GB2172542A

GB2172542A - Moulds

Info

Publication number: GB2172542A
Application number: GB08528645A
Authority: GB
Inventors: James A Bennett
Original assignee: HAZELDINE IND GROUP Ltd
Current assignee: HAZELDINE IND GROUP Ltd
Priority date: 1985-03-19
Filing date: 1985-11-21
Publication date: 1986-09-24
Also published as: GB8507073D0; GB8528645D0

Abstract

A mould comprises a first portion 16 defining the moulding surface and carrying electrical heating elements 18 and a second portion 32 capable of supplying a degree of structural rigidity and thermal insulation to the first portion. The first portion comprises a metal layer which defines the mould surface and a backing layer of silicone rubber 16, 20 or plastics in which the heating elements are embedded. The second portion 32 comprises foamed plastics. The mould is used in conjunction with a vacuum bag (44), Figure 3 (not shown), which is sealingly placed over the mould cavity and which is drawn by vacuum against a preform in the mould. <IMAGE>

Description

SPECIFICATION Mould This invention relates to moulds, and in particular relates to a mould construction suitable for hotcuring of fibre/adhesive structural laminates.

Structural laminate components, for example for light aircraft, boats and the like, can be produced using cold-cure epoxy resin systems impregnated into fibrous sheets and laid up by hand. Such cold-cure systems are suitable for small production quantities because the tooling costs for hot-cured resin systems would make their use uneconomical.

However, hot-cured resin systems have improved strength, better strength retention athighertemper- atures, improved quality control and reduced labour cost.

The invention seeks to provide a mould suitable for hot-cure structural laminates which is simple and economical to produce, and is economical in use.

According to the present invention there is provided a mould which comprises a first portion defining the moulding surface and carrying electrical heating elements and a second portion capable of supplying a degree of structural rigidity and thermal insulation to the first portion.

The first portion may be made of two layers: a first layer e.g. of metal, defining the moulding surface; and a second layer of flexible plastics or elastomeric material attached to the first layer on the side away from its mould surface. The second layer, being of an insulating material, would then carry the electrical heating elements. Alternatively, the first portion may be produced from a high temperature resistant resin, e.g a plastic resin, and the heating elements would be fixed to or embedded in this.

The mould may be supported within a framework, for example of wood and/or metal to improve its structural rigidity and ease handling. Furthermore, the mould will preferably carry vacuum and/or pressure pipes connecting to outlets in the first layer.

The mould of the invention is designed to be used in conjunction with a vacuum bagging arrangement and, to this end, may be provided with a resilient sealing ring around the moulding surface of the first layer which, in use of the mould, will co-act with the vacuum bag to seal the mould cavity and allow a vacuum to be drawn thereby causing the bag to exert pressure on the components being moulded.

The mould of the invention may be made as follows. Firstly, a former, or "plug", of the exact shape and dimensions of the eventual article to be produced is formed, e.g by machine, from suitable material such as a pattern making timber or glass reinforced plastics material. The former or plug can be filled and sprayed with, for example, paint to give a smooth finish to the required standard. The plug is then coated with a release coating, for example a polyvinyl alcohol release coating, and sprayed with a metal using a spraying tool such as that available from Metalisation Ltd. Typical metals which may be sprayed according to this method are zinc, tin/zinc mix, or aluminium. It is preferred to use zinc for most of the first layer but a final course coat of aluminium may be sprayed to act as a keen surface.

The metallic coating gives a durable lining to the mould with a good surface finish. Secondly, since zinc is a good thermal conductor, the mould surface rapidly reaches an even temperature, thus avoiding any hot spots or uneven temperature distribution across the mould. Alternatively an initial thin layer of zinc can be built up with an aluminium alloy, e.g Metco SF alloy.

The second layer contains a heating element which may consist of an array of electric resistance wires embedded in a silicone rubber insulation and bonded to the back of the metallic mould shell. The heating element may be built up in the following way. A coat of silicone rubber is sprayed or painted on the first layer of the mould shell. This forms a good adhesive bond with the coarse aluminium layer on the mould shell and is of sufficient thickness to give adequate elctrical insulation between the heater wires and the mould shell. For a particular mould the required spacing and type of heater wire is established to give the appropriate watts density over the mould surface. The heater wire is laid in place on top of the silicone rubber coating at the appropriate spacing, and further insulating layers of silicone rubber are applied over the heater wires.

This method of building up a heater mat ensures that there are no voids in the complete assembly and allows the heater array to be tailored to suit each mould.

The mould temperature may be monitored via a thermocouple bonded into the second layer heater mat during assembly and controlled by a temperature controller which applies or shuts off electrical power when temperature goes above or below pre-set limits.

If desired the heating array in the mould can be wired up in different parts of the mould with different heater wire spacings, giving different watts densities in selected areas. Each section can then be controlled independently with its own temperature controller.

This method can be used where the component to be cured with the mould has large variations in thermal mass. If the watts density is constant over the mould surface there may be an unacceptable difference in heat up rate between parts of the component. This can be overcome by altering the wiring spacing and increasing the watts density where the thermal mass is largest to increase the heat output to an acceptable level. Once cure temperature has been reached the only difference between separate heating areas will be the ratio of power-on, to power-off time for the heating array as controlled by each temperature controller. The mould temperature will be the same for all heater areas. An additional heater may be used in the mould flange to act as a "buffer" and prevent the mould edge being cooler than the middle.

At this stage the vacuum bleed system and pressure ejection systems may be built into the mould. The vacuum system will be used in conjunction with a vacuum bag to apply bonding pressure to the component to be cured. The pressure is used to assist releasing the component from the mould after curing. Fittings may be bonded in the holes in the mould with silicone rubber and connected via high temperature flexible tubing to an external manifold and then to the vacuum pump.

The backing material for the first and second layers of the mould shell, in the second portion, may be formed plastics material such as poly-isocyanurate foam, which is foamed in situ. This foam also acts as a thermal insulation for the mould and protects the pressure tappings, and piping systems for the vacuum and pressure lines. The preferred foam has been chosen for its ability to withstand the mould temperatures and to be formed in situ.

The whole mould may be edged with plywood for additional insulation and rigidity, and mounted on a steel frame to ensure that the mould does not distort during curing. Since the mould will expand when heated it is not rigidly attached to the frame, but is clamped in such a way as to allow expansion to take place. Similarly a release layer, e.g of coated glass fibre cloth, may be inserted between the first and second layers to allow for expansion of the mould shell during heating.

A latex rubber vacuum bag may be used to apply the bonding pressure. A bleed cloth and release film are used inside the vacuum bag as with normal vacuum bag methods. Sealing ofthevacuum bag around the edges is preferably achieved with a solid circular section of neoprene rubber sealing ring.

Neoprene rubber is preferred since the seal will be in contact with the mould surface and it needs to be able to withstand moulding temperatures. The sealing ring seals on the mould face and the vacuum bag, and may be adhered in place with silicone rubber in a groove formed by plywood sheets on either side. These plywood sheets are bonded on the mould flanges with silicone rubber and also act as additional insulation between the mould and the vacuum bag.

The temperature controller controls the temperature of the mould surface itself and the layers of pre-preg in contact with the mould. When curing laminates such as sandwich panels containing core materials which are good thermal insulators, e.g balsa wood, it is necessary to ensure that the layers of pre-impregnated resin cloth ("PM-PMG") away from the mould surface reach the cure temperature.

This can be achieved by very efficient thermal insulation placed externally over the top of the vacuum bag. An insulation system consisting of a metal coated mylarfilm (i.e "Space Blankets") and polystyrene foam blocks has been found to be sufficient. When curing thick G.R.P.-balsa wood G.R.P. sandwich panels the outer layer temperatures are within a few degrees of the mould surface temperature and show only a small lag in temperature during heat up.

In situations where it is required to cure very thick sandwich sections an alternative to the variable wiring spacing outlined above is to use a flexible resistance heating mat inside the vacuum bag on top of the outer layers of pre-preg. This heater mat is controlled in the same way as the mould heater with its own temperature control relay. Several types of resistance mats are available e.g PTFE coated carbon type or resistance wire sandwiched between silicone rubber sheets.

If the mould shape can be made accurately from flat or single curved panels it is possible to use a simpler method of mould manufacture. Instead of making a plug which is then metal sprayed, the mould can be fabricated from aluminium sheet or extrusion. The aluminium which is also a good thermal conductor serves the same purpose as the zinc mould shell and the construction of the complete mould is identical for both types of mould shell starting from the first insulating layer of silicone rubber. For the simpler shapes this method can save both time and cost in mould manufacture.

A suitable mould first portion may also be manufactured from high temperature resistant resin such as a Phenolic resin, or other plastics material preferably reinforced with glass or other similar fibre. The resin system known as XDF 4151 manufactured by Ciba Giegy PLC is a suitable material for this method of manufacture.

Because of the resistivity of the resin material it is unecessary to apply the heater wires to the back surface of the mould skin as these can be embedded in the actual mould itself. For suitably even skin heating a high level of thermal insulation to both sides of the mould together with a modest level of power dissipation per unit of mould area must be obtained. Dissipations in the region of 0.5 to 1.5 KW/M2 have been found to be effective.

In addition, powered metal may be introduced into the resin between the heater wire layer and the mould surface in order to improve heat dissipation throughout the mould skin.

When moulds are constructed using this method, heating may be obtained by the laminating into the mould skin construction layers of metal coated cotton or other fabric to which and electric current may be supplied. Bayer UK Ltd manufacture such a material under the trade name "BAYMETEX".

Because of the relatively low coefficient of thermal expansion of these materials fewer problems of differential expansion will be encountered and in particular, no release film need be introduced between the reverse of the mould surface and the insulating foam.

In order to increase mould utilisation and production rates it is possible to lay up the pre-pregs on a laying up sheet outside the mould. The complete pre-preg lay up on the sheet is then loaded into the mould.

This method has a number of advantages. It allows the operators to lay up the next batch of components while the previous batch of components are being cured. Also the moulds can be loaded and unloaded at relatively high temperatures e.g 60 C whereas when laying up directly in the mould it is necessary to allow the mould to cool down to room temperature to allow the pre-pregs to be positioned in the mould without premature melting of the resin and other associated problems.

Reducing the temperature cycle range greatly speeds up the heat-up and cool-down times and reduces heating costs.

A further advantage of the laying up sheets is that it protects the mould surface. The laying up sheet is coated with release agent but if there are any problems with releasing the worst that can happen is that a laying up sheet has to be scrapped. This is preferable to having a component adhering to the mould shell which could result in damage to the mould during removal or possible distraction of the mould.

A suitable material for moulds with single curvatures is 26 SWG clad aluminium alloy sheet which gives a mirror finish to the moulded component.

Materials for laying up sheets for double curved moulds include cured sheets of high temperature pre-preg or sprayed silicone rubber sheets.

In order to ensure that there is no restraint on these thermal expansions of the mould shell, a release layer may be incorporated between the silicone rubber insulation on the heater element and the insulating foam backing the mould shell. The foam is also trimmed away aroung the vacuum tappingsto allow free expansion of the shell.

For moulds with complex shapes or which have a deep "draw" a flat sheet of vacuum bag material may not be able to conform to the mould shape.

Instead there may be used a vacuum sheet formed by spraying RTV silicone rubber into the mould. The sheet can then be made to conform to the shape of the mould and the component to be moulded. The method of sealing the vacuum bag is unchanged.

The sprayed metal sheet is porous to air and the layer of silicone rubber insulating the heater element also acts as an impermeable membrane sealing the mould shell and minimises leakage into the vacuum bag.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which, Figure 1 is a partial diagramatic perspective view of one stage in the construction of a mould according to the invention; Figure 2 is a similar view to Figure 1, partly cut away, of the finished mould; and Figure 3 is a similar view to Figure 2 of the other side of the mould.

Referring to the drawings, a mould in accordance with the invention is constructed by first forming a plug 10, conforming to the size, shape and surface finish of the products eventually to be produced from the finished mould. The plug 10 is then placed on a splitter plate 12 which acts as a temporary support and a metal shell 14 is built up as a first layer as previously described. Referring now to Figure 2, once the shell 14 has been produced it is coated with silicone rubber 16 and heating elements 18 are laid in accordance with the desired heat distribution.

Further silicone rubber 20 is applied to encapsulate the heating elements 18 and together with the first application of silicone rubber 16 produce a second mould layer containing the heating element. Pressure fittings 22 culminating in pressure holes 24 in the mould cavity and vacuum fittings 26 culminating in vacuum holes 28 about the edges of the mould cavity are also formed at this stage, and a plywood frame 30 is adhered to the second layer. A layer of foamed polyisocyanurate plastics material 32 is then built up within the plywood frame 30, with or without an intervening sheet of release material such as a PTFE coated glass fibre fabric. The plywood frame may be given additional structural rigidity by the use of an outer steel frame 34 attached thereto.

As can be seen betterfrom Figure 3, the mould cavity 36 is surrounded by a flange 38 which may be strengthened by additional plywood members 40, and a sealing ring 42 is adhered thereto.

In use of the mould, the structural laminate can be laid up in the mould cavity 36 directly, or may be laid up on a separate laying up sheet as previously described and placed in the cavity 36. A vacuum bag 44 carried on a support frame 46 is then placed over the mould cavity where it seals with a near sealing ring 42, and the vacuum is drawn through the vacuum connections 26 causing the bag 44 to press against the layer and bias it into the mould cavity 36 to which it will then conform. Heat is applied by supplying an electrical current to the heating elements 18 until the mould has reached the desired temperature as measured by a thermacouple (not shown) thereby causing the layer of laminate to cure. Once curing has taken place, the hole may be allowed to cool down after which the vacuum may be released and the vacuum bag removed. Should the moulded article be difficult to remove, pressure may be applied through the pressure fitting 22 thereby separating the mould from the moulded article.

The mould of the invention forms a simple and economical way of producing hot-cure structural laminates suitable for use with aircraft, boats or the like without requiring elaborate tooling or expensive apparatus.

Claims

1. A mould which comprises a first portion defining the moulding surface and carrying electrical heating element and a second portion capable of supplying a degree of structural rigidity and thermal insulation to the first portion.

2. A mould as claimed in claim 1 in which the first portion is made of two layers, a first layer defining the moulding surface and second layer attached to the first layer on the side away from the mould surface.

3. A mould as claimed in claim 2 in which the first layer is of metal and the second layer is of a flexible plastics material, the second layer carrying the electrical heating elements.

4. A mould as claimed in claim 1 in which the first portion is made from a high temperature resistant resin and the heating elements are fixed thereto or embedded therein.

5. A mould as claimed in claim 4 in which the resin is a phenolic resin reinforced with glass fibres.

6. A mould as claimed in claim 4 or Sin which powdered metal is introduced into the resin between the heater wire layer and the mould surface in order to improve heat dissipation throughout the mould skin.

7. A mould as claimed in claim 4 or 5 in which the heating elements comprise layers of metal coated fabric laminated into the mould skin.

8. A mould as claimed in any one of claims 1 to 7 additionally supported within a framework to improve its structural rigidity and provided with vacuum and/or pressure pipes connecting to outlets in the first portion.

9. A mould as claimed in any of claims 1 to 8 in which the heating elements are wired up to different parts of the mould with different heater wire spacing as giving different watts densities in selected areas.

10. A mould as claimed in any of claims 1 to 9 in which the second portion is a foamed plastics material formed in situ.

11. A mould as claimed in claim 10 in which the foam plastics material is a poly-icosynurate foam.

12. A mould as claimed in any one of claims 1 to 11 additionally comprising a flexible vacuum bag having an edge seal to contact sealing relationship with the mould surface.

13. A mould as claimed in claim 12 in which the edge seal comprises a solid circular section of neoprene rubber sealing ring.

14. A mould as claimed in any one of claims 1 to 13 in which extra insulation is provided in the form of insulating blankets being placed overthe mould.