GB2193464A - A mould for use in rotational moulding - Google Patents

Abstract

An element (10) for use in rotational moulding comprises a base (11) having, projecting internally thereof, a pair of elements (16, 17) each of which supports a respective projecting member (18, 19). The base (11) supports side walls (12, 13, 14, 15) to form an open-mouthed container. A recess (20) is provided in the wall (15). The members (18, 19) are made from 8 metal or metal alloy the thermal conductivity of which is greater than that of the rest of the element (10). The element (10) is adapted to mate with a similar element to form a mould with the free ends of the members (18, 19) separated from similar opposing members of the other element by a gap of about 0.5 mm. <IMAGE>

Description

SPECIFICATION A mould element for use in rotational moulding This invention relates to a mould element for use in rotational moulding.

More particularly, it relates to a mould constructed from said mould element for use in a rotational moulding process.

Rotational moulding is a well-known technique. It is a technique particularly suited for the production of hollow parts especially those with complex and varied shapes. The process of rotational moulding involves the steps of loading plastics material into a closed mould, heating and fusion of the material and cooling and unloading the mould. After the charged mould is moved into an oven for heating and fusion of the material, the mould is generally rotated on axes at a relatively low speed. As heat penetrates the mould, the material adheres to the mould's inner surface until the material is completely fused. The mould is then cooled by air or by water spray or combination of both while still rotating thereby lowering the temperature in a gradual manner.

The mould is then opened, the finished product removed, and the mould recycled for the next cycle.

A variety of plastics materials may be used more particularly polyethylenes and plastisols.

Other materials include nylon, fluoropolymers, polypropylene, polycarbonates, cellulose, acetates, butyrates, elastomers, polyurethanes and EVA.

A problem arises in the use of rotational moulding for the manufacture of a product in which it is necessary to provide reinforcing elements between opposing walls thereof.

It is an object of the present invention to overcome this problem.

The invention therefore provides a mould element for a mould for use in rotational moulding which element comprises a base and side walls forming an open-mouthed container adapted for engaging with a similar element to form the mould; the base of the container having a member projecting therefrom in the direction of the mouth of the container wherein the member is made from a metal or metal alloy having a thermal conductivity which is substantially greater than that of the base or side walls of the container.

The invention will be understood in greater detail from the following description of preferred embodiments thereof given by way of example only and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a first element according to the invention for use in the construction of a mould for use in a rotational moulding process; Figure 2 is a perspective view of a second element according to the invention being a mirror image of the first element of Figure 1; Figure 3 is a perspective view of a storage tank produced by rotational moulding from a mould constructed of the elements of Figures 1 and 2 of the drawings; Figure 4 is the tank of Figure 3 of the drawings showing the interior thereof; Figure 5 is a side elavation of the tank of Figure 3 of the drawings; Figure 6 is an end elavation of the tank of Figure 3 of the drawings;; Figure 7 is a plan view of the tank of Figure 3 of the drawings; and Figure 8 is a cross sectional view of part of the tank of Figure 3 of the drawings.

Referring now to the drawings, there is shown an element 10 for use in the construction of a mould which element 10 comprises a base 11 having walls 12, 13, 14 and 15 projecting therefrom.

With particular reference to Figure 1 of the drawings (Figure 2 being a mirror image element 10a of the element 10 of Figure 1 of the drawings having features similar thereto and denoted by similar numerals ending in the suffix letter "a"), it will be noted that the base 11 has, projecting internally thereof, a pair of elements 16, 17 each of which supports a respective projecting member 18, 19.

Each projecting member 18, 19 tapers off in the direction of its free end. The cross sectional area of each projecting member 18, 19 at its free end relative to the fixed end is about 1:4.

The wall 15 has a slit 20 therein defined by inwardly projecting arms 21, 22. The side wall 13 has a recess portion 23. In the vicinity of the interface between the wall 12 and the wall 14 is a second recess portion 24.

Each of the walls 12, 13, 14, and 15 terminates in a flange 25 the face of which, when mated against the flange 24a of the element 10a, provides a closed mould.

Except for the members 18, 19 which are made from copper having a thickness of about 4mm, the rest of the element 10 is made from steel having a thickness of about 2mm.

Essentially, therefore, each of the members 18, 19 is made from a metal or metal alloy the thermal conductivity of which is greater than that of the rest of the element 10.

The open-mouth of the mould element 10a is placed so as to cover the open-mouth of the mould element 10a so that the side walls 12, 12a, 15, 15a, 12, 13a, and 13, 12a are in a butting relationship. The member 18 should now be in line or in register with the member 18a, the member 18 should be in line or register with the member 19a. The elements 10, 10a, are then clamped together to form a mould. There should now be a gap of about 0.5mm between the ends of the member 18, 19 and the member 18a, 19a respectively.

To manufacture a tank 30 as shown in Fig ure 3 of the drawings, about 70kg of medium density suitable polymer beads having an average diameter of about 500 microns is placed into the element 10. The mould is then indexed, by well-known means associated with rotational moulding techniques, into an oven (not shown) the temperature of which is about 300 C. The melting point of the beads is about 110 C. Prior to indexing into the oven, the hole generated by the slots 20, 20a is closed with a heat resistant member (not shown).

As the thermoconductivity of copper is approximately eight times greater than that of steel, the members 18, 18a, 19 and 19a reach a temperature relatively quickly when compared with the rest of the mould which termperature is sufficient to melt the beads which contacts them. It is important that the members 18, 18a, 19 and 19a reach the desired minimum working temperature of about 150 C. relatively quickly because it is only during the very early part of the moulding cycle that sufficient polymer passes in the vicinity of said members 18, 18a, 19 and 19a.

During the moulding cycle, the mould is rotated simultaneously about the axes of rotation as is known in rotational moulding techniques. The number of revolutions may be in the range of about 4 revolutions per minutes (rpm) to 20 rpm. Preferably, the number of rpm is about 4.

As the moulding cylce continues, the temperature of the rest of the mould will reach a working temperature to enable the rest of the beads to melt and cover the entire internal surface of the mould. At the end of the moulding cycle, which takes about 15 minutes, the mould is indexed into a cooling station (not shown) still rotating. The mould is cooled gradually with a combination of air and water until it reaches a handling temperature.

The mould is then moved into an unloading station (not shown) and the elements 10, 10a are separated to reveal the tank 30. The tank 30 has two substantially flat walls 31, 31a, a base 32 and a curved wall 33 having a hole 34 closed by a stopper 35. The stopper 35 is made by a conventional moulding technique which is well-known. Into the wall 33 is inserted a stop value 36. Having regard to the elements 16, 16a and 17, 17a of the mould, the side walls 31, 31a have reinforcing internally projecting elements 37 linked by connecting ribs 38, 38a and 39, 39a. One of the ribs 38, 38a is shown in cross-section in Figure 8 of the drawings. The rib 38 is generated by the element 18; the rib 38a is generated by the element 18a. The wall 40 connecting ribs 38, 38a is about 0.5mm in thickness.The degree of slop of each rib 38, 38a, 39, 39a from the outside to the walls 40, is about 1:4.

The recess portion 23 produces a recess 43 in the tank 30 to facilitate lifting thereof. If desired a similar recess portion (not shown) may be incorporated in the elements 10, 10a so as to produce a recess in the tank 30 opposite to the recess 43.

The recess portion 24 produced a recess of step 44 in the tank 30. The step 44, above which is located the stop valve 36, prevents any sludge in the tank 30 from passing from the tank 30 via the stop valve 36.

The tank 30 is preferably used for storing oil for oil fired domestic central heating purposes but may, if desired, be used to store other liquids as appropriate.

It will be appreciated that the use of the element 10 according to the invention may be adapted as required to produce other containers.

The invention is not limited by or to the specific embodiments described which can undergo considerable variation without departing from the scope of the invention.

Claims (17)

1. A mould element for a mould for use in rotational moulding which element comprises a base and side walls forming an open-mouthed container adapted for engaging with a similar element to form the mould; the base of which element having a member projecting therefrom in the direction of the mouth of the container wherein the member is made from a metal or metal alloy having a thermal conductivity which is substantially greater than that of the base or side walls of the container.

2. A mould element as claimed in claim 1 wherein the base and side walls are made from steel and the member is made from cooper.

3. A mould element as claimed in claim 1 or claim 2 wherein one of the side walls has a recess portion adapted for forming an opening when the mould element is in engagement with a similar mould element.

4. A mould element as claimed in any of claims 1-3 wherein each of the side walls terminates in a respective flange member, the face of which is adapted for abutting engagement with the face of a similar mould element.

5. A mould element as claimed in any of claims 1-4 wherein the cross-sectional area of the member at its free end when compared with the cross-sectional area at its fixed end is in the ratio of about 1:4.

6. A mould element as claimed in any of claims 1-5 wherein there is provided two projecting members in parallel spaced apart relationship.

7. A mould element substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

8. A mould for use in rotational moulding which mould comprises two mould elements as claimed in any of claims 1-7 releasbly secured in abutting engagement so that the projecting member of one of the elements is in register with the projecting member of the other element but seperated therefrom by a gap of about 0.5mum.

9. A mould as claimed in claim 5 substantially as hereinbefore described.

10. A process for making a plastics tank using a mould as claimed in 8 or claim 9 which process comprises charging at least one of the containers with an appropriate weight or volume of polymer beads; clamping the elements together to form the mould; moulding the beads in the mould to form the container by means of a rotational moulding technique incorporating heating means and rotating means; simultaneously cooling and rotating the mould until the mould reaches a handling temperature; and removing the thus formed container from the mould.

11. A process as claimed in claim 10 wherein the beads have a melting point of about 110 C.

12. A process as claimed in claim 10 or claim 11 wherein the beads have an average diameter of about 500 microns.

13. A process as claimed in any of claims 10-12 wherein the heating means raises the temperature of the mould to between 150 C.

and 300 C.

14. A process as claimed in any of claims 10-13 wherein the rotating means rotates the mould at between 4 rpm and 20 rpm.

15. A process as claimed in any of claims 10-12 wherein the rotating means rotates the mould at about 4 rpm.

16. A process for making a plastics tank using a mould as claimed in claim 8 or claim 9 substantially as hereinbefore described.

17. A plastics tank whenever made by a process as claimed in any of claims 10 16.