GB2172240A

GB2172240A - Injection moulding

Info

Publication number: GB2172240A
Application number: GB08605204A
Authority: GB
Inventors: Malcolm Terence Barrett
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-03-01
Filing date: 1986-03-03
Publication date: 1986-09-17
Also published as: GB2172240B

Abstract

A hot runner system unit 3 for use in thermoplastic injection moulding comprises a main flow path 7 which is connected at one end with a cold runner 9 in a mould 1 and which is connected at its outer end 13 with an injection device (not shown). To enable the material in a mould to be maintained in a pressurised state after the injection device has been disconnected and whilst the material achieves physical stability, moulding material also flows into a lateral cavity 17 which branches off the main flow path 7 and houses a piston 19 which is rearwardly displaced by the pressure of the material. Operation of an actuator to displace the piston 19 to its extended position maintains the pressurised supply of hot melt to the mould cavity from the lateral cavity 17, after disconnection of the injection device. <IMAGE>

Description

SPECIFICATION Improvements relating to injection moulding The present invention relates to injection moulding and in particular to the injection moulding of thermoplastic materials.

In the injection moulding of thermoplastic material, the raw material is fed, usually in granulated form, into a heated screw and barrel which converts it into a hot melt. This hot melt is then fed or injected, under pressure, from an injection device into a closed mould via a flow path which is termed a runner system.

To reduce the percentage of material which is wasted or must be regranulated for re-cycling it is common practice to provide a hot runner system to provide a reservoir of hot melt material between the injection device and the mould cavities so that the length of the cold runner finally leading into the mould cavities is kept to a minimum. This hot runner system is normally separated from the mould proper which has to be cooled, by some insulating mechanism.

To cool the mould a cooling medium e.g.

water, is usually passed through a system of passages in the mould to cool and thus solidify the thermoplastic material within the mould, prior to the opening of the mould and the extraction of the moulded article. This cooling time varies according to the material, temperature of the cooling medium, thickness of article etc. Also the thermoplastic material can shrink in the mould cavity as it cools, causing deformation and voids in the moulded article. To counteract this shrinking problem it is normal to keep the injection device in operational contact with the mould, to thus keep the thermoplastic material within the mould under pressure, until the material filling the mould cavity has cooled sufficiently to solidify and achieve physical stability. At this time the injection device can be switched off and the mould can be subsequently opened and the moulded article extracted.After extraction the mould can be closed and the injection device activated to provide a further moulding.

To achieve greater productivity from moulds and machines some manufacturers make use of systems in which one injection device sequentially fills a series of moulds. The moulds may be mounted on a turntable or in-line with the injector static. Alternatively the moulds may be in static stations with a moving injector. Such systems allow for the injection device to be processing a fresh charge of material and filling an empty mould whilst filled moulds are cooled.

With multi-station and even single station machines, it is important economically that the injection device should have as little wasted time i.e. dead time, as possible in its operational cycle. It is thus of prime importance that the injection device is kept in operational contact with a mould for a minimum of time after the mould has been filled.

To optimise the operational cycle of the injection device, moulds have been provided with their own after pressure arrangements so that after the mould has been filled, the injection device can be disconnected and used to fill a further mould, whilst the cavity pressure in the filled mould is maintained by the after pressure arrangement.

The known after pressure arrangement is provided in the hot runner system and applies pressure to the hot melt store in the hot runner system, when the injection device is disconnected from the mould. The after pressure arrangement basically comprises a lateral cavity leading off from the main flow path in the hot runner system, with an axially displaceable piston located in the lateral cavity. The piston is controlled by a suitable actuator e.g. spring, and when the mould cavity has been filled and pressurised the hot melt in the lateral cavity forces the piston back along the lateral cavity.

Subsequently the injection device is disconnected and the actuator can be used to apply pressure to the hot melt in the hot runner system and to the mould cavity, a one way valve preventing ejection of the material out of the injection inlet to the hot runner system.

Whilst the above after pressure arrangement can operate as required, subsequent connection of the injection device to the hot runner system after the mould has been emptied, will clear the main flow path of old melt but will leave old melt in the lateral cavity. As time elapses the old melt in the lateral cavity will degrade and if it at some time escapes from the lateral cavity into the mould cavity it can affect the quality of the finished moulded article. This is especially undesirable if, for example, pressurised pipes, or pipe joints, are being manufactured, the degraded material forming a weakness in the finished product.

The aim of the present invention is to overcome the above problem so that the thermoplastic material is never stored sufficiently long enough in the hot runner system, to degrade.

According to the present invention there is provided a hot runner system unit for use in thermoplastic injection moulding, said unit providing a main flow path which is connectible at one end with a cold runner in a mould when the unit is operationally connected to a mould, and which is connectible at its other end with an injection device, a cavity branching off from the main flow path and housing a piston which is axially displaceable by an actuator and which, in its fully extended position, closes the cavity entrance and forms a part of the wall of the main flow path.

According to a further feature of the present invention there is provided a mould assembly for use in thermoplastic injection moulding, said assembly comprising a mould which defines a mould cavity, and a hot runner system unit which is attached to the mould though thermally insulated therefrom, the hot runner system unit providing a main flow path which connects at one end with a cold runner in the mould, said cold runner leading to the mould cavity, and which is connectible at its other end with an injection device, a cavity branching off from the main flow path and housing a piston which is axially displaceable by an actuator and which, in its fully extended position, closes the cavity entrance and forms a part of the wall of the main flow path.

Thus, by virtue of the present invention, the lateral cavity branching off from the main flow path can be maintained completely closed whilst the mould is filled and pressurised.

When the mould is filled and pressurised to a certian pressure, the piston is moved to open the lateral cavity, hot melt filling the lateral cavity. Then with the lateral cavity filled and pressurised, the injection device can be disconnected and the actuator can move the piston in the lateral cavity to maintain the pressure in the mould cavity, a one-way valve preventing the egress of hot melt from the inlet to the hot runner system unit.

After a suitable period of time, when the thermoplastic material in the mould cavity has cooled sufficiently to attain physical stability, the actuator pressure is released and the mould is opened. The moulded article is then extracted, the solidified thermoplastic material in the cold runner of the mould being extracted with said moulded article. Subsequently, the mould is closed and the piston is moved along the lateral cavity to a position in which the lateral cavity is completely closed, all hot melt being thus expelled from the lateral cavity.Thus, by virtue of the present invention no hot melt remains in the hot runner system long enough to degrade during continuous usage and even if the mould assembly of the present invention is left for a period of time, all of the degraded material retained in the hot runner system can be easily expelled from the main flow path before moulding actually commences.

In a preferred embodiment of the present invention, the actuator is hydraulically operable and is located outside the hot runner system unit to avoid the obvious problems due to the effect of heat in hydraulic fluid. Alternatively, however, the actuator may be pneumatically operable, electrically operable, or even mechanically operable.

As will be appreciated, the end of the piston can be profiled to be complementary to the shape of the wall of the main flow path.

For example, a cylindrical flow path will require a complementarily curved end face for the piston.

Whilst seals provided on the piston are intended to prevent the leakage of hot melt past the piston and into the lateral cavity, a more positive seal may be provided by forming the end of the lateral cavity contiguous with said main flow path, with an inwardly directed lip, with which a complementarily profiled edge of the piston engages when the piston is in the fully extended position. The piston is thus seated on the inwardly directed lip preventing any leakage during initial filling and pressurising of a mould.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a schematic cross-sectional view of one embodiment of the present invention, and Figure 2 is an enlarged schematic view of part of a modified piston and cylinder arrangement for use in the present invention.

The mould assembly constructed according to the present invention and illustrated in the accompanying drawing, comprises a mould 1 and a hot runner system unit 3. The mould 1 is attached to the hot runner system unit 3 with a thermally insulating layer 5 located therebetween.

The hot runner system unit 3 has a main flow path 7 extending therethrough, which connects at one end with a cold runner 9 in the mould 1; the cold runner 9 leading to the mould cavity 11. The other end 13 of the main flow path 7 is connectible to an injection device (not shown) and incorporates a oneway check valve 15.

A lateral cavity 17 branches off the main flow path 7 and houses a piston 19 which is connected via a piston rod 21 to an actuator (not shown) located externally of the hot runner system unit 3. The actuator (not shown) is preferably hydraulically operable, though it may alternatively by pneumatic, electrical or mechanical in its operation. It is, however, located preferably externally of the hot runner system unit 3 to prevent any heat problems.

The lateral cavity 17 is cylindrical and the piston 19 is a complimentary cylindrical shape, so that it can be slid axially within the cavity 17; suitable seals 23 preventing the leakage of hot melt material past the piston 19. Alternatively, the lateral cavity 17 and the piston 19 can have any other desired transverse cross-section.

In the illustrated fully extended position, the piston 19 completely closes off the lateral cavity 17, the end face 25 of the piston 19 being profiled to provide a smooth continuous surface with the wall of the main flow path adjacent to the branch opening into the lateral cavity 17. Whilst the illustrated end face 25 is curved, the end face may, in other embodiments, equally well be part cylindrical or planar, dependent upon the configuration of the adjacent wall of the main flow path 7. With this construction, no hot melt can ever remain in the lateral cavity once the piston 19 has been fully extended.

In use with an injection device (not shown) connected to the inlet end 13 of the main flow path 7, and the piston 19 in the fully extended position, closing off the lateral cavity 19, thermoplastic material is injected through the main flow path 7 and into the mould cavity 11. When the mould cavity 11 is full of thermoplastic material and pressurised to a limited extent, the piston 19 then automatically moves back into the lateral cavity 17 due to the differential pressure created between the mould cavity and an adjustable preset basic back pressure acting on the piston 19 prior to operation of the actuator. This retraction of piston 19 allows hot melt to fill the lateral cavity 17.With the piston 19 in its fully retracted position the injection device (not shown) is disconnected from the inlet end 13 of the main flow path 7 and the actuator (not shown) is activated to apply pressure to the hot melt in the lateral cavity 19, the main flow path 7 and the mould cavity 11; check valve 15 preventing the egress of hot melt via inlet end 13. When a sufficient period of time has elapsed to allow the moulded article to cool enough to achieve physical stability, the actuator pressure is released and the mould is opened. In extracting the moulded article from the mould cavity 11, the solidified thermoplastic material from the cold runner 9 is also extracted. This can be trimmed from the finished article and recycled.Following extractions of the moulded article, the mould is closed and the piston 19 is moved to its fully extended position to both completely empty the lateral cavity 17 of hot melt and also close the lateral cavity for a subsequent moulding operation.

Thus, by virtue of the present invention the lateral cavity 17 is always completely emptied of thermoplastic material before any moulding occurs, preventing any degraded material from being moulded in the moulded article. Further, if the mould has been out of use for a period of time, the present invention enables the mould to be flushed completely free of any residual, possibly degraded, material.

Whilst the seals 23 are provided in the embodiment of Fig. 1 both to locate the piston 19 coaxially within the cylindrical lateral cavity 17 and to prevent the leakage of hot melt material past the piston 19, a more positive seal during initial filling and pressurisation of the mould can be obtained by the modification schematically illustrated in part in Fig. 2. This modification basically comprises the provision of an annular inwardly directed lip 27 on the end of the lateral cavity 17 adjacent to the main flow path 7, and the provision of a complementary chamfer 29 on the peripheral edge of the piston 19. In the fully extended position illustrated in Fig. 2, the chamfered region 29 is seated against the complementary profile of the inwardly directed lip 27 so that it is imposible for hot melt material to leak past the piston 19 in this position. Any complementary profiles of chamfer and lip can, of course, be substituted with the same effect.

The mould of the present invention thus improves the consistent quality of moulded articles, especially in a multi-mould arrangement which is sequentially filled by a fewer number of injection devices.

Claims

1. A hot runner system unit for use in thermoplastic injection moulding, said unit providing a main flow path which is connectible at one end with a cold runner in a mould when the unit is operationally connected to a mould, and which is connectible at its other end with an injection device, a cavity branching off from the main flow path and housing a piston which is axially displaceable by an actuator and which, in its fully extended position, closes the cavity entrance and forms a part of the wall of the main flow path.

2. A hot runner system unit as claimed in claim 1, in which the piston is profiled to form a continuous smooth surface with the wall of the main flow path when in the fully extended position.

3. A hot runner system unit as claimed in claim 1 or claim 2, in which the lateral cavity is cylindrical and the piston has a complementary axial shape.

4. A hot runner system unit as claimed in any one of claims 1 to 3, in which the end of the lateral cavity adjacent to the main flow path is provided with an inwardly directed lip having a part profile against which a complementarily shaped peripheral region of the piston can engage when the piston is in the fully extended position.

5. A hot runner system unit as claimed in any one of claims 1 to 4, in which the actuator is hydraulically operable.

6. A hot runner system unit as claimed in any one of claims 1 to 5, in which a one-way valve is provided in the main flow path to prevent egress of hot melt from said unit when the injection device is disconnected.

7. A hot runner system as claimed in any one of claims 1 to 6, when in combination with a mould which defines a mould cavity, the hot runner system unit being attached to the mould though thermally insulated therefrom, with the main flow path connected to the cold runner which leads to the mould cavity.

8. A hot runner system unit constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.