GB2048151A - Rotary moulding of hollow articles - Google Patents

Abstract

In a method and apparatus for rotationally moulding hollow articles, moulds 1 are clamped between a pair of beams 3 and are rotated therewith simultaneously about two different axes 7 and 17. Outer surfaces 2 of the moulds 1 are in contact with platens 4 of the beams 3 to ensure good heat conduction between them, and, during moulding, the temperature in the moulds 1 is varied as required by controlling the temperature of the beams 3, such as by using electrical heating elements and fluid cooling ducts located in the platens 4. <IMAGE>

Description

SPECIFICATION Rotary moulding of hollow articles This invention relates to the rotational moulding of hollow articles, such as balls, floats, tanks, boat hulls, and containers of various kinds.

The articles may be rigid or flexible, and may be made from either thermoplastic or thermosetting moulding material, that is any such material which is liquid or is liquefiable by application of heat to the material in a powdered, granular or other solid form, and which is able to set or cure after being moulded to the required shape. In the case of thermosetting materials the set or cure takes place automatically after the material has been heated and moulded, whereas in the case of thermoplastic materials the moulded article must be cooled in order to effect the set or cure.

The rotational moulding of hollow articles as described above is well known, and as presently practiced it involves:~ a) charging a two part mould with a measured quantity of the desired moulding material; b) closing the mould and holding it closed; c) heating and rotating the closed mould about two mutually perpendicular axes so that the moulding material, in liquid form, spreads over the internal surfaces of the mould; d) cooling the closed mould using a suitable cooling agent; and, finally, e) opening the mould and removing the moulded article.

When using a thermoplastic moulding material, rotation of the mould must be maintained during the cooling stage, at least until the initial set of the material has occurred, in order to prevent slumping of the material within the mould. When using a thermosetting moulding material however, rotation of the mould during the cooling stage may not be necessary.

Apparatus for carrying out such rotational moulding processes is also well known, the apparatus comprising a machine which is able to rotate the mould simultaneously about two different axes and which includes means for controlling the temperature of the mould (that is by heating or cooling the mould, or both).

However, while the known forms of this apparatus are able to carry out a rotational moulding process satisfactorily, they do nevertheless possess certain.

disadvantages, particularly in connection with the means provided for controlling the temperature of the mould.

In one form of apparatus the moulds are provided with heating and cooling chambers, which itself complicates the apparatus, but more importantly leads to an excessive consumption of thermal energy. In another case it has been proposed to spray the moulds with hot oil or molten salt in order to heat the moulds, although this method has been largely abandoned (in favour of hot air treatment) due to the danger of injury to the machine operators, machine deterioration, and the difficulty of recovering the sprayed liquid for re-use. In another form of the apparatus the moulds are of jacket construction so that fluid media can be circulated through the jackets in order to control the mould temperature. It is also known to construct the moulds with electric heating elements and stainless steel tubes (for the circulation of a cooling fluid) embedded in the moulds.However, these constructions are economically prohibitive due to the expense of having to manufacture individual moulds having the special and complex construction.

The present invention aims to provide a method and apparatus for the rotational moulding of hollow articles, in which the above mentioned disadvantages are avoided in a simple and relatively inexpensive manner.

According to one aspect of the invention, in a method of moulding a hollow article in a mould which is mounted in a moulding machine and is rotated about two different axes by the machine, the mould is held closed and is clamped in the machine by a pair of beams which bear against two opposite outer surfaces of the mould in heat exchange relationship therewith, and the temperature of the beams is controlled so as to establish, by heat exchange between the beams and the mould, a required temperature in the mould and its contents.

According to a further aspect of the invention, apparatus for rotationally moulding a hollow article comprises a mould box having two oppositely facing outer surfaces, a pair of beams which are arranged to be brought to bear against said outer surfaces of the mould box and which include means for clamping the mould box betweeen the beams and in heat exchange contact therewith, means for rotating the beams and the clamped mould box about two different axes, and means for controlling the temperature in the beams to establish a required temperature in the mould box by conduction through the contacting surfaces of the beams and the mould box.

Preferably the two oppositely facing outer surfaces of the mould box will be flat, (for simple machining) and parallel to each other (to avoid sidewardly directed clamping force components), and constitute the largest area external surfaces of the mould box. Alternatively, these outer surfaces of the mould box, may, if preferred, be part spherical or of some other shape, provided that the beam surfaces which are intended to make heat exchange contact therewith are shaped in a complementary manner so as to make closely enveloping superficial area contact with the mould box when the box is clamped between the beams.

The clamping means may comprise through bolts and nuts by which the beams may be drawn towards each other to clamp the mould between them. Alternatively the beams may be arranged to slide freely on columns and to be clamped by hydraulic or mechanical means which moves one beam towards the other, thus ensuring good thermal contact between the beams and the mould and, if necessary, providing the required sealing pressure for the contents of the mould cavities.

The temperature of the beams may be controlled by circulating a fluid medium through a system of ducts in the beams so as to heat or cool the mould as desired. Alternatively, the beams may be provided with electrical heating elements for heating the beams (and hence the mould) and with a system of ducts or tubes for the circulation of a fluid medium for cooling the beams. Since these temperature control means are required to operate during rotation of the beams with the mould in a moulding operation, electrical power for the heating elements will be supplied through slip ring connections, and fluid for circulation through the duct systems will be supplied through rotary pipe joints in a known manner.

Preferably the beams each comprise a platen for contacting the mould box and clamping bars attached to the platen. In this case the clamping means preferably acts between the clamping bars of the opposite beams, and the temperature control means of the beams (i.e. the electrical heating elements and/or the fluid circulation ducts) are provided in the platens.

As will be appreciated, an important advantage of the present invention is that, as a result of the moulds being heated and cooled as required by controlling the temperature of the beams, the moulds used may be of simple, light-weight and hence inexpensive construction. Also, the moulds are easily removed from the moulding machine as and when necessary. Thus, if the interior of a mould requires pretreatment before use, or precise filling with the moulding material, or the introduction of any other components, the operation is facilitated by the easy removal of the mould from the machine. if the moulds were all in one block, or themselves are equipped with heating and cooling means, as in the case of known apparatus, this could not be arranged so easily.Moreover, one set of moulds can be so treated while another set is being used in the machine, thus significantly improving production rate.

An example of apparatus in accordance with the invention and by which the method can be carried out is illustrated schematically in the accompanying drawing.

Referring to the drawing, the apparatus comprises a number of moulds 1 (in this case two part moulds for forming hollow balls) having oppositely facing flat outer surfaces 2 which are parallel to each other and which are the largest external facial surfaces of the moulds. The apparatus also comprises a pair of beams 3 each consisting of a platen 4 and clamping bar 5 having associated therewith through bolts 6 and nuts by which the beams 3 can be clamped together with the moulds 1 held firmly between them, the flat outer surfaces 2 of the moulds 1 being in face to face contact with the platens 4 of the beams, as shown in the drawing.

The apparatus is arranged to rotate the moulds 1, while clamped between the beams 3, orbitally about a major axis 7 and a minor axis 17 simultaneously. For the purpose of rotating the moulds about the major axis 7, a variable speed motor 8 is arranged to drive a tubular shaft 9 through gears 10. The shaft 9 has a pair of radially oppositely extending arms 11 and 12 fixed to it for rotation therewith. The arm 12 carries a counterweight 12a, and the arm 11 carries a cantilever structure 13 which supports at its outer end support means 14 carrying bearings 15 and a mounting sleeve 16. The clamped mould assembly is fixed to a cage 25 which in turn is fixed to a shaft 24 mounted within the sleeve 16.

Simultaneous rotation of the clamped mould assembly about the minor axis 17 is effected by means of a motor 18, a sprocket transmission 19, a shaft 20 mounted coaxially within the shaft 7, a second sprocket transmission 21, a shaft 22 mounted in the cantilever structure 13, and bevel gears 23 which rotate the shaft 24, and hence the cage 25 carrying the clamped mould assembly, about its own axis, which is coincident with the axis 17.

The platens 4 are preferably made of cast aluminium or other thermally conductive material to provide a good heat exchange between the platens 4 and the moulds 1. The platens 4 are internally furnished with electrical resistance heating elements, and tubes, preferably of stainless steel, for the circulation of cooling fluid.

The supply arrangements for the heating current and the cooling fluid to the platens 4 are of more or less conventional design.

The current supply is effected through a control panel 26 and brushes 27 to a slip ring disc 28 fixed on the shaft 7 and wired to a companion slip ring disc 29 which is fixed on the mounting sleeve 16. Since the sleeve 16 is fixed relative to the shaft 7, the wiring between the two slip ring discs 28 and 29 is simply run along the arm 11, the structure 13, the support 14, and the sleeve 16.

Brushes 30 ride on the disc 29 to pick up the current supply for conduction to the platen heating elements through a terminal box 31. The current supply is thermostatically controlled in a suitable manner.

The supply of cooling fluid (usually water) to the platens is effected, when required, through a rotary join 32, a pipe 33 extending through the shaft 20 to a pipe 34, through a pipe 35 extending through the shaft 24 to a second rotary joint 36, and through hose connections 37 to the tubes in the platens 4.

With this arrangement, the temperature in the moulds 1 can be varied as required throughout a rotational moulding operation simply by suitably controlling the temperature of the platens 4 using the electrical heating system and the water cooling system as necessary.

If desired, all or some of the components of the temperature control system of the apparatus, including those parts of the moulds not contacted by the platens 4, may be lagged in a conventional manner to prevent unwanted heat loss or unwanted heat gain as the case may be.

Claims (12)

1. In a method of moulding a hollow article in a mould which is mounted in a moulding maching and is rotated about two different axes by the machine, the mould is held closed and is clamped in the machine by a pair of beams which bear against two opposite outer surfaces of the mould in heat exchange relationship therewith, and the temperature of the beams is controlled so as to establish, by heat exchange between the beams and the mould, a required temperature in the mould and its contents.

2. A method according to claim 1, in which the temperature of the beams is controlled by circulating a fluid medium through ducts in the beams and controlling the temperature of the fluid medium.

3. A method according to claim 1, in which the temperature of the beams is controlled by means of electrical resistance heating elements which are incorporated in the beams for heating the beams, and by circulating a fluid cooling medium through ducts in the beams when the beams are to be cooled.

4. A method according to any one of the preceding claims, in which the mould is orbitally rotated about two mutually perpendicular axes.

5. Apparatus for rotationally moulding a hollow article comprising a mould box having two oppositely facing outer surfaces, a pair of beams which are arranged to be brought to bear against said outer surfaces of the mould box and which include means for clamping the mould box between the beams and in heat exchange contact therewith, means for rotating the beams and the clamped mould box about two different axes, and means for controlling the temperature in the beams to establish a required temperature in the mould box by conduction through the contacting surfaces of the beams and the mould box.

6. Apparatus according to claim 5, in which the two oppositely facing outer surfaces of the mould box are flat and parallel to each other, ahd are the largest external surfaces of the mould box.

7. Apparatus according to claim 5 or claim 6, in which the beams each comprise a platen for contacting the mould box, and clamping bars attached to the platen, the clamping means comprising means for clamping the clamping bars of one beam to the clamping bars of the other beam so that the mould box is firmly sandwiched between the platens of the two beams.

8. Apparatus according to claim 7, in which each platen has a system of internal ducts, and the apparatus includes means for circulating a fluid medium through the ducts of the platens for controlling the temperature thereof.

9. Apparatus according to claim 7, in which each platen has an electrical heating element incorporated therein, and the apparatus is provided with an electrical circuit for energising the heating elements.

10. Apparatus according to claim 9, in which each platen also contains a system of ducts, and the apparatus includes means for circulating a fluid cooling medium through the ducts.

11. A method according to claim 1, substantially as described herein with reference to the accompanying drawing.

12. Apparatus according to claim 5, substantially as described herein with reference to the accompanying drawing.