GB2157226A

GB2157226A - Changing synthetic plastics materials for casting

Info

Publication number: GB2157226A
Application number: GB08509270A
Authority: GB
Inventors: Michael Fraser
Original assignee: NYLACAST OILON Ltd
Current assignee: NYLACAST OILON Ltd
Priority date: 1984-04-13
Filing date: 1985-04-11
Publication date: 1985-10-23
Also published as: GB2157226B; GB8509270D0

Abstract

The invention relates to the moulding of synthetic plastics material, for example, nylon. The problem of voids and cavities occuring in moulded materials is avoided by placing within a vessel containing the liquid material or its mixed constituents an inlet of a mould member having a cavity consisting at least in part of an elongate passage. A piston in the passage is drawn therealong in a direction away from the liquid material by a vacuum device and the liquid is thus drawn into the mould cavity.

Description

SPECIFICATION Improvements in or relating to the casting of synthetic plastics materials.

The invention is concerned with improvements in or relating to the casting of synthetic plastics materials of the kinds including polymers, for example polyamides.

The casting of plastics materials has always presented problems in the avoidance of cavities and other imperfections resulting from the flow of liquid material into a mould. This problem is especially difficult where the desired moulded shape is that of a rod or bar, since when the liquid moulding material is poured into the open-topped mould, the flow patterns involve vortices and the like, making voids and other irregularities in the material. Where the moulding material comprises the the constituents for the formation of a polyamide, the onset of the polymerisation process precludes the use of time-consuming flow manipulation to minimise such irregularities. Moreover, the size of castings to be made in this way is limited, especially in the case of formation of rods of plastics material, the length of which is very restricted, if imperfections are to be avoided.

The invention therefore provides a method of casting synthetic plastics materials comprising procuring a receiving vessel for the materials or their constituents in liquid form, placing within the vessel an inlet of a mould member, a cavity of said mould member comprising an elongate passage containing a piston received therein in an air-tight fit, causing the receiving vessel to be filled so as to submerge said inlet, moving said piston along the passage in a direction away from the liquid in the vessel so as to draw liquid into the mould cavity.

In an example of the invention the cavity of the mould member consists of an elongate passage into which the liquid is drawn in at least substantial contact with one surface of the piston as the piston is drawn through the passage.

In another example, the cavity of the mould member comprises a first portion including said inlet and a second portion including said elongate passage, movement of the piston away from the liquid causing a partial vacuum within the first cavity portion into which the liquid is drawn.

It has been found that the rate of entry into the cavity using the above method is such that the formation of voids and irregularities due to the presence of air and/or the setting or polymerisation of the materials is reduced to a minimum or eliminated.

There will now be described two examples of apparatus suitable for use with the method according to the invention. It will be understood that the description, which is to be read with reference to the drawings, is given by way of example only and not by way of limitation.

In the drawings: Figure 1 is a diagrammatic view of a first apparatus; Figure 2 is a fragmentary view of a lower portion of the apparatus of Figure 1; and Figure 3 is a fragmentary view similar to Figure 2 but of an alternative arrangement of a lower portion of the apparatus.

The apparatus shown in Figure 1 is adapted to cast polyamide (nylon) in rod form and comprises a heated receiving vessel 2 into an upper portion of which lead inlet pipes 4 and 6. Pipe 4 conveys suitable activator material from a tank 8 and the pipe 6 conveys catalyst from a tank 10. It will be understood that the production of the melt of polyamide constituents in this vessel is entirely conventional and forms no part of the present invention. Leading upwardly from the interior of the vessel 2 is an elongate mould member 12 comprising a tubular cavity 14 which in the present example is 3 metres high. A lower end portion of the cavity comprises an inlet 16 which in use is submerged in the melt 18 within the vessel 2 (see Figure 2).Within the cavity is received a piston 20 in an air-tight manner, the arrangement being such that the mould cavity forms an elongate passage through which the piston may be drawn by suitable means in the manner of a pistoned-cylinder arrangement. However, because of the length of the rod to be cast, perhaps 2-3 metres,it is not usually convenient for the piston to be raised by a piston rod. Therefore, in the present example, the piston is raised at a rate of approximately 1 metre/5 sec by partially evacuating the cavity region above it using a vacuum pump 22 in conjunction with a vacuum reserve tank 24 to enhance continuity and smooth operation. If desired, a suitable quantity of liquid mould release agent, for example, a silicone oil, may be added to the cavity 14, the wall of which will become coated with the oil through movement of the piston.

It will be understood that in the case of nylon casting the constituent materials are immediately miscible and therefore no mixing devices are required. However, where the materials to be cast are more viscous, a suitable stirring device may be incorporated.

In the present example, the material to be moulded is drawn up from the vessel 2 at a temperature of approximately 130 - 140 C and it will be found advantageous to apply heating to the mould member 12 to maintain the walls thereof at approximately 1550C This may be achieved in any suitable manner, for example, in a hot air oven.

After the cavity is filled to the desired height, which may be for example 2 metres, the member 12 is removed. After polymerisation is complete (4 minutes approximately) and the casting has undergone shrinkage, it may be removed easily from the member 12 by allowing the solid rod to slip out of the inlet 16.

It will be found that the rod contains no voids or flaws due to vortices in the material flow.

In Figure 3, an alternative form of mould member 22 is shown having a first portion 24 of rectangular plan and an elongate portion 26 including a cavity 28. The cavity 28 contains a piston 20, simi lar to the piston 20 of the first example. The other portions of the apparatus are identical with corre sponding portions.

In use, the liquid in the vessel 2 is drawn into the portion 24 of the mould member by raising the piston 20' and a block of polyamide is formed, moulding material not necessarily being allowed to enter the cylindrical cavity 28. It will be understood that the quantity of air trapped between the liquid in the vessel and the piston 20' is not sufficient to impair the quality of the casting and is, in any event, under much reduced pressure as the piston rises.

In each case, the piston may be returned to its initial position by means of compressed air.

Various modifications may be made within the scope of the invention as defined in the claims.

Claims

1. A method of casting synthetic plastics materials comprising procuring a receiving vessel for the materials or their constituents in liquid form, placing within the vessel an inlet of a mould member, a cavity of said mould member comprising an elongate passage containing a piston received therein in an air-tight fit, causing the receiving vessel to be filled so as to submerge said inlet, moving said piston along the passage in a direction away from the liquid in the vessel so as to draw liquid into the mould cavity.

2. A method as claimed in claim 1, wherein the liquid is maintained in at least substantial contact with one surface of the piston as the piston is drawn through said passage.

3. A method as claimed in claim 1, wherein the cavity of the mould member is arranged to comprise a first portion including said inlet and a second portion including said elongate passage, movement of the piston away from the liquid causing a partial vacuum within the first cavity portion into which the liquid is drawn.

4. A method as claimed in any one of the preceding claims wherein a partial vacuum is applied to the region of the cavity at the side of the piston remote from the liquid.

5. A method as claimed in any one of the preceding claims wherein a liquid mould release agent is added to the cavity in order to coat the walls thereof through movement of piston.

6. A method as claimed in any one of the preceding claims wherein the walls of the cavity are heated to a temperature above the temperature of the materials to be moulded.

7. Apparatus for carrying out the method as claimed in claim 1, comprising a receiving vessel for containing the liquid material to be moulded, a mould member having a cavity comprising an elongate passage containing a movable piston received therein in an air-tight fit and means to draw said piston along the passageway in a direction away from the contents of the receiving vessel.

8. Apparatus as claimed in claim 7, wherein said inlet lies at the mouth of the elongate passage.

9. Apparatus as claimed in claim 7, wherein the cavity of the mould member comprises a first portion including said inlet and a second portion including said elongate passage, movement of the piston away from the liquid causing a partial vacuum within the first cavity portion into which the liquid is drawn.

10. Apparatus as claimed in any one of claims 7 to 9, wherein the receiving vessel is provided with a plurality of supply inlets through which constituents of the materials to be moulded are introduced into the vessel.

11. Apparatus as claimed in claim 10, wherein a stirring device is provided within the vessel.

12. Apparatus as claimed in any one of claims 7 to 11, wherein heating means are provided for heating the vessel and the cavity.

13. Apparatus as claimed in any one of claims 7 to 12, wherein the movement of the piston is controlled pneumatically, means being provided to provide a partial vacuum in the passageway acting on a surface of the piston remote from the receiving vessel to move the piston in one direction and to provide compressed air acting on said surface to move the piston in the opposite direction.

14. Apparatus as claimed in claim 13, wherein said means includes a vacuum pump having a vacuum reserve device to ensure smooth movement of the piston.

15. A method of casting synthetic plastics material substantially as hereinbefore described with reference to the drawings.

16. Apparatus for carrying out the method of claim 1, constructed and arranged substantially as hereinbefore described with reference to the drawings.