GB2514141A

GB2514141A - Injection moulding

Info

Publication number: GB2514141A
Application number: GB201308669A
Authority: GB
Inventors: Christopher James Whiteley
Original assignee: Oxford Plastic Systems Ltd
Current assignee: Oxford Plastic Systems Ltd
Priority date: 2013-05-14
Filing date: 2013-05-14
Publication date: 2014-11-19
Anticipated expiration: 2033-05-14
Also published as: GB201308669D0; GB2514141B

Abstract

A method of injection moulding an article comprises supplying mouldable material into a supply chamber, injecting mouldable material 24 from the supply chamber 21b into the mould via a delivery chamber 21e using a first piston 21a, and subsequently injecting mouldable material 24 present in the delivery chamber 21e into the mould and sealing off the delivery chamber 21e from the supply chamber 21b using a second piston 21d. Mouldable material 24 is allowed to solidify within the mould thereby forming the article and subsequently removed from the mould. The method further comprises, when the second piston 21d has sealed off the delivery chamber from the supply chamber, withdrawing the first piston 21b and supplying further mouldable material 24 into the supply chamber. The first piston 21b may be used to pressurize the mouldable material 24 in the supply chamber 21b to force any volatile materials out of the mouldable material 24.

Description

INJECTION MOULDING

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

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 11, a mould assembly 12, and a clamp/press 13.

The injection unit 11 is provided by an extruder that comprises a heated barrel ha housing an extruder screw lib that is driven by a screw motor lic. Mouldable material 12 is fed into the central bore of the extruder barrel 1 la via a hopper lid, and can exit a distal end of the barrel ii a via a nozzle lie. The mould assembly 12 includes two mould sections 1 2a, 1 2b 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 through is which mouldable material 14 is injected into the mould cavity i2c from the nozzle lie of the injection unit ii. The clamp/press 13 comprises a moveable platen i3a on which the second mould section i2b 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 clamp motor 13b drives the moveable platen 13a towards the injection unit 11, and thereby moves the second mould section i2b toward the first mould section i2a 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 llb is rotated by the screw motor llc thereby forcing the raw material towards the nozzle lie.

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 lb causes the raw material to melt as it moves towards the nozzle lie. The melted raw material then accumulates adjacent to the nozzle lie, where it is held until an appropriate volume of material (referred to as a shot) has accumulated. When sufficient material has accumulated, the screw llb is used as an injection ram, and is driven towards the nozzle lie to force the melted material through the nozzle lie 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 hold the mould 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 and 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 the mould via a delivery chamber using a first piston, subsequently injecting mouldable material present in the delivery chamber into the mould and sealing off the delivery chamber from the supply chamber using a second piston, allowing the mouldable material to solidify within the mould thereby forming the article and subsequently removing the article from the mould. The method further comprises, when the second piston has sealed off the delivery chamber from the supply chamber, withdrawing the first piston and supplying further mouldable material into the supply chamber.

The first piston can inject a first volume of mouldable material whilst the second piston injects a second volume of mouldable material. The first volume may be between 60% and 90% of the total mouldable material injected, and the second volume may 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, the first direction being 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.

The method may further comprise, when compressing the mouldable material in the supply S 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.

According to a second aspect there is provided a moulded article manufactured using the method according to the first aspect.

According to a third aspect this is provided an injection moulding apparatus. The injection moulding apparatus comprises a mould, a supply chamber configured to receive a supply of mouldable material, and a delivery chamber having an inlet from the supply chamber and an outlet to the mould. The apparatus further comprises 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 to seal off the delivery chamber 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 comprise a clamp configured to open and close the two or more mould sections.

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.

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, and a clamp/press 23.

The mould assembly 22 includes two mould sections 22a, 22b that when brought together (i.e. closed) 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 closed against the pressure of the material injected into the mould.

The injection unit 21 comprises a first piston 21a housed within a first cylinder 21b and driven by a first piston driver 21c, and a second piston 21d housed within a second cylinder 21e and driven by a second piston driver 21f. The proximal ends of both the first cylinder 21b and the second cylinder 21e are connected to a common delivery chamber 21g, and the common delivery chamber 21g is provided with a nozzle 21h through which material can pass into the mould 22 via the sprue 22d. In this regard, the first cylinder 21b and the first piston 21a are disposed perpendicularly relative to the nozzle 21h and the sprue 22d, whilst the second cylinder 21e and the second piston 21d are linearly aligned with the nozzle 21h and the sprue 22d. The first cylinder 21b and the first piston 21a are therefore perpendicular to the second cylinder 2ie and the second piston 21d.

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 first piston 21a is initially in a retracted position, whilst the second piston 21d is in a fully inserted position so as to seal/close off the first cylinder 21b from the delivery chamber 21 g, as illustrated in Figure 3a. Mouldable material 24 is then fed into the first cylinder 21b (Si), which therefore acts as a supply chamber for supplying material into the apparatus. For example, a raw material can be fed through a hopper into a heated extruder.

An extruder screw would then be rotated in order to mix and force the material through the extruder and onto a chute/channel that feeds into the first cylinder 21 b/supply chamber.

After a sufficient supply of mouldable material has been added to the supply chamber/first cylinder 21b, the first piston 21a is then driven partially into the first cylinder 21b 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 21a into the first cylinder 21b provides a pie-compression step, before any material is injected into the mould, that degasses the mouldable material by forcing any volatiles (e.g. air and other gases) out of the mouldable matenal and allowing these to escape from the open, distal end of the first cylinder 21b (as illustrated by the arrows in Figure 3b).

The second piston 21d is then retracted (S3), thereby unsealing the supply chamber/first cylinder 21b from the delivery chamber 21g. An initial/first injection procedure is then implemented, as illustrated in Figure 3c, in which the first piston 21a is driven into the first cylinder 21b in order to force mouldable material 24 from the supply chamber/first cylinder 21b into the mould via delivery chamber 21g (S4). The volume of mouldable material that can be delivered into the mould during this initial injection procedure is defined by volume of material present in the supply chamber/first cylinder 2ib, 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 procedure will typically inject between 60% and 90% of the total mouldable material that is required.

A further/second injection procedure is then implemented, as illustrated in Figure 3d, in which the second piston 21d 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 procedure) into the mould (S6). The volume of mouldable material that can be delivered into the mould during this further injection procedure is defined by volume of delivery chamber and is sufficient to fill any voids remaining in the mould cavity following the initial injection procedure.

Consequently, the combined volume of material injected into the mould by the combination of the initial injection procedure and further injection procedure is 100% of the total mouldable material that is required.

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 (Si).

As soon as the second piston 21d is inserted far enough into the delivery chamber 21g to seal off the first cylinder 21b from the delivery chamber 21g (S5), the first piston 21a is retracted from the first cylinder 21 b (SB). A further batch of mouldable material 24 is then fed into the first cylinder 21b in preparation for the next injection moulding operation (Si), as illustrated in Figure 3e. Therefore, during the time taken for the further injection procedure and subsequent solidification of the moulded article, the apparatus is primed/made ready to perform a further injection moulding operation. This is possible because, when the second piston 21d is inserted into the delivery chamber 21g, the second piston 21d seals off the first cylinder 21b from the delivery chamber 21g. 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.

S 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.

is 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.

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

CLAIMS1. A method of injection moulding an article, the method comprising: supplying mouldable material into a supply chamber (Si); injecting mouldable material from the supply chamber into the mould via a delivery chamber using a first piston (54); subsequently, injecting mouldable material present in the delivery chamber into the mould and sealing off the delivery chamber from the supply chamber using a second piston (55, 56); allowing the mouldable material to solidify within the mould thereby forming the article and subsequently removing the article from the mould (57); and when the second piston has sealed off the delivery chamber from the supply chamber, withdrawing the first piston and supplying further mouldable material into the supply chamber.
2. A 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. A 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. A 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 perpendicular to the second direction.
5. A 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 (52).
6. A 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).
7. A moulded article manufactured using the method according to any of the preceding claims.
8. An injection moulding apparatus (20) comprising: a mould (22); a supply chamber (21b) configured to receive a supply of mouldable material; a deliveiy chambei (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 to seal off the delivery chamber from the supply chamber.
9. An injection moulding appalatus accoiding to claim 8, wherein the appalatus 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.
10. An injection moulding apparatus according to any of claims B or 9, wherein the first piston is substantially peipendicular to the second piston.
11. An injection moulding apparatus according to any of claims 8 to 10, wherein the mould comprises two or more mould sections (22a, 22b), and the apparatus further comprises a clamp (23) configured to open and close the two oi more mould sections.