WO2004082921A1

WO2004082921A1 - Process and apparatus for gas-assisted injection moulding process with expulsion of plastics material

Info

Publication number: WO2004082921A1
Application number: PCT/GB2004/001255
Authority: WO
Inventors: Terence Colwyn Pearson
Original assignee: Cinpres Gas Injection Limited
Priority date: 2003-03-20
Filing date: 2004-03-22
Publication date: 2004-09-30
Also published as: GB2399533A; GB0306426D0

Abstract

The present invention relates to a process for injection moulding a hollow plastics article. The process includes the step of injecting a quantity of molten plastics material (20) into a mould cavity (25) to fill fully the mould cavity; and injecting pressurised fluid (31) into the plastics material in the mould cavity. At least one valve (29) is opened to allow a portion of the molten plastics material in the mould cavity to be expelled into at least one secondary cavity (26) coupled to the mould cavity, sufficient plastics material being expelled to fill the at least one secondary cavity. The remaining plastics material in the mould cavity is permitted to solidify and the fluid is exhausted from the mould cavity. The plastics article is then removed from the mould. The process includes determining when the step of filling full the mould cavity is at or near completion, and initiating the step of injecting pressurised fluid in response to that determination.

Description

PROCESS AND APPARATUS FOR GAS-ASSISTED INJECTION OULDING PROCESS ITH

EXPULSION OF PLASTICS MATERIAL

The present invention relates to a process and apparatus for injection moulding hollow plastics articles.

Injection moulding of plastics materials is well known and widely practised as a means of manufacturing an ever- increasing diversity of plastics components for industrial and consumer use. During the last two decades, versions of the process globally referred to as "gas assisted injection moulding" have been developed and used to overcome some of the problems inherent in conventional injection moulding, and to reduce costs and improve the quality of the final products .

In conventional gas assisted injection moulding, a gas, such as nitrogen, is injected into the molten plastics material after it has entered the mould cavity. The low viscosity gas flows into the paths of least resistance within the more viscous plastics, thereby forming hollow channels within the plastics. The process is particularly beneficial for thick section mouldings, such as handles, and weight savings of up to 45% or more can result. Also, the moulding time cycles can be substantially reduced. In multi -section mouldings, the injected gas tends to flow into the thicker sections, again forming hollow continuous channels through which pressure may be transmitted through the medium of the gas .

It is a fundamental principal of internal gas assisted injection moulding that the gas provides a medium for transmitting pressure to the plastics as it cools and solidifies, instead of the moulding machine which applies the pressure in conventional injection moulding. When a gas pressure is applied at the exit from the gas controller, a similar pressure is applied virtually instantaneously throughout the length of the continuous gas stream or channel . Therefore the gas enables a similar internal pressure to be exerted throughout the moulding. The pressure is transmitted more uniformly and efficiently and therefore lower pressures can be used than in conventional injection moulding. This adds to the scope of the designer and removes some of the design restrictions of conventional injection moulding.

One known gas assisted plastics moulding process employs filling the mould cavity with plastics material

(known as a "full shot" process) . Instead of injecting or packing more plastics into the mould cavity to compensate for the volumetric shrinkage of the plastics as it cools and solidifies, pressurised gas is injected into the plastics so that the gas expansion compensates for the plastics contraction. Thereby, the initial gas penetration continues to expand during the cooling cycle whilst the plastics shrinks in volume .

It is sometimes difficult to achieve sufficient gas penetration along intended gas channels. In such cases, a method of enabling some of the molten plastics to flow from the mould cavity into an overflow or secondary cavity is helpful to provide space for the required gas expansion.

A method of at least partially filling the mould cavity before injection of the gas is described in US Patent No. 5098637. However, this process requires accurate control of the shot volume of plastics, because there is no prevention of plastics flowing into and filling the overflow cavity before the gas is injected.

Another method is disclosed in Japanese Patent Application No. 50-74660 in which a shut-off valve is provided in a runner connecting the mould cavity and a secondary cavity. In one embodiment of this process, the valve may be opened by a pressure of injection of the gas, the gas being injected whilst resin is expelled from the mould cavity into the secondary cavity.

A further known process allows a portion of the molten plastics material to be expelled from the mould cavity into at least one secondary cavity through a valve controlled runner. With the valve closed, the mould cavity is filled with molten plastics material. Pressurised gas is injected into the plastics in the mould cavity with the valve maintained closed, whereby the pressure of the gas on the plastics in the mould cavity can be held for a period of time in order to apply a packing pressure to the plastics material to be expelled into the or each secondary cavity. Subsequently the valve is opened to allow a portion of the molten plastics in the mould cavity to be expelled into the secondary cavity. The remaining plastics material in the mould cavity is permitted to solidify before exhausting the gas from the mould cavity. The mould is then opened and the article removed.

A principal advantage of the above process is that the provision of the valve in the runner between the mould cavity and the secondary cavity allows the plastics to be pressurised, immediately after filling the mould cavity, with pressure applied by the moulding machine. Increasing pressure within the plastics serves to obtain a good replication of the mould cavity surface onto the surface of the article.

It is further known to employ a pressurised liquid, e.g. water, instead of the pressurised gas.

According to the invention there is provided a process for injection moulding a hollow plastics article comprising the steps of: injecting a quantity of molten plastics material into a mould cavity to fill fully the mould cavity; injecting pressurised fluid into the plastics material in the mould cavity; opening at least one valve to allow a portion of the molten plastics material in the mould cavity to be expelled into at least one secondary cavity coupled to the mould cavity, sufficient plastics material being expelled to fill the at least one secondary cavity; permitting the remaining plastics material in the mould cavity to solidify; exhausting the fluid from the mould cavity; and removing the plastics article from the mould; the process including determining when the step of filling full the mould cavity is at or near to completion, and initiating the step of injecting pressurised fluid in response to that determination. Preferably completion or near completion of the filling of the mould cavity is determined when a selected plastics pressure is reached during or after the injection of the plastics material, and the selected pressure is employed to initiate the step of injection of pressurised fluid.

In another embodiment completion or near completion of the filling of the mould cavity is determined when a piston or screw for injecting the plastics material into the mould cavity reaches a selected position, and the selected position is employed to initiate the step of injection of pressurised fluid.

The process preferably includes the step of holding the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the at least one valve to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity.

It is also preferred that the process includes determining a selected pressure of the pressurised fluid, and opening the valve which allows a portion of the molten plastics material in the mould to be expelled into the at least one secondary cavity in accordance with that determination.

The invention also provides an apparatus for injection moulding a hollow plastics article comprising: a mould with a mould cavity therein defining at least a portion of the article; means for injecting a quantity of molten plastics material into the mould cavity to fill fully the mould cavity; means for injecting pressurised fluid into the plastics material in the mould cavity; at least one secondary cavity coupled to the mould cavity by at least one valve controlled connection whereby when the valve means in the at least one connection is closed, no plastics or fluid can flow into the at least one secondary cavity, but with the valve means open a portion of the molten plastics in the mould is allowed to be expelled into the at least one secondary cavity, and control means for opening the valve means to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity, prior to the remaining plastics material in the mould cavity solidifying and the fluid being exhausted from the mould cavity to allow the removal of the plastics from the article from the mould; the apparatus including means for determining when the step of filling full the mould cavity is at or near to completion, and for initiating injection of pressurised fluid in response to that determination.

Preferably there are provided monitoring means for determining when a selected plastics pressure is reached during or after the filling step is complete or near to completion, and for initiating the means for injecting the pressurised fluid into the plastics material in the mould cavity.

In another embodiment a piston or screw is provided for injecting plastics material into the mould cavity. and positional sensing means are provided which monitor the movement of the piston or screw, the sensing means determining the position of the piston or screw to indicate that the step of filling the mould cavity is complete or near to completion, and activating the means for injecting the pressurised fluid.

The apparatus preferably includes means for opening the valve means which are activated by a selected pressure of the pressurised fluid.

It is also preferred that the control means are adapted to hold the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the valve to allow a portion of the molten plastics material in the mould to be expelled into the secondary cavity.

By way of example, specific embodiments in accordance with the invention will be described with reference to the accompanying diagrammatic drawings in which :-

Figures 1A - IE illustrate the sequence of steps of a first embodiment;

Figure 2 is a pressure/time graph of the first embodiment, in which a selected plastics pressure initiates gas injection and a selected gas pressure activates opening of the valve between the mould cavity and the secondary cavity;

Figures 3A - 3E illustrate the sequence of steps of a second embodiment;

Figure 4 is a pressure/time graph of the second embodiment in which a selected position of the plastics injection piston or screw initiates gas injection and a selected gas pressure activates opening of the valve between the mould cavity and the secondary cavity; and

Figures 5A and 5B illustrate an alternative arrangement of the secondary cavity, showing the valve member of the valve between the mould cavity and the secondary cavity in its valve closed and valve open positions.

Figures 1A - IE illustrate the sequence steps of a first embodiment of the process referred to generally by the reference numeral 10. In Figure 1A, a quantity of molten plastics material 20 is positioned in the barrel or cylinder 21 of an injection moulding machine. The injection moulding machine can be of any conventional type and can expel the plastics material from the nozzle 22 of the cylinder 21 in any conventional manner, such as by a helical screw or a piston or ram 23, as illustrated.

As shown in Figure 1A, the nozzle 22 of the injection moulding machine is coupled via a sprue or runner 24 to a mould cavity 25. The mould cavity 25 is positioned in a conventional- mould which in turn is mounted in a conventional moulding machine. In this embodiment, the mould cavity 25 represents a thicker section of the article to be moulded. A secondary cavity 26 is also provided within the mould in communication with the mould cavity 25, and coupled to the mould cavity by a flow runner or conduit 27. A valve member 28 is positioned in the runner 24 between the nozzle 22 and the mould cavity 25, while a second valve member 29 is positioned in the flow runner or conduit 27 leading to the secondary cavity 26. The valve members 28, 29 can be of any conventional type, but in this embodiment are shut-off type valve members, and can be operated in any conventional manner, such as pneumatically, hydraulically or electrically.

In Figure 1A, the quantity of plastics material in the cylinder 21 of the injection moulding machine is sufficient to fill fully the volume of the mould cavity 25. The plastics material 20 may be of any conventional type such as a thermoplastics or thermosetting resin and may be a filled material, that is mixed with a glass or mineral material.

The second step in the process is shown in Figure IB. The plastics material 20 is injected by the piston 23 into the mould cavity 25 to fill fully the mould cavity. For this process step, valve member 28 of the valve in the sprue or runner 24 is opened, but valve member 29 of the valve in the runner 27 is maintained closed in order to prevent any plastics from flowing out of the mould cavity 25 into the secondary cavity 26. In this embodiment, the plastics pressure is maintained for a short period of time, for example, 1 - 5 seconds. This assists to ensure good reproducibility of the mould surface on the moulded article in the mould cavity 25.

When a selected plastics pressure is reached (shown as 35 in the graph of Figure 2) , a signal representing the selected plastics pressure initiates the commencement of the gas cycle in which pressurised gas, e.g. nitrogen, is injected through inlet conduit 30. This is possible due to the volumetric contraction or shrinkage of the plastics material in the mould cavity 25, and is shown in Figure IC. It will be appreciated that the signal representing the selected plastics pressure can be applied to initiate the step of injecting the pressurised gas either before the completion of the step of filling the mould cavity with plastics material, or on completion of filling the mould cavity, or after the completion of filling the mould cavity, as in this embodiment. However, in each case, the gas pressure needs to overcome the plastics pressure before it can penetrate the plastics and begin creating a hollow in the plastics material. Thus, at whatever pressure of the plastics material is selected to initiate the step of injecting the gas, and whether it is before, at, or after the filling of the mould cavity with plastics material, the mould cavity will be filled fully with plastics material before the commencement of gas injection into the plastics material .

The signal of plastics pressure in the mould may be picked up at a variety of different positions. For example, the plastics pressure signal may be taken from sensing means which act as monitoring means within the moulding machine. The sensing means may include a pressure transducer or other sensor in the plastics extruder of the moulding machine. In other embodiments, the plastics pressure signal is taken from a pressure transducer or other sensor positioned in the runner 24 to the mould cavity or in the mould cavity 25, for example, at a position near to the valve member 29 controlling the outflow through runner 27 to the secondary cavity 26. In a further embodiment illustrated in Figures 5A and 5B, an in-mould pressure transducer or other sensor is located inside the valve member 29 of the valve controlling the outflow of plastics through the runner 27 to the secondary cavity 26. In Figure 5A, the valve member 29 is shown in its valve closed position and in Figure 5B the valve member 29 is in its valve open position. The valve member 29 acts in line with the runner 27, and the pressure transducer or other sensor (not shown) is flush with the end of the valve member 29 adjacent the mould cavity 25.

Alternatively, a signal corresponding to the completion or near completion of the filling of the mould may be taken from a predetermined pressure of a further pressurised medium employed to generate the supply of pressurised plastics material. For example, the further pressurised medium may be in an hydraulic or pneumatic circuit for operating the plastics extruder. In this case the signal taken from the hydraulic or other fluid is employed to initiate the commencement of gas injection.

In a further embodiment the plastics extruder may have an electric drive. In this case, the monitoring means are in the electric drive, and a predetermined power demand of the electric drive, corresponding to the completion or near completion of the filling of the mould cavity, may be employed to initiate the commencement of gas injection.

In each case, there may be provided a microprocessor having a software programme for controlling the monitoring means of the particular embodiment.

Initially, on gas beginning to enter the plastics in the mould cavity 25, the valve member 29 remains closed until receiving a signal activated by a selected pressure of the gas. The pressure of the gas creates a hollow cavity in the molten material and applies an internal pressure to the plastics which forces the plastics against the inner wall surfaces of the mould cavity 25 until an outer skin of the plastics has solidified, whilst a core portion of the plastics remains molten, thereby assisting to provide a good surface finish on the article.

Thereafter, as shown in Figure ID, the nozzle valve member 28 is closed and the secondary cavity valve member 29 is opened. The selected pressure of the gas 31 in the molten plastics material in the mould cavity 25 activates the opening of the valve member 29, which allows the gas to expel a portion of the plastics into the secondary cavity 26 through the runner 27. The portion expelled is from the more fluent core of plastics in the thicker section or sections in the article.

Conveniently, the valve member 29 is activated by a signal from monitoring means in the circuit supplying pressurised gas to the mould cavity 25. The monitoring means may be a pressure sensor, for example, a pressure transducer.

Gas pressure is maintained in the mould cavity 25 whilst the remaining plastics cools and solidifies.

The amount of plastics expelled is sufficient to fill the secondary cavity 26. Accordingly, the volume of the secondary cavity 26 is predetermined so that it is of an appropriate size to accept and be filled by the amount of plastics 32 which is required to be expelled to ensure that the gas path within the plastics extends throughout the thicker section or sections of the article. Adjustment of the size of the secondary cavity 26 is performed during mould test trials, and may be achieved by metal machining or removal from the mould to increase the volume, or by the addition of metal to reduce the volume.

Once the remaining plastics material in the mould cavity 25 has sufficiently cooled and solidified, the gas 31 is exhausted, for example, back through conduit 30. This is shown in Figure IE. The expelled gas can be collected and reclaimed for further use, but is generally expelled to atmosphere. Mechanisms and systems for venting the gas are well known in gas assisted injection moulding, and a conventional mechanism or system may be used. At the same time as the gas 31 is exhausted, the piston 23 of the moulding machine is typically retracted to its rest position, as illustrated, in preparation for the next moulding cycle .

The mould is then opened and the moulded article is ejected or otherwise removed from the mould cavity 25. At the same time, the expelled plastics 32 is similarly ejected or removed from the secondary cavity 26. The plastics 32 in the secondary cavity 26 can be re-ground for further use, or the secondary cavity can be used to form another plastics article.

Finally the valve member 29 in the runner 27 between the mould cavity 25 and the secondary cavity 26 is closed before the next moulding cycle. A graph illustrating the sequence of steps described above is shown in Figure 2. The graph charts the moulding cycle time against the plastics pressure and the gas pressure in the mould. The plastics pressure 34 initially increases as the plastics 20 is injected into the mould cavity 25. In this embodiment, when the mould cavity is full of plastics, the plastics pressure increases further until it reaches a selected pressure 35. At this plastics pressure 35, the gas injection cycle is activated to commence. At the same time, the plastics pressure continues along the path as indicated in the graph. In this embodiment, the process involves determining when the step of filling the mould cavity with plastics material has been completed, and initiating injection of the pressurised gas in response to that determination. In other embodiments, the process may involve determining when the step of filling the mould cavity with plastics material is near to completion, whether that is before or after completion of the filling step, and then initiating the step of injecting the gas in response to that determination.

For gas injection to occur, the gas pressure 36 increases at a selected rate or ramp. When the gas pressure exceeds the plastics pressure, the gas begins to create a hollow in the plastics in the mould cavity 25 and, in this embodiment, applies an internal packing or hold pressure indicated by the line 37. The gas pressure is again increased to a predetermined pressure 38 which activates the opening of the valve member 29 in the runner 27 connecting the mould cavity 25 and the secondary cavity 26. There is then a small drop in gas pressure, but the gas pressure is raised further, as illustrated, whilst the gas expels a portion of the plastics 32 from the centre of the thicker section of the article through the runner 27 to fill the secondary cavity 26. The secondary cavity 26 has a volume predetermined to receive the required amount of plastics which allows the gas to extend throughout the thicker section.

Gas "hold-on" or "packing" pressure 39 is maintained in the mould cavity 25 either at a constant pressure or first constant and then at a reduced pressure, as illustrated, whilst the remaining plastics in the mould cavity cools and solidifies. This may take a period of 10 to 25 seconds.

The gas pressure then reduces substantially as shown by line 40 to atmospheric pressure as the gas 31 is vented from the mould cavity through conduit 30, prior to the mould being opened to allow the article to be ejected or otherwise removed from the mould.

It will be appreciated that in the above described process, the moulding machine is first programmed to inject sufficient plastics to fill the mould cavity 25, and then to further increase the pressure within the plastics to obtain good replication of the mould cavity surface on the surface of the article.

Plastics pressure continues to rise to a programmed maximum. On reaching the maximum, or preferably at a preselected intermediate pressure 35, a signal is relayed to the gas controller to commence the introduction of gas into the mould, in the manner explained above. The gas pressure continues to rise until it exceeds the plastics pressure, thereby initiating penetration of the gas into the still molten plastics to create a hollow. This is possible due to the volumetric shrinkage of the plastics as it begins to cool. The gas pressure is programmed to continue to rise and thereby applies an internal pressure to the plastics. It is also a predetermined gas pressure which activates the opening of the valve member 29 which allows a portion of the plastics to be expelled by the gas from the mould cavity 25 into the secondary cavity 26 so that the gas path within the plastics extends throughout the thicker section or sections of the article. Signalling the start of introducing the gas by a specified plastics pressure, whether that is before, at, or after the mould cavity is filled with plastics material, is more precise and repeatable than signalling by timing.

Similarly, employing a specified gas pressure to signal the opening of the valve member 29 controlling the runner 27 between the mould cavity 25 and the secondary cavity 26 is advantageous in the balancing of pressures during the moulding cycle, and can be preferred to timing the pressure changes with signals from time programmers.

In particular, the above process is very effective in moulding articles with an excellent surface finish and a consistent expulsion of plastics into the secondary cavity 26, the secondary cavity having a pre-selected volume which determines the amount of plastics expelled. The process is repeatable and consistent in operation.

In a second embodiment illustrated in Figures 3A - 3E, instead of employing a selected plastics pressure to actuate the introduction of gas, there is employed a selected position of the piston or ram 23 or moulding machine injection screw during the forward movement of the piston or screw, which is its direction of movement during the injection of plastics into the mould. In this embodiment, the selected position is at or close to its forward position. It can be achieved by employing a software programme which accepts one or more signals from a positional sensor which, in this embodiment, is a linear displacement transducer (LDT)40. The transducer conveniently comprises an electric coil 42, which is mounted longitudinally on the cylinder 21 of the injection moulding machine, and a moving core 43, which is mechanically linked (shown diagra matically as link 44) to the piston rod 45 of the piston 23 or to the screw, if provided, for injecting plastics material 20 into the mould cavity 25. Thereby, the resistance of the core 43 moving through the coil 42 monitors the forward movement of the piston 23 or screw, and gives a signal which determines the selected position of the piston 23.or screw within the cylinder 23, and activates the gas controller equipment according to the selected position of the piston or screw to commence the gas cycle. Sometimes there is a time delay after the signal from the transducer 40 and before the gas injection starts, but the transducer 40 may be used to signal directly the start of gas injection when the plastics has filled the mould cavity 25, or just before the completion of filling occurs (as illustrated by line 41 in the graph of Figure 4) , or after filling and as the plastics starts to cool and shrink when the piston or moulding machine screw continues to move slowly forward. As before, selecting a position of the transducer movement eliminates the need for a timer, and can be more precise in interpreting the operation of the process compared with the use of a delay timer.

The graph of Figure 4 is similar to the graph of Figure 2 except that the gas injection cycle is signalled to commence by the linear displacement transducer 40 just before the end of the forward stroke of the piston 23 or extruder screw completes the filling of the mould cavity 25 with plastics material 20, as indicated by line 41. Thereafter, the plastics pressure continues along the path indicated in the graph. At the same time, the gas pressure increases at a selected rate until it exceeds the plastics pressure. The gas can then penetrate the plastics and begin to create a hollow in the plastics. The gas applies an internal packing or hold pressure indicated by the line 37. A further rise in gas pressure activates the opening of the valve member 29, which allows a portion of the plastics 32 to be expelled to fill the secondary cavity 26, and the gas path to extend throughout the thicker section. Gas pressure is maintained during the cooling stage, which is then followed by venting of the gas 31, before opening the mould to remove the article.

In the embodiments described herein the gas or fluid injection cycle is commenced when the mould cavity 25 is full or nearly full of plastics material. The skilled person will appreciate that the pressure of the fluid to be injected into the plastics material may be built up before commencement of the injection cycle to minimise or avoid any delay between commencement of the fluid injection cycle and the pressure of the fluid exceeding that of the plastics material in the mould cavity 25. The pressurisation of the fluid may commence when a predetermined quantity of plastics material has been introduced into the mould cavity 25. For example, when the mould cavity 25 is 80% full of plastics material the pressurisation of the fluid may commence. (Of course, the mould cavity 25 may be less than 80% full of plastics material when the pressurisation commences.) The pressure of the fluid to be injected may therefore build up while more plastics material is injected into the mould cavity 25. The pressurised fluid only enters the plastics material when its pressure exceeds that of the plastics material. Thus, the pressure of the fluid to be injected preferably exceeds the pressure of the plastics material only after the moulding cavity 25 is full or nearly full of plastics material; the quantity of plastics material injected into the moulding cavity before the fluid pressurisation cycle is initiated is preferably selected to ensure that this is the case. However, a valve (not shown) may be provided in the inlet conduit 30 so that the pressure of the fluid to be injected may exceed that of the plastics material in the mould cavity 25. The valve may be opened to facilitate injection of the fluid only when the mould cavity is full or nearly full of plastics material.

The means for determining when the mould cavity 25 has been filled by a predetermined quantity of plastics material to determine when to commence pressurisation of the fluid to be injected into the plastics material may operate in the same way as the means for determining when the mould cavity 25 is full or nearly full of plastics material. For example, a pressure sensing means, a timer, or a positional sensing means may determine when the predetermined quantity of plastics material has been injected into the moulding cavity 25 and, thus, when pressurisation of the fluid commences. Indeed, the same sensing or timing means may determine when a predetermined quantity of plastics material has been introduced into the mould cavity 25 to commence pressurisation of the fluid to be injected into the plastics material , and subsequently determine when the mould cavity is full or nearly full of plastics material to initiate injection of the pressurised fluid.

The invention is not limited by the specific details of the embodiments described above. For example, a pressurised liquid, e.g. water, may be employed as a pressurised fluid, instead of a pressurised gas.

There may also be more than one secondary cavity 26 connected to the mould cavity 25, each connecting runner 27 containing a valve member 29, which is actuated to open by a specified pressure 38 of the pressurised fluid within the plastics in the mould cavity 25. It will also be appreciated that a plurality of connecting runners 27 may connect the mould cavity 25 to each secondary cavity 26.

Claims

CLAIMS :

1. A process for injection moulding a hollow plastics article comprising the steps of.: injecting a quantity of molten plastics material into a mould cavity to fill fully the mould cavity; injecting pressurised fluid into the plastics material in the mould cavity; opening at least one valve to allow a portion of the molten plastics material in the mould cavity to be expelled into at least one secondary cavity coupled to the mould cavity, sufficient plastics material being expelled to fill the at least one secondary cavity; permitting the remaining plastics material in the mould cavity to solidify; exhausting the fluid from the mould cavity; and removing the plastics article from the mould; the process including determining when the step of filling full the mould cavity is at or near completion, and initiating the step of injecting pressurised fluid in response to that determination.

2. A process as claimed in Claim 1, wherein completion or near completion of the filling of the mould cavity is determined when a selected plastics pressure is reached during or after the injection of the plastics material, and the selected pressure is employed to initiate the step of injection of pressurised fluid.

3. A process as claimed in Claim 2, wherein the selected plastics pressure is taken at a position within a pressurised supply of the plastics material flowing to the mould cavity.

4. A process as claimed in Claim 2, wherein the selected plastics pressure is taken at a position within a runner leading to the mould cavity.

5. A process as claimed in Claim 4, wherein the selected plastics pressure is taken at a position within the mould cavity.

6. A process as claimed in Claim 2, wherein the selected plastics pressure is taken at a position within the mould cavity close to the valve, or one of the valves, controlling the outflow to the at least one secondary cavity.

7. A process as claimed in Claim 2, wherein the selected plastics pressure is taken from a position within the valve, or one of the valves, controlling flow of plastics into the at least one secondary cavity.

8. A process as claimed in Claim 1, wherein the supply of pressurised plastics material is generated using a further pressurised medium, the process comprising predetermining a pressure of the further pressurised medium corresponding to the completion or near completion of the filling of the mould cavity, and employing the selected pressure of the further pressurised medium to initiate the step of injection of pressurised fluid.

9. A process as claimed in Claim 8, wherein the further pressurised medium is a hydraulic circuit of the moulding machine .

10. A process as claimed in Claim 1, wherein the supply of pressurised plastics material is generated by an electric drive, the process comprising predetermining a power demand of the electric drive corresponding to the completion or near completion of the filling of the mould cavity, and employing the selected power demand of the electric drive to initiate the step of injection of pressurised fluid.

11. A process as claimed in any one of Claims 2 to 10, wherein the selected pressure or power demand is controlled by a software programme of a microprocessor.

12. A process as claimed in Claim 1, wherein completion or near completion of the filling of the mould cavity is determined when a moulding machine piston or screw for injecting the plastics material into the mould cavity reaches a selected position, and the selected position is employed to initiate the step of injection of pressurised fluid.

13. A process as claimed in Claim 12, wherein the selected position is close to the end of the movement of the piston or screw in the direction of filling the mould cavity with plastics material indicating that the filling step is near to completion.

14. A process as claimed in any one of the preceding claims, including the step of holding the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the valve to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity.

15. A process as claimed in any one of the preceding claims, including determining a selected pressure of the pressurised fluid, and opening the at least one valve which allows a portion of the molten plastics material in the mould to be expelled into the at least one secondary cavity in accordance with that determination.

16. A process claimed in Claim 15, wherein opening the at least one valve is activated by a signal taken from the circuit supplying the pressurised fluid to the mould^' cavity.

17. A process as claimed in any preceding claim further comprising the step of determining when a predetermined quantity of plastics material has been injected into the mould cavity and commencing pressurisation of the fluid to be injected into the plastics material in the mould cavity in response to that determination.

18. A process as claimed in any one of the preceding claims, wherein the pressurised fluid is a gas, e.g. nitrogen or air.

19. A process as claimed in any one of Claims 1 to 17, wherein the pressurised fluid is a liquid, e.g. water.

20. Apparatus for injection moulding a hollow plastics article comprising: a mould with a mould cavity therein defining at least a portion of the article; means for injecting a quantity of molten plastics material into the mould cavity to fill fully the mould cavity; means for injecting pressurised fluid into the plastics material in the mould cavity; at least one secondary cavity coupled to the mould cavity by at least one valve controlled connection whereby when the valve means in the at least one connection is closed, no plastics or fluid can flow into the at least one secondary cavity, but with the valve means open a portion of the molten plastics in the mould is allowed to be expelled into the at least one secondary cavity, and control means for opening the valve means to allow a portion of the molten plastics material in the mould cavity to be expelled into the at least one secondary cavity, prior to the remaining plastics material in the mould cavity solidifying and the fluid being exhausted from the mould cavity to allow the removal of the plastics from the article from the mould; the apparatus including means for determining when the step of filling full the mould cavity is at or near to completion, and for initiating injection of pressurised fluid in response to that determination.

21. Apparatus as claimed in Claim 20, wherein there are provided monitoring means for determining when a selected plastics pressure is reached during or after the injection of the plastics material indicating that the filling step is complete or near to completion, and for initiating the means for injecting the pressurised fluid into the plastics material in the mould cavity.

22. Apparatus as claimed in Claim 21, wherein the monitoring means is a pressure sensor positioned in a pressurised supply of the plastics material flowing to the mould cavity.

23. Apparatus as claimed in Claim 21, wherein the monitoring means is a pressure sensor positioned in a runner leading to the mould cavity.

24. Apparatus as claimed in Claim 21, wherein the monitoring means is a pressure sensor positioned in the mould cavity.

25. Apparatus as claimed in Claim 21, wherein the monitoring means is a pressure sensor positioned in the valve means controlling the outflow to the at least one secondary cavity.

26. Apparatus as claimed in any one of Claims 22 to 25, wherein the sensor is a pressure transducer.

27. Apparatus as claimed in Claim 20, wherein a piston or screw of a moulding machine is provided for injecting the plastics material into the mould cavity and monitoring means are provided for determining a selected machine pressure corresponding to the plastics pressure when filling of the mould cavity is completed or near to completion.

28. Apparatus as claimed in Claim 27, wherein the piston or screw is fluid, e.g. hydraulically, operated and the monitoring means are pressure sensing means in the fluid circuitry of the moulding machine.

29. Apparatus as claimed in Claim 27, wherein the piston or screw has an electric drive and the monitoring means are in the electric drive of the moulding machine.

30. Apparatus as claimed in any one of Claims 21 to 29, including a microprocessor having a software programme for controlling the monitoring means.

31. Apparatus as claimed in Claim 20, wherein a piston or screw is provided for injecting plastics material into the mould cavity and positional sensing means are provided which monitor the movement of the piston or screw, the sensing means determining the position of the piston or screw to indicate that the step of filling the mould cavity is complete or near to completion, and activating the means for injecting the pressurised fluid.

32. Apparatus as claimed in claim 31, wherein the positional sensing means is a linear displacement transducer.

33. Apparatus as claimed in any one of Claims 20 to 32, including means for opening the valve means which are activated by a selected pressure of the pressurised fluid.

34. Apparatus as claimed in Claim 33, including monitoring means in the circuit supplying pressurised fluid to the mould cavity, for determining the selected pressure which activates the means for opening the valve means .

35. Apparatus as claimed in Claim 34, wherein the monitoring means is a pressure sensor, for example, a pressure transducer.

36. Apparatus as claimed in any one of Claims 20 to 32, wherein the control means are adapted to hold the pressure of the fluid on the plastics material in the mould cavity to apply a packing pressure to the plastics material, before opening the valve means to allow a portion of the molten plastics material in the mould to be expelled into the at least one secondary cavity.

37. Apparatus as claimed in any one of claims 20 to 36 comprising means for determining when a predetermined quantity of plastics material has been injected into the mould cavity and commencing pressurisation of the fluid to be injected into the plastics material in the mould cavity in response to that determination.

38. A process for injection moulding a hollow plastics article substantially as hereinbefore described with reference to the accompanying drawings.

39. Apparatus for injection moulding a hollow plastics article substantially as hereinbefore described with reference to and as shown in the accompanying drawings .