GB2025302A

GB2025302A - Perforating Sheets

Info

Publication number: GB2025302A
Application number: GB7829998A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1978-07-15
Filing date: 1978-07-15
Publication date: 1980-01-23

Abstract

The invention concerns forming of perforations in heat curable resin- impregnated fibrous sheets. An uncured impregnated sheet 1 is held, e.g., between top and bottom plates whilst hot needles 13 or the like pierce it. The needles stay in position long enough to at least partially cure the resin immediately surrounding them, and are then withdrawn. The curing of the resin surrounding the perforations prevents ingress of hot fluid resin during subsequent moulding and curing operations. <IMAGE>

Description

SPECIFICATION Hot Needled Perforates The present invention relates to the manufacture of perforated sheet materials of the fibre-reinforced heat-cured resin type. In particular, it relates to an improved method of forming the perforations in such materials.

Good quality perforated sheet materials are required, e.g. for use in gas turbine aeroengines as facings for acoustic linings in the flow ducts of the engines. Although perforated sheet metals can be and are used in such situations, it is advantageous to substitute the fibre-reinforced resin type of sheet material where conditions allow since its weight per unit area can be made less than that of metal.

In the manufacture of fibrereinforced resin sheets having perforations therein, it is desirable to form the perforations before the sheet is cured, since otherwise they must be punched in the cured sheet. Punching holes in the cured sheets cuts the fibres and tends to cause cracking or chipping of the resin around the holes, thus reducing the strength and mechanical integrity of the sheet. However, if the perforations are formed before curing, the resin migrates into them and blocks them up during subsequent moulding and curing of the sheet unless positively prevented from doing so by pins or other means positioned in the perforations during the moulding and curing process.Though feasible, the use of such means for preventing blocking of the perforations complicates manufacture; furthermore they can be difficult to remove from the perforations, especially when the perforations are of small diameter.

It is an object of the present invention to provide an improved method of forming perforations in fibrereinforced resin materials before curing thereof. The method facilitates retention of the perforations in a state substantially free from ingress of resin during subsequent moulding and curing operations.

According to the present invention, a method of manufacturing perforated sheet materials of the fibrereinforced heat-cured resin type includes the steps of: a) impregnating a fibrous sheet with heatcurable resin; b) piercing said impregnated fibrous sheet with hot piercing means to form perforations in said sheet so that the resin in the walls of the said perforations is at least partially cured thereby; c) withdrawing said piercing means from said sheet; and d) completing the curing process for the whole sheet by the application of further heat thereto.

During the piercing step the impregnated fibrous sheet is preferably sandwiched between a perforated base plate and a perforated cover plate whose perforations are in registration with the perforations in the base plate, said hot piercing means passing through the perforations in the cover plate and base plate to form the perforations in the sheet.

The hot piercing means should be retained in position in the impregnated fibrous sheet for a time sufficient to produce the desired degree of curing of the resin in the regions of said sheet surrounding the perforations.

After the forming of the perforations in the impregnated fibrous sheet, the sheet may be retained between the cover plate and the base plate and further moulded and cured in site. The sheet may be removed from between said plates before it is fully cured and may be further moulded and cured in a mould if desired.

Alternatively, the sheet may be removed from between said plates after the forming of the perforations and before further curing has occurred and transfered to a mould for further moulding and curing. The mould may be configured to form the the sheet to a different configuration from that which it received when sandwiched between the base plate and the cover plate.

Preferably, a plurality of perforations are formed simultaneously by said piercing means.

Preferably, the piercing means comprises a plurality of needles or the like movable simultaneously to pierce said sheet. The piercing means may be incorporated in a heated platten which moves from a retracted position to a working position to effect piercing of the sheet.

The platten may also apply moulding pressure to the cover plate/sheet/base plate assembly.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an enlarged section scrap view of a sheet of uncured fibre-reinforced resin material in position between two perforated plates; Figure 2 is a view similar to Figure 1 showing perforations being formed in the sheet by hot needles; Figure 3 illustrates a further moulding and curing step which can be performed on the sheet after it is removed from between the perforated plates.

The drawings are not to scale.

Referring now to Figure 1, it is desired to form perforations in the sheet 1, which is made of a fibrous resin impregnated material. The fibrous component of the material may be in either the woven "fabric" form or the randomly oriented chopped fibre form and may comprise, for example, glass or carbon/graphite fibres. The resin component of the material is the heat-curable type, for example a thermosetting epoxy or polymide resin. The fibrous component is impregnated with the resin by any suitable means as known.

The steps involved in forming the perforations in sheet 1 and producing a finished perforated sheet of fibrereinforced resin material are described in the following paragraphs.

After impregnation of the fibrous sheet with heat-curable resin, the resulting sheet 1 is sandwiched between a base plate 3 and a cover plate 5, both of which have perforations 7 and 9 respectively formed through them. The perforations 7 and 9 are in registration with each other and the base plate 3 rests on a flat unyielding surface 11. Both plates 3 and 5 are coated with a suitable release agent on the faces which contact the sheet. The plates are made from a suitable metal such as steel, bronze, etc.

A large number of perforations are simultaneously formed in sheet 1 by means of needles 13 (Figure 2) fixed in a heated platten 15, the needles being of a size to pass through perforations 7, 9 and pierce sheet 1 when platten 1 5 descends (arrow A). The descent of the platten 1 5 and size and spacing of the needles 13 is of course such as to prevent contact between needles 13 and surface 11 and ensure their unimpeded passage through the perforations 7, 9.

However, contact between platten 1 5 and the top of cover plate 5 may be desired as shown in order to apply controlled moulding pressure to sheet 1.

Since platten 1 5 is heated (e.g. by electric elements, not shown), needles 1 3 are hot. As they penetrate sheet 1 to form the required perforations in it, they push aside the fibres in the sheet, preferably without breaking them, and at the same time initiate curing of the heat-curable resin in the walls of the perforations thus formed in the sheet.

Hot needles 13 are retained in the position shown in Fig. 2 for a time sufficient to produce a desired degree of curing of the resin in the walls of the perforations and in the regions of the sheet surrounding the perforations. The resin in and immediately adjacent to the walls of the perforations should be cured sufficiently to obviate ingress of the hot fluid resin from surrounding areas of the sheet under hydrostatic pressure during subsequent moulding and curing operations, but curing should not proceed in the regions of the sheet surrounding the perforations to an extent which would render the sheet unsuitable for a subsequent moulding operation in which it is desired to change the configuration of the sheet, e.g. bend it into a curve.

After platten 1 5 has dwelt at the bottom of its stroke for an appropriate period of time, the platten is caused to perform its return stroke and needles 13 are withdrawn from the sheet 1 and plates 3 and 5. Alternatively a mechanism (not shown) may be provided in the platten 1 5 whereby needles 13 can be retracted into the platten whilst the platten remains in contact with the cover plate 5 in order to restrain the coverlbase plate/sheet assembly against the upward pull of the needles as they are retracted.

Such an upward pull would be due, e.g. to harden resin from the sheet sticking to the needles.

After forming of the perforations in the sheet 1, it may be retained between the cover plates 5 and base plate 3 for further moulding and partial or full curing in situe, e.g. by means of pressure and heat from heated platten 1 5 with needles retracted as mentioned above. In the arrangement shown in Figure 2, full curing would result in the production of a flat sheet.

In order to produce a sheet having single or compound curvature, for example, it would be necessary to remove the sheet 1 from between plates 3 and 5 whilst it is still in a bendable and mouldable state (i.e. before substantial curing of the main bulk of the resin between perforations has occurred) and transfer it to a mould of the correct shape. In Figure 3 this has been done, the perforated but still plastically deformable sheet 1 being placed between the two halves of mould 1 7 and subjected to heat and moulding pressure to form the finished perforated plate with the required curvature and surface finish.

To ensure that the resin-impregnated sheet is still sufficiently plastically deformable to allow moulding to a different configuration after forming of the perforations by the hot needles 1 3 (Figure 2), it may be necessary to minimise transfer of heat from heated platten 1 5 through the cover plate 5 to the sheet 1 whilst the perforations are being formed. This may be done by arranging that instead of contacting cover plate 5 whilst hot needles 1 3 are forming the perforations, the platten 15 is separated from cover plate 5 by an air gap. When the retraction of the needles is required the platten would then descend a small amount to break bonding between hardened resin and the needles, and would then be withdrawn.Alternatively, if moulding pressure is required from platten 1 5 whilst the perforations are being formed, cover plate 5 could be composed of a hard heat insulating material and/or a layer of heat insulating material could be incorporated at the bottom of platten 1 5 or interposed between platten 1 5 and cover plate 5.

In cases in which sheet 1 is insufficiently thin and/or the perforations are sufficiently closely spaced to cause rapid build up of heat in the sheet due to conduction from the needles, it may be necessary to form the perforations in two or more piercing operations, e.g. only each alternate perforation in a row being formed in a first operation, the other perforation being formed in a second operation spaced in time from the first sufficiently to allow adequate dissipation of heat from the sheet 1.

As a further precaution to avoid excessive build-up of heat in sheet 1 during the piercing operation, surface 11 may, for example, be water cooled to extract heat from base plate 3.

Claims

1. A method of manufacturing perforated sheet materials of the fibre-reinforced heat-curing resin type, including the steps of: a) impregnating a fibrous sheet with heatcurable resin; b) piercing said impregnated fibrous sheet with hot piercing means to form perforations in said sheet so that the resin in the walls of said perforations is at least partially cured thereby; c) withdrawing said piercing means from said sheet; and d) completing the curing process for the whole sheet by the application of further heat thereto.

2. A method according to claim 1, the impregnated fibrous sheet being sandwiched between a perforated base plate and a perforated cover plate whose perforations are in registration with the perforations in the base plate, said hot piercing means passing through the perforations in the cover plate and base plate to form the perforations in the sheet.

3. A method according to claim 2 in which after forming of the perforations in the impregnated fibrous sheet, the sheet is retained between the cover plate and the base plate and further moulded and cured in situe.

4. A method according to claim 3 in which the sheet is removed from between the plates before it is fully cured and transferred to a mould for further moulding and curing.

5. A method according to claim 2 in which the sheet is removed from between the plates after forming of the perforations and before further curing has occurred and transferred to a mould for further moulding and curing.

6. A method according to any one of claims 1 to 5 in which a plurality of perforations are formed simultaneously by said piercing means.

7. Apparatus for perforating a fibrous sheet impregnated with heat-curable resin, the apparatus comprising a perforated base plate and a perforated cover plate whose perforations are in registration with the perforations in the base plate, and hot piercing means moveable to pass through the perforations in the cover plate and the base plate to form perforations in the fibrous sheet when said sheet is held in position between the plates.

8. Apparatus according to claim 7 in which the piercing means comprises a plurality of needles or the like moveable simultaneously to pierce the sheet.

9. Apparatus according to claim 7 or claim 8 in which the piercing means is incorporated in a heated platten moveable from a retracted position to a working position to effect piercing of the sheet.

10. Apparatus according to claim 9, adapted to apply mould pressure through the platten to the coverplate/sheet/base plate assembly.

11. A method of manufacturing perforated sheet materials of the fibre-reinforced heat-cured resin type, substantially as described in this specification.

12. Apparatus for perforating a fibrous sheet impregnated with a heat-curable resin, substantially as described in this specification.

13. A resin-impregnated fibrous sheet having perforations therein produced by a method or apparatus according to any one of the preceding claims.