GB2453339A

GB2453339A - Varying volume of a mould cavity in injection moulding apparatus

Info

Publication number: GB2453339A
Application number: GB0719232A
Authority: GB
Inventors: Peter Reginald Clarke
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-10-02
Filing date: 2007-10-02
Publication date: 2009-04-08
Anticipated expiration: 2027-10-02
Also published as: GB2468602B; GB201007189D0; GB2453339B; GB2468602A; WO2009044142A1; GB0719232D0

Abstract

First and second mould parts (platens 57, 58) of an injection mould 2 are adapted to be connected together in a fully closed configuration so as to define a mould cavity 10 for moulding an article. In the fully closed configuration, the cavity is further defined by a core 20, a back plate 6 and a cavity plate 8, which forms an outer surface 12 of the cavity 10 and which in use defines the outer shape of the article to be injection moulded. An injector, in the form of a feed nozzle 4 and a gate 5, injects molten material into the cavity 10. At least one movable portion (core 20) of one of the first and second mould parts is movable so as to vary the volume of the mould cavity 10 in the fully closed configuration. An actuator (piston and cylinder 45; pneumatic fluid sources 51, 53; switches 60, 62; switch actuators 64, 66), which actuator is operated by a pressurized gas, controls the movement of the movable portion 20. Fluid 51 is at a low pressure relative to fluid 53.

Description

Injection Moulding Apparatus and Injection Moulding Method The present invention relates to an injection moulding apparatus and to a method of injection moulding an article, or a plurality of articles. In particular, the present invention relates to the injection moulding of containers, and preforms for containers, from plastics material.

Injection moulding of articles from plastics materials, in particular thermoplastic polymers, is well known in the art. In particular, the injection moulding of containers, and preforms for containers, from plastics material is conventional in the art.

It is often desired to injection mould plastics containers having a small wall thickness, for example to reduce material costs. When it is necessary to injection mould a container having a high L/T ratio (where L is the flow length of the molten plastics material from the injection gate and T is the wall thickness), a high injection pressure at the gate is needed to ensure that the mould cavity is filled with the molten plastics material. The gate acts to restrict material flow therethrough, and the wall section directly opposite the gate also restricts the material flow into the cavity.

The conventional approach to attempt to reduce the high injection pressure at the gate is to inject the molten plastics material at a faster injection rate, and to raise the melt temperature to lower the melt viscosity, to enable the mould to be filled by the molten plastics material.

It is also well known that in order to reduce the fill pressure, it is possible when designing a container to increase the base thickness, particularly in the gate area. This gate area is also the hottest area of the injection moulding. As all of the material in the sidewall has to flow across the base, within an interior gap defined between the static exterior skin layers laid down during the first phase of filling, base cooling is always a problem. Another problem with such laminar flow across the base is that the skins are progressively solidifying and therefore getting thicker, narrowing the flow channel.

This causes a further restriction on the material flow.

All of this adds up to the need to flow the molten material into the cavity at a faster rate, and to do that one needs to increase the fill pressure. The higher fill pressure will, in turn, require a higher clamp pressure to counter the hydraulic force on the end of the core. It should be readily apparent to the skilled reader why injection moulding machines for the manufacture of plastics packaging need to have very high injection speeds and pressures, and very rigid platens, to make what appears to be a simple container or preform.

Over the years a significant development in the packaging art to try to address these problems has been in increase the melt flow index of the plastics materials, while maintaining their rigidity and impact resistance. This merely required adjustment of the machine and mould specifications to suit the newer materials. In other words, some attempts to solve the high gate pressure problems discussed above of injection moulding of plastics materials into thin-walled bodies has primarily been focused on the nature of the plastics material, rather than the fundamental injection moulding processes and machines.

A particularly important application for injection moulding in the packaging industry is to make injection moulded polyester, particularly polyethylene terephthalate (PET), preforms for subsequent blow moulding into containers, typically bottles for carbonated beverages. It is understood in the art that in order to make good quality blow moulded bottles from PET injection moulded preforms, the preforms must be injection moulded with the minimum of moulded-in stress. This is because any stress pattern resulting from the preliminary injection moulding process would affect the way in which the preform stretches during the subsequent blow moulding process.

The optimum preform would have no visible signs of stress when viewed under polarised light; however, this is very difficult to achieve with conventional injection moulding techniques.

This is due to the requirement of continuing to add material to the preform as it cools during the filling and packing phases of the injection moulding cycle. Internal shrinkage causes the preform to collapse away from the mould cavity wall creating surface "ripples", which are unacceptable to the blow moulding process. To overcome internal shrinkage, it is necessary to continue to add material into the preform during this shrinkage phase. This requires the maintenance of a fill pressure and material flow sufficient to compensate for the changing density caused by the cooling of the material.

However, such pressure and forced flow cause residual stress to be present in the ultimate injection moulded article.

Polyethylene terephthalate bottles currently in commercial production are too thick in the base, which increases material costs and degrades the mechanical properties of the bottle. It is believed in the art that it is not possible to significantly reduce the base thickness due to the increase in stress caused by the increased injection pressure needed to produce the correspondingly required thin preforms, particularly in the base-forming region thereof.

The injection moulding of preforms requires a delicate balance of pressure and flow to achieve the low stress required to blow mould the preform into a good quality bottle.

Preforms have tended to become shorter and fatter over the years to assist in the reduction of gate stress, by attempting to avoid the higher pressures required to fill the longer and thinner wall section mouldings. Such higher pressures also tended to cause core shift, where the central core defining the preform cavity is shifted off its axis by the higher-pressure fill, which creates a wall section differential around the circumference of the preform that cannot be accommodated during the subsequent blow moulding stage.

It is also known to inject the molten plastics material before the clamp pressure has been fully applied to the injection mould, i.e. before the two mould halves have been urged together under sri applied clamp force so as to fully close the mould cavity.

Husky, Netstal and BMB all have this as a standard procedure in their software for operating their injection moulding machines. This effectively increases the cavity base thickness opposite the gate during the first phase of injection to improve the molten material flow. The problem with this method is that the mould loses concentricity when the mould is not fully closed. The two mould halves are normally mutually engaged by a taper lock to ensure that in the closed configuration the two mould halves are coaxial.

However, the taper lock becomes ineffective to ensure concentricity before the clamp pressure has been fully applied to the injection mould, and this can cause the initially injected material to become circumferentially asymmetrically distributed within the cavity.

There is also known in the art a process of injection compression moulding (1CM) that has been proven to overcome many of these problems. In particular, the injection compression moulding process can allow increased flow-length:wall thickness ratios in injection moulded parts, and can reduce clamping forces and injection pressures, and thereby reduce internal material stresses.

There are four injection compression moulding processes.

In sequential injection compression moulding (Seq-ICM), the material is injected into the mould cavity when the mould is not fully closed. This creates a larger mould cavity portion of about twice the final wall thickness into which the injected material is received. Thereafter, the mould is closed by the clamp. This closing action causes the material to be distributed by the closing clamp pressure throughout the entire mould cavity.

In simultaneous injection compression moulding (Sim-ICM), the material is injected into the mould cavity when the mould is not fully closed, and simultaneously the mould is closed by the clamp.

In breathing injection compression moulding (Breath-ICM), the mould is fully closed before the material is injected into the mould cavity. Then the mould is progressively * opened during the injection to create a larger mould cavity portion than the final wall thickness. When nearly all of the material volume has been injected, the mould is closed by the clamp. This closing action causes the material to be distributed by the closing clamp pressure throughout the entire mould cavity.

In all of these three injection compression moulding processes the mould is required to be at least partly open during the moulding process. These processes therefore encounter the coaxial non-alignment problems for the mould halves discussed above.

The present inventor has earlier devised a modified sequential injection-compression moulding process that is the subject of US-A-7,090,800. This teaches the use of the core to be pushed back by the incoming material under a light spring pressure and then using the machine to clamp up to finish the flow and pack the moulding. This process used a precise shot volume control to meter the material into a variable cavity and then compress to achieve fully packed and low stress mouldings. This modified sequential injection-compression moulding process injected all of the material into the cavity volume between the end of the core and the base of the cavity. The injection pressure causes the core to be pushed back, thereby enlarging the volume, and reducing the material pressure in the cavity. Thereafter, the core is rapidly forced forwardly to reduce the volume and to displace the material to the end of cavity flow. This force comes from the machine clamp.

The inventor's earlier modified sequential injection-compression moulding process provided a number of advantages as compared to conventional injection moulding processes, in particular that the material can be processed at lower temperatures; lower clamp pressures are needed; 30% faster cycle time are achievable and very thin mouldings can be made.

However, the process also suffers from some disadvantages, in particular: there is a need to use an individual shooting pot per cavity; as for other 1CM processes, complex and expensive mould alignment systems are required, and the moulds are very expensive. In addition, since the clamp pressure causes closure of the mould and controlled movement of the core, the process can only be carried out on modified injection moulding machines, and in particular is not suitable to be carried out on all types of machines (for example, the process is not suited to direct lock machines).

A fourth injection compression moulding process is known as selective injection compression moulding (Select-ICM), or coining. The mould is completely closed and a separate core is pressed locally into the mould during or after injection of the material.

This reduces the cavity volume and distributes the injected material throughout the mould cavity. However, coining is not known for use in the manufacture of containers.

Coining is traditionally used for locally thinning an area of a moulding, such as a live hinge to improve its service life. This would only need a small hydraulic cylinder in the mould using a standard core-pulling valve on the machine. Compressing a minor area of a flat moulding to complete the fill is used but only where the product design will allow.

The viable commercial use of injection compression moulding processes for containers is in its infancy and has a long way to go before it is production viable and cost effective.

There is a need in the art for a cost effective, robust injection moulding process that at least partly overcomes the various problems with known processes as discussed above.

In particular, there is a need for an injection moulding process, and an apparatus therefor, that is suitable for producing containers, or preforms for containers, having high flow-length:waIl thickness ratios, and/or low material stress, which can be produced using conventional injection moulding machines and therefore can be interfaced with the minimum of problems into conventional production practices.

The present invention aims at least partly to meet these needs in the art of container manufacture.

It is common practice within the injection moulding industry to use pneumatic cylinders to move cavity members that have low forces applied to them or have a low projected area. However, because factory air supplies are normally limited to a pressure of 10 bar for safety reasons, and air cylinders are rated to a maximum pressure of 10 bar, if higher forces are needed then it is common practice to use hydraulic cylinders instead of pneumatic cylinders.

Accordingly, most injection moulding machines are hydraulically operated and have additional valves and progranuning for mould actuation. Electric moulding machines have the option of a hydraulic power pack for performing some mould functions.

For safety reasons hydraulic pressures are normally limited to less than 230 bar, and most are less than 200 bar. There are also concerns over potential leakage of hydraulic fluid, which could cause product contamination.

In another sector of the injection moulding industry, gas assisted moulding (GAM) is used. This applies a highly pressurized controlled injection of gas to form continuous hollow channels in the thicker sections of a moulding. The gas is injected directly into the mould cavity at very high pressure, directly to act on the molten material and urge it against the moulding surfaces. Gas assisted moulding (GAM) typically uses nitrogen gas pressurised to 670 bar or more to make hollow mouldings and most moulding companies in automotive, teletronics and large packaging containers commonly use GAM. By virtue of its use in GAM, high pressure nitrogen gas is accepted by the plastics industry as safe and free from contamination.

The present invention accordingly provides an injection moulding apparatus for injection moulding an article, the apparatus comprising: first and second mould parts of an injection mould which are adapted to be connected together in a fully closed configuration so as to define a mould cavity therebetween for moulding an article, in the fully closed configuration the first and second mould parts defining a cavity outer surface which defines the outer shape of the article to be moulded in the mould cavity, an injector for injecting into the mould cavity molten material to be moulded into the article, at least one movable portion of one of the first and second mould parts being movable so as to vaiy the volume of the mould cavity in the fully closed configuTation, and an actuator for controlling the movement of the at least one portion of one of the first and second mould parts, the actuator being operated by a pressurised gas.

Preferably, the article is a hollow article having a base and a sidewall, and the mould cavity has a base-fonning portion and a sidewall-forming portion for respectively forming a base and a sidewall of an article to be moulded.

In a preferred embodiment, the pressurised gas actuator is a gas spring adapted selectively to permit the at least one movable portion to move in a first direction under a filling pressure in the cavity and to urge the at least one movable portion to move in a second opposite direction under an applied gas pressure.

The apparatus may further comprise a switching valve adapted selectively to connect different gas pressures to the gas spring.

More preferably, the gas spring is adapted selectively to permit the at least one movable portion to move in the first direction when supplied with a relatively low gas pressure and to urge the at least one movable portion to move in the second direction when supplied with a relatively high gas pressure.

The apparatus may further comprise a switch mechanism adapted to be switched by the at least one movable portion to control the supply of gas pressure to the gas spring.

Preferably, the switch mechanism is adapted to cause reversal of the direction of movement of the at least one movable portion.

Preferably, the switch mechanism comprises a first switch to detect a first particular rearward position of the at least one movable portion.

More preferably, the first switch is adapted to terminate the injection by the injector.

The switch mechanism may further comprise a timer that is triggered by operation of the first switch and the timer is arranged to control the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.

Alternatively, the switch mechanism may further comprise a second switch to detect a second particular rearward position of the at least one movable portion, more rearward than the first rearward position, and the second switch is arranged to control the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.

Preferably the injector opens into the base portion of the mould cavity and the at east one movable portion is a core or portion thereof.

The present invention further provides a method of injection moulding an article, the method comprising the steps of: (a) providing an injection mould comprising first and second mould parts, and having at least one movable portion of one of the first and second mould parts; (b) disposing the first and second mould parts in a fully closed configuration so as to define a mould cavity therebetween for moulding an article, in the fully closed configuration the first and second mould parts defining a cavity outer surface which defines the outer shape of the article to be moulded in the mould cavity; (c) injecting molten material into the cavity at an injection inlet of the cavity; (d) moving at least one movable portion of one of the first and second mould parts thereby to vary the volume of the mould cavity in the fully closed configuration; and (e) controlling the movement of the at least one movable portion of one of the first and second mould parts by an actuator which is operated by a pressurised gas.

Preferably, the injecting step (c) commences after disposing step (b) has commenced but before the first and second mould parts are in a fully closed configuration. This can reduce the cycle time of the injection moulding process.

Preferably, the article is a hollow article having a base and a sidewall, the mould cavity has a base-fonning portion and a sidewall-forming portion for respectively forming a base and a sidewall of an article to be moulded.

In one embodiment, in step (d) the at least one movable portion is moved in a first direction away from the injection inlet during the injection of molten material and further comprising a subsequent step (f) of moving the at least one movable portion in a second opposite direction towards the injection inlet after the injection of molten material thereby to mould the article to a predetermined shape.

Preferably, the pressurised gas actuator is a gas spring adapted selectively to permit the at least one movable portion to move in step (d) in the first direction under a filling pressure in the cavity and to urge the at least one movable portion to move in step (f) in the second direction under an applied gas pressure.

The method may further comprise selectively connecting different gas pressures to the gas spring.

Preferably, the gas spring is selectively supplied with a relatively low gas pressure to permit the at least one movable portion to move in the first direction and supplied with a relatively high gas pressure to urge the at least one movable portion to move in the second direction.

Preferably, the relatively low gas pressure is up to 150 bars and the relatively high gas pressure is up to 350 bars. More preferably, the relatively low gas pressure is from 25 to 150 bars and the relatively high gas pressure is from 100 to 350 bars.

Preferably, the gas is nitrogen.

The method may further comprise controlling the supply of gas pressure to the gas spring by a switch mechanism which is switched by movement of the at least one movable portion in the first direction.

Preferably, the switching of the switch mechanism causes reversal of the direction of movement into the second direction of the at least one movable portion.

The direction of movement may be reversed after a preset distance of movement of the at least one movable portion in the first direction subsequent to the switching of the switch mechanism.

Alternatively, the direction of movement may be reversed after a preset time period subsequent to the switching of the switch mechanism.

Preferably, the switch mechanism comprises a first switch which detects a particular rearward position of the at least one movable portion.

Preferably, the first switch terminates the injection by the injector.

In one arrangement, the switch mechanism further comprises a timer that is triggered by operation of the first switch and the timer controls the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.

In another arrangement, the switch mechanism further comprises a second switch that detects a second particular rearward position of the at least one movable portion, more rearward than the first rearward position, and the second switch controls the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.

Preferably, the injector opens into the base portion of the mould cavity and the at least one movable portion is a core or portion thereof Preferably, the injection mould has a plurality of mould cavities each defined by a respective pair of the first and second mould parts, and each cavity having a respective injector and a respective actuator.

This invention in one particular embodiment employs a mould in an injection moulding machine in which the filling pressure of the injected material to be moulded is controlled by moving the core, or a part of the core, away from the injection gate.

Alternatively, it is possible to move the cavity, or part the cavity, in the same way. In either case, this movement of a mould part can vary the volume of the base portion of the mould cavity, and increase the base thickness opposite the gate, or close to the gate, and thereby reduce the filling pressure of the injected material.

A high pressure gas, preferably nitrogen but other gases, such as air, could alternatively be employed, may be utilised to move the core or core part backs towards the injection gate, in order to overcome the filling pressure of the injected material, and other forces that may be present within the mould cavity.

In accordance with this aspect of the invention, a high-pressure gas is employed to perform mould functions where one or more cavity parts, for example the core or part thereof, need to move against the forces present within the mould cavity during and after filling with molten plastic material.

In accordance with a further aspect of the invention, the moving cavity part is used to reduce the filling pressure of the molten plastic material within the cavity. The mould part can be configured to operate a switch when the desired amount of plastic material has entered the cavity through a valve gate, which material filling in turn causes movement of the cavity part away from the incoming material. The switch signals the valve gate to close, thereby shutting off the flow of molten plastic material, after the cavity part has moved sufficiently away from the valve gate to define a desired enlarged mould cavity volume. A time delay is then employed to allow the valve gate to close before the forward force is applied to the cavity part to reduce the mould cavity volume to the defined volume for moulding the desired article. This time delay can be provided by a timer, or may alternatively be achieved by using two linearly separated switches or a linear transducer, which is or are configured to permit the moving cavity part to continue to move rearwardly for a preset time period or distance until the moving cavity part is triggered to reverse its direction of movement and move forwardly to compress the molten plastic material within the cavity.

A gas spring is employed to control the cavity part movement, by providing at least two gas pressures. A first pressure in the gas spring is set low to allow the moving cavity part to be pushed rearwardly away from the incoming material during the filling phase.

The movement triggers the switch and shuts the valve gate. After a short time delay or a second switch position, a second higher pressure in the gas spring is applied forcing the cavity part forward to complete the cavity fill and packing of the molten plastic material.

The switches are adjustable and may be set after trial and error to establish the correct cavity volume. It may be possible to accurately calculate the switch positions when the material and processing properties, such as the material grade, manufacturer and process temperature, are known.

In an alternative embodiment, the mould is provided with a gate that does not have a valve. With such a gate, there is no need to provide a delay between rearward and subsequent forward movements of the moving cavity part. Accordingly, the control mechanism for controlling the gas spring to control the cavity part movement needs only one switch and no delay, because the single switch can trigger substantially instantaneous switching between the first, lower, pressure and the second, higher, pressure. Any excess molten plastic material within the cavity would be forced out of the gate against the pressure in front of the machine screw, leaving the required amount of molten plastic material within the cavity of predetermined volume.

Each cavity of a multi cavity mould would be separately switched, where valve gates are used the switches would control the corresponding vale gate to that cavity. This will cause a random firing order over a narrow time period.

In a further embodiment of the present invention, the moving cavity part may be used to apply force to an area of the cavity after filling, in particular to displace molten material to be moulded within the cavity. Such a process is known in the art as "coining". In particular, in such a coining embodiment a high-pressure gas spring is used as an actuator for such a moving cavity part. A device to move the cavity part back after the previous cycle, ready for the next application, is also provided. Switches are not needed, only a time delay after start of injection or a pressure transducer installed near end of flow, or an infrared sensor installed near end of flow. These trigger the high-pressure gas (or hydraulic) to force the cavity part to displace material.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-section through an injection mould having a movable core in accordance with a first embodiment of the present invention; and Figure 2 is an enlarged schematic cross-section through a gate of the injection mould of Figure 1.

Referring to Figure 1, there is shown an injection mould (2) in accordance with a first embodiment of the present invention for injection moulding a preform, for example from polyester, particularly polyethylene terephthalate (PET), for subsequent blo w moulding to form a container. The preform is a hollow body and has a base and a sidewall. The injected material to be moulded is injected through a feed nozzle (4) in a first back plate (6) of the injection mould (2). A cavity plate (8) is adjacent to the first back plate (6) and defines an injection mould cavity (10). A gate (5) of the feed nozzle (4) opens into the cavity (10). The cavity plate (8) forms an outer surface (12) of the cavity (10) which in use defines the outer shape of the article to be injection moulded.

A plurality of neck splits (14) is provided at the end (15) of the cavity (10) remote from the feed nozzle (4). The neck splits (14) are shaped to mould the outer shape of one end of the article to be injection moulded (in this embodiment the neck finish of a preform for subsequent blow moulding to form a bottle). The neck splits (14) also support the injection moulded article as it is removed from the cavity (10) after the injection moulded material has solidified.

A core bearing (16) is adjacent to the plurality of neck splits (14) and has a central bore (IS) in which an elongate core (20) is slidably received. The elongate core (20) can be translated in a longitudinal direction coaxial with the axis of the cavity (10) and with the feed nozzle (4). Accordingly, the core (20) can selectively be slid in the core bearing (16) forwardly in a direction into the cavity (10) towards the feed nozzle (4) or rearwardly in a direction out of the cavity (10) away from the feed nozzle (4). Such forward and backward movement can vary the distance of the free end (22) of the core (20) from the feed nozzle (4). The core bearing (16) is received within an annular core bearing support (24).

The neck splits (14) have a tapering male portion (25) (which is typically frustoconical) which is fitted into a complementary tapering female portion (26) in the cavity plate (8). Correspondingly, the core bearing (16) has a tapering male portion (28) (which is typically frustoconical) which is fitted into a complementary tapering female portion (30) in the neck splits (14). Thereby the core (20) and the neck splits (14) are axially centered with respect to the axis of the cavity (10).

A pressure plate assembly (32) is fitted to the end (34) of the core (20) remote from the free end (22) that is within the cavity (10). The pressure plate assembly (32) is axially fixed relative to the core (20) so that longitudinal movement of the pressure plate assembly (32) correspondingly causes longitudinal movement of the core (20) within the cavity (10). The pressure plate assembly (32) comprises a pair of adjacent plate members (35, 36) between which the end (34) of the core (20) is locked. The end (34) of the core (20) includes an annular flange (38) that is radially outwardly directed relative to the cylindrical outer surface (40) of the remainder of the core (20). A planar end plate member (35) of the pressure plate assembly (32) is disposed against the transverse end surface (39) of the flange (38) and an intermediate plate member (36) of the pressure plate assembly (32) has a central hole (40) therein in which the end (34) of the core (20), including the flange (38), is snugly received, so that the flange (38), and thereby the core (20), is captive in the pressure plate assembly (32). The pressure plate assembly (32) is disposed within a chamber (42) defined by an annular housing (44) adjacent to the core bearing support (24).

A pneumatic piston and cylinder assembly (45) is mounted adjacent to the housing (44). The piston (46) in the pneumatic piston and cylinder assembly (45) is mounted for translational forward and backward longitudinal movement along a direction coaxial with the axis of the core (20), and bears against an end face (47) of the pressure plate assembly (32). The cylinder (48) of the pneumatic piston and cylinder assembly (45) includes a pneumatic chamber (50) having a fluid inlet (52) for selective connection to two sources of pressurised pneumatic fluid (51, 53), each at a respective pressure via a switching valve (54) One source of pressurised pneumatic fluid (51) is a relatively low pressure, whereas the other source of pressurised pneumatic fluid (53) is a relatively high pressure. The relatively low gas pressure may be up to 150 bars and the relatively high gas pressure may be up to 350 bars, or even higher, using nitrogen pressures such as those used for gas assisted moulding (GAM) (670 bar or higher). Typically, the relatively low pressure is from 25 to 150 bars and the relatively high gas pressure is from 100 to 350 bars.

The pressurised pneumatic fluid (51) is preferably nitrogen gas, but it may comprise another gas or mixture of gases, for example air.

A transducer (not shown) can be provided to measure the pressure of the pneumatic fluid in the pneumatic chamber (50).

Accordingly, the pneumatic fluid in the pneumatic chamber (50) can be selectively pressurised by connection to the respective source of pressurised pneumatic fluid (51, 53). As described in detail hereinbelow, during one early phase of the injection moulding cycle the pneumatic fluid is at the relatively low pneumatic pressure and the pneumatic piston (46) can be urged by the injection pressure, which is higher than the relatively low pneumatic pressure, in a direction inwardly of the pneumatic chamber (50). This permits the core (20) to move back smoothly against the injection pressure, thereby to keep the injection pressure at a low value. When the injection pressure in the cavity (10) applies a rearward force on the core (20) that is greater than the forward force on the core (20) as a result of the applied pneumatic pressure on the pressure plate assembly (32), the core (20) is urged rearwardly in a direction out of the cavity (10) away from the feed nozzle (4).

In contrast, during one later phase of the injection moulding cycle the pneumatic fluid is at the relatively high pneumatic pressure and the pneumatic piston (46) can be urged by the relatively high pneumatic pressure, which is higher than the injection pressure, in a direction outwardly of the pneumatic chamber (50). This latter movement in turn urges the core (20), via the pressure plate assembly (32), forwardly in a direction into the cavity (10) towards the feed nozzle (4), subject to the forward pressure applied to the core (20) overcoming any reverse injection pressure in the cavity (10).

Two switches (60, 62) and respective switch actuators (64, 66) are provided in conjunction with the pressure plate assembly (32). The switches (60, 62) and the respective switch actuators (64, 66) control the switching of the pneumatic pressure between the relatively low and high pneumatic pressures from the respective sources (51, 53) via the switching valve (54) and control the valve gate. One switch (60) initially closes the valve gate, and the other switch (62) subsequently controls the switching valve (54), as described hereinafter.

The switches (60, 62) are fitted to the static pneumatic piston and cylinder assembly (45). The switch actuators (64, 66) are fitted to the movable pressure plate assembly (32). The switch actuators (64, 66) are adapted or arranged to actuate the respective switch (60, 62) at a respective translational position of the movable pressure plate assembly (32). In the illustrated embodiment, as the movable pressure plate assembly (32) moves rearwardly from an initial forward position of the core (20), the first switch (60) is actuated by the first switch actuator (64). Thereafter, as the movable pressure plate assembly (32) progressively moves further rearwardly, the second switch (62) is actuated by the second switch actuator (66).

The switches (60, 62) and respective switch actuators (64, 66) can be employed to provide a dynamic control of the pneumatic pressure, and thereby the movement and position of both the piston (46) and the core (20). In this way, the position of the core (20) relative to the cavity (10) is dynamically controlled in order to maintain a specific pressure condition within the injection mould cavity (10) during the injection moulding process.

The switch actuators (64, 66) are adjustable, whereby the position of the core (20) relative to the cavity (10) at which the respective switch (60, 62) is switched can readily be varied by adjustment of the actuator (64, 66). In the illustrated embodiment the switch actuators (64, 66) comprise threaded nuts on elongate bolts, with the longitudinal position of the nut on the respective bolt determining the actuating position of the actuator on the respective switch.

When the core (20) is moved forward against the injected material, the melt pressure maintained by the injection system (not shown) supplying molten plastics material to the feed nozzle (4) is increased to balance the increasing pressure within the mould cavity (10). This causes either more material to be injected into the cavity (10) to help completely fill the mould cavity (10), or some of the material to move back though the gate (5) and the feed nozzle (4) if the cavity pressure is higher than the injection pressure within the feed nozzle (4).

In use, the injection mould (2) is kept in a closed configuration -the back plate (6) and the pneumatic cylinder (48) of the pneumatic piston and cylinder assembly (45) are disposed between two platens (57, 58) of an injection moulding machine. The platens (57, 58) are urged together throughout the entire injection moulding process so that the mould cavity is completely closed, even though the core (20) can move to vary the volume of the mould cavity. This retains the core bearing (16), neck splits (14) and cavity plate (8) engaged together until the injected material has solidified sufficiently within the cavity (10) to permit the injection moulded article safely to be ejected therefrom. This is achieved by opening up of the cavity (10) and removal of the injection moulded article supported on the neck splits (14), without any subsequent significant, e.g. greater than about 5%, change in shape or dimensions (for example post-moulding shrinkage).

The operation of the injection mould of Figure 1 over the course of a single injection moulding cycle is now described. Throughout the entire injection moulding cycle, subject to the option of commencing injection of the material just before the operation of closing the mould is terminated, the injection mould is fully closed, and the platens (57, 58) are urged together under a predetermined clamp pressure throughout the entire injection moulding process. However, the core (20) is permitted to move under dynamic control to vary the volume of the cavity (10).

Initially, the mould is caused to close. The pneumatic fluid in the pneumatic chamber (50) is selectively pressurized to the relatively low pressure by connection to the respective low pressure source of pressurised pneumatic fluid (51).

In one embodiment, after closing the mould and a full locking force is applied, the valve gate (4) is opened to start material injection. In an alternative embodiment, just before the mould is fully closed, the valve gate (4) is opened and to start material injection whilst the closing operation is completed and the full locking force applied.

In a first phase, the material is injected through the gate (4) at a constant volumetric rate. In the gate (4), the material is maintained at a constant injection pressure controlled by the machine. As the material is injected through the gate (4), the injection of the material pushes the core (20) back against the gas spring constituted by the pneumatic piston and cylinder assembly (45) when the pneumatic gas is at low pressure. The pressure is set at a value that allows the material to move the core (20) with minimal ingress up the sides of the core (20).

Correspondingly, the core pressure falls as the core (20) is moved rearwardly away from the gate (4), since the rearward movement of the core (20) tends to lower the core pressure.

The first switch (60) and its corresponding actuator (64) are set to switch the operation of the valve gate (4) thereby to initiate the valve gate (4) to close when there is sufficient material in the cavity to form a complete moulding. It is assumed that material will continue to flow until the valve gate (4) has closed but material inflow may be terminated by accurate control of the moulding machine (aithougli this is unlikely to be effective in a multi cavity mould).

Accordingly, the switch (60) acts to controllably close the valve gate (4) with the gas spring at its lower pneumatic pressure. As shown in Figure 2, typically the valve gate (4) has valve pins (80, 82) with two diameters. It is particularly important to close such a valve gate with the gas spring at its lower pneumatic pressure, otherwise the subsequent change to higher pneumatic pressure would act on the larger pin diameter, thereby dramatically increasing the force required to close the gate (4).

The second switch (62) and its corresponding actuator (66) are set to be actuated after the first switch (60) and its corresponding actuator (64) to allow the valve gate (4) to close at the lower pneumatic pressure. This may alternatively be achieved by timer instead of the second switch/actuator, the timer measuring a preset time delay after actuation of the first switch (60).

When the second switch (62) is switched, or after a preset time delay, valve (53) is actuated to cause the pneumatic gas pressure to be raised to the higher level. A second phase of the moulding cycle is initiated. The elevated pneumatic pressure forces the core (20) forward to displace the material up the sidewalls to complete the filling and packing of the cavity. The pneumatic gas flow rate, and therefore the material pressure increase, is controlled to match the optimum compression speed/pressure to suit the cavity configuration and the flow characteristics of the material. It is important not to allow the core (20) to stop moving, as otherwise this would cause a shudder line that would stay in the memory of the moulding and may affect its performance. A smooth continuous transition from backward to forward movement of the core (20) is preferred.

Thereafter, in final third phase the material is allowed to solidify and to cool sufficiently to allow removal from the cavity. After a cooling time has expired, the mould opens and the moulding is ejected by the use of the neck splits being forced forward by the ejector system of the injection-moulding machine. This completes the cycle.

The present inventor has found that the method and injection mould of the present invention employing switched gas pressures for controlling movement of the core in order to control injection pressure within the cavity can yield very significant technical advantages as compared to known methods and moulds. In particular, a mould having a 50mm free core stroke with a switch set at 10mm was employed. When the core was pushed back under low pressure, it triggered the switch, and this initiated the high pressure nitrogen to force the core forward. However, the core continued to move back to 35mm before moving forward. This was due to the time delay for the pressure to build up in the gas spring. The high core pressure was balanced against the holding injection pressure of the machine, set at 185 bar (whereas a normal pressure for preforms is 300 bar). The preforms fabricated had no measurable stress, and were of very high quality and uniformity, with a very thin (2mm thick) sidewall and a very thin (1.5mm thick) base.

The core action was such that after the rearward movement of the core triggered the first switch the core continued backwards against low gas pressure until the high gas pressure took over and pushed the core forwardly in the opposite direction. The valve gate was left open to allow the excess material to back flow against the 185 bar holding pressure until the core had moved fully forward. This meant the injection moulding machine was used as a pressure relief valve to stop over packing and to minimise the packing pressure. This mould had only one switch to control the switching valve to switch the nitrogen gas pressure from low to high at the preset distance of movement of the core. Since no gate valve was present, there was no need for a second switch or a timer, to control the operation of a gate valve.

The net result is an injection moulding technology that can reduce the weight of, for example, a polyethylene terephthalate (PET) bottle preform of any constuction by as much as 5%.

In alternative embodiments of the present invention that employ multi-cavity moulds, corresponding respective switches/actuators are provided that are correlated to the individual cavity in order to actuate the valve gate correlated to that cavity. This allows each cavity to self regulate the amount of material that is allowed to enter the cavity.

This elevates the need for finely balanced hot runner material distribution systems.

High cavitation moulds will have a random "firing order" as each cavity closes each valve gate to suit its own needs.

One problem with injection compression moulding (1CM) and multi-cavity moulds is the lack of resistance against the material being injected into the cavities, this why some multi-cavity moulds use shooting pots to predetermine the material dose.

Resistance is needed for the hot runner distribution system to achieve pressure balance during injection. Low resistance causes the hot runner to become unbalanced as the material takes the route of least resistance. Even when the hot runner is "perfectly" balanced with all run lengths being equal and all gates are identical, slight differences in temperature will cause the material to start flowing though one gate first. The first cavity to start filling is the last to complete filling due to the material cooling as it enters the cold cavity. This causes the pressure in that cavity to increase and therefore this directs the flow to the next gate that starts to flow. This pattern continues until the pressure has reached a level that re-starts the flow in the first cavity.

The above problem is know to those skilled in the art of injection-moulding but is not general knowledge, with most believing that molten material flows like water and then cools to become a solid.

When a conventional, long flow path, cavity is filled the material flows using a flow mechanism known as "fountain" flow. The surface skins are laid against the walls of the core and cavity and the material flows through the central region between these opposed surface skins. The skins act as an insulator and the material takes the hottest path. This is similar to a lava flow from a volcano when a crust is formed. If the material's molten front become too cold (freezing) it starts to "plug" flow with the pressure building up behind it and shunting it forward in short bursts. This is highly undesirable as it causes shudder lines in the memory of the material that are also visible and increases stress in the moulding.

In accordance with the present invention, allowing the core, or cavity, to move away from the incoming material in a high aspect ratio moulding such as a PET preform or deep container, helps overcome the problem of material ingress up the sidewall before the start of compression. As long as the material ingress is less than the core movement, the movement itself will keep the inner skin moving relative to the outer skin, which will prevent "freezing" by keeping the material moving.

This is all relative to the wall section and the flow characteristics of the material. Thin cups would need fast backward and forward compression movement to prevent this "freezing" phenomenon.

Having established that some ingress is acceptable, it is understandable that a multi-cavity mould can pressure balance against the cavity pressure, as the cores are moving away from the incoming flow. This is only practicable on higher aspect ratio mouldings where there is a substantial difference between the material in the displaced area and the sidewall.

When designing a multi-cavity mould for a PET preform, or a closure, it is important to take into consideration the above. Groups of four, six or eight cavities should be treated as a sub-mould with common gas springs behind a plate, so that the grouped cores are mounted to with the relevant group of valve gates switched by the plate movement.

The smaller the group number, the more accurate the flow control. Therefore for some mouldings all the cavities could be in one group with only one switching system controlling all the valve gates and one set of gas springs plumbed into common gas lines.

An alternative to this method is to allow all the cores to move back to a pre-determined position. This would be calculated based on the total cavity volume being equal to the volume created by the core moving back, the volume between the end of the core and the cavity base with some ingress up the side wall. Sidewall ingress should be sufficient to raise the cavity fill pressure to assist hot runner balance. Pressure transducers sensing cavity pressure could switch the valve gates or core position switches could be used.

When allowing ingress up the sidewall it is important not to allow a hesitation of more than a small percentage of the filling time before advancing the core. This would prevent the melt front freezing -if this were allowed to happen it would cause the material to burst through when the core is advanced, leaving an undesirable witness mark on the mouldings.

It will be apparent to those skilled in the art that when setting up the process parameters for a moulding process, the melt temperatures, injection pressures and speed profile for the injection moulding machine can readily be adjusted to set the core movements, pressures and speeds to achieve the technical effects described herein. In turn the core movements and pressures would be selected to suit the melt temperatures, injection pressures and speed profile of the injection moulding machine. Trial and error would readily be used to establish the best combination to suit the mouldings being produced on any given machine. Core movement distance and valve gate switching, pressure or position, can readily be adjusted to determine the optimum settings.

The valve gate actuator cylinder may use pressurised nitrogen instead of compressed air. The purpose of this is to improve the reaction time of the valve gate and increase the available force from the standard valve gate actuator cylinder. By increasing the operating pressure in the valve gate actuator cylinder from 10 bar pneumatic to 25 bar nitrogen, this can correspondingly increase the valve closing force by a ratio of typically 2.5:1.

The exhausted nitrogen gas from the pneumatic system is re-compressed to the highest pressure being used (100 to 350 bar) using a high-pressure pump and stored in a suitable high-pressure vessel to be drawn from as required. The gas could be released to atmosphere but this would not be energy efficient.

This technology is applicable to the manufacture of all injection-moulded containers from test tubes to storage bins with reasonably symmetrical sidewall geometry, with any foot print shape (base area) and any number of mould cavities.

It is within the scope of the technology for flatter mouldings such as lids, lenses and compact disks to be manufactured because the switch position can be adjusted to allow the continuous flow of material across the faces of the core and cavity.

Allowing the core to move back as the material enters the cavity greatly reduces the filling pressure. As the contact area increases then so does the hydraulic force applied by the incoming material. This reacts against the low gas pressure in the gas spring, which in turn allows the core to move back until it triggers the switches.

An important lechnical feature is the smooth transition from "core back" to "core forward", eliminating flow "shudder" lines. Also the reduction in injection pressure allowed by the core movement greatly reduces the required clamp force.

Using pressunsed nitrogen gas as the energy source allows for very high-speed compression movement, because the valve and pipe bore size can be considerably smaller than that required for hydraulic oil.

For very thin walled mouldings with long flow paths it is essential to achieve very high-speed compression movement, and in particular speeds that would be very difficult and very expensive to achieve with hydraulics and impossible to achieve using an injection-moulding machine with open mould technology. If the compression speed is too slow for the moulding, this would dramatically increase the pressure required to displace the material causing very high cavity pressure (known in the art as "pack pressure"). This would create an over pack condition that on a shallow tapered container would cause the mould to lock together and resist opening, or cause the mould to open during the compression force as the internal pressure overcomes the machine locking force.

Although switches and actuators are disclosed in the illustrated embodiment, switching can be achieved in a number of ways including the use of transducers; any suitable switching device detecting movement, pressure or time, for example, may be used, as would be apparent to those skilled in the art.

A programmable control unit could be used to coordinate all the switches, transducers and solenoid valves used for the system.

There may be mould configurations in accordance with the present invention where it would be necessary to provide an additional drive mechanism to retract the mould part.

If the movable mould part has high mass and/or mechanical drag, because the pressure of the injected material may not be sufficient alone to overcome this mass and mechanical drag to cause rearward movement of the mould part, at least during the initial phase of the injection, the additional drive mechanism can be employed at least to initiate retraction of the mould part. This can be achieved by using an actuator rather than a gas spring and applying gas pressure to the return side.

There may also be mould configurations in accordance with the present invention in which the valve gate is closed after the core has moved fully forward, and optionally also after the machine holding time at the high pressure (e.g. 185 bar) has been commenced. This can be controlled automatically from the machine controller. This configuration would however only work for single impression moulds.

It would be readily apparent to the skilled person knowledgeable of injection moulding methods and apparatus how to apply these general principles to a specific mould configuration to manufacture a specific injection moulded article using the methods and injection moulds of the present invention disclosed herein. In particular, although a movable core, opposite a gate, is disclosed in the illustrated embodiment, in alternative embodiments of the present invention a movable cavity part, or movable part of a fixed core is provided that, as described herein, can be reciprocated backwardly from a gate to reduce the pressure on the injected molted material and then urged forwardly to compress the molten material into the desired shape and configuration within the mould cavity.

For the avoidance of doubt, it should be understood that various features of the illustrated embodiments may be used interchangeably, and that other embodiments of the present invention may be provided using one or more combined features from two or more different embodiments.

Although various embodiments of the present invention have been described in detail, it will be apparent to those skilled in the art that other modifications of the injection mould and the injection moulding process may be employed that are within the scope of the invention as defined in the appended claims.

Claims

Claims 1. An injection moulding apparatus for injection moulding an article, the apparatus comprising: first and second mould parts of an injection mould which are adapted to be connected together in a fully closed configuration so as to define a mould cavity therebetween for moulding an article, in the fully closed configuration the first and second mould parts defining a cavity outer surface which defines the outer shape of the article to be moulded in the mould cavity, an injector for injecting into the mould cavity molten material to be moulded into the article, at least one movable portion of one of the first and second mould parts being movable so as to vary the volume of the mould cavity in the fully closed configuration, and an actuator for controlling the movement of the at least one portion of one of the first and second mould parts, the actuator being operated by a pressurised gas.
2. An apparatus according to claim 1 wherein the article is a hollow article having a base and a sidewall, and the mould cavity has a base-forming portion and a sidewall-forming portion for respectively forming a base and a sidewall of an article to be moulded.
3. An apparatus according to claim 1 or claim 2 wherein the pressurised gas actuator is a gas spring adapted selectively to permit the at least one movable portion to move in a first direction under a filling pressure in the cavity and to urge the at least one movable portion to move in a second opposite direction under an applied gas pressure.
4. An apparatus according to claim 3 further comprising a switching valve adapted selectively to connect different gas pressures to the gas spring.
5. An apparatus according to claim 4 wherein the gas spring is adapted selectively to permit the at least one movable portion to move in the first direction when supplied with a relatively low gas pressure and to urge the at least one movable portion to move in the second direction when supplied with a relatively high gas pressure.
6. An apparatus according to any one of claims 3 to 5 further comprising a switch mechanism adapted to be switched by the at least one movable portion to control the supply of gas pressure to the gas spring.
7. An apparatus according to claim 6 wherein the switch mechanism is adapted to cause reversal of the direction of movement of the at least one movable portion.
8. An apparatus according to claim 6 or claim 7 wherein the switch mechanism comprises a first switch to detect a first particular rearward position of the at least one movable portion.
9. An apparatus according to claim 8 wherein the first switch is adapted to terminate the injection by the injector.
10. An apparatus according to claim 9 wherein the switch mechanism further comprises a tinier that is triggered by operation of the first switch and the timer is arranged to control the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.
11. An apparatus according to claim 9 wherein the switch mechanism further comprises a second switch to detect a second particular rearward position of the at least one movable portion, more rearward than the first rearward position, and the second switch is arranged to control the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.
12. An apparatus according to any foregoing claim wherein the injector opens into the base portion of the mould cavity through a cavity part and the at least one movable portion is a core or portion thereof.
13. An apparatus according to any one of claims 1 to 11 wherein the injector opens into the base portion of the mould cavity through the core and the at least one movable portion is a cavity part or portion thereof.
14. An apparatus according to any foregoing claim wherein the injection mould has a plurality of mould cavities each defined by a respective pair of the first and second mould parts, and each cavity having a respective injector and a respective actuator.
15. A method of injection moulding an article, the method comprising the steps of: (a) providing an injection mould comprising first and second mould parts, and having at least one movable portion of one of the first and second mould parts; (b) disposing the first and second mould parts in a fully closed configuration so as to define a mould cavity therebetween for moulding an article, in the fully closed configuration the first and second mould parts defining a cavity outer surface which defines the outer shape of the article to be moulded in the mould cavity; (C) injecting molten material into the cavity at an injection inlet of the cavity; (d) moving at least one movable portion of one of the first and second mould parts thereby to vary the volume of the mould cavity in the fully closed configuration; and (e) controlling the movement of the at least one movable portion of one of the first and second mould parts by an actuator which is operated by a pressurised gas.
16. A method according to claim 14 wherein the injecting step (c) commences after disposing step (b) has commenced but before the first and second mould parts are in a fully closed configuration.
17. A method according to claim 15 or claim 16 wherein the article is a hollow article having a base and a sidewall, the mould cavity has a base-forming portion and a sidewall-forming portion for respectively forming a base and a sidewall of an article to be moulded.
18. A method according to any one of claims 15 to 17 wherein in step (d) the at least one movable portion is moved in a first direction away from the injection inlet during the injection of molten material and further comprising a subsequent step (f) of moving the at least one movable portion in a second opposite direction towards the injection inlet after the injection of molten material thereby to mould the article to a predetermined shape.
19. A method according to claim 18 wherein the pressurised gas actuator is a gas spring adapted selectively to permit the at least one movable portion to move in step (d) in the first direction under a filling pressure in the cavity and to urge the at least one movable portion to move in step (1) in the second direction under an applied gas pressure.
20. A method according to claim 19 further comprising selectively connecting different gas pressures to the gas spring.
21. A method according to claim 20 wherein the gas spring is selectively supplied with a relatively low gas pressure to permit the at least one movable portion to move in the first direction and supplied with a relatively high gas pressure to urge the at least one movable portion to move in the second direction.
22. A method according to claim 21 wherein the relatively low gas pressure is up to bars and the relatively high gas pressure is up to 350 bars.
23. A method according to claim 22 wherein the relatively low gas pressure is from to 150 bars and the relatively high gas pressure is from 100 to 350 bars.
24. A method according to any one of claims 19 to 23 wherein the gas is nitrogen.
25. A method according to any one of claims 19 to 24 further comprising controlling the supply of gas pressure to the gas spring by a switch mechanism which is switched by movement of the at least one movable portion in the first direction.
26. A method according to claim 25 wherein the switching of the switch mechanism causes reversal of the direction of movement into the second direction of the at least one movable portion.
27. A method according to claim 26 wherein the direction of movement is reversed after a preset distance of movement of the at least one movable portion in the first direction subsequent to the switching of the switch mechanism.
28. A method according to claim 23 wherein the direction of movement is reversed after a preset time period subsequent to the switching of the switch mechanism.
29. A method according to any one of claims 25 to 28 wherein the switch mechanism comprises a first switch which detects a particular rearward position of the at least one movable portion.
30. A method according to claim 29 wherein the first switch terminates the injection by the injector.
31. A method according to claim 30 wherein the switch mechanism further comprises a timer that is triggered by operation of the first switch and the timer controls the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.
32. A method according to claim 30 wherein the switch mechanism further comprises a second switch that detects a second particular rearward position of the at least one movable portion, more rearward than the first rearward position, and the second switch controls the switch mechanism to urge the at least one movable portion to move in the second direction under the applied gas pressure.
33. A method according to any one of claims 15 to 32 wherein the injector opens into the base portion of the mould cavity and the at least one movable portion is a core or portion thereof.
34. A method according to any one of claims 15 to 33 wherein the injection mould has a plurality of mould cavities each defined by a respective pair of the first and second mould parts, and each cavity having a respective injector and a respective actuator.
35. An injection moulding apparatus substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.
36. A method of injection moulding substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.