer vent hole of the valve body (42).

Description

AN EVACUATION CHAMBER

The present invention relates to an evacuation chamber by which is meant a chamber which can be at least partly evacuated.

The invention is particularly concerned with, but not restricted to, an evacuation chamber for manufacturing a composite material, such as for example a composite material which is suitable for forming an outer surface material of an aircraft.

The term "material" includes any suitable component or structure, such as for example a laminate joint, depending on the application of the invention.

A previously proposed composite material has been manufactured by embedding high strength fibres having a high modulus of elasticity within a homogeneous matrix.

In one example of such a material, the fibres are made of carbon, aramid or boron, and the matrix can be formed of material such as an epoxy, bismalemide or polyimide resin.

The fibres are supplied in uni-directional, woven or fibrous mat format which is pre-impregnated with slightly cured resin to form an intermediate fabric material. This intermediate fabric material is flexible and tacky, and the positions of the fibre elements therein are not fixed.

Fig 1 of the accompanying drawings is a diagrammatic illustration of the manufacture of a composite material.

Referring to Fig 1, layers of intermediate fabric material are assembled one upon the other in a mould tool The stack 4 is covered by a vacuum bag which makes an airtight seal with the mould tool to so as to form an evacuation chamber. This vacuum bag is in the form of a flexible membrane which is capable of withstanding high compression forces at high temperature. The stack 4 is then compressed and heated to cure the resin and form a composite laminate material 6 illustrated on the right-hand side of Fig 1. Air and volatiles are removed from the evacuation chamber by means of an evacuation valve.

Fig 2 is a sectional elevation of an evacuation chamber including one conventional evacuation valve installation.

Fig 3 is an exploded view (partly in section) of the evacuation valve of Figure 2. Referring to Figures 2 and 3, a stack 8 of layers of intermediate fabric material is placed in an evacuation chamber including a base plate 10.

A vacuum bag 12 is placed over the base plate 10 and an airtight seal is made between the base plate 10 and the vacuum bag 12 by means of a sealer strip 14 located at the periphery of the vacuum bag 12 where it contacts the base plate 10. An edge breather 16 is located on the base plate 10, and a surface breather 18 and extra surface breather 20, both of plate form, are located near the upper portion of the vacuum bag 12. The surface breather 18 has an area slightly less than the area of the top of the vacuum bag 12. The space inside the vacuum bag 12 is evacuated by withdrawing air through an evacuation valve 22 located near one edge of the vacuum bag 12. The edge breather 16 and the surface breathers 18 and 20 assist in ensuring that air is withdrawn from substantially the entire region covered by the vacuum bag 12, and that the vacuum bag 12 does not wrinkle or distort unduly during evacuation.

The evacuation valve 22 includes a threaded shaft 24 which extends from a circular base member 26 having a raised upper face.

A pressure plate 28 is located on the shaft 24 to sit on the base member 26 over a gasket 30. A locking ring 32 and a quick release coupling 34 are screwed onto the shaft 24, and a vent passage extends through the shaft and the coupling. A compression ring 36 extends downwardly from the lower face of the pressure plate 28.

To secure the evacuation valve to the evacuation chamber at the desired location, the vacuum bag 12 is slightly stretched over the quick release coupling 34 to leave a circular stretch mark in the vacuum bag 12. The bag in then pinched at the region of the stretch mark, and the bag is cut to form a hole of approximately the size of the threaded shaft 24. Although this hole need not be circular its largest diameter must be less than the inside diameter of the compression ring 36 on the pressure plate 28. This hole in the vacuum bag 12 must be large enough to slip over the threaded shaft 24 without the need to stretch the vacuum bag 12. The threaded shaft 24 is located in the newly cut hole in the vacuum bag 12, any wrinkles in the bag are smoothed out, the pressure plate 28 is mounted on the threaded shaft 24 and the locking ring 32 is screwed onto the threaded shaft 24 to secure the shaft 24 to the vacuum bag 12 with the base member 26 sitting on the extra surface breather 18. The quick release coupling 34 is then screwed on to the upper region of the threaded shaft 24.

It has been found that the above - mentioned prior evacuation chamber may have the following disadvantages:- The required cutting of the vacuum bag 12 gives rise to the possibility of bag leakage:-

The vacuum seal has to rely on the pressure between the pressure plate 28 and the gasket 30:-

It is not possible to correct the positioning of the evacuation valve after the initial cut has been made in the vacuum bag:-

Errors can easily arise in assembling the component parts of the vacuum valve, and wrinkling and twisting of the vacuum bag 12 can easily occur during the tightening of the vacuum bag.

Fig 4 is a sectional elevation of another previously known evacuation valve installation.

The evacuation chamber of Fig 4 is substantially similar to the evacuation chamber of Fig 2, and for clarity corresponding components will be given the same reference numerals. Referring to Fig 4, a vacuum bag 12 is secured on a base plate 10 by a sealer strip 14 so as to define an evacuation chamber. An evacuation valve 22 is connected to the base plate 10 to enable air to be evacuated from the evacuation chamber through a passage 38 in the base plate 10. A fine mesh gauze 40 is secured to the base plate 10 to cover the passage 38 to prevent the vacuum bag 12 from bridging over the region of the passage 38.

The evacuation chamber of Fig 4 has been found to have the following disadvantages:-

The evacuation valve 22 is a permanent installation, and it is not possible to re-position the evacuation valve in the base plate 10;-

The possibility exists of blockage of the evacuation valve due to excessive bleeding of resin from the stack 8 of the intermediate fabric material. Because of the construction of the evacuation chamber and valve of Fig 4, removal of the resin blockage is expensive and time-consuming thereby resulting in wasteful tool down time.

It is an aim of the invention to provide an evacuation chamber in which the above mentioned disadvantages are alleviated.

According to the present invention there is provided an evacuation chamber comprising a base plate, a vacuum cover located over the base plate, an edge sealant providing an airtight seal between the base plate and the vacuum cover at their periphery, a breather located in the chamber, and an evacuation valve to enable the chamber to be evacuated wherein the valve includes a valve body having a vent passage extending therethrough from an inner vent hole to an outer vent hole, and a valve coupling communicating with the outer vent hole, and the valve body is located between opposed portions of the edge sealant at the periphery of the chamber between the vacuum cover and the base plate with the coupling outside the chamber.

By "breather" is meant a material to assist in ensuring that air is withdrawn from the evacuation chamber during evacuation.

In a preferred embodiment of the invention the breather comprises a first portion located beneath at least the inner part of the valve body, and a second portion covering the inner vent hole. The first portion may be an edge breather in the form of an edge strip extending around the base plate at its periphery, and the second portion may be a surface breather of plate form having an area slightly less than the area of the top of the vacuum bag. The inner vent hole may be covered with mesh material to reduce the risk of blockage of a vent passage.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Fig 5 is a perspective view of one evacuation valve of the invention;

Fig 6 is a perspective cut-way view of an evacuation chamber of the invention including the valve of Fig 5;

Fig 7 is a side elevation, partly in section, of an evacuation chamber of the invention including another evacuation valve;

Fig 8 is a perspective view of another evacuation valve of the invention, and

Fig 9 is a perspective view of a further evacuation valve of the invention.

Referring to Fig 5, an evacuation valve includes a quick-release coupling 40 extending from a valve body 42. Five passages extend through the valve body 42 from inner vent holes 43, in the upper portion 52 of the inner side of the body 42. These five passages communicate with a vent passage extending through the coupling 40. The valve body 42 is generally trapezoidal in cross-section with generally flat upper and lower surfaces 44 and 46. Recessed portions 48 and 50 extend across these upper and lower surfaces 44 and 46 respectively. The upper portion 52 of the inner side of the valve body 42 is connected to the coupling 40 by the five passages through the valve body 42. If for any reason the vacuum bag 12 should not pressed tightly against the upper portion 52 there is a risk that the bag may enter the vent holes 43 thereby causing at least a partial blockage of the air flow through the valve. A wire mesh 54 is secured to the upper portion 52 to cover the inner vent holes 43. This wire mesh 54 is sufficiently fine to prevent the vacuum bag from entering the vent passages and causing blockage.

Referring to Fig 6, the evacuation valve is placed on a base plate 56 with the upright outer side 58 of the valve body 42 flush with the edge of the base plate 56. The outer part of an edge breather strip 60 extends beneath the inner part of the valve body lower surface 44 and the outer part of a surface breather 62 of plate form rests on the inner side upper portion 52, and the inner vent holes 43. A lower edge sealant strip 64 extends around the periphery of the base plate 56, and passes through the recessed portion 50 the valve body lower surface 46. An upper edge sealant strip 66 is located in the recessed portion 48 in the valve body upper surface 44, and extends along the opposite inclined sides 68 and 70 of the valve body 42 so as to rest on the lower edge sealant strip 64 at opposite sides of the valve body 42.

A vacuum bag 72 is placed on the base plate 56 to define therewith an enclosed space constituting the evacuation chamber, and an airtight seal is made between the vacuum bag 72 and the base plate 56 by means of the edge sealant strips 64 and 66. The edge breather strip 60 extends completely around the base plate 56 adjacent to its periphery, and the surface breather 62 of plate form is located near the upper portion of the vacuum bag 72. This surface breather 62 has an area slightly less than the area of the upper portion of the vacuum bag 72. The edge breather strip 60 and surface breather 62 assist in ensuring that air is withdrawn from substantially the entire region covered by the vacuum bag 72, and that the vacuum bag does not wrinkle or distort unduly during evacuation.

A release film 74 is located between the edge breather strip 60 and the surface breather 62 to prevent these breathers adhering to one another.

The evacuation chamber of Fig 6 includes the evacuation valve of Fig 5 having a straight quick release coupling 40.

Fig 7 is a side elevation of the evacuation chamber of Fig 6 using a modified evacuation valve. The modified evacuation valve comprises the valve body 42 illustrated in Fig 5, and a T-shaped quick release coupling 76 extending from the valve body 42 in place of the straight coupling 40.

In another coupling construction the three arms of the coupling can extend at 120° to one another.

Referring to Fig 8 another evacuation valve includes a valve body 78 which is longer and narrower than the valve body 42 illustrated in Figure 5. The valve body 78 has an upright outer side 79.

For clarity, corresponding parts of the valve bodies 42 and 78 have been given the same reference numerals. A straight quick release coupling 80 extends from the upper surface 44 of the valve body 78 perpendicularly to the vent passage through the valve body 78.

Figure 9 is a perspective view of a modified version of the evacuation valve of Figure 8 located in the evacuation chamber of Figure 6. For clarity, corresponding components of the evacuation valve and evacuation chamber of Figures 5 to 9 will be given the same reference numerals.

Referring particularly to Figure 6, difficulty arises in removing the evacuation valve from the evacuation chamber. This difficulty is caused by the tenacious grip exerted by the edge sealant strips 64 and 66 on the valve body 42. To assist in releasing the evacuation valve, the valve of Figure 9 has been designed. This valve is the same as the valve of Figure 8 except that two blind holes 82 are formed in the upright outer side 79. An elongate hand tool 84 has two studs 86 dimensioned and arranged to fit into the holes 82 with the hand tool 84 abutting the valve body outer side 79. A suitable twisting action is then applied to the hand tool 84 which applies sufficient leverage to prise the valve body 78 from the sealant strips 64 and 66 and the other components of the evacuation chamber. It is found that the described and illustrated embodiments of the invention possess the following advantages over previously known evacuation chamber constructions:-

(a) The possibility of a bag leak is reduced since a cut in the bag in not required and consequently a costly rebagging operation is avoided.

(b) The vacuum seal does not rely on pressure between a coupling ring and a silicon rubber gasket.

(c) The number of vacuum vent holes and their combined surface area is much greater than with conventional ports and results in improved air paths through the adjacent breather material. (d) Incorrect positioning of the valve body by the operator is easily corrected since the edge sealant may be peeled back allowing valve body removal and insertion in a more strategic place on the base plate. This is impossible with through the bag vacuum porting and obviously also with the permanent in-tool porting.

(e) The proposed evacuation valve is a one piece item much less complex than through the bag ports, which are in themselves, prone to operator errors, ie by placing the silicone gasket on the bag outer surface or wrinkling and twisting of the bag during the tightening operation.

(f) With fixed in-tool vacuum port systems the possibility of blockage due to excessive resin bleed out always exists. This results in expensive reworking of the built in vacuum lines which leads to wasteful tool down time. The proposed valve port is easily cleared or substituted.

(g) A number of options exist as to the means of fixing the valve in position. For a single cure cycle, edge sealant under and over is sufficient. For several cures then the valve body 42 will be bonded to the tool surface with a silicone adhesive. For multiple cures, then brazing or welding might be employed.

(h) One further problem relating to poor installation of through the bag vacuum ports is toppling of the port in the autoclave caused-by connection of the vacuum line. Once a vacuum is applied the toppled port causes bag bridging and a restricted flow path. The problem does not occur with the through the edge sealant vacuum ports as described and illustrated.

(i) The design of the proposed valve, illustrated in Fig 7, allows for multi-angled positioning of the valve coupling. This is a distinct advantage when attaching vacuum lines in the autoclavfclto tools at various heights.

(j) Manufacture of the valve requires a few simple milling, drilling and tapping operations. The multi-piece through the bag vacuum port consists of two or three precision machined parts in order to achieve full vacuum integrity.

(k) The valve design is particularly suitable to carbon fibre reinforced tooling since through the tool porting leads to delamination and loss of vacuum integrity.

SUBSTITUTESHEET

Claims

CLAIMS :

1. An evacuation chamber comprising a base plate (56) , a vacuum cover (72) located over the base plate, an edge sealant (64,66) providing an airtight seal between the base plate and the vacuum cover at their periphery, a breather (60,62) located in the chamber, and an evacuation valve to enable the chamber to be evacuated characterised in that the valve includes a valve body, (42) having a vent passage extending therethrough from an inner vent hole (43) to an outer vent hole, and a valve coupling (40) communicating with the outer vent hole, and the valve body (42) is located between opposed portions of the edge sealant (64,66) at the periphery of the chamber between the vacuum cover (72) and the base plate (56) with the coupling (40) outside the chamber.

2. An evacuation chamber as claimed in claim 1, wherein the breather comprises a first portion (60) located beneath at least the inner part of the valve body (42) , and a second portion (62) covering the inner vent hole.

3. An evacuation chamber as claimed in claim 2, wherein the first portion may be an edge breather in the form of an edge strip extending around the base plate (56) at its periphery.

4. An evacuation chamber as claimed in claim 2, wherein the second portion may be a surface breather of plate form having an area slightly less that the area of the top of the vacuum cover (72) .

5. An evacuation chamber as claimed in any preceding claim, wherein the inner vent hole (43) may be covered with mesh material (40) to reduce the risk of blockage of a vent passage.

SUBSTITUTESHEET

6. An evacuation chamber as claimed in any one of claims 1 to 4, wherein the edge sealant is of strip form.

7. An evacuation chamber as claimed in claim 6, wherein opposed faces (44) and (46) of the valve body have recessed portions (48) and (50) to accommodate the edge sealant strips (66) and (64) .

8. An evacuation chamber as claimed in any one of claims 1 to 4, wherein the coupling (40) extends from the outer side of the valve body (42) .

9. An evacuation chamber as claimed in claim 8, wherein the coupling (40) is T-shaped.

10. An evacuation chamber as claimed in any one of claims 1 to 4, wherein the coupling (80) extends from the upper surface (44) of the valve body (78) .

11. An evacuation chamber as claimed in any one of claims 1 to 4, including a hand tool (84) to engage the valve body (78) to facilitate release of the evacuation valve from the evacuation chamber.

12. An evacuation chamber as claimed in claim 11 in which at least one engageable projection and recess are located in the hand tool and valve body respectively.

SUBSTITUTESHEET