WO1992003273A1 - A machine to form articles from thermoplastic plastics materials - Google Patents

Abstract

An extrusion/moulding machine (10) to form product from thermoplastic materials. The machine (10) includes a feed hopper (11) which receives a supply of polystyrene granules. Extending from the hopper (11) is the barrel (12) surrounding a screw conveyor (13). The granules from the hopper (11) are delivered to the barrel (12) and are conveyed by the conveyor (13) to a mould assembly (16). Delivered to the barrel (12) at a position downstream of the hopper (11), is a blowing agent. The mould assembly (16) includes a plurality of mould cavities (19) which are arranged for rotation or reciprocal movement so that each cavity (19) is sequentially brought into communication with the barrel (12) to receive the thermoplastic material therefrom.

Description

A Machine to Form Articles From Thermoplastic Plastics Material

The proposed invention relates to plastics, extrusion and moulding machines and processes, and more particularly but not exclusively to such apparatus and processes when adapted to mould products from thermoplastic materials. Extrusion machines consist of a gravity-feed hopper which delivers raw material to a barrel. Located within the barrel is a screw conveyor which forwards the material along the barrel. The barrel is heated so that as material travels along the barrel, it picks up heat by conduction to supplement the heat generated by the shearing action of the screw flights as they force the material forward. In some instances, the shearing action generates sufficient heat and may actually require cooling of the barrel rather than heating. However, the barrel needs to be heated when the process is initiated.

The molten plastic is forced through a die and cooled into either a flat or tubular sheet form. This material is subsequently delivered to a forming machine which produces the finished article. This process has the disadvantage that there is considerable scrap and there is a two-stage process which is time consuming, costly and requires two separate machines, (Extrusion and thermoforming). When foaming the plastic, two extruder machines are normally required. The first to mix and heat the plastic and to introduce the gas, the second to cool the molten material before it is forced through a die. The sheet produced normally needs to be stored prior to thermofor ing. Whether foamed or unfoamed a large capital investment is required. An alternative to this extrusion method for foamed plastic is the "steam-chest" moulding method where pre-gassed plastic beads are-used. This raw material is relatively more expensive than plastic which is not pre-gassed and used in the extrusion method. Another alternative for moulding plastic articles is injection moulding.

Injection moulding is characterised by its step-by-step method of operation, so that when the plattens are open the moulds are no longer able to receive further material, significantly slowing the process. Additionally, extremely high pressures are required to clamp and keep the mould parts together during injection.

It is the object of the proposed invention to overcome or substantially ameliorate the above disadvantages. There is disclosed herein an extrusion/moulding machine to form product from thermoplastic materials, said machine comprising: a feed hopper into which the thermoplastic material is delivered; a barrel communicating at one end with said hopper to receive material therefrom; a mould assembly communicating with the other end of said barrel to receive said material from said barrel; a screw conveyor within said barrel to transport said material from said hopper to said mould assembly; means to heat said barrel; means to deliver a blowing agent to said barrel adjacent to said other end; and wherein said mould assembly includes a mould cavity, a branch duct extending from said cavity to said barrel to deliver blown or unblown material from said barrel to the cavity.

There is further disclosed herein an extrusion/moulding process comprising: providing a supply of thermoplastic material; screw conveying the material to a mould cavity; heating the material while being conveyed to said cavity; and introducing into said material a blowing agent at a downstream position during conveying of said material.

In the above described extrusion/moulding machine and process, the mould or moulds could be closed and mounted for rotational or reciprocating movement.

The preferred method of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a schematic side elevation of an extrusion/moulding machine; and Figure 2 is a schematic side elevation of the machine of Figure 1. In the accompanying drawings there is schematically depicted an extrusion/moulding machine 10. The extrusion/moulding machine 10 is adapted to receive a supply of thermoplastic granular material such as polystyrene granules. The granules are not pre-expanded. The raw material could also contain starch or other naturally renewable resources or materials to make the articles produced, degenerate with time.

The raw material is fed to a feed hopper 11 which delivers the material under the influence of gravity to a barrel 12. Located within the barrel is a screw conveyor 13 which is rotatably driven to convey the raw material in the direction of the arrow 14.

Preferably the barrel 12 is heated so that the granular material becomes molten by the time it reaches the downstream end 15 of the barrel 12. It may be necessary once the process has reached a steady state condition, to cool the barrel 12 as shearing of the raw material due to the action of the screw conveyor 13 will generate sufficient heat to cause the material to reach a molten state.

The above portion of the machine 10 comprises the extrusion apparatus 23.

The molten material is delivered to a mould assembly 16, which in this embodiment includes a rotatably driven hub 17 from which there extends branch ducts 18 to mould cavities 19. The mould cavities 19 are defined between male die members 20 and female die members 21. The machine 10 is provided with a further duct 22 through which a blowing or foaming agent may be introduced into the molten material.

The molten material is conveyed to the mould cavities 19 by the pressure generated by the screw conveyor 13. Alternatively the branch ducts 18 could be provided with smaller screw conveyors to aid in conveying the material. It may also be advantageous to heat the branch ducts 18. Alternatively, or in conjunction with these smaller screw conveyors a plunger arrangement may be used to meter and convey a correct amount of material .

The mould assembly 16 includes a rotatably driven hub 17 as described above. The hub 17 rotates so as to sequentially provide communication between the branch ducts 18 and the barrel 12. During a first pre-determined angle, as selected one of the branch ducts 18 communicates with barrel 12 and receives material therefrom to be delivered to the mould cavity 19. As it passes through a second pre- determined angle, the branch duct 18, is isolated from the barrel 12. During this period of rotation, the mould cavity 19 is cooled together with the product located therein. The product is subsequently ejected after passing through a further angle. The cavity 19 then rotates to its initial start position where it communicates with the barrel 12 so as to receive a fresh charge of molten material.

The above described preferred embodiment of the present invention offers several advantages. Firstly, polystyrene granule material is used rather than pre-gassed bead material. The foamed material is delivered directly into the mould cavity rather than firstly to an extruder from where it is rolled and possibly stored. There is substantially no scrap produced in comparison to other methods which employ an intermediate sheet product from which the finished product is formed. Still further, by forming the product in a "rotary manner" the efficiency of the process is considerably increased as it is not a "step-by-step" operation. In addition the proposed process will operate at relatively low moulding pressures compared to conventional injection moulding.

The process also offers the opportunity to print the moulded product in-line, rather than off-line as is normally the case.

Claims

1. An extrusion/moulding machine to form product from thermoplastic materials, said machine comprising: a feed hopper into which the thermoplastic material is delivered; a barrel communicating at one end with said hopper to receive material therefrom; a mould assembly communicating with the other end of said barrel to receive said material from said barrel; a screw conveyor within said barrel to transport said material from said hopper to said mould assembly; means to heat said barrel; means to deliver a blowing agent to said barrel adjacent to said other end; and wherein said mould assembly includes a mould cavity, a branch duct extending from said cavity to said barrel to deliver blown or unblown material from said barrel to the cavity.

2. The machine of claim 1, wherein mould assembly includes a plurality of moulds mounted for rotation or reciprocal movement with the moulds sequentially brought into communication with said barrel to receive material therefrom.

3. An extrusion/moulding process comprising: providing a supply of thermoplastic material; screw conveying the material to a mould cavity; heating the material while being conveyed to said cavity; and introducing into said material a blowing agent at a downstream position during conveying of said material.

4. The process of claim 3, wherein said thermoplastic material is polystyrene granules.

5. The process of claim 3, wherein there is a plurality of mould cavities which are arranged for rotational or reciprocal movement, and are sequentially delivered thermoplastic material.