WO1995001862A1

WO1995001862A1 - A method of rotational moulding and a moulding made thereby

Info

Publication number: WO1995001862A1
Application number: PCT/GB1994/001424
Authority: WO
Inventors: Ronald Arthur Easterlow; Gordon Frederick Smith
Original assignee: Rover Group Limited
Priority date: 1993-07-03
Filing date: 1994-06-30
Publication date: 1995-01-19
Also published as: GB9313800D0

Abstract

The moulding is formed by a method comprising introducing into a mould (10) a first plastics material (15) and then rotating the mould (10) to cause the plastics material in a melted condition to coat a surface (15a) of the mould. There is then introduced into the mould (10) a second plastics material (16) and the mould is rotated to cause the second plastics material in a melted condition to coat the first plastics material to form the component (17). One of the first and second plastics materials used in the method is formed from or is in the form of a powdered plastics paint.

Description

A METHOD OF ROTATIONAL MOULDING AND A MOULDING MADE THEREBY

The invention relates to a method of rotational moulding and a moulding made thereby.

Rotational moulding is widely used for producing large hollow plastics components. It is a technique applied particularly to the production of plastics containers such as fuel tanks.

Figs.1 and 2 of the accompanying drawings illustrate diagrammatically a typical rotational moulding method.

In Fig.1, a rotary mould 10 comprises separable halves 11, 12. Suitable heating means (not shown) is provided for the mould.

In use, the mould is heated and a polymer powder 14 is introduced into the cavity 13. The mould 10 is then rotated, normally about more than one axis, so that the polymer powder coats the surface of the mould and melts as shown at 14a to form the component. The mould is then allowed to cool, the mould halves 11, 12 separated and the component removed.

If the component is to be made in a particular colour, an appropriately coloured plastics material is used as described in GB-A-1 521 121 and GB-A-1 140 384 where layers having different colours are formed by rotational moulding. Alternatively, the finished component can be provided with a coat of paint after being taken from the mould 10. If containers of many shades of colour are required, the manufacturer needs to keep in stock very large quantities of appropriately coloured polymer powder. If the component is to be painted, there can be difficulties in providing paints which will adhere to the surface of the plastics components sufficiently well.

An object of the present invention is to provide a method of rotational moulding which will enable a component to be provided with a paint coating in an improved manner, and a component made by such a method.

According to one aspect of the invention there is provided a method of rotational moulding to form a component comprising introducing into a mould a first plastics material; rotating the mould to cause the plastics material in a melted condition to coat a surface of the mould, introducing into the mould a second plastics material, and rotating the mould to cause the second plastics material in a melted condition to coat the first plastics material to form a moulding, one of the first and second plastics materials being formed from or being in the form of a powdered plastics paint material.

With such a method, the component and paint bond together within the mould to form a component having a strongly adhering painted surface. The density of pigmentation in the plastics paint will be high compared to that of plastics material used to form the component itself. Therefore, the thickness of the paint coating required to give the desired finish is expected to be small when compared to the amount of plastics material necessary to form the component or to form constructionl layers for example as described in the above prior specifications. Therefore, the quantity of powdered paint required to produce various finishes is expected to be relatively small compared to the quantities of plastics material of different colour that would be required to construct complete components in the same range of coloured finishes.

The powdered paint may be introduced into the mould in powdered form and then be caused to melt by heat from the mould.

To provide a desired external finish to the component, the first said plastics material may comprise or be formed from the said powdered plastics paint material. On the other hand, if the second plastics material comprises or is formed from a powdered plastics paint, the inside surface of the component will then be coated with the paint finish which may help to improve resistance to permeability. That can be particularly useful where the container is a fuel tank.

In one embodiment, the powdered plastics paint material comprises a thermosetting material. Such material will normally have a thermoplastic phase so that heat applied thereto will melt the powdered paint material to enable it to coat the mould and continued applied heat will cause the onset of curing as the paint moves into its thermosetting phase.

Preferably, the second plastics material is introduced into the mould while the first plastics material is unset. In that way, a good bond is achieved between the two materials.

The other of the first and second plastics materials may also be in the form of a powder, for example, a polymer powder.

The mould is preferably allowed to cool before the moulded component is removed therefrom.

In accordance with a further aspect of the invention, there is provided a moulding made by a method according to the first aspect of the invention or any of the consistory clauses related thereto.

A method of rotational moulding and a moulding formed thereby will now be described by way of example with reference to the remaining accompanying drawings in which:-

Fig.3 is a cross section through a mould for use with a method in accordance with the present invention, powdered plastics paint material having been initially introduced into the mould,

Fig.4^" is a view similar to Fig.3 showing the way in which the powdered paint material melts and coats the inside of the mould, and

Fig.5 is a view similar to Fig.4 with a polymer powder introduced into the mould and

Fig.6 shows the way in which the polymer powder melts and coats the paint after rotation of the mould to produce the component, and Fig.7 is a cross section of the component removed from the mould in Fig.6.

As in Figs.1 and 2, the mould 10 comprises mould halves 11, 12 which are separable and which include heating means not shown.

The mould 10 is heated and a quantity of cross-linking thermosetting or thermoplastic powdered paint material is introduced.

An example of a suitable thermosetting material is Interpon DT1 10 available from Courtaulds Coatings (Holdings) Limited of Felling Gateshead, Tyne and Wear, England which will have a thermoplastic condition at temperatures between 80 and 100°C but which will begin to cure at temperatures above 120°C. The temperature of the mould 10 is sufficient to cause the thermosetting powdered paint 15 to enter its thermoplastic phase so as to melt the paint.

A suitable thermoplastic powder paint may be based on a mixture of thermoplastic material such as polypropylene and a concentrate of pigment to provide filling and/or coloration. In general moulding techniques, it is usual for such a material to be dispersed in a bulk polymer to provide a material for moulding having desired qualities. Such a mixture is known in the art as Master Batch. A bulk polymer with the master batch dispersed therein will not exhibit the properties normally required of a paint, in particular the necessary depth of image and hardness/ resistance to abrasion. To enable the thermoplastic material/pigment concentrate mixture to provide a paint coating in accordance with the present invention , an additive is combined with the mixture to cause cross linking. A suitable additive is an acrylic- modified polyolefinic ionomer. An ionomer is a thermoplastic material provided with a low degree of crosslinking in the solid state by the addition of a metal salt which provides ionic bonds between the polymer chains. By using the cross linking additive, a mould having a smooth surface will enable the paint coating to provide a surface finish having a depth of image and hardness which would not be exhibited by a moulded bulk polymer having the aforesaid mixture dispersed therein. Such a thermoplastic paint material suitable for coating a polypropylene substrate may comprise the following composition: 91.6% polypropylene 2.5% carbon black 0.5% antioxidant 0.4% UV stabilizer 5.0% acrylic-modified polyolefinic ionomer Other suitable cross linking- thermoplastic compositions for providing the paint coating are described in WO 94/06612 to which the reader is directed and are incorporated herein by reference.

Having introduced the powdered paint material, the mould 10 is then rotated about more than one axis so that the melted paint coats the inside surface 10a of the mould to form a paint film 15a.

Where the powdered plastic paint material is thermosetting, continued heat applied by the mould causes the paint film 15a to enter its thermosetting phase but before the paint film sets, a polymer powder 16 eg ABS having a melt temperature of 250°C, is introduced into the mould as shown in Fig.5 and melts due to the mould temperature.

Where the powdered plastic paint material 15 is thermoplastic as described above, continued heat applied by the mould causes the paint film 15 to remain in a molten condition. A polymer powder 16 eg polypropylene is then introduced into the mould and melts due to the mould temperature.

The mould is then rotated so that the liquid plastics formed from material 16 flows over the inside surface of the paint film 15a to form a coat 16a constituting the bulk or substrate of the component being moulded. A paint film thickness of 0.15 mm has been found to be suitable and it is envisaged that typical thicknesses of the paint film are likely to be in the range 0.1 mm to 2 mm. Normally, the wall thickness of the plastic component 17 will be considerably thicker than the paint film 15a.

Where the paint material is thermosetting, heat applied by the mould continues to cure the paint film 15a. Where the paint material is thermoplastic the mould is allowed to cool in order to cure the paint film.

After the mould 10 has cooled, it is opened to allow the component indicated generally at 17 to be removed from the mould so that it can cool and allow the melted polymer substrate to cure. If necessary, on removing the component having the thermosetting coating from the mould, any post curing required to complete the curing of the paint film can be carried out by applying heat H in an oven. The component can then be removed from the oven and allowed to cool whereby the polymer substrate layer will cure. The component comprises a hollow body having a exterior surface formed by the paint film 15a.

If desired, the order can be reversed so that the polymer powder 16 is introduced first into the mould followed by the powdered paint material 15. In such a case, the inside surface of the component 17 will have the painted surface. As the paint is particularly impervious, such a method could be useful in the production of fuel tanks.

Where the paint is initially introduced into the mould as in Fig.1, the surface gloss of the paint will be dependent upon the finish of the surfaces defining the mould cavity 13.

If desired, the component could be made such that having reached the point shown in Fig.6 with the component unset, powdered paint is then introduced into the component 17 is then melted and the mould rotated so as to coat the inside surface of the layer of component 17. In that way, the component will have both internal and external paint coated surfaces.

Likewise, if the polymer powder were introduced initially into the mould to form an outer layer of the component and then a layer of paint were produced on the inside surface of the polymer layer followed by a further inner polymer layer, the component would have inner and outer surfaces formed by the polymer layer and an inner painted surface sandwiched therebetween.

The powdered paint material 15 may incorporate finely divided metallic or pearlescent materials to provide a metallic finish. Examples of such materials are disclosed in WO 94/06612.

The term "powdered paint material" used herein embraces paint in powdered or granulated form.

Claims

1. A method of rotational moulding comprising introducing into a mould (10) a first plastics material (15), rotating the mould (10) to cause the plastics material (15) in a melted condition to coat a surface (10a) of the mould (10), introducing into the mould a second plastics material (16), and rotating the mould (10) to cause the second plastics material (16) in a melted condition to coat the first plastics material (15) to form a moulding (17), characterised in that one of the first and second plastics materials (15) is formed from or in the form of a powdered plastics paint.

2. A method according to Claim 1 characterised in that the powdered paint (15) is introduced into the mould (10) in powdered form and is caused to melt by heat from the mould.

3. A method according to Claim 1 or 2 characterised in that the first said plastics material (15) comprises or is formed from the said powdered plastics paint material.

4. A method according to any preceding claim characterised in that the powdered plastics paint (15) is a thermosetting material having a thermoplastic phase.

5. A method according to Claim 4 characterised in that the powdered plastics paint (15) is Interpon UT1410.

6. A method according to any preceding Claim including introducing the second plastics material (16) into the mould while the first plastics (16) material is unset or uncured.

7. A method according to any preceding claim including curing the first plastics material (15) in the mould (10) after introduction of the second plastics material (16).

8. A method according to any of Claims 1 to 6 including partially curing the first plastics material (15) in the mould (10) and completing curing the first plastics material (15) by removing the moulding (17) from the mould (10) and applying heat (H) thereto.

9. A method according to any preceding claim including allowing the moulding (17) to cool so as to cure the second plastics material (16).

10. A method according to any preceding Claim characterised in that the other of the first and second plastics materials (16) is in the form of a polymer powder.

11. A method according to any preceding Claim including allowing the mould (10) to cool prior to removing the moulding (17) therefrom.

12. A moulding made by a method of rotational moulding according to any preceding Claim.