WO2004113048A2

WO2004113048A2 - Liquid assisted injection moulding method and apparatus

Info

Publication number: WO2004113048A2
Application number: PCT/GB2004/002670
Authority: WO
Inventors: Kenneth Crow; Ulrich Stieler; Terence Pearson
Original assignee: Cinpres Gas Injection Ltd
Current assignee: Cinpres Gas Injection Ltd
Priority date: 2003-06-20
Filing date: 2004-06-21
Publication date: 2004-12-29
Anticipated expiration: 2005-12-20
Also published as: WO2004113048A3; GB0314451D0

Abstract

The present application relates to a method of producing an injection moulded plastics article (2). The method involves introducing a charge of plastics material (6) through an injection aperture finto a mould space (4) and then supplying pressurising liquid (46) finto the mould space to form a cavity (58) in the molten plastics material and to cause the plastics material to conform to the shape of the mould space. A pressurised supply (44) of gas is then communicated with the pressurised liquid in the cavity. At least one outlet (47) is opened to allow pressurised gas in said cavity to expand and discharge of least some of the liquid from the cavity. The application also relates to apparatus (1) for injectio:n moulding a plastics article.

Description

LIQUID ASSISTED MOULDING METHOD AND APPARATUS

The present invention relates to a process and apparatus for injecting liquids into a molten plastics material during an injection moulding cycle.

It is well known to inject gas into molten plastics material provided^' in a mould space to expel the hotter, less viscous, material from the interior of the moulding to form a cavity. These so-called "gas-assisted" moulding techniques reduce the time required for cooling and conversion of the plastics material from a liquid to a solid state and therefore reduce cycle times. The weight of the moulded article is also reduced.

However, there are certain limitations associated with known gas -assisted moulding processes. For example, once the gas is injected into the plastics material it is comparatively static and therefore is very poor at conducting heat from the plastics material. Furthermore, the specific heat of gas is very low compared with the plastics material and therefore the gas absorbs very little heat from the plastic. Thus, in known gas assisted methods the majority of cooling of the plastics material is derived from the reduced volume of plastic and reduced wall sections and removal of heat through the walls of mould tool. Accordingly cooling of the plastic occurs only from the outside contours of the plastic material. The cycle times to ensure adequate cooling of the plastics material may be undesirably long.

It may also be desirable to produce mouldings having smooth internal surfaces, for example where the moulding is to be used to convey fluids such as water, oil or other chemicals. The quality of the internal surface finish obtained by gas assisted moulding is dependent on a number of factors such as the plastic type or grade, and whether the plastic contains fillers such as talc or glass fibre reinforcement. If higher than normal gas pressures are used, there may be a tendency for the gas to penetrate the soft or semi-molten plastics material, thereby creating a rough or semi-porous surface which may be detrimental to the flow of fluids or may retain absorb some of the fluid with resulting difficulty in cleaning the tubular component .

To address some of the limitations of gas assisted moulding, it has been proposed to inject liquids, such as water, into the molten plastics material. Liquids generally have a higher specific heat than gases and, thus, provide a more effective means of conducting heat from within the plastics material. The cycle time for each moulding may therefore be reduced for these liquid assisted moulding methods. Liquid assisted moulding may also allow greater quantities of plastics material to be expelled from the mould cavity and allow mouldings having have thinner walls to be produced. The surface finish on the interior of the moulded article is also generally improved for liquid assisted moulding.

However, there are also limitations associated with liquid assisted methods. For example, the increased cooling effect of the liquid means that it may be difficult to inject the liquid sufficiently quickly to create the desired cavity. Consequently, the cavity may not form properly in the plastics material and the desired weight savings may not be achieved. Moreover, if insufficient liquid is introduced the plastics material may not be displaced throughout the mould space and the article may be not be properly formed.

Furthermore, as liquids generally have a lower specific heat than the molten plastics material, the temperature of the liquid will quickly rise when it contacts the molten plastics particularly if the amount of liquid injected is small relative to the amount of plastic. Indeed, the liquid may rise to its boiling temperature and vaporise . The pressure exerted on the plastics material by the vapour is more difficult to control accurately and the desired cavity may not be formed in the plastics article.

A further problem associated with injecting a liquid into the molten plastics material is to ensure that the mould is dry prior to the injection of plastics material for each moulding cycle. If any liquid is present in the mould when the plastics material is injected, the surface finish of the resulting article will be spoiled. Further, it is undesirable for moulded articles to be substantially full of liquid when ejected from the mould for practical moulding purposes.

Moulding processes utilising a pressurised gas to form a cavity in the molten plastics material and then introducing a liquid to effect cooling are also known, for example from EP 1072384, US 5,127,814 and Japanese publication No. 09309126. However, these arrangements may be subject to some of the limitations associated with gas assisted moulding, such as poor quality finishes on the cavity walls, as well as those associated with liquid assisted moulding, such as the need to ensure that the mould is dry prior to introducing molten plastics material . The present invention, at least in preferred embodiments, attempts to overcome at least some of the limitations outlined above for known moulding processes and apparatus . Viewed from a first aspect, the present invention provides a method of producing an injection moulded plastics article comprising the steps of:

(a) introducing a charge of plastics material through an injection aperture into a mould space; (b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space;

(c) communicating a pressurised supply of gas with the pressurised liquid in the cavity; and

(d) opening at least one outlet to allow pressurised gas in said cavity to expand and discharge at least some of the liquid from the cavity.

Thus, the present invention at least in preferred embodiments utilises liquid injection to form articles having relatively thin sidewalls and interior sidewalls with a comparatively good interior finish. The formation of a pressurised gas pocket in the mould cavity allows the cooling liquid to be discharged from the moulding cavity and therefore helps to maintain the mould space dry ready for the next moulding cycle.

Viewed from a further aspect the present invention provides a method of producing an injection moulded plastics article comprising the steps of : (a) introducing a charge of plastics material through an injection aperture into a mould space; (b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space; (c) introducing a piercing element into the mould space to pierce the plastics article and form at least one outlet; and

(d) introducing gas into the cavity to discharge liquid from the cavity through said at least one outlet. Again, articles having relatively thin sidewalls and a cavity with a good interior finish may be formed whilst ensuring that the mould space is kept dry for the next moulding cycle.

The plastics article is preferably pierced at a point distal from the liquid inlet. The gas may be introduced through an injection inlet located proximal to the liquid inlet to expel the liquid through the outlet formed when the plastics article is pierced. The outlet is preferably formed at the lowermost portion of the mould cavity to assist the discharge of liquid from the cavity.

The liquid introduced into the cavity may be maintained under pressure in the cavity for a period of time and the plastics article pierced whilst the liquid is under pressure. The supply of liquid may be continued after the article has been pierced and liquid allowed to flow through the cavity to effect cooling and reduce cycle times. The liquid preferably exits the cavity through the outlet formed in the plastics article when it is pierced. The pressurised gas may subsequently be introduced into the cavity to effect drying. Viewed from a still further aspect, the present invention provides a method of producing an injection moulded plastics article comprising the steps of :

(a) introducing a charge of plastics material through an injection aperture into a mould space;

(b) supplying a pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space; (c) piercing the plastics article to form at least one inlet, and introducing a pressurised gas through said at least one inlet into the cavity to discharge liquid from the cavity through at least one outlet .

The sidewall is preferably pierced by pressure penetration of the pressurised gas. Alternatively, however, a piercing element, such as a pin, may be introduced into the cavity to pierce the plastics article.

In processes where the plastics article is pierced to form an inlet or an outlet, the liquid may be allowed to flow through the cavity for a period of time to effect cooling of the plastics material. The liquid is preferably maintained under pressure as it is passed through the cavity to ensure that the plastics material is packed against the mould walls (i.e. the liquid preferably applies a packing pressure) . Once the liquid has passed through the cavity for a predetermined period of time or the plastics material has been cooled to a predetermined temperature, the pressurised gas may be introduced into the cavity to discharge liquid from the cavity. The gas introduced into the cavity may be chilled prior to its introduction to improve cooling of the plastics materials and to reduce cycle times. The gas may, for example, be chilled nitrogen. In processes where the plastics article is pierced to form an inlet or an outlet, pressurised gas may be allowed to flow through the cavity for a period of time to effect cooling of the plastics material and/or drying of the interior of the cavity. The step of flowing gas through the cavity may be performed in addition to or instead of flowing liquid through the cavity.

Preferred steps common to all of the above aspects of the present invention will now be described.

The liquid is preferably pressurised before the plastics material is introduced into the mould space.

More preferably, the liquid is pressurised and held under pressure at a position adjacent to the plastic before and whilst the plastic is introduced into the mould space. The pressurised liquid may then be released for injection instantaneously when signalled. This advantageously minimises any delay in the process and helps to prevent hesitation marks on filling the mould cavity and the application of pressure to the plastic by the liquid helping to prevent hesitation marks forming on the moulded article if the mould is initially only partially filled with plastics material (the so-called "short-shot" process) . Preferably for a moulding having a weight less than 1kg, the liquid is introduced in less than one second. Water injection rates may be up to 1.8 litres per second and at pressures up to 300 Bar, although typically it is sufficient to exert water pressures of between 50 and 200 Bar. Although gas may be introduced into the liquid before the plastics material has solidified (the combined pressure of the liquid and gas ensuring that the plastics material conforms to the shape of the mould space) , the introduction of the gas is preferably delayed until the plastics material has solidified and cooled sufficiently so that the article can sustain the form dictated by the mould space. This ensures that the liquid may effect substantially uniform cooling of the plastics material before the gas is introduced.

The liquid pressure, or the combined liquid and gas pressure, is preferably maintained sufficiently high to provide a packing pressure to force the plastics material into contact with the mould walls to help ensure that a good exterior surface is achieved.

Some of the liquid contained in the cavity is preferably released prior to the step of introducing pressurised gas into the mould space. The release advantageously reduces the pressure in the cavity and allows the gas to be introduced at a lower pressure. The release of liquid is preferably performed under controlled back pressure to ensure that a packing pressure is continued to be exerted on the plastics material to push it against the sidewalls of the mould space .

The reduction of pressure in the cavity by releasing some of the liquid contained therein may allow some of the remaining liquid to be vapourized. The vapour may subsequently assist in the discharge of liquid from the cavity. Indeed, if sufficient liquid is vapourized in the cavity, the resulting pressure may be sufficient to expel all of the remaining liquid from the cavity without the need to introduce pressurised gas into the cavity.

Equally, if all of the liquid is vapourized, it may not be necessary to introduce pressurised gas into cavity.

The reliance on vaporization alone to expel the remaining liquid from the mould (i.e. without the need to introduce pressurised gas into the cavity) is considered in itself to be independently patentable.

As mentioned above, the mould space may initially only be partially filled with plastics material (the short-shot process) such that the introduction of liquid to form a cavity is required to distribute the plastics material over the interior of the mould space to form the moulded article.

Alternatively, the mould space may initially be filled with plastics material (the so-called "full-shot" process) . The excess plastics material may be expelled into a secondary cavity or back through the plastics injection aperture. Of course, a secondary cavity may also be provided for the short-shot process if desired. Preferably, at least one valve is provided to control the expulsion of plastics material from the mould cavity. A vacuum may be provided to assist in discharging liquid or vapour from the mould cavity.

Viewed from a further aspect, the present invention relates to a method of producing an injection moulded plastics article comprising the steps of:

(b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space; (c) allowing at least some of the liquid in the cavity to vaporize; and

(d) opening at least one outlet to allow vapour in said cavity to expand and discharge at least some of the remaining liquid from the cavity.

Preferably, the step of allowing at least some of the liquid in the cavity to vaporize comprises reducing the pressure of the liquid in the cavity (for example by releasing some of the liquid from the cavity) and thereby lowering the temperature at which the liquid is vapourized.

This method is preferably utilized for moulding processes utilizing plastics materials having relatively high melt temperatures to ensure that a sufficient quantity of the liquid in the cavity is vapourized. Of course, it will be appreciated that the melt temperature required to vaporize sufficient liquid will depend on the liquid being utilised. For example, when the liquid is water, the melt temperature for the plastics material is preferably greater than or equal to 280°C to ensure that sufficient water is vapourized. The temperature of the water injected into the mould space may rise to 150 °C if the melt temperature of the plastics material is 280°C. Of course, plastics materials having melt-temperatures greater than or equal to 200°C, 225°C, 250°C, 275°C or

300°C may be employed with varying levels of vaporization of water resulting.

The processes described herein preferably further include the step of forming an integral seal between the plastics materials and the liquid inlet. The liquid inlet preferably comprises a nozzle which extends into the mould space. When the charge of molten plastics material is introduced into the mould space it preferably surrounds the portion of nozzle extending into the mould space and, as the plastics materials cools around the nozzle, a seal is formed between the nozzle and the plastics material . The shrinkage of the plastics material as it cools causes it to contract around the nozzle and further helps to form the aforementioned seal. The formation of this seal is especially advantageous as it helps to ensure that when pressurised liquid is introduced into the mould space the liquid is introduced directly into molten plastics material to form a cavity therein. If the liquid inlet does not have a nozzle which extends into the mould space, the liquid may not penetrate the skin of the plastics material formed as it cools and instead the liquid may form a pocket or effect leakage of gas and/or liquid between the plastics material and the mould surface.

Viewed from a still further aspect, the present invention provides apparatus for injection moulding a plastics article, the apparatus comprising a liquid injection inlet for supplying a pressurised liquid into a molten plastics material provided in a mould space, and a gas injection inlet for supplying a pressurised gas into the pressurised liquid, wherein said liquid injection inlet and said gas injection inlet are co-axial and are closable independently of each other.

The independent operation of the liquid injection inlet and the gas injection inlet advantageously prevents gas bubbles being introduced into the liquid supply. Moreover, as the liquid injection nozzle can be shut independently, the liquid may be pre-pressurised ready for injection. Preferably, the liquid is pressurised and held under pressure at a valve arrangement provided at the gas injection inlet proximal to the mould cavity ready for injection into the molten plastics material. Thus, delays in the moulding cycle may be minimised. Preferably the liquid injection nozzle is provided radially outwardly of the gas injection inlet and more preferably it has a larger cross-sectional area than the gas injection inlet to facilitate more rapid introduction of the cooling liquid into the plastics material provided in the mould space. The larger cross-sectional area of the liquid injection nozzle may also help to maintain the liquid free of turbulence as far as possible. The apparatus preferably also comprises pressurisation apparatus for pressurising the fluid to be supplied through the liquid injection inlet.

Preferably, the liquid injection inlet is provided in a first member and the gas injection inlet is provided in a second member. The second member is preferably provided inside the first member and the first and second members are preferably displaceable in an axial direction relative to each other. Most preferably the first and second members are arranged concentrically with the second member provided in the middle of the liquid flow through the liquid injection nozzle when liquid is injected.

In use, the first and second members preferably cooperate with each other to control the supply of pressurised liquid through the liquid injection inlet. The pressurised liquid is preferably a cooling liquid and the liquid injection is preferably a cooling liquid injection inlet. The first and/or second members are preferably made of plasma-nitrided stainless steel as this has been found to be particularly suitable for withstanding the high loads which may be experienced during use and to prevent corrosion of the nozzle members.

The second member is preferably provided with at least one side member cooperable with the first member to provide support in a transverse direction. The side member (s) may advantageously prevent or reduce deformation of the second member due to loading when liquid is introduced into the first member under pressure. The second member preferably has three side members equally spaced around the circumference thereof. A valve member may be provided for controlling the supply of pressurised gas through the gas injection inlet. This allows the gas to be pre-pressurised ready for introduction into the cooling liquid.

The apparatus preferably further comprises at least one outlet, separate from the cooling liquid and,gas injection inlets, for the release of cooling liquid from said mould cavity. The at least one outlet may be located proximal to the cooling liquid and gas injection inlets and opened, for example, when the first member is retracted. Alternatively, the at least one outlet may be provided in communication with a piercing element to facilitate the release of the cooling liquid through an outlet created by piercing the plastics article.

The cooling liquid may alternatively be allowed to exit the mould cavity through the cooling liquid inlet. This outlet is preferably opened by displacing the first and second members relative to each other. Preferably means are provided to control the backpressure of the cooling liquid as it exits the mould cavity.

The at least one outlet is preferably lower than the gas inlet and, most preferably, is provided at the lowermost point of the mould space to assist the release of cooling liquid.

The gas injection inlet is preferably provided on a distal end of the second member and the second member is preferably displaceable in a first direction to extend the gas injection inlet into the mould space. This arrangement allows the pressurised gas to be introduced directly into the cavity in the plastics article. Most preferably, the cooling liquid injection inlet is closed when the second member is displaced fully in said first direction to prevent the pressurised gas escaping through the cooling liquid injection inlet.

The apparatus may further comprise a secondary cavity for receiving molten plastics material expelled from the moulded plastics article. A valve is preferably provided for controlling the flow of plastics material into the secondary cavity. The valve is preferably closed when the plastics material is introduced into the mould space and then opened when the cooling liquid is introduced or some time thereafter.

The apparatus preferably also comprises a plastics injection apparatus for injecting molten plastics material into the mould space through a plastics inlet. Viewed from a yet still further aspect, the present invention provides apparatus for injection moulding a plastics article, the apparatus comprising a liquid injection inlet for supplying a pressurised liquid into a molten plastics material provided in a mould space, and a gas injection inlet for supplying a pressurised gas into a cavity formed in the plastics material; wherein the liquid injection inlet comprises a nozzle for extending into the mould space such that, in use, a seal is created between the plastics material and the nozzle. The seal may advantageously prevent leakage of liquid and/or gas between the plastic and mould surface.

Viewed from a still further aspect, the present invention provides a method of producing an injection moulded plastics article comprising the steps of:

(b) supplying a pressurised gas into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space;

(c) supplying a liquid into the cavity and thereby further pressurising the gas in the cavity; and (d) opening at least one outlet to allow pressurised gas in said cavity to expand and discharge at least some of the liquid from the cavity.

This arrangement advantageously facilitates formation of the mould cavity prior to introducing a pressurised liquid to cool the plastics material. By maintaining the pressurised gas in the mould cavity, the cooling liquid may subsequently be discharged by releasing the pressure in the mould cavity and allowing the pressurised gas, which may have formed a gas pocket, to expand.

Viewed from a further aspect, the present invention provides a method of producing an injection moulded plastics article comprising the steps of: (a) introducing a charge of plastics material through an injection aperture into a mould space; (b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space; (c) communicating a pressurised supply of gas with the pressurised liquid in the cavity; and (d) opening at least one outlet to allow pressurised gas in said cavity to discharge at least some of the liquid from the cavity.

The method preferably includes expelling plastics material from said cavity into at least one secondary cavity when the pressurised liquid is supplied into the mould space. The method may further comprise piercing the plastics material in the secondary cavity.

The step of piercing the plastics material in the secondary cavity may open the at least one outlet in accordance with step (d) of the method. Alternatively, the step of piercing the plastics material in the secondary cavity may form at least one inlet through which the pressurised supply of gas is communicated with the pressurised liquid in the cavity.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows schematically apparatus in accordance with a first embodiment of the present invention;

Figures 2a to 2j show the sequential steps of the first embodiment of the present invention utilising the apparatus of Figure 1; Figure 3 show an alternative arrangement of the nozzle system for use in accordance with the first embodiment of the present invention;

Figures 4a to 4j show the sequential steps of a second embodiment of the present invention utilising a secondary overflow cavity;

Figures 5a to 5j show the sequential steps of a third embodiment of the present invention utilising a modified version of the apparatus of Figure 1; Figure 6 show a modified arrangement of the nozzle system for use in accordance with the third embodiment of the present invention;

Figures 7a to 7i show the sequential steps of a fourth embodiment of the present invention; Figures 8a to 8i show the sequential steps of a fifth embodiment of the present invention;

Figures 9a to 9i show the sequential steps of a sixth embodiment of the present invention;

Figures 10a to lOi show the sequential steps of a seventh embodiment of the present invention;

Figures 11a to Hi show the sequential steps of a eighth embodiment of the present invention relying on vapourisation of the liquid injected into the plastics material; and Figures 12a to 12i show the sequential steps of a ninth embodiment of the present invention again utilising vapourisation of the liquid introduced into the plastics material .

Injection moulding apparatus 1 for producing an injection moulded plastics article 2 in accordance with the present invention is shown in Figure 1. The apparatus 1 comprises a nozzle system 3 and a mould 5. The shape of the plastics article 2 is defined by a mould space 4 formed in the mould 5. Molten plastics material 6 is introduced into the mould space 4 through a plastics inlet 7. The nozzle system 3 has an outer housing 9 inside of which is housed a first piston 11 and a second piston 13. The first and second pistons 11, 13 are co-axial and the second piston is provided inside a first cylindrical channel 14 provided in the first piston. The first and second pistons 11, 13 are hydraulically actuated and may be operated independently of each other.

The second piston 13 comprises a central shaft 15 having a diameter less than that of the first channel 14. A rear member 17 and a front member 19 are provided on the central shaft 15 and they each create a seal around their circumference with the first channel 14. The diameter of the rear member 17 is greater than that of the front member 19 and the first channel 14 is stepped to accommodate this change. The increased diameter of the rear member 17 limits the forward movement of the second piston 13 as it abuts the step in the channel 14 when it is in its forwardmost position.

A collar 21 is provided on the central shaft 15 between the front and rear members 17, 19. A seal is also created between the collar 21 and the first channel 14. A first annular chamber 23 is defined between the rear member 17 and the collar 21; and a second annular chamber 25 is defined between the front member 19 and the collar 21. A front surface 27 of the front member 19 is conical in shape and forms a fluid tight seal when it abuts a corresponding conical valve seat 29 formed on the interior of the first piston 11. A first inlet 31 for the mould space 4 is formed at the front of the first piston 11. The abutment of the front surface 27 of the second piston 13 against the valve seat 29 closes the first inlet 31.

A plurality of axial grooves 30 are provided on the outer surface of the front member 19 such that when the second piston 13 is retracted relative to the first piston 11, a fluid pathway is created between the second annular chamber 25 and the mould space 4 via the grooves 30. The grooves 30 may be replaced with one or more openings extending through the front member 19 radially outwardly of the first inlet 31.

A tubular member 33 is provided at the front of the second piston 13 having a second inlet 41 at the end distal from the front member 19 of the second piston 13. When the second piston 13 is in its forwardmost position the cylindrical member extends through the first inlet 31 into the mould space 4. A second axial channel 35, extending along the length of the central shaft 15, is provided in the second piston 13 in fluid communication with the second inlet 41. A radial opening 37 is formed in the shaft 15 to facilitate fluid communication between the second channel 35 and the first annular chamber 23. An axially movable valve pin 39 extends along the length of the channel 35. A conical portion 42 is provided at the distal end of the valve pin 39. The retraction of the valve pin 39 causes the conical portion 42 to abut the tubular member 33 and seal the second inlet 41. The conical portion 42 is biased towards its closed position by a spring 40 provided at the end of the valve pin 39 distal from the conical portion 42. A gas inlet 43 is provided through the housing 9 and the first piston 11 to facilitate the introduction of pressurised gas 44 into the first annular chamber 23. The pressurised gas 44 communicates with the axial channel 35 in the central shaft 15 via the radial opening

37.' The flow of pressurised gas 44 into the mould space

4 is controllable by displacing the valve pin 39 axially relative to the second piston 13. The gas is preferably air and it is preferably dried prior to being introduced into the mould cavity. Other gases, such as nitrogen, may be used in place of air.

A water supply inlet 45 is provided through the housing 9 and the first piston 11 to facilitate the introduction of pressurised water 46 into the second annular chamber 25.

A discharge connection 47 is provided in the housing 9 to facilitate the drainage of water 46 from the mould space 4. The discharge connection 47 is closed when the first piston 11 is in its forwardmost position. The first piston 11 is connected to a first hydraulic ram 49 by a threaded connection. The first ram 49 is provided with a first connection 51 and a second connection 53. Supplying oil to the first connection 51 causes the first ram 49 to perform a forward stroke and, conversely, supplying oil to the second connection causes the ram 49 to perform a return stroke.

The second piston 13 is mounted on a second hydraulic ram 55. A third connection 55 is provided to the second ram 55 and the supply of oil to the third connection causes the second ram 55, and thus the second piston 13, to perform a forward stroke. The return stroke of the second piston 13 is facilitated by reducing the pressure in the third connection 55 until it is less than the water pressure in the second annular chamber 25 (the water pressure acting on the valve seat 27 may then displace the second piston 13) . By maintaining pressure in the third connection 55 above the water pressure in the second annular chamber 25, the second piston 13 may be displaced together with the first piston 11 with substantially no relative movement therebetween. The operation of the apparatus 1 in a first arrangement will now be described with reference to Figures 2a to 2j . This process is the aforementioned short-shot process whereby the mould space is initially only partially filled with molten plastics material 6.

As shown in Figure 2a, the first step is to displace the first piston 11 to its forwardmost position. A fluid tight seal is thereby created between the first piston 11 and the mould 5. In this position, the drain connection 47 is closed by the first piston 11.

The second piston 13 is then displaced to its forwardmost position to create a fluid tight seal between the mating surfaces 27 and 29, as shown in Figure 2b. High pressure water 46 is then introduced into the second annular chamber 29 through the water inlet 45. The water 46 is held in the second annular chamber 25 at high pressure and prevented from entering the mould space 4 by the seal created between the conical surface 27 of the front member 19 and the valve seat 29.

The first and second pistons 11, 13 are held in their forward positions and a predetermined quantity of plastics material 6 is introduced through the plastics inlet 7 into the mould 5 to partially fill the mould space 4, as shown in Figure 2c. The oil pressure in the second connection 55 is then reduced and the high pressure water 46 in the second annular cavity 25 acts on the collar 21 to displace the second piston 13 rearwardly relative to the first piston 11. The rearward movement of the second piston 13 opens the fluid pathway between the first and second pistons 11, 13 and the high pressure water 46 passes through the grooves 30 and first inlet 31 and enters the mould space 4. The continued retraction of the second piston 13 fully withdraws the tubular member 33 into the first channel 14 such that the first inlet 31 is unobstructed, as shown in Figure 2d.

The introduction of the water 46 commences as soon as practicable after the plastics material 6 has been introduced into the mould space 4 in order to ensure that the expulsion distribution of plastics material is completed as quickly as possible to avoid premature and intermittent solidification of the plastics inner surface of the moulded article 2. The high pressure water 46 forms a liquid filled cavity 58 in the plastics material 6. The introduction of water 46 into the mould space also displaces the plastics material 6 so as to conform to the shape of the mould space 4, as shown in Figure 2d. The water injection rate into the mould space 4 may be up to 1.8 litres per second and at pressures up to 300 Bar, although typically it is sufficient to exert water pressures between 50 and 200 Bar. The water 46 is held under pressure inside the mould cavity 58 during commencement of the change in the plastics material 6 from a molten to a solid state and thereby applies a packing pressure. The water 46 has a higher specific heat than a gas and the cooling time of the plastics material 6 is thereby greatly reduced compared to methods utilising pressurised gas to form a cavity in the plastics material 6. As shown by the arrows 59 in Figure 2e, the water pressure in the water inlet 45 is reduced by allowing some to return through the water inlet 45. The reduction in pressure may be under a controlled back pressure to maintain a packing pressure on the plastics material 6. The oil pressure in the third connection 55 is then increased to displace the second piston 13 in a forward direction to close the fluid pathway between the second annular chamber 25 and the mould space 4, as shown in Figure 2f. High pressure gas 44 is then introduced into the first annular chamber 23 through the gas inlet 43. The gas 44 enters the axial chamber 35 in the second chamber piston 13 through the radial opening 37. When the gas pressure exceeds that of the water in the cavity 58 in the plastics material 6 the valve pin 39 moves forward, against the bias of the spring 40, to unseat the conical sealing member 41 and allow the gas 44 to enter the mould cavity. As shown in Figure 2g, the gas 44 bubbles through the water 46 and forms one or more pockets 57 of pressurised gas 44. The location of the gas pocket 57 is determined by the shape of the mould space and also the location of the gas inlet 31. The mould 5 is arranged such that the gas pocket 57 is remote from the discharge outlet 47. After a predetermined amount of gas 44 has been introduced into the cavity 58 the gas supply pressure is reduced and the pressure of the liquid in the cavity 58 closes the conical sealing member 41, as shown in Figure 2h, thereby trapping the water 46 and compressed gas 44 inside the cavity.

Oil is then introduced under pressure into the second connection 53 of the hydraulic ram 49, whilst maintaining oil pressure in the third connection 55, to return the first piston 11 to its return position with the second piston 13 held in its forward position relative to the first piston 11 to keep the first inlet 31 closed. The retraction of the first piston 11 opens the discharge connection 47 and allows at least some of the water 46 to exit the cavity 58 and this escapes drain through the discharge connection 47. The movement of the first piston 11 also reduces the pressure inside the cavity 58 and advantageously this allows the compressed gas 44 in the pocket 57 to expand. The expansion of the gas pocket 57 discharges at least some of the remaining water 46 from inside the mould cavity, as shown in Figure 2i. If necessary, the first and second pistons 11, 13 may be advanced to their forward positions and additional pressurised gas 44 introduced through the inlet 33 and the expulsion process repeated.

As shown in Figure 2j , the pressure inside the moulding is released when all of the water 46 has been expelled. The mould 5 is then opened and the moulded plastics article 2 ejected from the mould space 4. The concentric arrangement of the first and second inlets 31, 41 advantageously results in only one opening being formed in the plastics article 2. The preferred arrangement of the present invention is to provide the discharge connection 47 at the bottom of the mould space 4 to maximise the volume of liquid expelled from inside the cavity 58 when the gas pocket 57 expands. However, the skilled person will appreciate that the expansion of the gas pocket 57 enables the discharge connection to be located elsewhere provided the gas pocket 57 is formed remotely from the discharge connection 47 so that at least some of the liquid between the gas pocket and the discharge connection is expelled when the gas pocket expands. To this end, particular mould shapes may benefit from locating the gas inlet 33 remotely from the discharge connection 47, provided the gas pocket 57 does not form proximal to the discharge connection. It will be appreciated also that more than one gas pocket 57 may be formed, for example, in mouldings having more than one channel in which the plastics material 6 is displaced by liquid.

An alternative arrangement of the nozzle system 3 of the first embodiment of the present invention is shown in Figure 3. Like reference numerals have been used for like components in the following description of this arrangement .

The only modification to the nozzle system 3 is to provide the first piston 11 with an inlet nozzle 32. The inlet nozzle 32 is cylindrical and extends into the mould space 4 when the first piston 11 is in its advanced position. The inlet nozzle 32 preferably extends further into the mould space 4 than the tubular member 33 when the first and second pistons 11,13 are in their forward positions. Thus, the inlet nozzle 32 may help shield the second inlet 41 from plastics material and help prevent it becoming blocked.

When plastics material 6 is introduced into the mould space 4, it comes into contact with the outer surface of the inlet nozzle 32 and tends to cool. As the plastics material 6 cools it forms a skin on the surface of the inlet nozzle 32 and a seal is formed between the plastics material 6 and the inlet nozzle 32. The further cooling of the plastics material 6 typically causes the plastics material to shrink and this may further improve the seal between the plastics material and the inlet nozzle 32.

Subsequently, when liquid, such as water, is introduced into the mould space 4 the seal between the plastics material 6 and the inlet nozzle 32 advantageously helps to prevent the liquid forming a pocket between the surface of the mould cavity and the plastics material. Rather, the liquid may be introduced directly into the molten plastics material 6 away from the surface of the mould space 4.

The different stages in the moulding processes utilising this alternative arrangement of the nozzle system 3 are unchanged from those described above for the first embodiment of the present invention.

A second embodiment of the present invention will now be described with reference to Figures 4a to 4j . The nozzle system 3 for use in this embodiment is unchanged from the apparatus described in respect of the first embodiment and the same reference numerals are used for like components. However, this embodiment is directed to a full-shot process whereby the mould space 4 is initially filled with plastics material 6. The mould 5 is provided with a secondary cavity 8 and a control valve 10 is provided to facilitate control of the flow of molten plastics material 6 into the secondary cavity, as shown in Figure 4a. In this embodiment the control valve 10 for the secondary chamber 8 is initially closed to prevent plastics material 6 entering the secondary cavity. The molten plastics material 6 is then introduced into the mould 5 substantially to fill the mould space 4, as shown in Figure 4b. As in the previous embodiment, the water supply is preferably pressurised prior to the introduction of plastics material 6 into the mould space 4 to minimise cycle times. The oil pressure in the third connection 55 is then reduced to allow the second piston 13 to be displaced rearwardly and allow the high pressure water 46 to enter the mould space 4 through the inlet 31, as shown in Figure 4c. The pressurised water 46 may then begin to form a water filled cavity 58 in the plastics material 6. As shown in Figure 4d, the control valve is then opened to allow molten plastics material 6 to be expelled from the centre of the plastics material 6 by the high pressure water 46 into the secondary cavity 8. The water 46 is held under pressure in the cavity to provide a packing pressure to maintain the plastics material 6 in contact with the mould walls to help ensure that a good surface finish is attained whilst the plastics material 6 undergoes further cooling. As shown in Figure 4e, the water pressure in the cavity 58 is then reduced by allowing some of the water to escape back through the inlet 31. The second piston 13 may then be advanced to its forward position to create a seal with the first piston 11, as shown in Figure 4F. As shown in Figure 4g, a gas pocket 57 is formed by introducing pressurised gas 44 through the gas inlet 33 and, as shown in Figure 4i, the first and second pistons 11, 13 retracted to open the discharge connection 47 and allow the water 46 to be discharged in the same way as described above with regards to the first embodiment of the present invention. The gas and liquid may optionally be held in the cavity 58 for a period of time to allow the plastics material to cool whilst maintaining a packing pressure, as shown in Figure 4h.

Alternatively, the plastic wall in the secondary cavity may be pierced to enable the outflow of high pressure water 46 to commence. The outflow is preferably against a back pressure control in order to retain water pressure within the moulded article 2. The oil pressure may then be applied to the second connection 55 to create a seal between the first and second pistons 11, 13. As before, gas 44 may then be injected into the water containing cavity 58 in the moulded article 2 through the gas inlet 33. The first and second pistons 11, 13 may then be retracted to reduce the pressure inside the moulded article 2 and open the discharge connection 47. The introduction of gas 44 into the mould preferably continues to discharge the water 46 from the inside of the moulded article 2 through the opening pierced in the sidewall of the secondary cavity. Finally, the pressure within the moulding is relieved to atmospheric pressure and the mould is opened and the moulding ejected from the mould space.

A third embodiment of the present invention will now be described with reference to Figures 5a to 5j . The nozzle system 3 for use in the third embodiment is unchanged from the arrangement described above for the first embodiment and like reference numerals have again been used for like components. The apparatus for use in accordance with the third embodiment further comprises a piercing element 61 having a channel 63 defined therein in fluid communication with a second drain connection 65. The piercing element 61 is mounted on a hydraulic ram 67 having fourth and fifth connections 69,71. The supply of oil under pressure to the fourth connection 69 causes the third ram 67 to perform a forward stroke and, conversely, supplying oil under pressure to the fifth connection 71 causes the third ram 67 to perform a return stroke. Thus, the forward and return motion of the piercing element 61 can be controlled.

The operation of the apparatus in accordance with the third embodiment of the present invention will now be described.

The steps shown in Figures 5a to 5e correspond to the steps shown in Figures la to le. As before, a predetermined quantity of plastics material 6 is initially introduced through the plastics inlet 7 to partially fill the mould space 4. High pressure water 46 is then introduced through the first inlet 31 to form a water filled cavity 58 in the plastics material 6. Throughout these steps, the piercing element 61 is held in its retracted position. As shown in Figure 5f, pressurised oil is supplied through the fourth connection 69 to displace the third ram 67 in a forward direction causing the piercing element 61 to pierce the side wall of the plastics article 2 in the mould space 4. The piercing element 61 extends into the interior of the cavity 58 and brings the third channel 63 into fluid communication with the pressurised water 46 inside the cavity. Once the sidewall has been pierced the high pressure water 46 escapes through the third channel 63 and out through the second drain connection 65. The first and second inlets 29,31 are held in a closed position when the piercing element 61 is actuated.

A high pressure gas 44 is then supplied through the gas inlet 43 and opens the second inlet 31 into the mould space 4. The gas 44 enters the cavity 58 and discharges the water 46 contained therein through the second drain connection 65, via the third channel 63, as shown in Figure 5g.

When the water 46 has all been discharged from the cavity 58, the piercing element 61 is retracted, as shown in Figure 5i . The first and second pistons 11,13 are then retracted together, as shown in Figure 3j , and the mould 5 opened and the moulded article 2 removed.

An alternative arrangement of the nozzle system 3 employed in the third embodiment of the present invention is shown in Figure 6. Like reference numerals have again been used for like components.

This arrangement of the nozzle system 3 corresponds generally to the alternative arrangement of the first embodiment of the present invention, as described above with reference to Figure 3. Again, the first piston 11 is provided with an inlet nozzle 32 which extends into the mould space when the first piston is in its forward most position.

The different stages in the moulding processes utilising this alternative arrangement of the nozzle system 3 are unchanged from those described herein for the third embodiment of the present invention. A fourth embodiment of the present invention is shown in Figures 7a to 7i. The nozzle system 3 is generally unchanged from the previous embodiments described herein and like reference numerals have been used for like components. The first cylinder 11 of the nozzle system 3 is provided with an inlet nozzle 32 projecting into the mould space 4 when the first cylinder is in its advanced position.

The apparatus 1 further comprises a gas injection nozzle 73 projecting into the interior of the mould space 4. The gas injection nozzle 73 is located remotely from the first and second inlets 31, 41. The operation of the apparatus 1 in accordance with the fourth embodiment of the present invention will now be described. The first piston 11 is advanced to its forward position, as shown in Figure 7a. Pressurised oil is then supplied to the third inlet 55 to displace the second piston 13 to its forward position to create a seal between the first and second pistons 11, 13. Pressurised water is then introduced into the second annular cavity 25 ready for injection into the mould space 4, as shown in Figure 7b.

A charge of plastics material 6 is then introduced into the mould space 4. The plastics material 6 partially fills the mould space 4 (i.e. this is a short- shot process), as shown in Figure 7c. The molten plastics material 6 comes into contact with the walls of the mould space 4 and begins to cool and form a skin. In the region of the first inlet 31 and the gas inlet 43, the plastics material surrounds the inlet nozzle 32 and begins to cool. As the plastics material 6 cools it shrinks and a seal is formed between the plastics material and the outside of the inlet nozzle 32.

The oil pressure in the third inlet 55 is then reduced and the second piston 13 is allowed to be displaced to its rearward position by the pressure of the water in the second annular cavity 25. The rearwards displacement of the second piston 13, relative to the first piston 11, creates a fluid pathway from the second annular chamber 25 to the interior of the mould space 4 via the inlet nozzle 32. The pressurised water enters the mould space 4 and forms a mould cavity 58 in the plastics material. The introduction of the pressurised water into the mould space 4 also causes the plastics material 6 to conform to the interior space of the mould space 4, as shown in Figure 7e. The plastics material covers the gas injection nozzle. A control valve (not shown) may be provided to close the gas injection nozzle prior to introduction of plastics material 6 into the interior of the mould cavity.

The supply of pressurised liquid into the cavity 58 is maintained for a period of time sufficient to allow the plastics material to cool and become self-supporting. The pressure in the cavity 58 is then reduced by allowing some of the water to return through the inlet 45, as shown in Figure 7e . The reduction of the liquid pressure is preferably controlled, for example by a valve (not shown) , to ensure that a packing pressure is maintained on the plastics material in the mould. As shown in Figure 7f, the second piston 13 is then returned to its forward position to create a seal between the first and second pistons 11, 13.

As shown in Figure 7g, gas is introduced into the interior of the cavity 58 through the gas injection nozzle. A pocket 57 of pressurised gas is formed in the cavity 58. The combined pressure of the liquid and gas in the cavity 58 may be maintained for a period of time to allow the plastics material to cool further. The first and second pistons 11, 13 are then retracted to open the drain outlet 47. As shown in Figure 7h, the pocket of gas expands and, together with the continuing introduction of gas through the gas injection nozzle 73, causes the liquid to be expelled from the cavity 58. The supply of gas may be continued after the liquid has been expelled to effect further drying and/or cooling of the plastics material in the mould. The gas may also be cooled prior to its introduction into the cavity 58 to improve cooling of the plastics material and thereby reduce cycle times.

The supply of gas is subsequently stopped and the moulded article is then ready for removal from the mould, as shown in Figure 7i.

The fourth embodiment of the invention is particularly well suited for moulding articles having a shape which does not readily lend itself to the introduction of gas through an inlet proximal to the water inlet 31. Indeed, it will be appreciated that the nozzle system 3 may be simplified by omitting the gas injection feature for use in accordance with this embodiment .

A fifth embodiment of the present invention is shown in Figure 8a to 8i. The nozzle system 3 is generally unchanged from that of the fourth embodiment and like reference numerals have been used for like components. This embodiment is directed to a full-shot process whereby the mould cavity is initially filled with molten plastics material 6. A secondary cavity 8 is provided for receiving plastics material expelled from the mould cavity 4. A flow channel is provided between the mould cavity 4 and the secondary cavity. An hydraulically operated control valve 10 is provided in the flow channel to allow the flow of plastics material and fluid through the flow channel to be controlled. A piercing pin is provided for piercing plastics material in the second cavity. The operation of this embodiment of the invention will now be described.

As in the previous embodiments, liquid is held under pressure in the second annular chamber 25 with the first and second pistons 11, 13 in their advanced positions, as shown in Figure 8a. A charge of plastics material 6, sufficient at least substantially to fill the mould space 4, is then introduced through the plastics inlet 7. The control valve 10 between the mould space 4 and the secondary cavity is held in a closed position whilst the plastics material is introduced, as shown in Figure 8B. A packing pressure may be applied by the molten plastics material 6 injected into the mould space 4.

The second piston 13 is then allowed to retract allowing pressurised water to be introduced into the plastics material 6 through the inlet nozzle 32. As the mould space 4 is initially at least substantially filled with plastics material, water is primarily able to enter into the mould space 4 due to the shrinkage of the plastics material. Thus, the cavity 58 in the plastics material may be formed only partially, as shown in Figure 8c.

The control valve 10 between the mould space 4 and the secondary cavity 8 is subsequently opened to allow plastics material 6 to be discharged into the secondary cavity, as shown in Figure 8d. The expulsion of plastics material from the mould space 4 allows the cavity 58 to form fully. The liquid pressure in the cavity 58 is typically maintained until the plastics material has cooled sufficiently so that the article is self- supporting.

As shown in Figure 8e, a piercing pin 61 is advanced into the secondary cavity 8 to pierce the plastics material collected therein. The plastics material in the secondary cavity is pierced whilst the water in the mould space 4 is under pressure and the supply of water to the interior of the mould cavity 4 is maintained after the secondary cavity has been pierced. The piercing of the plastics material in the secondary cavity may allow a relatively small quantity of plastics material to escape from the secondary cavity 8 through the third channel 63 formed in the piercing pin 61. Pressurised water is then permitted to flow through the plastics article 2 and the secondary cavity 8 to effect further cooling of the plastics material. The escape of water from the secondary cavity is preferably controlled by a back pressure regulator to ensure that a packing pressure is maintained on the plastics material as it cools. As shown in Figure 8f, the supply of pressurised water into the mould space 4 is stopped once the plastics material has cooled sufficiently. The pressure in the cavity 58 is reduced by retracting the first and second pistons 11, 13 and allowing some of the water to escape through the water inlet 45. The water pressure in the cavity 58 may be returned almost to atmospheric pressure .

As shown in Figure 8g, the second piston 13 is advanced to its forward position to create a seal with the first piston 11 (also in its forward position) . A supply of pressurised gas is then introduced into the first annular chamber 15. Once the gas pressure is above that of the liquid in the cavity 58, the valve member 41 is displaced in a forwards direction and gas is introduced into the cavity 58. The introduction of gas displaces the liquid through the pierced opening in the secondary cavity. Again, the displacement of liquid from the system is preferably controlled by a back pressure regulator. The flow of gas through the cavity 58 may be continued after the water has been expelled to effect additional cooling and/or drying of the plastics material . As shown in Figure 8i, the moulded article is then ready for removal from the mould space 4.

A sixth embodiment of the present invention is shown in Figure 9a to 9i. Again, the nozzle system 3 is generally unchanged from that of the earlier embodiments. Indeed, this embodiment of the present invention is similar to that of the previous embodiment insofar as a secondary cavity 8 is provided together with a control valve 10 for controlling the discharge of plastics material from the mould cavity into said secondary cavity. However, rather than a piercing pin 61 for piercing the plastics material in a secondary cavity, this embodiment utilises a gas injection nozzle 13 which projects into the interior of the secondary cavity 8 for effecting piercing of the plastics material . The steps shown in Figures 9a to 9d correspond generally to steps 8a to 8d. Specifically, molten plastics material 6 is introduced into the mould cavity 4 as least substantially to fill the mould space 4 and pressurised liquid is then introduced into the molten plastics material via the inlet nozzle 32. The control valve 10 provided between the mould space 4 and the secondary cavity 8 is maintained closed whilst the plastics material is injected into the mould cavity and is subsequently opened to allow plastics material to be expelled into the secondary cavity. The secondary cavity 8 is typically filled with plastics material expelled from the mould space 4 and, therefore, the size of the cavity may be used to control the amount of plastics material displaced.

The liquid pressure in the mould cavity 58 is then reduced by allowing some of the water to return through the water inlet, as shown in Figure 9e . The second piston 13 is then advanced to form a seal with the first piston 11 (already in its forward position) , as shown in Figure 9f.

Subsequently, gas is introduced through the gas injection nozzle 13 into the secondary cavity 8, as shown in Figure 9g . The introduction of pressurised gas pierces the plastics material in the secondary cavity 8 and forms a flow passage through the secondary cavity into the mould space 4. A small quantity of plastics material may be displaced from the secondary cavity back into the mould space 4 when the gas is introduced into the secondary cavity. The introduction of gas into the secondary cavity forms a gas pocket 57 which may assist in the application of a packing pressure on the plastics material .

The first and second pistons 11, 13 are then retracted and the gas introduced into the secondary cavity causes the water to be expelled through the discharge outlet. The flow of gas may be continued to effect further cooling of the plastics material and/or drying of the interior of the cavity 58. The moulded article is ready for removal from the mould, as shown in Figure 9i.

A seventh embodiment of the present invention is shown in Figures 10a to lOi. This embodiment utilises apparatus 101 comprising a modified nozzle system 103. The nozzle system 103 has first and second pistons 105, 107 which operate in the same manner as the pistons of the embodiments described above. The collar 21 of the previous embodiments is omitted from the second piston 107 such that a single annular cavity 109 is formed between the first and second pistons 105, 107. The apparatus further comprises a fluid heater 111 and a high pressure water supply 113. First and second ball valves 115, 117 are provided for controlling the supply of liquid to the first annular chamber 109. The fluid heater 111 initially circulates hot water through the annular chamber 109 ready for introduction into a mould space 119 defined in a mould 121, as shown in Figure 10a. With the first and second pistons 105, 107 in their forward most positions, a charge of molten plastics material 123 is introduced at least substantially to fill the mould space 119. The supply of plastics material is continued to maintain a packing pressure on the plastics material 123 in the mould space 119. As shown in Figure 10c, pressurised water is supplied to the annular chamber 109 and the first piston 105 is allowed to retract. A supply of high pressure water is thereby introduced into the annular chamber 109 causing the hot water supplied by the fluid heater 107 to be introduced into the molten plastics material 123 and to form a cavity 125 therein. During this step, water is cycled through the fluid heater 107 in a closed loop.

An hydraulically actuated control valve 127 provided between the mould cavity 115 and a secondary cavity 129 is then opened to allow plastics material to be expelled into the secondary cavity, as shown in Figure lOd. The water pressure is maintained in the cavity 125 by the high pressure water supply 109 until the plastics material in the mould cavity has cooled sufficiently to allow it to support itself.

As shown in Figure lOe, the second piston 107 is then retracted to allow liquid to escape from the mould space 119 to reduce the pressure inside the cavity 125. The hot water from the fluid heater 107 is cycled through the annular cavity 109. The second piston is then advanced to form a seal with the first piston 107, as shown in Figure lOf .

As shown in Figure lOg, pressurised gas is then introduced through a gas pin 131 provided in communication with the secondary cavity 129. Molten plastics material in the secondary cavity may be discharged back into the cavity 125 by the introduction of the pressurised gas. A pocket 133 of pressurised gas is formed in the secondary cavity and the cavity 125.

Subsequently, the first and second pistons 105, 107 are retracted to allow liquid to escape from the cavity 125 through a drain connection 135, as shown in Figure lOh. The expansion of the pocket 133 of pressurised gas and/or the continued introduction of gas into the secondary cavity expels liquid from the mould. Pressurised gas may continue to be supplied into the secondary cavity to effect further drying and/or cooling of the plastics material 123. The moulded plastics article may be removed from the mould once the plastics material has cooled sufficiently. The use of hot water, or any other heated liquid or gas, advantageously provides an improved surface finish for the walls of the cavity 125. This is particularly desirable when moulding components where the cavity 125 will be used as a fluid pathway when the articles are used. If water is used, it may be heated to at least 40, 50, 60, 70, 80 or 90°C ready for introduction into the plastics material . The temperature may be even higher if the water is pressurised.

An eighth embodiment of the present invention is shown in Figures 11a to Hi. The apparatus 1 for this embodiment corresponds to that described above for the first embodiment and like reference numerals have been used for like components. Indeed, the steps shown in Figures HA to 11F correspond to those shown in Figures 2A to 2F and, for the sake of brevity, the description of these steps shall not be repeated for the eighth embodiment . In the first embodiment of the present invention, the liquid pressure in the cavity 58 is reduced, as shown in Figure 2e, and then gas is introduced into the cavity 58 in the plastics article. However, in the eighth embodiment, gas is not introduced into the cavity 58 at this stage. Instead, this embodiment relies on the heat of the plastics material in the mould to vaporise the liquid introduced into the mould cavity. The vaporisation of the liquid causes an increase in pressure in the cavity 58 and this vapour may subsequently be used to expel at least some of the remaining water from the cavity 58 in the plastics article. The melt temperature of the plastics material in this embodiment is 280 °C, and this causes the temperature of the water introduced into the plastics material to be raised to approximately 150 °C. This increase in temperature is sufficient to allow at least some of the remaining water to vaporise when the pressure in the cavity 58 is reduced. Thus, in the eighth embodiment of the present invention, rather than introduce gas into the cavity 58 to expel water, at least some of the water therein is allowed to vaporise, as shown in Figure llg. The first and second pistons 11, 13 are then retracted, as shown in Figure llh, and the vapour in the cavity 58 is allowed to expand and expel at least some of the remaining water from the cavity 58. The expulsion of the water may be under a controlled back pressure to maintain the plastics material in contact with the surface of the mould. If necessary, gas may subsequently be introduced to expel any remaining water from the cavity 58 and/or to cool the plastics material further. Once the plastics material has cooled, the moulded plastics article is ready for removal from the mould, as shown in Figure Hi.

A ninth embodiment of the present invention is shown in Figures 12a to 12i. This embodiment is similar to the second embodiment described above but relies on the heat of the plastics material in the mould to vaporise the liquid introduced into the mould cavity. The vaporisation of the liquid causes an increase in pressure in the cavity 58 and this increased pressure may subsequently be used to expel the remaining water from the cavity 58 in the plastics article. The apparatus 1 for this embodiment corresponds to that described above for the second embodiment and like reference numerals have been used for like components. Indeed, the steps shown in Figures 12a to 12f correspond to those shown in Figures 4A to 4F and the description of these steps shall not be repeated for the ninth embodiment . Rather than introduce gas into the liquid remaining in the cavity 58 after the pressure has been reduced, at least some of the remaining liquid is allowed to vaporise, as shown in Figure 12g.

The first and second pistons 11, 13 are then retracted, as shown in Figure 12h, and the vapour in the cavity 58 is allowed to expand and expel at least some of the remaining water from the cavity 58. The expulsion of the water may be under a controlled back pressure to maintain the plastics material in contact with the surface of the mould. If necessary, gas may subsequently be introduced to expel any remaining water from the cavity 58 and/or to cool the plastics material further. Once the plastics material has cooled, the moulded plastics article is ready for removal from the mould, as shown in Figure 12i.

It will be appreciated that in each of the embodiments described herein the outlet (s) through which the liquid is released may be the same connection through which the pressurised liquid is initially supplied. Thus, the same connection may function as an inlet and as an outlet for the liquid at different stages in the cycle. The backpressure of the liquid is preferably controlled irrespective of which outlet it is exiting through.

Claims

CLAIMS :

1. A method of producing an injection moulded plastics article comprising the steps of : (a) introducing a charge of plastics material through an injection aperture into a mould space;

(b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space;

2. A method of producing an injection moulded plastics article comprising the steps of :

(b) supplying pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space; (c) introducing a piercing element into the mould space to pierce the plastics article and form at least one outlet; and

(d) introducing gas into the cavity to discharge liquid from the cavity through said at least one outlet.

3. A method as claimed in claim 2, wherein liquid is allowed to flow through the cavity after the plastics article has been pierced and prior to the introduction of gas into the cavity.

4. A method as claimed in claim 3, wherein the liquid is maintained under pressure as it flows through the cavity to provide a packing pressure on the plastics article.

5. A method as claimed in claim 2, 3 or 4 , wherein gas is introduced into the cavity and allowed to escape through said at least one outlet to effect cooling of the plastics material and/or drying of the interior of the cavity.

6. A method as claimed in claim 5, wherein the flow of gas through the cavity is maintained for a predetermined period of time.

7. A method of producing an injection moulded plastics article comprising the steps of :

(b) supplying a pressurised liquid into the mould space to form a cavity in the molten plastics material and to cause the plastics material to conform to the shape of the mould space;

(c) piercing the plastics article to form at least one inlet, and introducing a pressurised gas through said at least one inlet into the cavity to discharge liquid from the cavity through at least one outlet.

8. A method as claimed in claim 7, wherein the plastics article is pierced by pressure penetration of the pressurised gas.

9. A method as claimed in claim 7 or 8, wherein liquid is allowed to escape through the inlet pierced in the plastics article to facilitate the flow of liquid through the cavity prior to the introduction of pressurised gas into the cavity.

10. A method as claimed in claim 7, 8 or 9, wherein gas is introduced into the cavity through said at least one inlet and allowed to escape through said at least one outlet to effect cooling of the plastics material and/or drying of the interior of the cavity.

11. A method as claimed in claim 10, wherein the flow of gas is maintained for a • predetermined period of time.

12. A method as claimed in any one of claims 2 to 11, wherein the liquid is under pressure when the plastics article is pierced.

13. A method as claimed in any preceding claim, wherein the liquid in the cavity is maintained under pressure until the plastics material has solidified and cooled sufficiently so that the article can sustain the form dictated by the mould space.

14. A method as claimed in any preceding claim, wherein the pressurised gas is supplied after the plastics material has solidified and cooled sufficiently so that the article can sustain the form dictated by the mould space.

15. A method as claimed in any preceding claim further comprising releasing some of the pressurised liquid from the cavity to reduce the pressure in the cavity prior to the step of introducing pressurised gas into the cavity.

16. A method as claimed in claim 15, wherein the release of pressurised liquid is performed under controlled back pressure .

17 A method as claimed in claim 15 or 16, wherein the liquid pressure is reduced sufficiently to allow at least some of the liquid in the cavity to be vapourized.

18. A method as claimed in any preceding claim, wherein the pressurised liquid is supplied through an opening functioning as an inlet and at least some of the liquid is subsequently discharged through the same opening functioning as an outlet.

19. A method as claimed in any preceding claim wherein said liquid is pressurised prior to the step of supplying the plastics material into the cavity.

20. A method as claimed in claim 19, wherein said liquid is held under pressure until it is supplied into the mould space .

21. A method as claimed in any preceding claim, wherein said mould space is substantially filled by plastics material during the step of introducing a charge of plastics material.

22. A method as claimed in any one of claims 1 to 20, wherein the mould space is partially filled with plastics material during the step of introducing a charge of plastics material.

23. A method as claimed in any preceding claim further comprising expelling plastics material from said cavity into at least one secondary cavity when the pressurised liquid is supplied into the mould space.

24. A method as claimed in claim 23 further comprising piercing the plastics material in the secondary cavity.

25. A method as claimed in claim 24, when dependent directly or indirectly on claim 1, wherein the step of piercing the plastics material in the secondary cavity opens the at least one outlet in accordance with step (d) of claim 1.

26. A method as claimed in claim 24, when dependent directly or indirectly on claim 1, wherein the step of piercing the plastics material in the secondary cavity forms at least one inlet through which the pressurised supply of gas is communicated with the pressurised liquid in the cavity.

27. A method as claimed in any one of claims 24, 25 or 26, wherein the step of piercing the plastics material in the secondary cavity is performed by introducing a piercing element into the secondary cavity.

28. A method as claimed in any one of claims 1 to 22 further comprising expelling plastics material from said cavity back through the injection aperture when the pressurised liquid is supplied into the plastics material .

29. A method as claimed in any one of claims 23 to 27, wherein at least one valve is provided to control the expulsion of plastics material from the cavity.

30. A method as claimed in claim 29, wherein the at least one valve is provided in at least one connection from said mould space to said secondary cavity.

31. A method of producing an injection moulded plastics article comprising the steps of: (a) introducing a charge of plastics material through an injection aperture into a mould space;

(c) allowing at least some of the liquid in the cavity to vaporize; and

32. A method as claimed in any preceding claim further comprising the step of heating said liquid prior to its introduction into the mould space.

33. A method of producing an injection moulded plastics article comprising the steps of .-

(c) supplying a liquid into the cavity;

(d) allowing at least some of said liquid to vaporise;

(e) opening at least one outlet to allow vapour in said cavity to expand and discharge at least some of the remaining liquid from the cavity.

34. A method as claimed in claim 31, 32 or 33 wherein the step of allowing at least some of the liquid in the cavity to vaporize comprises reducing the pressure of the liquid in said cavity.

35. A method as claimed in any one of claims 31 to 34 further comprising the step of introducing gas into the cavity to discharge at least some of the remaining liquid from the cavity.

36. A method of producing an injection moulded plastics article comprising the steps of: (a) introducing a charge of plastics material through an injection aperture into a mould space;

(c) supplying a liquid into the cavity and thereby further pressurising the gas in the cavity; and

37. A method as claimed in any preceding claim, wherein the pressurised liquid is introduced into the mould space through a nozzle projecting into the mould space.

38. A method as claimed in claim 37, wherein a seal is allowed to form around said nozzle when the molten plastics material begins to solidify.

39. A method as claimed in claim 33 or 36 further comprising the step of heating the gas prior to its introduction into the mould space.

40. Apparatus for injection moulding a plastics article, the apparatus comprising a liquid injection inlet for supplying a pressurised liquid into a molten plastics material provided in a mould space, and a gas injection inlet for supplying a pressurised gas into the pressurised liquid, wherein said liquid injection inlet and said gas injection inlet are co-axial and are closable independently of each other.

41. Apparatus as claimed in claim 40 further comprising pressurisation apparatus for pressurising fluid to be supplied through the liquid injection inlet.

42. Apparatus as claimed in claim 40 or 41, wherein the liquid injection inlet is provided in a first member and the gas injection inlet is provided in a second member, the second member being provided inside the first member and the first and second members being displaceable in an axial direction relative to each other.

43. Apparatus as claimed in claim 42, wherein the gas injection inlet is provided on a distal end of the second member and the second member is displaceable in a first direction such that the gas injection inlet extends into the mould space .

44. Apparatus as claimed in any one of claims 42 or 43, wherein, in use, the first and second members cooperate with each other to control the supply of pressurised liquid through the liquid injection inlet.

45. Apparatus as claimed in any one of claims 41 to 44, further comprising at least one valve member for controlling the supply of pressurised gas through the gas injection inlet.

46. Apparatus as claimed in any one of claims 41 to 45 further comprising at least one outlet for releasing liquid from said mould cavity.

47. Apparatus as claimed in claim 46 when dependent directly or indirectly on claim 43, wherein said at least one outlet is opened by displacing said first member relative to the mould space.

48. Apparatus as claimed in claims 46 or 47, wherein at least one outlet is provided lower than the gas inlet.

49. Apparatus as claimed in any one of claims 46, 47 or 48 wherein at least one outlet is provided at the lowermost point of the mould space.

50. Apparatus as claimed in any one of claims 41 to 49 further comprising a piercing element for piercing a sidewall of a moulded plastics article to create at least one outlet for the release of liquid.

51. Apparatus as claimed in any one of claims 41 to 49 further comprising a piercing element for piercing a sidewall of a moulded plastics article to create at least one further inlet for injecting pressurised gas into the liquid.

52. Apparatus as claimed in any one of claims 41 to 51 further comprising a secondary cavity for receiving molten plastics material expelled from the moulded plastics article.

53. Apparatus as claimed in claim 52 further comprising a piercing element for piercing the plastics material received in the secondary cavity to create at least one outlet for the release of liquid.

54. Apparatus as claimed in claim 52 further comprising a piercing element for piercing the plastics material received in the secondary cavity to create at lest one further inlet for injecting pressurised gas into the liquid.

55. Apparatus as claimed in any one of claims 52, 53 or 54 further comprising a valve for controlling the flow of plastics material into the secondary cavity.

56. Apparatus as claimed in claim 55 wherein the at least one valve is provided in at least one connection from the secondary cavity to the mould space .

57. Apparatus as claimed in any one of claims 41 to 56 further comprising plastics injection apparatus for injecting molten plastics material into the mould space through a plastics inlet.

58. Apparatus as claimed in any one of claims 41 to 57, wherein the liquid inlet is also operable as an outlet for releasing liquid from said mould cavity.

59. Apparatus as claimed in any one of claims 41 to 58 further comprising means to control the backpressure of liquid as it is released from the mould cavity.

60 . Apparatus as claimed in any one of claims 41 to 59, wherein the liquid is water.

61. Apparatus as claimed in any one of claims 41 to 59, wherein the pressurised gas is air or nitrogen.

62. Apparatus as claimed in any one of claims 41 to 61, wherein the liquid injection inlet comprises a nozzle for projecting into a mould space.

63. Apparatus for injection moulding a plastics article, the apparatus comprising a liquid injection inlet for supplying a pressurised liquid into a molten plastics material provided in a mould space, and a gas injection inlet for supplying a pressurised gas into a cavity formed in the plastics material; wherein the liquid injection inlet comprises a nozzle for extending into the mould space such that, in use, a seal is created between the plastics material and the nozzle for preventing or reducing leakage of gas and/or liquid between the plastic and mould surfaces.

64. Apparatus as claimed in any one of claims 40 to 63 further comprising a fluid heater for heating the liquid and/or gas.