GB2121345A

GB2121345A - Method of rotary moulding a plastics article

Info

Publication number: GB2121345A
Application number: GB08215659A
Authority: GB
Inventors: John Lloyd
Original assignee: Glasdon Ltd
Current assignee: Glasdon Ltd
Priority date: 1982-05-28
Filing date: 1982-05-28
Publication date: 1983-12-21
Also published as: GB2121345B

Abstract

The disclosure relates to a method of rotary moulding a plastic article (18) using a rotary mould (10) having a male former (12) and an encircling reservoir (13) for the raw material from which the moulding is to be formed. Raw plastics material is introduced into the mould including a proportion of coarse particles and the mould is rotated to tumble the raw materials over the male form to cause an adherent layer of the material to be created on the form with the coarse particles at the outer surface of the layer. The coarse particles form a rough or textured surface on the moulded article. <IMAGE>

Description

SPECIFICATION Improvements in rotational moulding of plastics The process of producing hollow plastics articles by rotational moulding is well established and is achieved by introducing a measured amount of the raw material in finely ground powder form into a metal mould, the interior wall of which defines the exterior form of the article to be moulded, closing the charging aperture with a suitable cover plate and heating the mould whilst simultaneously rotating it.

Any suitable method of mould heating may be used but it is commonly achieved either by placing the mould in an oven or by playing gas flames onto its exterior surface.

The rotation of the mould is commonly either about two co-planar axes at right angles to each other, or about a single axis the disposition of which oscillates in a vertical plane so as to incline the axis alternately between limits either side of a horizontal median. In either case the speed of rotation of the mould is only such as to ensure that the body of powder within comes into contact with all parts of the moulds internal surface as it moves and it is not intended to contribute any centrifugal effect to the powder distribution.

As the mould temperature reaches the melting point of the plastic material, the powder begins to fuse forming an homogeneous layer of molten material against the internal surface of the mould.

The heating and rotation continue until all the free powder within the mould has been exhausted by the growing thickness of the plastic 'moulding'.

The mould and moulding are allowed to cool to the point where the plastic is fully solidified and self-supporting at which time some detachable portion of the mould is removed to permit the withdrawal of the moulding. It is normally - necessary to coat the internal surface of the mould with a release agent to prevent excessive adherence of the plastic material to the metal, so facilitating withdrawal of the finished moulding.

This invention provides a method of rotary moulding a plastics article using a rotary mould having a male former and an encircling reservoir for the raw material from which the moulding is to be formed, wherein the mould is charged with raw plastics material including a proportion of coarse particles and the mould is rotated to tumble the raw material over the form to cause a coherent layer of the material to be created on the form with the coarse particles on the outer surface of the layer.

The coarse particles may be formed from a thermoplastics and the male former may be heated under controlled conditions so that the coarse particles are trapped in the outer surface of the plastics layer deposited on the male former but do not melt and become dispersed through the layer.

The following is a description of a specific embodiment of the invention, reference being made to the accompanying drawings, in which: Figure 1 is a partly cut-away perspective view of a rotational mould for use in performing the invention; Figure 2 is a sectional view of the mould of Figure 1 taken through the mould axis; Figure 3 is a sectional view through a modified male former of the mould of Figure 1 with a moulding formed thereon; Figure 4 is a view of the area A on Figure 3 enlarged to show greater detail; and Figure 5 is a sectional view of part of a further modified male formed of the mould.

With reference to Figures 1 and 2, a mould 10 mounted to rotate about axis 1 1 comprises two sections; that is a male cylindrical section 12 coaxial with axis 11 and an encircling cylindrical reservoir section 13. The reservoir section provides an outer shell which serves both to retain the moulding material when at rest and, during rotation, to direct a continuous flow of material over the surface of the forming section whilst recovering any free material falling past the forming surface. The reservoir 12 has an open end 14 through which the reservoir may be charged and the moulded article removed and a cover plate 15 closes the end when the mould is in use.

For the purpose of moulding, it is unnecessary and undesirable for the reservoir section 1 3 of the mould to be heated. The forming section 12 of the mould must however be heated as with conventional rotational moulding and this may be effected by any suitable system which could include direct or indirect electrical surface heating, the circulation of a hot gas or liquid from a remote heat source, a direct gas flame and the insertion of a preheated core.

If at the time of or subsequent to charging the mould with powder, measured quantities of larger particles of material, selected for size and colour, are introduced into the mould these will remain free until near the end of the heating cycle and thus remain visible on the external surface of the finished moulding. If thermoplastics particles are used, careful control of the heating and cooling sequence will result in the particles being entrapped in the surface but not being allowed to melt to the point where they flow fully into it. Thus a decorative multi-coloured relief texture can be produced on the external surface of the moulding.

By the use of particles of suitable size and colours, the finished moulding may be given the appearance of exposed aggregate as the thermoplastic particles, however irregular initially, will tend to take on a smooth pebble like form in the semi-molten condition.

It is necessary to ensure that material flows evenly over the entire surface of the forming section if a uniform particle dispersion is required.

This may be achieved solely by the control of the speed and direction of rotation of the mould, and the orientation of its axis or axes of rotation, but with some shapes of a moulding, it may be necessary to install additional mechanical controls within the reservoir section. Such devices could include baffle plates, paddles and funnels to direct material preferentially to certain areas of the forming section.

On completion of the cooling of the mould and moulding it is necessary either to remove some part of the reservoir wall to gain access to the forming section or to separate the forming section from the reservoir section in order to withdraw the finished moulding 18 from the forming section of the mould. Because most moulding materials contract as they cool, the finished moulding will shrink onto the forming section and with some shapes of moulding a mechanical device may be necessary to assist in the separation of the moulding from the forming section of the mould.Such devices might include a valve for the introduction of pressurised gas at the interface of the moulding and forming surface, a movable plate or collar situated at the junction of the forming and reservoir sections which can be used to push the moulding off the former, or a forming section having a portion of its wall which is collapsible or removable so as to relieve the tension of the contracting mould. It is also possible to facilitate the separation of the moulding and former by the design of the moulding itself. If as shown in Figures 3 and 4, one or more female flutes 16 are arranged longitudinally in the direction of ultimate moulding withdrawal, they provide in effect a corrugated surface which can relieve shrinkage tension across the flutes by drawing material from the flute depressions which are thus reduced in depth.

Figure 5 shows an alternative more heavily corrugated form.

There are two other variants of the rotational moulding technique capable of utilising this invention. Certain materials such as P.V.C. are mouldable in heated rotating moulds, but are added to the mould in a liquid, slurry or paste form. This process is more commonly called rotational casting. Other thermo setting material such as polyurethanes can be moulded in a rotating mould but solidify as a result of a chemical reaction between two or more components of the material.

Here the chemical action will generate heat and although an external heat source will accelerate the process, it may not be necessary.

Claims

1. A method of rotary moulding a plastics article using a rotary mould having a male former and an encircling reservoir for the raw material from which the moulding is to be formed, wherein the mould is charged with raw plastics material including a proportion of coarse particles and the mould is rotated to tumble the raw material over the form to cause a coherent layer of the material to be created on the form with the coarse particles on the outer surface of the layer.

2. A method as claimed in claim 1 wherein the raw material includes a thermoplastic material in powder form and the male former of the mould is heated to cause the powder particles depositing on the male former to fuse together to form said layer.

3. A method as claimed in claim 1 wherein the raw material comprises a liquid, slurry or paste and the male former is heated during rotation of the mould to cause a layer of the liquid, slurry or paste depositing on the male former to solidify on the former.

4. A method as claimed in claim 1 wherein the raw material with which the mould is charged includes the reactive components of a thermosetting plastics and the mould is rotated to mix the components to cause them to react together and deposit on the male former to form said layer.

5. A method as claimed in claim 4 wherein the former is heated to accelerate the setting process of the layer.

6. A method as claimed in any of the preceding claims wherein the coarse particles are formed from a thermoplastics and the male former is heated under controlled conditions so that the coarse particles are trapped in the outer surface of the plastics layer deposited on the male former but do not melt and become dispersed through the layer.

7. A method as claimed in any of the preceding claims wherein the article is stripped from the male former after the plastics material has set.

8. A method as claimed in any of claims 1 to 7 wherein the male former is collapsed to release the moulded article.

9. A method as claimed in any of claims 1 to 7 wherein the male former has longitudinally extending flutes or corrugations around its outer periphery so that the layer of plastics material as created on the male former extends into the corrugations and, on cooling, contracts and is drawn at least partly away from the flutes or corrugations to enable the moulded article to be stripped from the male former.

10. A method as claimed in any of the preceding claims wherein the male former is hollow and a heated fluid is supplied to the hollow former to heat the former and cause the plastic materials depositing on the former to form a coherent layer on the former.

11. A method as claimed in claim 10 wherein a gas flame is applied on the inner surface of the former to heat the former.

12. A method as claimed any of claims 1 to 9 wherein direct or indirect electrical surface heating is used for heating the male former.

13. A method as claimed in any of claims 1 to 9 wherein a pre-heated core is inserted in the male former to heat the former.