WO2016059358A1

WO2016059358A1 - Method and apparatus for injection moulding articles using pistons

Info

Publication number: WO2016059358A1
Application number: PCT/GB2014/053111
Authority: WO
Inventors: Christopher Whiteley
Original assignee: Oxford Plastic Systems Limited
Priority date: 2014-10-16
Filing date: 2014-10-16
Publication date: 2016-04-21

Abstract

There is provided a method of injection moulding an article. The method comprises supplying mouldable material (S1) into a supply chamber (21b), injecting mouldable material from the supply chamber (21b) into the mould via a delivery chamber using a first piston (21,a, S4), subsequently injecting mouldable material present in the delivery chamber into the mould and, during the injection of mouldable material into the mould using the second piston (21d,S6), the second piston causes the delivery chamber to be sealed off from the supply chamber (S5), allowing the mouldable material to solidify within the mould thereby forming the article and subsequently removing the article from the mould (S7). The method further comprises, when the second piston has caused the delivery chamber to be sealed off from the supply chamber, withdrawing the first piston (S8) and supplying further mouldable material into the supply chamber (S1).

Description

METHOD AND APPARATUS FOR INJECTION MOULDING ARTICLES USING PISTONS

The present invention relates to methods and apparatus for injection moulding and articles manufactured using injection moulding.

5 Injection moulding is a manufacturing process in which a pliable raw material is shaped into an article/item/product using a mould.

Figure 1 illustrates schematically a cross-section through a conventional injection moulding apparatus 10 that includes an injection unit 1 1 , a mould assembly 12, and a clamp/press 13. The injection unit 11 is provided by an extruder that comprises a heated barrel 11 a housing an0 extruder screw 11 b that is driven by a screw motor 1 1c. Mouldable material 12 is fed into the central bore of the extruder barrel 1 1a via a hopper 11 d, and can exit a distal end of the barrel 1 1a via a nozzle 1 1 e. The mould assembly 12 includes two mould sections 12a, 12b that when brought together (i.e. closed) define a cavity 12c, wherein the cavity 12c defines the shape of the article to be moulded. The first mould section 12a is provided with a sprue 12d through5 which mouldable material 14 is injected into the mould cavity 12c from the nozzle 11 e of the injection unit 1 1. The clamp/press 13 comprises a moveable platen 13a on which the second mould section 12b is mounted. The moveable platen 13a is driven by a clamp motor 13b via a number of tie bars 13c.

To manufacture an article using such a conventional injection moulding apparatus, the clamp0 motor 13b drives the moveable platen 13a towards the injection unit 1 1 , and thereby moves the second mould section 12b toward the first mould section 12a until the mould is closed. Raw material (usually in the form of pellets or granules of thermoplastic or thermosetting plastic) is then fed through the hopper 11 d into the central bore of the heated extruder barrel 11 a, and the extruder screw 1 1 b is rotated by the screw motor 1 1 c thereby forcing the raw material towards5 the nozzle 11 e.

The heat provided by the heated barrel 11 a together with the compression and shearing of the raw material by the rotation of the extruder screw 1 1 b causes the raw material to melt as it moves towards the nozzle 11 e. The melted raw material then accumulates adjacent to the nozzle 11 e, where it is held until an appropriate volume of material (referred to as a shot) has0 accumulated. When sufficient material has accumulated, the screw 11 b is used as an injection ram, and is driven towards the nozzle 1 1e to force the melted material through the nozzle 11 e into the closed mould. The mouldable material is then allowed to solidify within the mould (e.g. by cooling or curing) before the mould is opened and the shaped article is removed.

In order to manufacture products at a sufficient rate, and thereby make efficient use of such a conventional injection moulding apparatus, the above-described injection moulding process is performed at high rates of injection and high pressures. However, during the injection moulding process, it is vital that the clamp/press is able to prevent the mould sections from separating by holding the mould sections in the closed position, against the pressure of the injected mouldable material, so as to ensure that the mouldable material will fill the mould cavity without leaking and will therefore assume the desired shape. Consequently, by operating at high pressures, these conventional injection moulding apparatus must make use of more powerful clamps/presses than would otherwise be necessary, and the clamp/press therefore comprises a significant proportion of the size and cost of a conventional injection moulding apparatus.

It is therefore desirable to provide methods and apparatus for injection moulding that can operate at lower pressures, thereby reducing the required tonnage of the clamp/press, without compromising the speed of operation.

Therefore, according to a first aspect there is provided a method of injection moulding an article. The method comprises supplying mouldable material into a supply chamber, injecting mouldable material from the supply chamber into a mould via a delivery chamber using a first piston, subsequently injecting mouldable material present in the delivery chamber into the mould using a second piston, wherein during the injection of mouldable material into the mould using the second piston, the second piston causes the delivery chamber to be sealed off from the supply chamber. The method further comprises allowing the mouldable material to solidify within the mould thereby forming the article and subsequently removing the article from the mould, and when the second piston has caused the delivery chamber to be sealed off from the supply chamber, withdrawing the first piston and supplying further mouldable material into the supply chamber.

The first piston may inject a first volume of mouldable material whilst the second piston injects a second volume of mouldable material. The first volume can be between 60% and 90% of the total mouldable material injected, and the second volume can be between 40% and 10% of the total mouldable material injected. The first piston may force mouldable material through the delivery chamber from a first direction whilst the second piston forces mouldable material through the delivery chamber from a second direction, with the first direction being substantially perpendicular to the second direction.

The method may further comprise, prior to injecting mouldable material using the first piston, compressing the mouldable material in the supply chamber using the first piston in order to force any volatiles out of the mouldable material. When compressing the mouldable material in the supply chamber, the delivery chamber may be sealed off from the supply chamber by the second piston, and the second piston withdrawn prior to injecting mouldable material using the first piston. The step of supplying mouldable material into the supply chamber may cause the first piston to move towards a distal end of the supply chamber, and the rate at which mouldable material is supplied into the supply chamber may then be varied in dependence upon the rate at which the first piston moves towards the distal end of the supply chamber.

The step of supplying mouldable material into the supply chamber may cause the first piston to move towards a distal end of the supply chamber, and the step of supplying mouldable material into the supply chamber may then be continued until it is determined that the volume between the first piston and a proximal end of the supply chamber has reached a pre-defined volume.

The step of supplying mouldable material into the supply chamber may continue until it is determined that the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber. It may be determined that the pressure of the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber when movement of the first piston towards the distal end of the supply chamber is detected.

The injection of mouldable material into the mould may continue until it is determined that the volume of mouldable material injected is sufficient to fill the mould cavity. It may be determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting that a predefined volume has been injected by monitoring the distances moved by one or both of the first piston and the second piston. Alternatively, it may be determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting an increase in the force acting to push the second piston away from the mould. As a further alternative, the mould may comprise two mould sections that when brought together define the mould cavity, and it may then be determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting relative movement between the two mould sections.

According to a second aspect of the present invention there is provided moulded article manufactured using the method according to the first aspect. According to a third aspect of the present invention there is provided an injection moulding apparatus. The apparatus comprises a mould, a supply chamber configured to receive a supply of mouldable material, a delivery chamber having an inlet from the supply chamber and an outlet to the mould, a first piston configured to inject mouldable material from the supply chamber into the mould via a delivery chamber, and a second piston configured to inject mouldable material present in the delivery chamber into the mould and, during the injection of mouldable material by the second piston, to cause the delivery chamber to be sealed off from the supply chamber.

The apparatus may be configured such that the second piston seals off the delivery chamber from the supply chamber when the second piston is inserted into the delivery chamber. The first piston may be substantially perpendicular to the second piston.

The mould may comprise two or more mould sections, and the apparatus may then further comprises a press configured to open and close the two or more mould sections.

The apparatus may further comprise a feeder unit configured to feed mouldable material into the supply chamber at a location that is between a head of the first piston and a proximal end of the supply chamber.

The apparatus may further comprise a first piston sensor configured to monitor the rate at which the first piston moves towards a distal end of the supply chamber, and a feeder unit controller configured to vary the rate at which mouldable material is supplied into the supply chamber in dependence upon the rate at which the first piston moves towards the distal end of the supply chamber as indicated by an output of the first piston sensor.

The apparatus may further comprise a first piston sensor configured to detect that the first piston has moved a predefined distance towards a distal end of the supply chamber, and a feeder unit controller configured to stop the supply of mouldable material into the supply chamber when an output of the first piston sensor indicates that the first piston has moved a predefined distance towards a distal end of the supply chamber. The apparatus may further comprise a first piston sensor configured to detect when the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber, and a feeder unit controller configured to stop the supply of mouldable material into the supply chamber when an output of the first piston sensor indicates that the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber.

The apparatus may further comprise one or more sensors that are configured to detect that a predefined volume of mouldable material has been injected into the mould by monitoring the distances moved by one or both of the first piston and the second piston. The apparatus may further comprise a pressure sensor that is configured to detect an increase in the force acting to push the second piston away from the mould.

The mould may comprise two mould sections that when brought together define a mould cavity, and the apparatus may then further comprise a mould sensor that is configured to detect relative movement between the two mould sections. The mould may comprise two mould sections that when brought together define a mould cavity, and the apparatus may then further comprise a mould fastening mechanism on the mould that releasably fastens the mould sections to one another when in a closed position.

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

Figure 1 illustrates a conventional injection moulding apparatus;

Figure 2 illustrates an embodiment of an alternative injection moulding apparatus;

Figure 3a to 3de illustrates the injection moulding apparatus of Figure 2 during various stages of an injection moulding operation; and

Figure 4 is a flow diagram illustrating an injection moulding operation implementing using the apparatus of Figure 2; and

Figures 5a and 5b illustrates cross-sections through an embodiment of a mould comprising a first mould section and a second mould section

Referring now to Figure 2, there is shown an embodiment of an injection moulding apparatus that can operate at lower pressures, thereby reducing the required tonnage of the clamp/press, without compromising the speed of operation. In the embodiment of Figure 2, the injection moulding apparatus 20 comprises an injection unit 21 , a mould assembly 22, a clamp/press 23, and a feeder unit 24.

The mould assembly 22 includes two mould sections 22a, 22b that when brought together (i.e. the mould sections are moved into contact with one another) define a cavity 22c, wherein the cavity 22c defines the shape of the article to be moulded. The first mould section 22a is provided with a sprue 22d through which mouldable material 24 is injected into the mould cavity 22c. The clamp/press 23 comprises a moveable platen 23a on which the second mould section 22b is mounted. The moveable platen 23a is driven by a clamp motor 23b via a number of tie bars 23c. The clamp/press 23 is configured to open and close the mould, and to clamp/hold the mould against the pressure of the material injected into the mould so as to prevent the mould from opening. The injection unit 21 comprises a first piston 21 a housed within a first cylinder 21 b and driven by a first piston driver 21 c, and a second piston 21 d housed within a second cylinder 21 e and driven by a second piston driver 21f. The proximal ends of both the first cylinder 21 b and the second cylinder 21 e are connected to a common delivery chamber 21 g, and the common delivery chamber 21 g is provided with a nozzle 21 h through which material can pass into the mould 22 via the sprue 22d. In this regard, the first cylinder 21 b and the first piston 21 a are disposed perpendicularly relative to the nozzle 21 h and the sprue 22d, whilst the second cylinder 21e and the second piston 21 d are linearly aligned with the nozzle 21 h and the sprue 22d. The first cylinder 21 b and the first piston 21 a are therefore perpendicular to the second cylinder 21e and the second piston 21 d.

Raw material is fed into the injection unit 21 from the feeder unit 24. In the embodiment illustrated in Figure 2, the feeder unit 24 is provided by an extruder that comprises an extruder channel 24a that is connected to the first cylinder 21 b, and an extruder screw 24b within the extruder channel 24a that is driven by a screw motor 24c. The feeder unit 24 is configured such that mouldable material can be fed into the extruder channel 24a, and can exit a distal end of the channel 24a into the central bore of first cylinder 21 b between the head of the first piston 21a and the proximal end of the first cylinder 21 b.

Figure 4 is a flow diagram illustrating an injection moulding operation implementing using the apparatus described above. When manufacturing an article using the injection moulding apparatus of Figure 2, the apparatus 20 initially implements a priming phase. At the start of the priming phase, the first piston 21a is in a retracted position, whilst the second piston 21 d is in a fully inserted position so as to seal/close off the first cylinder 21 b from the delivery chamber 21g, as illustrated in Figure 3a. Mouldable material 25 is then fed into the first cylinder 21 b (S1), which therefore acts as a supply chamber for supplying material into the apparatus. For example, raw material can be fed through a hopper into the extruder channel 24a of the feeder unit 24. The extruder screw 24b would then be rotated in order to mix and force the material through the extruder channel 24a that feeds into the first cylinder 21 b/supply chamber.

In conventional injection moulding processes, this initial step of supplying mouldable material 25 into the injection moulding apparatus would typically involve running the extruder/feeder unit 24 at a constant speed for a pre-determined period of time, on the assumption that the amount of mouldable material 25 supplied by the extruder/feeder unit 24 would then be approximately correct and consistent on each occasion that the extruder/feeder unit 24 is run. However, depending upon the form of raw material used, the consistency/density of the material can be highly variable and inconsistent, such that merely running the extruder/feeder unit 24 at a constant speed for a pre-determined period of time will not provide an accurate and consistent volume of material. This is especially true when making use of recycled plastics as a raw material.

In order to overcome this problem, the injection moulding apparatus 20 of this embodiment is configured such that the outlet of the feeder unit 24 feeds mouldable material into the central bore of first cylinder 21 b between the head 21 i of the first piston 21 a and the proximal end of the first cylinder 21 b. In particular, the distal end of the extruder channel 24a joins the first cylinder 21 b between the head 21 i of the first piston 21a and the proximal end of the first cylinder 21 b. Consequently, as the feeder unit 24 supplies mouldable material into the central bore of first cylinder 21 b, the volume between the head of the first piston 21a and the proximal end of the first cylinder 21 b will fill with mouldable material such that eventually the pressure of mouldable material within the first cylinder 21 b will cause a force to act upon the head of the first piston 21 a that acts to push the first piston 21 a towards the distal end of the first cylinder 21 b.

The volume of mouldable material within the first cylinder 21 b can therefore be determined with sufficient accuracy by retracting the first piston 21 a to a position in which the volume of the first cylinder 21 b between the head of the first piston 21a and the proximal end of the first cylinder 21 b is equal to the desired volume of mouldable material, and then detecting when a force begins to act so as to push the first piston 21 a towards the distal end of the first cylinder 21 b, as this will be an indication that the volume between the head of the first piston 21a and the proximal end of the first cylinder 21 b is full of mouldable material.

In order to detect when a force begins to act so as to push the first piston 21a towards the distal end of the first cylinder 21 b, the injection moulding apparatus 20 of this embodiment is provided with a first piston sensor (not shown) that is configured to detect and/or measure movement of the first piston 21 a. By detecting movement of the first piston 21 a towards the distal end of the first cylinder 21 b the first piston sensor can detect when the volume between the head of the first piston 21 a and the proximal end of the first cylinder 21 b is full of mouldable material, and thereby detect when a pre-defined volume of mouldable material has been fed into the supply chamber/first cylinder 21 b. By way of example, the first piston sensor could be provided by a linear transducer/linear variable displacement transducer that is configured to measure the position of the first piston 21a.

The apparatus 20 can therefore also be provided with a feeder unit controller (not shown) that is configured to control the screw motor 24c. The output of the first piston sensor could therefore be provided as an input to the feeder unit controller which would be configured to stop the extruder when the output of the first sensor indicates that a pre-defined volume of mouldable material has been fed into the supply chamber/first cylinder 21 b. As an optional step, after a sufficient supply of mouldable material has been added to the supply chamber/first cylinder 21 b, the first piston 21a is then driven partially into the first cylinder 21 b until the material within the chamber can no longer be compressed (S2), as illustrated in Figure 3b. With the second piston 21d sealing off the first cylinder 21 b from the delivery chamber 21 g, this initial insertion of the first piston 21 a into the first cylinder 21 b provides a pre-compression step, before any material is injected into the mould, that degasses the mouldable material 25 by forcing any volatiles (e.g. air and other gases) out of the mouldable material and allowing these to escape from the open, distal end of the first cylinder 21 b (as illustrated by the arrows in Figure 3b).

Once the priming phase has been completed, the apparatus 20 implements an initial injection phase. In the initial injection phase, the second piston 21 d is retracted (S3) away from the mould assembly 22 towards the distal end of the second cylinder 21 e, thereby unsealing the supply chamber/first cylinder 21 b from the delivery chamber 21 g. An initial/first injection step is then implemented, as illustrated in Figure 3c, in which the first piston 21 a is driven into the first cylinder 21 b in order to force mouldable material 25 from the supply chamber/first cylinder 21 b into the mould via delivery chamber 21 g (S4). The volume of mouldable material that can be delivered into the mould during this initial injection step is defined by volume of material present in the supply chamber/first cylinder 21 b, and is sufficient to fill the most of the volume of the mould cavity, but is less than the total volume of material required. For example, this initial injection step will typically inject between 60% and 90% of the total mouldable material that is required.

After the initial injection phase has been completed, the apparatus 20 implements a final injection phase. In the final injection phase, a further/second injection step is implemented, as illustrated in Figure 3d, in which the second piston 21 d is driven into the delivery chamber 21g in order to force mouldable material 24 that is present in the delivery chamber 21g (i.e. following the initial injection step) into the mould (S5 and S6). The volume of mouldable material that can be delivered into the mould during this further injection step is defined by the volume of the delivery chamber and is sufficient to fill any voids remaining in the mould cavity following the initial injection step (S4). Consequently, the combined volume of material injected into the mould by the combination of the initial injection step (S4) and further injection step (S6) is 100% of the total mouldable material that is required.

The present inventors have also recognised that there are various approaches that could be implemented in order to determine that the volume of material that has been injected has filled the mould cavity.

In the most straightforward approach, given that the volume of mouldable material that is fed into the first cylinder 21 b can be determined with a high-degree of accuracy, as described above, it could be assumed that the mould cavity will be full once both the first piston 21 a and the second piston 21d have been drive into their corresponding cylinders to the fullest extent possible (i.e. until they reach the ends of their corresponding cylinders). However, this approach does not take into account the possible change in volume of the mouldable material present in the first cylinder 21 b as a result of the pre-compression step (S2) that degasses the mouldable material. Furthermore, in view of the possibility that the volume of mouldable material present in the first cylinder 21 b may decrease as a result of the pre-compression step, it may desirable to configure the apparatus 20 so that volume of raw material that is fed into the supply chamber/first cylinder during the priming phase is actually larger than the volume of mouldable material required to fill the mould cavity.

In order to overcome this problem, the first piston sensor of this embodiment could be further configured to monitor the distance moved by the first piston 21a during the initial injection phase, and a second piston sensor could be used to monitor the distance moved by the second piston 21 d during the final injection phase. By monitoring these distances, the total volume of mouldable material that has been injected into the mould cavity can be monitored (i.e. based on knowledge of the volumes of the first cylinder 21 b and the second cylinder 21e), and one or both of the injection steps can be stopped when it is determined that the volume of material is sufficient to fill the mould cavity.

Alternatively, a pressure sensor (not shown) could be provided on the second piston 21 d that is configured to detect a change in the pressure on the second piston 21d. In this regard, as the mould cavity eventually fills with mouldable material during the final injection phase the pressure of mouldable material within the delivery chamber 21 g/second cylinder 21e will cause a force to act upon the head 21 j of the second piston 21 d that acts to push the second piston 21 d towards the distal end of the second cylinder 21 e. Consequently, detecting an increase in the pressure on the second piston 21 d will be an indication that the mould cavity is full of mouldable material. By way of example, the pressure sensor could be configured to measure the pressure within the hydraulics of the second piston 21d as a means for detecting an increase in the pressure on the second piston 21d.

As a further alternative, the apparatus could be provided with a mould sensor (not shown) that is configured to detect any movement of the second mould section 22b relative to the first mould section 22a. Conventionally, relative movement between the two sections of a mould would be considered a fault, as this would mean that the mould is opening to leave a gap as the two sections separate from one another, such that mouldable material would leak from the mould cavity thereby preventing the material from accurately assuming the desired shape. However, the present inventors have recognised that the mould can be configured to allow a very small amount of relative movement (i.e. less than 1 mm) between the first mould section 22a and the second mould section 22b without the mould cavity opening to leave a gap. To do so, the edges of the first mould section 22a and the second mould section 22b should be configured to overlap when in the fully closed position, so that the mould cavity will remain enclosed (i.e. without any gaps) even if there is some relative movement between the first mould section 22a and the second mould section 22b, thereby ensuring that the material within mould cavity will still accurately assume the desired shape.

By way of example, Figures 5a and 5b illustrates cross-sections through an embodiment of a mould that is configured to allow a very small amount of relative movement between the first mould section 22a and the second mould section 22b without the mould cavity opening to leave a gap. In this embodiment, the external edges of the mould cavity of the second mould section 22b are configured with extensions 22e that form sheer overlapping portions with the first mould section 22a both when the mould is in the fully closed position (i.e. when contact between opposing surfaces of the mould sections prevent them moving any further towards one another) and when there has been a very small amount of relative movement between the first mould section 22a and the second mould section 22b. As illustrated in Figure 5b, even when there has been a very small amount of relative movement between the first mould section 22a and the second mould section 22b, the sheer overlapping portions provided by the extensions 22e at the edges of the mould sections prevent the mould cavity 22c from becoming open so as to leave a gap.

In an alternative embodiment, and depending upon the particular requirements of the article to be moulded, it may be desirable to configure the mould sections so that the extensions 22e at the edges of one of the mould sections are linearly aligned with a corresponding recess on the opposing mould recess, so that when the extensions 22e overlap with opposing mould recess the extensions 22e are located within a corresponding recess when

For a mould that is designed to form more complicated shapes than that illustrated in the example embodiment of Figures 5a and 5b, the mould sections 22a, 22b may be configured with internal projections for producing features, such as through holes, such that internal edges of the mould cavity will also require overlapping portions. In this case, any such internal projections on a mould section would be configured to extend into a corresponding, linearly aligned recess on the opposing mould section, so that the internal projection do not separate from the opposing mould section as a result of a very small amount of relative movement between the mould sections.

In the above described embodiments that make use of one or more sensors to determine when the mould cavity is full of mouldable material, the apparatus 20 can also be provided with a first piston controller (not shown) that is configured to control the first piston driver 21 c and/or a second piston controller (not shown) that is configured to control the second piston driver 21f. The output of the sensor(s) could then be provided as an input to the first piston controller/second piston controller, and the first piston controller/second piston controller would then be configured to stop the corresponding piston driver when the output of the sensor(s) indicates that the mould cavity is full.

By way example, if the apparatus 20 was configured to determine that the mould cavity is full using either a pressure sensor that is configured to detect a change in the pressure on the second piston 21 d or a mould sensor that is configured to detect any movement of the second mould section 22b relative to the first mould section 22a, then the output of the pressure sensor/mould sensor would be provided as an input to a second piston controller that is configured to control the second piston driver 21 f. The second piston controller would then be configured to stop the second piston driver 21f when the output of the pressure sensor/mould sensor indicates that the mould cavity is full. After the further/second injection step (S5 and S6) has been completed the mouldable material is then allowed to solidify within the mould (e.g. by cooling or curing) before the mould is opened and the shaped article is removed (S7).

During the final injection phase, as soon as the second piston 21 d is inserted far enough into the delivery chamber 21g to seal off the first cylinder 21 b from the delivery chamber 21 g (S5), the apparatus 20 begins implementing the next priming phase by retracting the first piston 21 a towards the proximal end of the first cylinder 21 b (S8). This next priming phase then continues concurrently with the remainder of the final injection phase. In particular, after the first piston 21a has been retracted, a further batch of mouldable material 25 is then fed into the first cylinder 21 b (S1) in preparation for the next injection moulding operation, as illustrated in Figure 3e. Therefore, during the time taken for the remainder of the final injection phase, including the second injection step (S6) and the subsequent solidification of the moulded article (S7), the apparatus is primed/made ready to perform a further injection moulding operation (i.e. by the implementation of step S1 and optional step S2 concurrently with steps S6 and S7). This is possible because, when the second piston 21d is inserted into the delivery chamber 21g, the second piston 21 d seals off the first cylinder 21 b from the delivery chamber 21 g. Consequently, the withdrawal of the first piston 21a and introduction of a further batch of mouldable material does not impact on the final stages of a preceding injection moulding operation.

The two-stage injection operation therefore allows the injection moulding process to be carried out a lower maximum pressure without compromising the speed of operation. In this regard, operating an injection moulding process at lower pressures generally results in slower injection of material into the mould, thereby lengthening the time required to mould an individual article. This is undesirable, as this decrease in the rate at which articles can be moulded impacts on the profitability of operating an injection moulding apparatus. However, the injection moulding apparatus and method described above provide that the initial stages of an injection moulding operation can occur concurrently with the final stages of a preceding injection moulding operation, such that lower pressures can be used without compromising the overall rate of production.

By providing an injection moulding process that can be implemented at lower pressures without compromising the overall rate of production, the injection moulding apparatus and method described above reduces the required tonnage of the clamp/press, thereby reducing the cost of the apparatus and consequently reducing the cost of injection moulding an article. Furthermore, the two-stage injection operation provides that the initial injection stage can inject mouldable material at a first pressure and/or rate, whilst the further initial injection stage injects mouldable material at a second, different pressure and/or rate.

Furthermore, the present inventors have recognised that when the extruder/feeder unit 24 is not running, the stationary raw material within the extruder/feeder unit 24 can degrade, and that it is therefore preferable that the extruder/feeder unit 24 is run for as long as possible during the step of supplying mouldable material into the apparatus (S1). To do so, the extruder/feeder unit 24 must be run at the lowest possible speed, wherein this lowest possible speed will by that which takes substantially the entire duration of the further injection step (S6) and subsequent solidification of the moulded article (S7) to deliver the required volume of mouldable material. Whilst conventional approaches would attempt to maximise the length of time for which the extruder/feeder unit 24 is running simply by running the extruder/feeder unit 24 at a constant speed that has been determined to be low enough to deliver the required volume over the required time period, this approach does not take into account the variability in the consistency/density of the material, which would again lead to significant inaccuracies in the volume delivered.

The present inventors have recognised that by providing the apparatus 20 with a first piston sensor that is configured to detect the position of the first piston 21 a as a means for accurately determining when a pre-defined volume of mouldable material has been fed into the first cylinder 21 b, the output of the piston sensor can provide feedback as a basis for controlling the speed of the extrude/feeder unit 24. The speed of the extrude/feeder unit 24 can thereby be variably controlled so as to ensure that the delivery of the required volume of mouldable material implemented during the priming phase takes substantially the entirety of the remaining duration of the final injection phase (i.e. including the second injection step (S6) and the subsequent solidification of the moulded article (S7)). In this regard, by locating the first piston 21a below a position in which the volume of the first cylinder 21 b between the head of the first piston 21a and the proximal end of the first cylinder 21 b is equal to the desired volume of mouldable material, the supply of mouldable material into the first cylinder 21 b by the feeder unit 24 will eventually cause the first piston 21 a to move towards the distal end of the first cylinder 21 b. The rate at which the first piston 21a moves during the supply of material into the first cylinder 21 b will then vary with the variability of the material, such that the feedback provided by the first piston sensor would allow the speed of the extrude/feeder unit 24 to be varied in proportion with any variance in the rate of movement of the first piston 21 a thereby taking into account the variability of the material. The output of the first piston sensor could therefore be provided to a feeder unit controller that is configured to control the speed of the screw motor 24c in dependence upon the rate of movement of the first piston 21a as indicated by the output of the piston sensor. In addition, the output of the first piston sensor could then also be used to determine when the first piston 21a has reached the position in which the volume of the first cylinder 21 b between the head of the first piston 21a and the proximal end of the first cylinder 21 b is equal to the desired volume of mouldable material, and the feeder unit controller would be configured to then stop the screw motor 24c.

Moreover, the present inventors have also recognised that, in order to further reduce the size of the clamp/press, it is also possible to make use of a separate fastening mechanism on the mould itself that releasably fastens the mould sections to one another in the closed position in order reduce the work required by the clamp/press. For example, the mould fastening mechanism could take the form of a latch comprising a catch provided on one of the mould sections that can engages with an engagement surface or projection provided on the other of the mould sections.

It will be appreciated that individual items described above may be used on their own or in combination with other items shown in the drawings or described in the description and that items mentioned in the same passage as each other or the same drawing as each other need not be used in combination with each other. In addition, the expression "means" may be replaced by actuator or system or device as may be desirable. In addition, any reference to "comprising" or "consisting" is not intended to be limiting in any way whatsoever and the reader should interpret the description and claims accordingly.

Furthermore, although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims.

Claims

1. A method of injection moulding an article, the method comprising:

supplying mouldable material into a supply chamber (S1);

injecting mouldable material from the supply chamber into the mould via a delivery chamber using a first piston (S4);

subsequently, injecting mouldable material present in the delivery chamber into the mould using a second piston (S5), wherein during the injection of mouldable material into the mould using the second piston, the second piston causes the delivery chamber to be sealed off from the supply chamber (S6);

allowing the mouldable material to solidify within the mould thereby forming the article and subsequently removing the article from the mould (S7); and

when the second piston has caused the delivery chamber to be sealed off from the supply chamber, withdrawing the first piston (S8) and supplying further mouldable material into the supply chamber (S1).

2. The method according to claim 1 , wherein the first piston injects a first volume of mouldable material and the second piston injects a second volume of mouldable material.

3. The method according to claim 2, wherein the first volume is between 60% and 90% of the total mouldable material injected, and the second volume is between 40% and 10% of the total mouldable material injected.

4. The method according to any preceding claim, wherein the first piston forces mouldable material through the delivery chamber from a first direction and the second piston forces mouldable material through the delivery chamber from a second direction, the first direction being substantially perpendicular to the second direction.

5. The method according to any preceding claim, and further comprising:

prior to injecting mouldable material using the first piston, compressing the mouldable material in the supply chamber using the first piston in order to force any volatiles out of the mouldable material (S2).

6. The method according to claim 5, wherein, when compressing the mouldable material in the supply chamber, the delivery chamber is sealed off from the supply chamber by the second piston, and the second piston is withdrawn prior to injecting mouldable material using the first piston (S3).

The method according to any of preceding claim, wherein the step of supplying mouldable material into the supply chamber causes the first piston to move towards a distal end of the supply chamber, and the rate at which mouldable material is supplied into the supply chamber is varied in dependence upon the rate at which the first piston moves towards the distal end of the supply chamber.

The method according to any preceding claim, wherein the step of supplying mouldable material into the supply chamber causes the first piston to move towards a distal end of the supply chamber, and the step of supplying mouldable material into the supply chamber continues until it is determined that the volume between the first piston and a proximal end of the supply chamber has reached a pre-defined volume.

The method according to any of claims 1 to 6, wherein step of supplying mouldable material into the supply chamber continues until it is determined that the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber.

The method according to claim 9, wherein it is determined that the pressure of the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber when movement of the first piston towards the distal end of the supply chamber is detected.

The method according to any preceding claim, wherein the injection of mouldable material into the mould continues until it is determined that the volume of mouldable material injected is sufficient to fill the mould cavity.

The method according to claim 1 1 , wherein it is determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting that a predefined volume has been injected by monitoring the distances moved by one or both of the first piston and the second piston.

The method according to claim 1 1 , wherein it is determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting an increase in the force acting to push the second piston away from the mould.

The method according to claim 11 , wherein the mould comprises two mould sections that when brought together define the mould cavity, and it is determined that the volume of mouldable material injected is sufficient to fill the mould cavity by detecting relative movement between the two mould sections.

A moulded article manufactured using the method according to any of the preceding claims.

An injection moulding apparatus (20) comprising:

a mould (22);

a supply chamber (21 b) configured to receive a supply of mouldable material;

a delivery chamber (21g) having an inlet from the supply chamber and an outlet to the mould;

a first piston (21a) configured to inject mouldable material from the supply chamber into the mould via a delivery chamber; and

a second piston (21d) configured to inject mouldable material present in the delivery chamber into the mould and, during the injection of mouldable material by the second piston, to cause the delivery chamber to be sealed off from the supply chamber.

The injection moulding apparatus according to claim 16, wherein the apparatus is configured such that the second piston seals off the delivery chamber from the supply chamber when the second piston is inserted into the delivery chamber.

The injection moulding apparatus according to any of claims 16 or 17, wherein the first piston is substantially perpendicular to the second piston.

The injection moulding apparatus according to any of claims 16 to 18, wherein the mould comprises two or more mould sections (22a, 22b), and the apparatus further comprises a press (23) configured to open and close the two or more mould sections.

20. The injection moulding apparatus according to any of claims 16 to 19, and further comprising a feeder unit (24) configured to feed mouldable material into the supply chamber at a location that is between a head (21 i) of the first piston and a proximal end of the supply chamber.

21. The injection moulding apparatus according to claim 20, and further comprising:

a first piston sensor configured to monitor the rate at which the first piston moves towards a distal end of the supply chamber; and

a feeder unit controller configured to vary the rate at which mouldable material is supplied into the supply chamber in dependence upon the rate at which the first piston moves towards the distal end of the supply chamber as indicated by an output of the first piston sensor.

22. The injection moulding apparatus according to claim 20, and further comprising:

a first piston sensor configured to detect that the first piston has moved a predefined distance towards a distal end of the supply chamber; and

a feeder unit controller configured to stop the supply of mouldable material into the supply chamber when an output of the first piston sensor indicates that the first piston has moved a predefined distance towards a distal end of the supply chamber.

23. The injection moulding apparatus according to claim 20, and further comprising:

a first piston sensor configured to detect when the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber; and

a feeder unit controller configured to stop the supply of mouldable material into the supply chamber when an output of the first piston sensor indicates that the mouldable material within the supply chamber is forcing the first piston towards the distal end of the supply chamber.

24. The injection moulding apparatus according to any of claims 16 to 23, and further comprising one or more sensors that are configured to detect that a predefined volume of mouldable material has been injected into the mould by monitoring the distances moved by one or both of the first piston and the second piston. The injection moulding apparatus according to any of claims 16 to 23, and further comprising a pressure sensor that is configured to detect an increase in the force acting to push the second piston away from the mould.

The injection moulding apparatus according to any of claims 16 to 23, wherein the mould comprises two mould sections that when brought together define a mould cavity, and further comprising a mould sensor that is configured to detect relative movement between the two mould sections.

The injection moulding apparatus according to any of claims 16 to 25, wherein the mould comprises two mould sections that when brought together define a mould cavity, and further comprising a mould fastening mechanism on the mould that releasably fastens the mould sections to one another when in a closed position.