GB2179291A - Panel - Google Patents

Abstract

A method of forming a panel, preferably for a vehicle body, comprising pressure and temperature moulding a sandwich construction of formable materials to form a panel. Moulding is preferably effected by vacuum forming using a mould. Also disclosed is a sandwich construction panel comprising an outer layer of thermo-formable material and central and inner layers of formable material.

Description

SPECIFICATION Panel This invention relates to a panel and to a method of forming a panel.

Hitherto, plastics material body panelsforvehicles have generally been formed by a conventional glass reinforced plastics moulding process. Body panels and vehicle bodies produced by such a method can be made adequately strong and with a satisfactory surface finish. However, such a process requires skilled labour and is rather slow due to the time required forpropercuring ofthe resins used. Thus such a process is generally only suitable for small scale pro- duction and it tends to increase the cost of a finished vehicle.

It is an aim of the present invention to reducethe above mentioned problem.

Accordingly, this invention provides a method of forming a panel comprising pressure and temperature moulding a sandwich construction offormable materials to form a panel.

The word 'panel' as used herein is not intended to be restricted to a flat product and the panel can be curvedandformed in any general shape,forexample depending upon where in a vehicle the product isto fit.

The method ofthe invention enables a combination of the benefits of low tooling costs with production cycletimesandsurfacefinishesthatarecomparable with panels pressed in steel or aluminium utilising matched dyes. The panels may exhibit structural characteristics that are comparable with metal panels, but the panels produced by the method of the invention may be significantly lighter. The low tooling costs are afforded by the use of the formable materials.

Preferably the moulding is effected by vacuum forming using a female ora male mould, depending upon the desired shape ofthe panel to be formed.

Advantageously, only a single mould is employed to again give low tooling costs. The vacuum forming can be effected using atmospheric pressure (forexample by evacuating a female mould) or by using air pressure,forexample by blowing a bubbleofmate- rial that is then finally formed by applying a vacuum.

The sandwich construction may comprise a multilayer construction having outer and inner layers of a thermo-formable material and a central layer of a formable material. Alternatively, the sandwich construction may comprise a multi-layer construction having an outer layer of a thermoformable material, and central and inner layers of a formable material.

The therm o-fo rma ble material is preferablyather- mo-formable impermeable material. Examples of thermo-formable materials that may be used are ABS, otherthermo-formable plastics materials, super plastic aluminium alloy, and epoxy compounds.

The central layer may be a neutral layer of a form able material. Examples of such materials are closed cell polyvinyl chloride foam sheet material, polyurethane foam material, and an aramid paper honeycomb material. The inner layer may be formed from a woven material.

Preferably, an adhesive is used to bond the layers of the sandwich construction together.

The adhesive material is preferably an epoxy resin.

The central layer is advantageously provided with through holes such that the adhesive material ties the construction together.

In one embodimentofthe invention, pressure and temperature are applied to the sandwich construction by evacuating the interior of the mould and applying high air pressure to the outside ofthe sandwich construction to create a pressure differential on the sandwich construction, the high pressure air being at an elevated temperature to heat the sandwich construction.

The layers of the sandwich construction may be provided in or on the mould individually or preassembled from sheet material and provided in or on the mould together.

If the sandwich construction is provided in or on the mould in pre-assembled form, then the adhesive used between the layers advantageously has a cure temperature higherthan the deform temperature of thethermo-formable material such that the panel may be completely formed in or on the mould before thetemperature ofthe panel is raised sufficiently to cure the adhesive.

The present invention also provide a sandwich construction panel comprising an outer layer of a ther mo4ormable material, and central and inner layers of formable material.

The formable material for the inner layer may be a thermo-formable material or it may be a woven formable material.

As indicated above, the layers may be bonded together with an adhesive material. A preferred adhesive material is an epoxy resin adhesive material.

The present invention further provides a panel when produced by the method ofthe invention.

Embodiments ofthe invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 is a section through part of a mould and a sandwich construction offormable materials; Figure2 is an exploded view of part of a firstsandwich construction; and Figure 3 is an exploded view of part of a second sandwich construction.

Referring to Figure 1, a panel is formed using a female mould 2. The mould 2 has a highqualitysur- face finish to ensure that the finished panel has a similarly high quality finish on its outer surface. The panel is built up in the mould 2 in layers to form a sandwichconstruction4offormable materials.Afirst layer 6 forms the finished outer surface of the panel and this first layer 6 is of a material which is capable of reproducing the high quality surface finish ofthe mould 2. The first layer 6 is of a thermo-formable impermeable material and is in fact ABS (acrylonitrile butadiene styrene).

The first layer 6 is placed over the mould 2 and air within the mould 2 is evacuated. The sandwich construction 4 is clamped at the edges 8 of the mould 2 by means of a clamping frame 10. Atube 12 is inserted between the edge 8 ofthe mould 2 and the clamping frame 10 before the seal is made and the tube 12 can then be used to evacuate airfrom the interiorofthe mould 2 after the seal has been made. The seal is made by the application of localised heat.

The first layer 6 ofthe sandwich construction 4 is drawn in tightconformitywith the surface of the mould 2 by means of a vacuum. The vacuum is applied from a vacuum pump (not shown) which operates via a tube 14.

A relatively high pressure hot air blanket can be applied to the top layer 16 ofthe sandwich construc tion 4. A heat source 18 for providing the required heat is shown somewhat schematically in Figure 1. As the sandwich construction is heated, it tends to deform and conformtothe innersurface ofthe mould 2.

The sandwich construction 4 is such that it has a neutral axis or core layer 20 of a formable material which is laid over the first layer 6. The core layer 20 is formed from a closed cell polyvinyl chloride foam sheetwhich is bonded to the first layer 6 with an epoxy adhesive. The layer 16 is of a thermo-formable material which may be bonded to the core layer 20 using an epoxy adhesive. The layer 16 forms an inner skin of the complete panel . The completed panel is substantially ready for use once it is removed from the mould.

In the above described method, the layers 6,16 and 20 may be separately laid in the mould 2.

In an alternative method and still referring to Figure 1 ,the various layers 6,16,20 may be pre-assembled in a cold condition as a sandwich construction. Such a method is of particular use for relatively large production rates. in this alternative method, the sandwich construction may comprise the layer 6 to form the panel skin, a film (not shown) of a selected adhesive, the neutral axis core layer 20, a furtherfilm (not shown) of adhesive, and the layer 16 to form the inner skin of the panel. Preferably, the layer 6 is an ABS layer, the layer 20 is a closed cell polyvinyl chloride foam sheet layer, and the layer 16 is a thermoformable layer. The adhesive films are preferably epoxy adhesive films.

A number of the sandwich constructions may be pre-assembled and stored readyforforming into complete panels. One ofthe pre-assembled sandwich constructions is placed in the mould 2 and the mould 2 is evacuated as described above. Heat is also applied as described above and the use of a vacuum via the tube 14 ensures that the sandwich construction be formed to take up the shape of the inner surface ofthe mould 2. The particular adhesive used is chosen to have a cure temperature which is greaterthan the deform temperature of the thermo-formable materials used in the sandwich construction. Thus, after the initial forming of the panel to shape by virtue of the pressure and the heat applied to the sandwich construction, further heat can be applied to raisethe temperatureto the curetemperatureforthe adhesive.

The heating is then discontinued and the panel is allowed to cool. Once the panel is cool, the application of pressure is also discontinued and the com pletepanel is then removed from the mould.

If desired, the panel as removed from the mould may have its edges trimmed, for example by cutting blades.

The layer 20 may be a woven filament layer. The woven filament layer is preferably pre-impregnated with the adhesive.

With the pre-assembled sandwich construction, it is obviously necessaryto heat the pre-assembled sandwich construction to its deform temperature before the applied vacuum will cause the sandwich construction to deform. Thus the sandwich construction is usually heated prior to evacuating the airfrom the mould 2. During this heating and the subsequent evacuation of the air, the cure cycle of the adhesive starts and, whilstthe sandwich construction is held within the mould 2, the cure cycle is completed. On cooling and release of the formed panel from the mould, the various layers ofthe sandwich construction will have strongly bonded together. Since the intermediate layer or layers 20 is/are of a softer material, the forming pressure causes the filaments par tially to embed within the film core, creating a secondary matrix.

Referring nowto Figure2,thereisshown inex ploded form a cross section through part of a first sandwich construction 22. The sandwich construction comprises a layer 24 of a thermo-formable material. The thermo-formable material is typically ABS PC (acrylonitrile butadiene styrene-polycarbonate). A layer 26 is also provided which may be of carbon or glass filaments lightlywoven together. This layer 26 may be impregnated with, for example, toughened acrylic material.

Figure 2 also shows the presence of a layer 28 which is of a relatively soft plastics film such for example as PET (polyethyleneterephthalate) or PBT (polybutylene terephthalate). The layer 30 may be the same as the layer 26 with, if required, filament orientation at an angle. The layer 32 may be the same as the layer 28, and the layer 34 may be the same as the layer 26. Generally, the layers 26 and 28 can be repeated as may be required. A layer 36 is of a thermoformable material.

The layer 24 may be, for example, 1.00-1 .55mm thick. The layer 28 may be, for example, 0.25 - 0.5 mm thick.

If desired, the beam strength and rigidity of a panel can be increased by allowing rivets of adhesive to form through holes in the neutral axis core layer or layers to effectively tie the panel together. This is illustrated in Figure 3 which shows an exploded view of part of a second sandwich construction 38. The sandwich construction 38 has a thermo-formable layer40,and an adhesive film layer42which may an epoxy orvinylester adhesive on a scrim carrier. A layer 44 is the same as the layer 42 and, positioned between the layers 42,44 is a polyvinyl chloride foam layer 46. The layer 46 is provided with a plurality of pierced holes 48. The pitch ofthe holes 48 determines the stiffness of the sandwich construction 38. The sandwich construction 38 also has a layer 50 of a thermo-formable material or, alternatively, of a woven glass cloth.

The adhesive may run into the holes 48 to form the above mentioned rivets.

In an alternative construction, the rivets of adhesive may be omitted and this enables the formed panel to have an increased abilityto deform under impact and recover without damage. The ability of the panel to recover its shape can be varied by selecting various combinations of materials and adhesives. In particular, adhesives with varying elongation to break ratios may be used.

By using various methods in accordance with the present invention, it is possible to produce panels which, compared to conventionally produced panels, are of lowweight, have good impact resistance and recovery from deflections caused by impacts, and have a beam strength and deflection resistance better than other materials for the same weight of panel.

The high quality moulds and the accuracy with which the moulding reproduces the mould shape, ensures that panels of accurate and consistent dimensions are produced.

The panels may have a surface finish which is of sufficient quality that they can be used withoutfurth erattentionorwithonlyminimal preparation if painting is required to be effected.

The panels can be produced simply and quickly.

The above mentioned methods are especially useful in low volume vehicle production applications. A complete vehicle body shell together with its various lids and apertures, ready forfitment of internal divisions, may be produced in approximately7 minutes.

Panels for uses other than vehicle parts can orcourse be produced and appropriate materials will be employed appropriate to the intended use of the panels, forexamplefor boats or homes.

It is to be appreciated that the embodiments of the invention described above have been given by way of example only and that modifications may be effected.

Thus, for example as an alternative to the use of ABS for the first layer, other therm o-fo rma ble plastics materials may be used, or a super plastics aluminium alloy material or an epoxy compound may be used.

The neutral layer or layers may be formed from a thin layerofpolyurethanefoam or an aramid paper honeycomb material, as alternatives to the polyvinyl chloride foam mentioned above. The inner layer may be one of a selection of thermo-formable materials or of woven materials. Adhesive resins of types other than epoxy adhesive resins are also suitable. The ABS or other material may be used alone or combined with other materials, for example polycarbonates, to improve surface performance. Aluminium can also be employed since it can be vacuum formed if heated to approximately 400"C. The types of ABS compounds usually employed will usually thermoform at approximately 160 C.

Unsupported ABS mouldings do not have great rigidity and, whilstfl#eir impact performance is adequate,they are only suitabieforvery light panels. The above mentioned reinforcing of the panels increases their performance and range of use. Generally,the performance ofthe panels in bending can be improved either by increasing the neutral axis (and hence the effective I-section) or by so arrangeing the panel that bending places the various elements in shear. If one or more elements of the sandwich construction are strong in tension and compression, the resultant resistance to bending across the axis ofthe panel is much improved. Obviously the sandwich construction is then also much better equiped to handle high tensile loads parallel to the axis.Long filaments are advantageously incorporated within the sandwich construction, so that the longitudinal axis of the long filaments can be arranged either in one or several layers to absorb and resist predicted stress. The formed panels can thus, if required, be given differing degrees of stiffness in differentdirections, or even in different parts of the same panel. The long filaments employed may be, for example, glass, carbon or aramid fibres. The long filaments should be supported within a compatible matrix in order that the long filaments can reach their design performance. A preferred sandwich construction is that utilising a thermo-formable outer skin, with layer of fiiament (with a lightsupportweave) impregnated with a toughened acrylic adhesive, alternating with PETfilm (forexample Mylar) and, athigherformingtempera- tures, high performance plastics films such for example as polysulphides which are temperature compatible.Similarly, if the surface skins of the panels have a highthermo4orm temperature, the composi- tion ofthetoughened acrylic may be altered so that it cures atthethermo-form temperature. At very high temperatures, specialised epoxides can be used.

Claims (13)

1. A method of forming a panel comprising pressure and temperature moulding a sandwich construction offormable materials to form a panel.

2. Amethod according to claim 1 inwhichthe moulding is effected by vacuum forming.

3. A method accordingto claim 1 or claim 2 in which the sandwich construction comprises a multilayer construction having outer and inner layers of a thermo-formable material, and a central layerofthe formable material.

4. A method according to claim 1 or claim 2 in which the sandwich construction comprises a multilayer construction having an outer layer of a thermoformable material, and central and inner layers of a formable material.

5. A method according to any one of the preceding claims in which an adhesive is used to bond the layers of the sandwich construction together.

6. A method according to any one of the preced- ing claims in which individual layers of the sandwich construction are separately placed in a mould forthe pressureandthetemperature moulding.

7. A method according to any one of claims 1 to 5 in which the sandwich construction is pre-assembled and is then placed in a mouldforthe pressure and the temperature moulding.

8. A method of forming a panel, substantially as herein described with reference to the accompanying drawings.

9. A panel when produced by a method as claimed in any one of the preceding claims.

10. A sandwich construction panel comprising an outer layer of thermo-formable material, and central and inner layers of formable material.

11. A sandwich construction panel according to claim 10 in which the inner layer is ofathermo- formable material.

12. A sandwich construction panel according to claim lOin which the inner layer is of a formable material which is foamed, expanded orwoven.

13. A sandwich construction panel substantially as herein described with reference to the accompanying drawings.