WO1992003272A1

WO1992003272A1 - A hollow body and a method for rotational moulding of same

Info

Publication number: WO1992003272A1
Application number: PCT/NO1991/000111
Authority: WO
Inventors: Sigmund Berg
Original assignee: Nordic Industrier A/S
Priority date: 1990-08-16
Filing date: 1991-08-15
Publication date: 1992-03-05
Also published as: NO176510C; NO176510B; AU8411291A; NO903626D0; NO903626L

Abstract

A hollow body comprising an inner wall (10) and an outer wall (11) is produced by means of rotational moulding with, e.g., thermoplastic as the moulding material, an inner mould (1) and an outer mould (3) being used. The first step in the invention is to produce an object (1) with an internal surface corresponding to the internal surface of the hollow body. Thereafter the object is connected in an airtight manner with an outer mould (3), in such a way that a space (6) is formed between these. The moulding material is poured into the space (6), and by means of a known per se technique it is arranged that the moulding material, which during the rotation of the mould assembly is distributed over the outer surface of the object, i.e. the surface of the object facing the outer mould (3), adheres to this, while the moulding material which is distributed over the internal surface of the outer mould (3) does not adhere to this. After removal of the outer mould (3) a hollow body is thereby provided with an outer wall of moulding material and an inner wall comprising the object (1) coated externally with moulding material. The invention comprises both the hollow body and the method of production.

Description

A hollow Body and a Method for rotational Moulding of same

The invention concerns a hollow body and a method for production of such hollow bodies with an inner wall and an outer wall, by means of rotational moulding with, e.g. thermoplastic as the moulding material, an inner mould and an outer mould being used.

A known method for moulding with thermoplastic consists in the inner and outer moulds being connected with each other so as t define an intermediate, airtight space into which the moulding material, e.g. plastic, is introduced in the form of a granulate or powder which, during the rotation and heating of the moulds, melts and is distributed over the walls of the mould. In order to facilitate the removal of the finished object from the moulds when these have been cooled, the walls can be coated with a non-adhesive material.

If the moulding material is plastic and the plastic granulate is required to be converted into foam, a foaming agent is added. In this way containers or tanks, e.g., can be produced with a thin outer layer, an inner layer of solid plastic and a intermediate layer of foamed plastic, the foaming agent being added to the remaining granulate material which has not yet melted, after the rest of the granulate material has initially been melted and deposited on the walls of the mould. However, the plastic material melts and adheres only to walls which hav been heated. If a mould wall, e.g. the wall of the inner mould is kept cool until the foaming agent is added, this results in a finished object comprising only of an outer layer of solid plastic which is coated on the inside with a layer of insulating, stiffening foamed plastic.

Such containers could be useful in the food industry, but sinc plastic is easily scratched and scratched containers cannot be cleaned in a simple and satisfactory manner, they are unsuitable for use in this industry. Containers made of, e.g., stainless steel are therefore used, which in order to withstan the strains to which they are exposed, must have relatively thick (2-3 mm) walls and in addition reinforcement ribs etc. attached thereto by welding, which makes them heavy and expensive.

Moreover, it could be advantageous to produce sealed hollow bodies, e.g. hot water tanks, by means of rotational moulding, but since these are sealed the inner mould cannot be removed after moulding.

The object of the invention is to provide a hollow body and a method whereby the above-mentioned disadvantages are eliminated.

The nature of the invention is characterized by the features in the claims presented.

In the following section the invention will be described in more detail with reference to the drawing.

Fig. 1 is a cross section through a mould assembly which is used during the exercise of the method according to the invention.

Fig. 2 is a cross section of a container according to the invention, produced by means of the mould assembly illustrated in fig. 1.

As illustrated in fig. 1 the mould assembly consists of an inner mould 1, the inside of which is shaped like the inside of the container which is to be produced. Along its upper edge the inner mould has a projecting flange portion 2.

The inner mould is arranged in an outer mould 3 which in the same way has a projecting flange portion 4 along its upper edge.. Between the inner mould's and the outer mould's flange portion 2 and 4 respectively, a seal 5 can be fitted and the flange portions can be pressed against each other by means of not shown clamps, so that the moulds define an airtight space 6 into which can be filled a moulding material, e.g. granulated, thermo-hardening plastic via not shown filling openings.

In order to facilitate the heating of the thermo-hardening plastic, it is advantageous for the moulds to be made of a material which is a good heat conductor, e.g. metal.

If the container is to be used for food, the inner mould is made of a stainless steel plate with a wall thickness of, e.g. between 0.3 mm and 1.0 mm.

During moulding the mould assembly is rotated and heated, thus causing the plastic material to melt and be deposited on the walls of the mould. In order to cause the moulding material to adhere to the inner mould, but to be easily separated from the outer mould after moulding, the wail of the inner mould can be coated with a known per εe agent which causes the moulding material to adhere to it easily, while the outer mould is coated with a non-adhesive agent.

If the moulding material is only required to form a layer along the mould walls without completely filling the space between the moulds, in order to stiffen the container's inner wall, the outer mould can have indentations or convex portions 7, so that the space between the moulds at these portions is not larger than approximately twice the layer thickness of the moulding material, causing the moulding material in the inner and outer moulds to touch each other and thus forming bridges of moulding material at these portions.

If the inner mould is kept cool during the heating of the oute mould, during which a part of the plastic material melts and i deposited on the latter mould, and thereafter heated up while foaming agent is added to tne remaining plastic, a container can be obtained with an outer wall of solid plastic and an inner wall of stainless steel with an intermediate layer of foamed plastic which insulates and supports the inner wall.

In this case, however, the outer mould can also have indentations, but these can be shallower than those described above, in order to provide portions which constitute stiffening portions or ribs in the container's outer wall.

In the production of large containers where the inner wall consists of a very thin plate, the inner wall can be supported during moulding by means of a jig which is attached to the rotating construction and/or outer mould, and which projects into the container.

The invention is also suitable for, and for the production of, e.g. hot water tanks where the inner mould is completely surrounded by the outer mould during the rotational moulding and attached to the outer mould by end pieces which are welded on or attached by other means to the inner mould and which are subsequently used as supply pipes or outlet pipes for the hot water tank.

Fig. 2 is a cross section of an embodiment of a container according to the invention which is produced by means of the mould assembly illustrated in fig. 1.

As this figure shows, the container consists of an inner wall 10 made of a thin plate of stainless steel, an outer wall 11 of solid plastic, and a core 12 of foamed plastic situated between the inner wall 10 and the outer wall 11.

Claims

PATENT CLAIMS

1. A hollow body comprising an inner wall (10) and an outer wall (11), produced by means of rotational moulding with, e.g. thermoplastic as the moulding material, in which an inner mou (1) and an outer mould (3) are used, the inner mould consisti of an object (1) with an internal surface corresponding to th internal surface of the hollow body, and the object being connected in an airtight manner with the outer mould (3) in such a way that between these is formed a space (6) , into whi the moulding material can be filled, and means have been provided to ensure that the moulding material which during th rotation of the mould assembly is distributed over the outer surface of the object, i.e. the surface facing the outer moul (3), can adhere to this, so that after removal of the outer mould (3) a hollow body is provided with an inner wall comprising the object coated externally with moulding material c h a r a c t e r i z e d in that the object (1) is made of plate of a good heat conducting material such as stainless steel so thin that the plate is unable alone and under the influence of its own weight to maintain the desired shape of the inner mould, and thus during moulding the object has to be supported internally by means of a removable jig.

2. A method for production of a hollow body according to claim 1, comprising an inner wall (10) and an outer wall (11), by means of rotational moulding with, e.g., thermoplastic as moulding material, an inner mould (1) and an outer mould (3) being used, where the inner mould consists of an object (1) with an internal surface corresponding to the internal surface of the hollow body, and the object being connected in an airtight manner with an outer mould (3) in such a way that between these is formed a space (6) , into which the moulding material can be filled, and means are provided to ensure that the moulding material which is distributed over the outer surface of the object, i.e. the surface facing the outer mould (3) during rotation, can adhere to this, whereby after removal of the outer mould (3) a hollow body is provided with an inner wall comprising the object coated externally with moulding material, c h a r a c t e r i z e d in that the object (1) is made of a thin plate of a good heat conducting material such as stainless steel, the plate thickness being so small that it is unable alone and under the influence of its own weight to maintain the desired shape of the inner mould, and the object is supported internally during moulding by means of a removable jig, thus providing it with the said desired shape.