Method of injection moulding

Abstract

A method of injection moulding using a mould 10 defining a cavity; the mould comprising a first part movable relative to a second part between a first relative position, giving the mould an uncompress

Description

BACKGROUND OF THE INVENTION This invention relates to a method of injection moulding and to an injection moulding system configured to carry out such a method. It is known to form small components by injecting molten material into moulds having a desired shape. The present invention provides an injection moulding method, and associated device, that are advantageous, particularly in the context of injection moulding large (e.g. thick-walled) parts. SUMMARY OF THE INVENTION From a first aspect, the invention provides a method of injection moulding using a mould defining a cavity, the mould comprising a first part and a second part, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a second relative position, giving the mould a compressed configuration, in which the cavity has a second shape, the method comprising: in an injection phase, injecting material into the cavity to produce a piece within the mould having the first shape; in a compression phase, following the injection phase, applying force to the first part, to move the first part such that the mould is moved from the uncompressed configuration to the compressed configuration, so that the piece is compressed to have the second shape. It will be understood that the injection phase is carried out using an injection moulding machine, e.g., with the mould arranged at the injection moulding machine. From a second aspect, the invention provides an injection moulding system, comprising: an injection moulding machine; a mould, defining a cavity, the mould comprising a first part and a second part, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a second relative position, giving the mould a compressed configuration, in which the cavity has a second shape; the injection moulding system configured to carry out the method of injection moulding as described above. Thus it will be seen that, in accordance with the invention, by applying force to a first part of the mould after the end of the injection phase (i.e. once injection has been completed to completely fill the cavity defined by the mould in the uncompressed configuration, giving a piece having the first shape), in order to compress a piece within the cavity, the piece is compressed (within the mould) at a time when it has started to cool (since the material has already been ejected from the injection moulding machine and injected into the mould). Whilst it is known to carry out both injection and compression during a process known as injection compression moulding, this is a very different process to the one described herein. Notably, the claimed compression phase compressing the piece to the second shape occurs after the injection phase, where during the injection phase the mould is completely filled with the injected material to give a piece having the first shape. In the claimed method, the compression is used to control shrinkage of the material during cooling, as set out below, as opposed to how compression is used in injection compression moulding in which it is used to improve filling of the mould during the injection phase, i.e. to assist in totally filling the mould. It will be understood that the mould is in the uncompressed configuration at the end of the injection phase, i.e. since at the end of the injection phase a piece has been produced which has a volume greater than the second shape, and substantially equal to the first shape (e.g. which has the first shape). The mould may be in the uncompressed configuration throughout the injection phase, or alternatively, may be compressed to the “uncompressed” configuration from an even further uncompressed (i.e. third) configuration, during the injection phase. Thus, whilst the process of the present invention differs in that further compression is applied to a filled mould, compressing the shape of the piece to a second, smaller shape, it will be appreciated that compression may also be applied during the injection phase, i.e. to move injected material into the first shape, e.g. the injection phase may comprise injection-compression moulding as described above, at the end of which a piece having the first shape is provided within the mould, filling the mould, which has the uncompressed configuration. In other words, in the injection phase, the first part and the second part may initially have a third relative position, in which the cavity has a third shape, larger than both the first shape and the second shape. The injection phase may comprise, after and / or simultaneously with injecting the material into the cavity, applying force to the first part and / or the second part to move the mould into the first relative position, in which the cavity has the first shape. In other embodiments, the injection phase may be carried out with the mould in the uncompressed configuration (i.e. throughout the injection phase). Many materials used for injection moulding have a positive expansion coefficient, meaning that the same mass of material has a larger volume when the material is at a higher temperature. Thus, the material injected into the cavity may have a positive expansion coefficient. As a result of this, the material may shrink in volume as it cools (or is actively cooled). The piece will cool from its outside surfaces first, meaning that these exterior surfaces will crystallise, or “set”, first. The material injected into the cavity may be polymer. Thus, the piece may comprise, or consist of, polymer material. If no compression were applied to the piece, after injection, then the exterior surfaces of the piece would set having the first shape. However, as the internal parts of the piece (away from the exterior surfaces) then subsequently cooled, the material in this internal part may shrink, e.g. owing to the positive expansion coefficient of the material, and this may leave voids within the piece. Instead, the piece is compressed after injection, and therefore as it (at least partly) cools. This helps to compensate for the shrinkage of the material during cooling, such that the formation of voids within the piece may be reduced or avoided altogether. This is particularly important where the injection moulding method is used to make large pieces, e.g. pieces weighing a few kilograms, e.g. tens of kilograms, e.g. hundreds of kilograms. This is particularly important where the injection moulding method is used to make thick-walled parts. In contrast, the kinds of small parts that are commonly injection moulded (e.g. bottle tops, syringes, and other small plastic parts), do not suffer from the problem of void formation, since the overall change in volume of the material in the piece is very small in real terms, and the walls are very thin, and thus voids of an appreciable size do not form. The mould may be configured to produce a piece weighing at least 5 kg, optionally at least 50 kg, further optionally at least 200 kg, further optionally at least 500 kg, and further optionally approximately (or at least) 1000 kg. The piece may have a width of at least at least 1 m, optionally at least 1.5 m, further optionally at least 2 m, further optionally at least 3 m. The piece may have a width of approximately 3 m. The piece may have a depth of at least 1 m, optionally at least 1.5 m, further optionally at least 2 m, further optionally at least 3 m. The piece may have a depth of approximately 3 m. The piece may have a height of at least 30 cm, optionally at least 50 cm, further optionally at least 1 m. The piece is formed in the shape of the cavity (initially the first shape, and then compressed to the second shape), thus the cavity may also have any of these dimensions and the mould may be configured to produce a piece having any of these dimensions. In some embodiments, the method of injection moulding is a method of injection moulding a compressive element for a mooring component. Thus, the piece may be a compressive element for a mooring component. The piece may be a shell, the shell comprising first and second annular portions which are joined together by a central section. The shell may be rotationally symmetrical about a central axis. The shell may have a profiled shape in a cross-section taken along the central axis. The cavity may define the profiled shape. The mould may have a width of at least 1 m, optionally at least 1.5 m, further optionally at least 3 m. The mould may have a depth of at least 1 m, optionally at least 1.5 m, further optionally at least 3 m. The mould may have a height of at least 50 cm, optionally at least 80 cm, further optionally at least 1 m. In some embodiments, the difference in volume between the first shape and second shape of the cavity is approximately equal to the change in volume of the piece due to shrinkage of the material during cooling. Thus, the volume of additional material that is injected into the cavity to form a piece having the first shape (as compared to the second shape) is approximately equal to the volume that the material will lose as it cools from the injection temperature to ambient temperature. The compression of the piece from the first shape to the second shape may be achieved by changing the overall shape of the cavity (e.g. by moving parts of the mould that form the cavity relative to one another), or it may be achieved by pushing on a particular part of the piece within the cavity, using a movable part inside the cavity - or it may be achieved by a combination of these two techniques. Thus, in some embodiments, the first part and the second part define the cavity (i.e. in a space between them when they are placed in contact). Such first and second parts are arranged to move relative to each other, so that the cavity defined between them changes shape, when a force is applied to at least the first part, as set out above. This allows compression of the piece to the second shape to be achieved without requiring any additional component of the mould, simply by causing relative movement between the mould parts. It will be appreciated that in such embodiments the first and second parts may have any of the dimensions discussed above in relation to the mould. Applying compression to the piece in this way advantageously applies compression throughout the entire piece, rather than in one localised area, which helps to prevent void formation throughout the entire piece. It is also possible to apply higher compression forces when using the entire first part (and optionally also second part) to apply them. In another set of embodiments, the first part is a movable part arranged within the cavity (i.e. it may internally compress a piece in the cavity from the first shape to the second shape by pressing directly on it). Thus, according to an embodiment, there is provided a method of injection moulding using a mould defining a cavity, the mould comprising a movable part arranged within the cavity, movable between an uncompressed position, in which the cavity has a first shape, and a compressed position, in which the cavity has a second shape, the method comprising: in an injection phase, injecting material into the cavity, with the movable part in the uncompressed position, to produce a piece within the mould having the first shape; in a compression phase, following the injection phase, applying force to the movable part, to move the movable part to the compressed position, so that the piece is compressed to have the second shape. Further embodiments may include both mould parts movable relative to one another, and in addition a movable part arranged within the cavity (e.g. within one of the mould parts). Thus, the first part and the second part may define the cavity, and the mould may further comprise a movable part, arranged within the cavity, wherein the movable part is movable between a first, uncompressed position within the mould, and a second, compressed position, within the mould. Thus, in some embodiments, both internal and external compression of the mould may be applied. In some embodiments, the method comprises applying a clamping force to an exterior of the mould. It will be understood that a clamping force comprises a pair of opposing forces, acting from approximately opposing sides of the mould, in a direction towards the mould. In embodiments in which the first part and the second part define the cavity, the clamping force will act to push the first and second parts together (i.e., to move the mould from the uncompressed configuration to the compressed configuration unless resisted by internal pressure within the cavity). Thus, the clamping force may provide the force applied to the first part to move the first part such that the mould is moved to the compressed configuration. The clamping force may also provide a force applied to the second part to move the second part such that the mould is moved to the compressed configuration (i.e. such that the first and second mould parts move mutually towards each other). In other embodiments, in which the first part is a movable part arranged within the cavity, in addition to the force applied to the movable part, as referred to above, such a method may further comprise applying a clamping force to the exterior of the mould. This exterior clamping force is different to the internal compression provided by movement of the movable part, which changes the shape of the cavity. The clamping force may be applied during the injection phase (and optionally - where it is not used to apply the compression force that moves the mould from the uncompressed configuration to the compressed configuration - only during the injection phase). Thus, the method may further comprise, during the injection phase, applying a clamping force to an exterior of the mould. This may help to keep the parts of the mould well sealed together, despite a high pressure from the material as it is injected into the cavity. Alternatively, the clamping force applied during the injection phase may move the mould parts into the uncompressed configuration (which may also be referred to as the first configuration), thus compressing (i.e. forming) the piece into the first shape, as described above. The method may comprise, additionally or alternatively, during the compression phase, applying a clamping force to an exterior of the mould. Optionally, the clamping force may be applied, in the compression phase, to move the first part such that the mould is moved to the compressed configuration (i.e. to apply the force referred to above to the first part). Where a clamping force is applied during both the injection phase and the compression phase, a different magnitude of clamping force may be used in each phase. Thus a different amount of force may be used in the injection phase, to keep the mould parts from being pushed apart, as compared to the force used in the compression phase (which may be used to move the mould from the uncompressed configuration to the compressed configuration). Thus, in some embodiments, the method comprises applying, during the injection phase, a first clamping force, to an exterior of the mould, and applying, during the compression phase, a second clamping force, to the exterior of the mould, the second clamping force having a different (e.g. higher) magnitude than the first clamping force. The second clamping force may be at least double the first clamping force, optionally at least triple. The clamping force may be applied by the injection moulding machine. Thus, the mould may remain at the injection moulding machine during the compression phase. The injection moulding machine may comprise a first platen and a second, opposing platen. The clamping force may be provided by the first platen and / or the second platen pushing towards the other platen. The clamping force may be applied by hydraulic pressure. Additionally or alternatively, a clamping force may be applied by a clamping mechanism (such as a clasp or latch) arranged to attach the two parts of the mould together. Thus the first part of the mould may comprise a first latch part, and the second part of the mould may comprise a second latch part, wherein the first latch part and the second latch part cooperate together to form a latch mechanism. The (first and / or second) clamping force may at least 10000 kN, optionally at least 20000 kN. The (first and / or second) clamping force may be approximately 20000 kN. The (second) clamping force, i.e. the force applied during the compression phase, may be at least 20000 kN, optionally at least 30000 kN, further optionally at least 50000 kN. The clamping force applied during the compression phase (e.g. to move the mould to the compressed configuration) may be approximately 50000 kN. In some embodiments, the injection phase comprises injecting material at a pressure of at least 0.5 MPa, optionally at least 5 MPa, further optionally at least 10 MPa. The injection phase may comprise injecting material at a pressure of approximately 5 MPa. In some embodiments, the injection phase comprises injecting material into the cavity at a temperature of at least 180°C, optionally at least 200°C, further optionally at least 230°C. In some embodiments, the injection phase comprises injecting material into the cavity at a temperature of approximately 240°C. In some embodiments, the injection phase lasts for at least 3 minutes, optionally at least 5 minutes, further optionally at least 10 minutes. In some embodiments, the injection phase lasts for approximately 5 minutes. In some embodiments, the injection phase lasts for less than 15 minutes, optionally less than 10 minutes, further optionally less than 5 minutes. Fast injection is advantageous since the material will begin cooling as soon as it is injected into the mould, and therefore the cavity must be filled before the material cools so much that it cannot subsequently be compressed, or cools so much that the injection process is compromised, for example by the injected material folding back on itself or forming cold weld failure points. It will further be appreciated that by moving the first part of the mould in order to compress the piece to the second shape, this allows compression of the piece to be achieved more easily than if a separate component is used to apply compression to the piece. It also enables the possibility for the mould to be removed from the injection moulding machine, after the injection phase is complete, and for compression to be applied to the piece with the mould elsewhere (away from the injection moulding machine). This frees up the injection moulding machine more quickly to mould another piece. This helps to reduce wastage, particularly in the event that the material used by the injection moulding machine can only be kept molten for a limited amount of time (e.g., 15-30 minutes), without degrading. In this case, it is important to make the injection moulding machine available to be used again as quickly as possible, otherwise, if the time taken between injections of material is too long, the molten material will degrade and then all of the material within the injection moulding machine will have to be cleared out before the machine can be used again. Thus, in some embodiments, the method further comprises removing the mould, containing the piece, from the injection moulding machine before the compression phase. Alternatively, the compression phase may also be carried out at (e.g. by) the injection moulding machine. In some embodiments, the compression phase comprises applying a force of approximately 20000 kN to the first part. In some embodiments, the compression phase comprises applying a packing pressure to the material of the first piece of approximately 40 MPa. The compression phase may last for at least 15 minutes, optionally at least 30 minutes, further optionally at least 45 minutes. The compression phase may last for up to 2 hours. The compression phase may last for approximately 45 minutes. The material of the piece will cool from the outside inwards during the compression phase. This forms a solid outer shell of material (e.g. polymer) around a molten core. This solid material is still compressible until it drops below the crystalline temperature. The compression can continue until enough of this solid shell falls below the crystalline temperature to resist further compression. The core may still be molten at this point. The compression phase may last until the material providing the piece (e.g. an external outer surface of the piece, or substantially all of the material providing the piece) is at a temperature of less than 200° C, optionally less than 180°C, further optionally less than 150°C. Thus, the temperature of the material providing the piece may be below the crystalline temperature of the material, i.e. such that deformation of the material into a new shape is no longer possible. In some embodiments, the method further comprises connecting a compression system to the mould, and applying force to the first part during the compression phase, using the compression system, so as to move the first part relative to the second part to move the mould to the compressed configuration. Thus, in some embodiments, the injection moulding system further comprises a compression system connectable to the mould so as to apply a force to the first part. The use of a compression system helps to achieve movement of the first part, so as to compress the piece. The compression system may be a mechanical compression system. The compression system may be a hydraulic pressure system. Thus, the method may further comprise applying force to the first part using hydraulic pressure. Hydraulic pressure is particularly effective at moving the first part against the high pressure of the material within the cavity, in particular hydraulic pressure may be beneficial in a case where mechanical actuation is not sufficient to move the first part, for example due to high pressures within the cavity. In some embodiments, the mould (e.g. the first part of the mould and / or the second part of the mould) comprises an actuation mechanism (e.g. a hydraulic actuation mechanism). The actuation mechanism may be arranged to drive movement of the first (e.g. movable) part (e.g. in cooperation with the compression system). The actuation mechanism may be connectable to the compression system. Thus, in some embodiments, the method further comprises connecting the compression system to the actuation mechanism of the mould, and applying force to the first part during the compression phase, using the compression system and the actuation mechanism, so as to move the first part to move the mould to the compressed configuration. In particular, in the context of the compression system being a hydraulic pressure system, the compression system may specifically be a hydraulic supply, and the actuation mechanism may be one or more hydraulic actuators. Thus, more generally, the compression system may supply the force to actuate the actuation mechanism, e.g., by supplying hydraulic fluid to the actuation system. It will be understood that the compression system is connected to the mould before the compression phase, i.e. so that it is able to actuate the first part during the compression phase. The compression system may be connected to the mould during the injection phase, but preferably the compression system is connected to the mould after the injection phase (and before the compression phase). Optionally, the compression system may be spaced apart from the injection moulding machine. Thus, the method may comprise removing the mould, containing the piece, from the injection moulding machine before the compression phase, and then connecting a compression system to the mould. It will be understood that this only defines that the step of connecting the compression system is done after the removing of the mould from the injection moulding machine, but does not exclude the presence of one or more intervening steps between these two steps. Alternatively, as set out above, the force applied to the first part during the compression phase may be applied with the mould still on the injection moulding machine, optionally it may be applied by the injection moulding machine (e.g. by the platens). In some embodiments of the method, the time between the end of the injection phase and the start of the compression phase is less than 30 minutes, optionally less than 15 minutes, further optionally less than 5 minutes. In some embodiments, the time between the end of the injection phase and the start of the compression phase is approximately 10 minutes. It is important that the compression phase is started as soon as possible after the end of the injection phase, since the material of the piece will be cooling throughout this time, and once large parts of the material cool below the crystalline temperature of that material, further compression will have little effect on the shape of the resulting piece. The piece may be cooled only by passive cooling (i.e. if left for long enough it will lose heat to its ambient environment until it reaches a temperature equilibrium with its ambient environment). However, preferably the overall time to form the piece by injection moulding can be reduced by accelerating the cooling of the piece using active cooling. Thus, in some preferred embodiments, the method further comprises, in an active-cooling phase, cooling the piece, within the mould, by circulating a coolant fluid through a part of the mould. Thus, the injection moulding system may further comprise a coolant circulation system, connectable to the mould so as to circulate fluid through a part of the mould (e.g. through the first part and / or the second part). The injection moulding machine may comprise the coolant circulation system, or may be separate from it. In some embodiments, the injection moulding machine comprises a first coolant circulation system and the injection moulding system further comprises a second coolant circulation system (i.e. separate from the injection moulding machine). In some embodiments, the method further comprises connecting a coolant circulation system to the mould, and activating the coolant circulation system to circulate fluid through a part of the mould. The coolant fluid may be water, which may be cooled to a suitable temperature. It may be pumped through the mould, so as to circulate around the piece. It will be understood that the coolant fluid does not flow into the cavity, i.e. so as to contact the piece, but is rather in only thermal contact with the piece. The coolant fluid may circulate in one or more coolant channels within the mould (e.g. within the first part and / or the second part). It will be understood that the coolant circulation system is connected to the mould before the active-cooling phase, i.e. so that it is able to circulate cooling fluid during the active-cooling phase. The coolant circulation system may be connected to the mould during the injection phase, but preferably the coolant circulation system is connected to the mould after the injection phase (and before the active-cooling phase). In embodiments in which the coolant circulation system is connected to the mould during the injection phase, the coolant circulation system may be used, during the injection phase, to circulate heating fluid (e.g. hot water), i.e., such that the material injected into the mould is kept at a higher temperature. In some embodiments, the first coolant circulation system (which is part of the injection moulding machine) is connected to the mould during a first stage of the active-cooling phase, and subsequently the second coolant circulation system, which is separate from the injection moulding machine is connected to the mould during a second stage of the active-cooling phase (and the first coolant circulation system is disconnected). Optionally, the (second) coolant circulation system may be spaced apart from the injection moulding machine. Thus, the method may comprise removing the mould, containing the piece, from the injection moulding machine (e.g. before or during the active-cooling phase), and then connecting the coolant circulation system to the mould. It will be understood that this only defines that the connecting the coolant circulation system is done after the removing of the mould from the injection moulding machine, but does not exclude the presence of one or more intervening steps between these two steps. For example, the coolant circulation system may be connected to the mould before or after (or at the same time as) the compression system is connected to the mould. Carrying out active-cooling separate to the injection moulding machine advantageously frees up the injection moulding machine for further injection operations as soon as possible, rather than keeping it occupied as the piece cools within the mould (which may take some time). The active-cooling phase may last for at least one hour, optionally at least 2 hours, further optionally at least 4 hours. The active-cooling phase may last for approximately 4.5 hours. As set out above, the active-cooling phase may comprise two stages - a first stage carried out at the injection moulding machine, and a second stage carried out away from the injection moulding machine (optionally both being carried out whilst the piece is still within the mould). During both stages, the piece is actively cooled (i.e. by coolant fluid circulation). As set out below, the first stage of the active-cooling phase may coincide with the compression phase. In some embodiments, during the active-cooling phase the piece (e.g. an external outer surface of the piece) cools from a temperature of approximately (or at least) 200°C to a temperature of approximately (or less than) 100°C. Since the piece has such thick walls the core of the piece may still be molten even once the outside surface has cooled sufficiently to form a solid outer skin. In some embodiments, the compression phase and the active-cooling phase overlap in time. By overlap it will be understood that there is a non-zero period of time during which compression is applied to the piece by applying a force to the first part, and during which also coolant fluid is circulated through a part of the mould to actively cool the piece within the mould. The compression phase and the active-cooling phase may have approximately the same start time and / or the same end time. The compression phase and the activecooling phase may be substantially (optionally fully) contemporaneous. The method may comprise a combined compression and active-cooling phase, wherein the combined compression and active-cooling phase comprises both the compression phase and (at least a part of, optionally all of) the active-cooling phase. Thus, the method of injection moulding may further comprise, in a combined compression and active-cooling phase, following the injection phase, applying force to the first part, using the compression system, to move the first part to a relative position with the second part to give the mould the compressed configuration, so that the piece is compressed to have the second shape and activating the coolant circulation system to circulate fluid through a part of the mould so as to cool the piece, within the mould, by circulating a coolant fluid through a part of the mould. The compression system and the (e.g. first) coolant circulation system may be located at substantially the same position, which may be referred to as a compression and active-cooling station. This may be provided by the injection moulding machine or may be spaced apart from (i.e. separate to) the injection moulding machine. The mould may be transferred to the compression and active-cooling station before the combined compression and active-cooling phase. The combined compression and active-cooling phase may last for at least 30 minutes, optionally at least 45 minutes. The combined compression and active-cooling phase may last, for example, for approximately 45 minutes. As set out above, the active-cooling phase may have the same start time as the compression phase, thus the active-cooling phase may comprise the combined compression and active-cooling phase. The active-cooling phase may then continue (in a second-stage active-cooling phase) after the end of the compression phase (i.e., after the end of the combined compression and active-cooling phase). The activecooling phase may thus comprise a first stage, the combined compression and activecooling phase, and a second-stage active-cooling phase, i.e. in which no compression of the piece occurs. In this second-stage active-cooling phase, the material may continue to be cooled by circulation of coolant. The second-stage active-cooling phase (without compression) may take place at the injection moulding machine, or away from the injection moulding machine. Thus, at least part of the active-cooling phase (i.e. specifically the second-stage active-cooling phase) may take place away from the injection moulding machine. Thus, in some embodiments, the combined compression and active-cooling phase takes place at the injection moulding machine, and the mould parts, containing the piece, are then moved away from the injection moulding machine during the active-cooling phase, to a second coolant circulation system, at which the second-stage active-cooling phase is carried out. Whilst it is stated that no compression takes place in the second stage of active cooling, it will be understood that compression refers specifically to a force which moves the mould parts (or a part of the mould parts) so as to change the shape of the piece. Thus, a clamping or pressurising force may be applied to the piece (e.g. to the mould parts) during the second stage, to keep the piece under pressure. In some embodiments, the method further comprises, in a removal phase, removing the piece having the second shape from the mould. It will be understood that the removal phase must occur after the compression phase, since the compression phase must have occurred in order for the piece to have been compressed to have the second shape. The removal phase may optionally also take place after the activecooling phase (or the combined compression and active-cooling phase). In some embodiments, removing the piece having the second shape from the mould comprises the injection moulding machine opening the mould (e.g. separating the first and second parts). It may also comprise returning the mould to the injection moulding machine, i.e. before the mould is opened by the injection moulding machine. This is particularly advantageous because the mould, having had the piece removed from it, is then positioned at the injection moulding machine, ready to be used again for another injection moulding process (optionally after also being cleaned, e.g. at the injection moulding machine). In some embodiments (i.e. where the first and second parts define the cavity) opening the mould comprises separating the first and second parts. Thus, in some embodiments, removing the piece having the second shape from the mould comprises returning the mould to the injection moulding machine, and the injection moulding machine separating the first part and the second part of the mould from each other. Where the first part is the movable part, arranged within the cavity, as discussed above, the mould may further comprise a third part (i.e. such that the second and third parts define the cavity), and the second part and the third part may provide the parts of the mould which are separated to open the mould. The movable part may be arranged at least partially (optionally fully) within the second part and / or the third part of the mould (where present). It will be appreciated that the labels of “first”, “second” and “third” part etc. may be interchangeable and are simply used for reference purposes. In some embodiments, the mould (e.g. the first part and / or the second part) comprises an ejection portion (e.g. a hydraulically actuated ejector). The mould may comprise a plurality of ejection portions. The ejection portion(s) may be in a different part of the mould (e.g. the other part of the mould) compared to the actuation mechanism. In some embodiments, the method comprises, in the removal phase, actuating the ejection portion of the mould, to apply an ejecting force onto the piece. This may be done after the mould is opened by the injection moulding machine, such that the method comprises returning the mould to the injection moulding machine, the injection moulding machine opening the mould (e.g., by separating the first part and the second part), and actuating the ejection portion of the mould, to apply an ejecting force onto the piece. The ejection portion(s) may be arranged to protrude into the cavity, when actuated, i.e. such that the piece is pushed out of the cavity. In some embodiments, the method further comprises, in an external-cooling phase, positioning the piece, no longer within the mould, onto a support. Thus, in some embodiments, the injection moulding system further comprises a support, arranged to support a piece that has been removed from the cavity of the mould. This allows the piece to be removed from the mould even whilst the internal material within the piece is still molten, provided that the external surfaces are set sufficiently to support the material within the interior of the piece as it cools. This makes the mould available more quickly for subsequent injection moulding processes. The support may support the piece from underneath (i.e. support it to resist the force of gravity acting on the mould). Supporting the piece from underneath is particularly advantageous since the piece can be more conveniently accessed and moved by automated vehicles, as discussed further below, while leaving the upper side of the piece exposed, for example, to aid cooling. It will be understood that the external cooling phase takes place after the removal phase (but as with other stages this does not exclude the possibility of intervening phases). In some embodiments, the external-cooling phase lasts for at least 6 hours, optionally at least 8 hours, further optionally at least 16 hours. The external-cooling phase may last for approximately 24 hours. In some embodiments, during the external-cooling phase the piece cools to approximately room temperature (i.e. approximately 18°C-21°C. The cooling during the external-cooling phase may be passive cooling (e.g. only passive cooling). By passive cooling it will be understood that the piece cools by dissipation of heat to its ambient environment. The cooling during the external-cooling phase may additionally (or even predominantly) comprise active cooling. For example, cooled air may be circulated around or over the piece, to accelerate the dissipation of heat from the piece. The external-cooling phase may be the last phase in the injection moulding method. Thus, once the piece is cooled to ambient (e.g., room) temperature during the external-cooling phase, the formation of the piece by the injection moulding machine may be considered to be complete. In some embodiments, the method further comprises transporting the mould and / or the piece, outside of the mould, using an automated vehicle. Thus, in some embodiments, the injection moulding system further comprises an automated vehicle arranged to transport the mould (e.g. the first part and / or the second part) and / or the piece. The injection moulding system may comprise a first automated vehicle and a second automated vehicle. In some embodiments, the method further comprises transporting the mould using a first automated vehicle and transporting the piece using a second automated vehicle. Using different automated vehicles to transport the mould and the piece allows each automated vehicle to be adapted more specifically to its particular purpose. For example, a smaller vehicle may be used to transport the piece as compared to the vehicle used to transport the mould. The method may further comprise transporting the mould from the injection moulding machine to the compression system using an automated vehicle (i.e. before the compression phase). The method may further comprise transporting the mould from the injection moulding machine to the coolant circulation system using an automated vehicle (i.e. before the active-cooling phase e.g. the second stage). The method may further comprise transporting the mould from the injection moulding machine to a compression and active-cooling station using an automated vehicle (i.e. before the combined compression and active-cooling phase). The automated vehicle may be a first automated vehicle. The method may further comprise transporting the piece from the injection moulding machine to the support using an automated vehicle (i.e. after the removal phase). The automated vehicle may be a second automated vehicle (i.e. different to the first vehicle that transports the mould). Thus, as laid out above, according to a preferred embodiment, there is provided a method of injection moulding using a mould defining a cavity, the mould comprising a movable part arranged within the cavity, movable between an uncompressed position, in which the cavity has a first shape, and a compressed position, in which the cavity has a second shape, the method comprising: in an injection phase, injecting material into the cavity using an injection moulding machine, with the movable part in the uncompressed position, to produce a piece within the mould having the first shape, further comprising applying a clamping force to an exterior of the mould during the injection phase; removing the mould, containing the piece, from the injection moulding machine; connecting a compression system to the mould; connecting a coolant circulation system to the mould; then, in a combined compression and active-cooling phase, following the injection phase, applying force to the movable part, using the compression system, to move the movable part to the compressed position, so that the piece is compressed to have the second shape and activating the coolant circulation system to circulate fluid through a part of the mould so as to cool the piece, within the mould, by circulating a coolant fluid through a part of the mould; removing the piece having the second shape from the mould by returning the mould to the injection moulding machine, and the injection moulding machine opening the mould; transporting the piece, outside of the mould, from the injection moulding machine to a support, using an automated vehicle; and in an external-cooling phase, positioning the piece, no longer within the mould, onto the support. Further, as also laid out above, according to another preferred embodiment, there is provided a method of injection moulding using a mould defining a cavity, the mould comprising a first part and a second part defining the cavity, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a second relative position, giving the mould a compressed configuration, in which the cavity has a second shape, the method comprising: in an injection phase, injecting material into the cavity using an injection moulding machine, with the mould in the uncompressed configuration, to produce a piece within the mould having the first shape, further comprising applying a first clamping force to an exterior of the mould (e.g. to the first part and the second part) during the injection phase, the clamping force having a first magnitude; connecting a coolant circulation system to the mould; then, in a combined compression and active-cooling phase, following the injection phase, applying a second clamping force to an exterior of the mould (e.g. to the first part and the second part) using the injection moulding machine (e.g. platens of the injection moulding machine), to move the first part such that the mould is moved to the compressed position, so that the piece is compressed to have the second shape and activating the coolant circulation system to circulate fluid through a part of the mould so as to cool the piece, within the mould, by circulating a coolant fluid through a part of the mould, the second clamping force having a second magnitude that is greater than the first magnitude; removing the piece having the second shape from the mould by the injection moulding machine opening the mould; transporting the piece, outside of the mould, from the injection moulding machine to a support, using an automated vehicle; and in an external-cooling phase, positioning the piece, no longer within the mould, onto the support. Further, as also laid out above, according to another preferred embodiment, there is provided a method of injection moulding using a mould defining a cavity, the mould comprising a first part and a second part defining the cavity, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a second relative position, giving the mould a compressed configuration, in which the cavity has a second shape, the method comprising: in an injection phase, injecting material into the cavity using an injection moulding machine, with the mould in the uncompressed configuration, to produce a piece within the mould having the first shape, further comprising applying a first clamping force to an exterior of the mould (e.g. to the first part and the second part) during the injection phase, the clamping force having a first magnitude; connecting a first coolant circulation system to the mould; then, in a combined compression and active-cooling phase, following the injection phase, applying a second clamping force to an exterior of the mould (e.g. to the first part and the second part) using the injection moulding machine (e.g. platens of the injection moulding machine), to move the first part such that the mould is moved to the compressed position, so that the piece is compressed to have the second shape and activating the first coolant circulation system to circulate fluid through a part of the mould so as to cool the piece, within the mould, by circulating a coolant fluid through a part of the mould, the second clamping force having a second magnitude that is greater than the first magnitude; removing the mould, containing the piece, from the injection moulding machine; connecting a second coolant circulation system to the mould; then, in a second-stage active-cooling phase, activating the second coolant circulation system to circulate fluid through a part of the mould so as to cool the piece, within the mould, by circulating a coolant fluid through a part of the mould; removing the piece having the second shape from the mould by the injection moulding machine opening the mould; transporting the piece, outside of the mould, from the injection moulding machine to a support, using an automated vehicle; and in an external-cooling phase, positioning the piece, no longer within the mould, onto the support. It will be appreciated that the injection moulding system referred to herein above may be arranged to carry out any of the steps referred to above as part of the method claim. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap. BRIEF DESCRIPTION OF THE DRAWINGS Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing showing an injection moulding system according to an embodiment of the present invention; Figure 2 is a cross-sectional side view of a mould that may be used in the injection moulding system of Figure 1; Figure 3 is an external side view of the mould of Figure 2; Figure 4 is a front view of a first part of the mould of Figure 2; Figure 5 is a perspective view of the first part, shown in Figure 4; Figure 6 is a cross-sectional side view of components of the first part, shown in Figure 4; Figure 7 is a perspective view of some of the components shown in Figure 6; Figure 8 is a front view of a second part of the mould of Figure 2; Figure 9 is a perspective view of the second part, shown in Figure 8; Figure 10 is a perspective view of a lower layer of the second part, shown in Figure 9; Figure 11 is a perspective view showing only the injection channels of the part shown in Figure 10; Figure 12 is a flow chart, representing a method of injection moulding according to an embodiment of the present invention; Figures 13-16 are cross-sectional side views of a second exemplary mould that may be used in the injection moulding system of Figure 1; Figure 17 is a flow chart, representing a method of injection moulding according to a second embodiment of the present invention. DETAILED DESCRIPTION Figure 1 is a schematic drawing showing an injection moulding system 1. The injection moulding system 1 includes a mould 10. The mould 10 can be positioned in various locations, or stations, within the injection moulding system 1, and is therefore shown with dashed lines. The injection moulding system 1 includes an injection moulding machine 2, at (or within) which the mould 10 can be positioned, to be filled by injection with molten material (e.g. molten polymer). The injection moulding machine 2 includes platens 16, which can be arranged on opposing sides of the mould 10, to secure the mould 10 in position within the injection moulding machine 2, and also to apply a compressive, or clamping, force to an exterior of the mould 10, pushing the two parts of the mould together. The injection moulding system 1 further includes a separate compression and activecooling station 3. Although combined together at a single station in this particular example, it will be understood that in other embodiments compression and active cooling may take place separately. This station 3 includes a compression system 4, and a coolant circulation system 6. Furthermore, although both the compression system 4 and the coolant circulation system 6 are shown separately from the injection moulding machine 2 in this particular example, it will be understood that one or both of these may be provided as part of the injection moulding machine 2 (either additionally or as an alternative to the illustrated arrangement). For example, the injection moulding machine 2 may provide both a compression system 4 and a coolant circulation system 6, and a separate, further coolant circulation system may also be provided. As described below, after injection, the mould 10 is removed from the injection moulding machine 2, and transported to the compression and active-cooling station 3, at which point the compression system 4 and the coolant circulation system 6 are connected to the mould 10. The mould 10 may alternatively not be removed from the injection moulding machine 2 where, as set out above, the cooling and compression functions are provided by the injection moulding machine 2. The injection moulding system 1 further includes a support 8. In this example the support 8 is separate from both the injection moulding machine 2, and the compression and active-cooling station 3. The support 8 is shaped and configured to support a piece 12 that is removed from within the mould 10, after it has been sufficiently cooled to be able to be removed. This then allows the piece 12 to be cooled further by releasing heat into the surrounding ambient environment. The injection moulding system 1 further includes automated vehicles 14a, 14b. One of the automated vehicles 14a is arranged to move the mould 10 (which in use may contain a piece 12) between the injection moulding machine 2 and the compression and active-cooling station 3. The other automated vehicle 12b is arranged to move a piece 12, outside of the mould 10, between the injection moulding machine 2 and the support 8. Embodiments of the mould 10, and certain of its components, are shown in greater detail in Figures 2-11. Figure 2 shows a cross-sectional side view of a mould that may be used in the injection moulding system of Figure 1. The mould 10 comprises a first part 20, and a second part 22. The outsides of these parts 20, 22 are seen in the side view shown in Figure 3. From this view it can be seen that a clasp 24 is provided, which attaches the two mould parts 20, 22 together, to keep them together during injection filling of the mould. When the two mould parts 20, 22 are fitted together they define a cavity 26 between them, as seen in Figure 2. The cavity 26 is bounded by the first part 20 and the second part 22. To this end, the first part 20 includes a recessed portion 28, which is circular (from a front view, as shown in Figures 4 and 5) and has a curved cross-sectional profile. This defines the profiled shape of the outer surface of the piece 12. Figure 6 is a cross-sectional side view of some of the components of the first part 20, shown in Figure 4. Figure 7 is a perspective view of some of the components shown in Figure 6. The first part 20 includes a movable part 32. The movable part 32 (which in this example is a ring, as seen in Figure 7), is movable between an uncompressed position (seen in Figure 6), in which the cavity 26 has a first shape, and a compressed position (represented by the vertical dashed line 34 in Figure 6), in which the cavity 26 has a second shape. The movable part 32 bounds an edge of the cavity 26 in which the piece 12 is formed. The first part 20 further includes an actuation mechanism 30, that is arranged to drive movement of the movable part 32. In this example, the actuation mechanism 30 is a hydraulic actuation mechanism 30 which is arranged, when connected to a hydraulic supply, to drive movement of the movable part 32 using hydraulic pressure. The actuation mechanism 30 and the movable part 32 are seen from a perspective view in Figure 7. As seen in Figures 2 and 6, when the first part 20 and the second part 22 are brought together, they form a cavity 26, within which the piece 12 is formed. The shape of the cavity 26 is defined partly by the shape of the recessed portion 28 of the first part 20. The shape of the cavity 26 is further defined by a protrusion portion 36 of the second part 22. This protrusion portion 36 can be seen in the side view of Figure 6, and can also be seen in Figures 8 and 9. Figure 8 is a front view of the second part 22 of the mould of Figure 2. Figure 9 is a perspective view of the second part 22, shown in Figure 8. This protrusion portion 36 defines the profiled shape of the inner surface of the piece 12 (i.e. the shape of the external surface that is closer to the central axis of the piece). Ejection portions 38 of the second part 22 are visible in the front view of Figure 8 and the perspective view of Figure 9. These ejection portions 38 are spaced circumferentially around the base of the protrusion portion 36. They are arranged so that when actuated they push upwards (i.e. out of the page, from the perspective of Figure 8) so as to project outwards from the face of the second part 22, and to push a piece 12, formed on the protrusion portion 36 away from the protrusion portion 36 and therefore away from the second part 22. This allows a piece 12 that has been formed inside the cavity 26, between the protrusion portion 36 and the recessed portion 28, to be released from the protrusion portion 36 after the mould 10 has been opened (i.e. the two parts 20, 22 have been separated from each other). Figure 10 is a perspective view of a lower layer of the second part 22, shown in Figure 9. Figure 10 shows that injection channels 40 are arranged beneath the protrusion portion 36. Beneath it will be understood that the injection channels 40 are arranged in a layer that is closer to an outer surface of the second part 22 than the protrusion portion 36, when the mould is assembled for use, the outer surface being the side that is furthest from the piece 12. Figure 11 is a perspective view showing only the injection channels 40 of the lower layer of the second part 22, shown in Figure 10. These injection channels 40 are seen on their own in Figure 11, and in position within the second part 22 in Figure 10. The injection moulding machine 2 is connected, in use, to these injection channels 40, so as to drive molten material into the cavity 26 through the injection channels 40, to form the piece 12. Figure 12 is a flow chart, representing a method of injection moulding, according to an embodiment of the present invention. This uses the components shown in Figures 1-11 and described above. The method of injection moulding will now be described with reference to the Figures. First, the two parts of the mould 20, 22 are secured together by the latch 24. Then the closed mould 10 is moved to the injection moulding machine 2, and secured between two platens 16, which apply a compression force, keeping the parts 20, 22 of the mould together. Then, during an injection phase 100, the injection moulding machine 2 injects molten material into the cavity 26, as the platens 16 continue to clamp the mould 10 together. Specifically, the molten material is injected into the cavity 26 via injection channels 40 in the second part 22 of the mould. In this injection phase 100, the movable part 32 of the first part 20 of the mould 10 is in the uncompressed position, e.g. as shown in Figure 2, giving the mould 10 an uncompressed configuration. As a result, a small “step” 42 forms at the top of the piece 12, e.g. as shown in Figures 2 and 6. The resulting piece 12 has what is referred to herein as the “first shape”. This “step” 42 is not present in the “second shape”, achieved when the movable part 12 is pushed to the compressed position, as represented by dashed line 34, seen in Figure 2. Next, at step 102 seen in Figure 12, the mould 10 is removed from between the platens 16 of the injection moulding machine 2, e.g. by an automated vehicle 14a, and moved to a separate compression and active-cooling station 3. A compression system 4, which in this example comprises a hydraulic supply, is connected to the first part 20 of the mould 10. Specifically, the hydraulic supply 4 is connected to the hydraulic actuation mechanism 30 of the first part 20 of the mould 10. A coolant circulation system 6 is also connected to the mould 10 (e.g. to the first part 20). Then, in a combined compression and active-cooling phase 104, following the injection phase 100, a force is applied to the movable part 32, using the external compression system 4 and the actuation mechanism 30 of the mould 10, to move the movable part 32 into the compressed position, as represented by the dashed line 34 seen in Figure 6. This causes the piece 12 to be deformed into the second shape as it cools. The combined compression and active-cooling phase 104 may last for a long period of time, for example at least 1 hour (e.g. approximately 1.5 hours), during which the piece 12 is gradually compressed from the first shape into the second shape, as it is being cooled. During this combined compression and active-cooling phase 104, the coolant circulation system 6 is activated to circulate fluid through a part of the mould (e.g. through cooling channels of the mould) so as to cool the piece 12, within the mould 10. It will be appreciated that many materials have a positive coefficient of thermal expansion. By this it is meant that their size increases at increased temperatures. Since the material is injected from the injection moulding machine at a high temperature (which is necessary to allow the molten material to be injected into the mould 10), and this material then cools, the dimensions of the piece 12, will decrease as the piece 12 cools, following injection. Furthermore, since the piece 12 that is being formed by the mould 10 is large, its outer surfaces (which have a large area for heat exchange and are closer to the cooled mould 10) will cool more quickly than the inner parts of the piece 12 (i.e. the central core, away from the outer sides). As a result the outside of the piece 12 will “set”, whilst the inside is still at a higher temperature. However, if this process is allowed to occur, then when the inner material subsequently cools it will also reduce in size when doing so, but the overall volume of the piece will have already been determined when the outside “set”. This may result in voids within the material. In contrast, by applying compression to the piece 12 after it is injected (and particularly advantageously by doing so during an active cooling phase so that the cooling process is better controlled) the piece 12 can be compressed to a smaller shape (i.e. the second shape) as it is cooled, thus reducing the risk of internal voids forming within the piece 12. Next, at stage 106, the mould 10 is opened. To do this, the mould 10, containing the piece 12, is returned to the injection moulding machine 2 (e.g. by an automated vehicle 14a). First the latch 24 is undone, then the injection moulding machine 2 is used to move the first part 20 and the second part 22 away from each other. When moved apart from each other, the piece 12 remains in the second part 22 of the mould, supported on the protrusion portion 36. Ejection portions 38 are then activated, to push upwards from the base of the protrusion portion 36, so as to lift the bottom edge of the piece away from the protrusion portion 36. This releases the piece 12 from the mould 10. An automated vehicle 14b then moves the piece 12, no longer inside the mould 10, to a support 8 (stage 108, Figure 12). The piece 12 is placed on top of the support 8, which is shaped to receive it. In this position, the piece 12 is supported from beneath as it cools further, until the entire shape is “set”. During this external-cooling phase 110, the piece 12 cools further by passive cooling, i.e. by dissipating heat to its ambient environment. This external-cooling phase 110 may last for approximately 8 hours. Figures 13-16 are cross-sectional side views of a second exemplary mould that may be used in the injection moulding system of Figure 1. Components which are alike with the mould of the first example are labelled with the same reference numerals as in the first example, but followed by an apostrophe, e.g. a mould 10’ rather than a mould 10. This example mould 10’ differs from the mould 10 of the first example in that instead of the first part including a movable part, to allow the shape of the cavity to be changed, instead the entire first part 20’ of the mould 10’ is arranged to move relative to the second part 22’, so as to change the shape of the cavity 26’ defined between them. Figure 13 shows the two mould parts 20’, 22’ arranged relative to each other to define a cavity 26’ between them having a first shape. In order to define the first shape between them, a height gap 11’, along a first direction 9’ parallel to a central rotational axis of the mould parts 20’, 22’, indicated by the pair of dashed lines and arrows, is left between the first part 20’ and the second part 22’. The first part 20’ includes a recess 23’, which is sized and shaped to accommodate a corresponding protrusion 25’ of the second part 22’. In this example, the height gap 11’ is approximately (or at least) 25 mm. This may allow approximately 2% more material to be injected into the cavity 26’ than the volume of the final, second cavity shape requires, as explained further below. As explained above, this helps to compensate for material shrinkage. In an example method of injection moulding, first the mould parts 20’, 22’ are arranged into this configuration, providing a cavity with the first shape. Next, in an injection phase 100’, material is injected into the cavity 26’ having the first shape, by the injection moulding machine 2’, to produce a piece 12’ having substantially the first shape, as illustrated in Figure 14. In this example the injection phase lasts for approximately 5 minutes. As material is injected into the cavity at stage 100’ simultaneously heating fluid (e.g. hot water) is circulated through the mould, through the coolant circulation channels, to keep the material warm. At this stage, the height gap 11’ between the mould parts is still the same as that illustrated in Figure 13. Then at stage 104’, as illustrated in Figure 15, a compression force 13’ is applied to at least the first part 20’ (and optionally also to the second part 22’) to move the mould parts 20’, 22’ together. This reduces the size of the height gap 11’ and thereby changes the shape of the cavity 26’ gradually from the first shape, shown in Figure 14, to the second shape, shown in Figure 16. As seen in Figure 15, the protrusion 25’ moves into the recess 23’, to allow the height gap between the first part 20’ and the second gap 22’ to be reduced. This movement of the mould parts relative to each other applies a compression force across the entire outer surface of the piece 12’ within the cavity 26’ (i.e. of the curved surfaces), therefore providing compression evenly to the piece 12’ and compressing it to have the second, smaller shape. During this stage 104’, coolant is circulated around the mould, to actively cool the piece 12’. This stage 104’ lasts for approximately 45 minutes in this example. Figure 16 shows the two mould parts 20’, 22’ at the end of the compression phase 104’, having been pushed into contact, such that the height gap 11’ between them is zero (or at least negligible), and the protrusion 25’ is fully accommodated within the recess 23’, i.e. such that the mould 10’ is in the fully closed configuration. Then, at stage 105’, the mould 10’, containing the piece 12’ now having the second (i.e. fully compressed) shape, is removed from the injection moulding machine, and transported to a separate, second coolant circulation system. It is connected to the coolant circulation system and the piece 12’ is further cooled by the circulation of more coolant fluid, this time away from the injection moulding machine. During this time the injection moulding machine is available for use producing another piece. This secondary active-cooling stage 105’ lasts for approximately 3-4 hours in this example. Next, at stage 106’, the mould 10’ is returned to the injection moulding machine and the piece 12’ is removed. At stage 108’ the piece 12’ is transported to a cooling area, where it continues to cool passively, outside of the mould 10’, during stage 110’. This external cooling phase in this example lasts for approximately 24 hours, at the end of which the piece 12’ is ready for further processing, e.g. for welding to other components. It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

1. A method of injection moulding using a mould defining a cavity, the mould comprising a first part and a second part, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a compressed configuration, in which the cavity has a second shape, the method comprising:in an injection phase, injecting material into the cavity to produce a piece within the mould having the first shape;in a compression phase, following the injection phase, applying force to the first part, to move the first part such that the mould is moved from the uncompressed configuration to the compressed configuration, so that the piece is compressed to have the second shape.

2. The method of claim 1, wherein the first part and the second part define the cavity.

3. The method of claim 1, wherein the first part is a movable part arranged within the cavity.

4. The method of any preceding claim, wherein the injection phase is carried out using an injection moulding machine, the method further comprising removing the mould, containing the piece, from the injection moulding machine before the compression phase.

5. The method of any preceding claim, further comprising connecting a compression system to the mould, and applying force to the first part during the compression phase, using the compression system, so as to move the mould to the compressed configuration.

6. The method of claim 5, wherein the compression system is a hydraulic pressure system, and wherein the method comprises applying force to the first part using hydraulic pressure.

7. The method of any preceding claim, further comprising, in an active-cooling phase, cooling the piece, within the mould, by circulating a coolant fluid through a part of the mould.

8. The method of claim 7, wherein the compression phase and the active-cooling phase overlap in time.

9. The method of claim 7 or 8, wherein the active-cooling phase lasts for at least one hour.

10. The method of any preceding claim, further comprising, in a removal phase, removing the piece having the second shape from the mould.

11. The method of claim 10, wherein the injection phase is carried out using an injection moulding machine, and wherein removing the piece having the second shape from the mould comprises the injection moulding machine opening the mould.

12. The method of claim 10 or 11, wherein the mould comprises an ejection portion, and wherein the removal phase comprises actuating the ejection portion of the mould, to apply an ejecting force onto the piece.

13. The method of any of claims 10 to 12, further comprising, in an external-cooling phase, positioning the piece, no longer within the mould, onto a support.

14. The method of claim 13, wherein the external-cooling phase lasts for at least 4 hours.

15. The method of any preceding claim, further comprising, transporting the mould and / or the piece, outside of the mould, using an automated vehicle.

16. The method of any preceding claim, comprising applying a clamping force to an exterior of the mould.

17. The method of claim 16, comprising applying, during the injection phase, a first clamping force, to an exterior of the mould, and applying, during the compressionphase, a second clamping force, to the exterior of the mould, the second clamping force having a different magnitude than the first clamping force.

18. The method of any preceding claim, wherein the material injected into the cavity is polymer.

19. The method of any preceding claim, wherein the mould is configured to produce a piece weighing at least 50 kg.

20. The method of any preceding claim, wherein the injection phase lasts for at least 3 minutes.

21. The method of any preceding claim, wherein the compression phase comprises applying a force of approximately 20000 kN to the first part.

22. The method of any preceding claim, wherein the compression phase lasts for at least 30 minutes.

23. The method of any preceding claim, wherein the difference in volume between the first shape and second shape of the cavity is approximately equal to the change in volume of the piece due to shrinkage of the material during cooling.

24. An injection moulding system, comprising:an injection moulding machine;a mould, defining a cavity, the mould comprising a first part and a second part, the first part movable relative to the second part between a first relative position, giving the mould an uncompressed configuration, in which the cavity has a first shape, and a second relative position, giving the mould a compressed configuration, in which the cavity has a second shape;the injection moulding system configured to carry out the method of any preceding claim.

25. The injection moulding system of claim 24, further comprising a first automated vehicle arranged to transport the mould and a second automated vehicle arranged to transport the piece.

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