GB2328901A - Equalised in-mould pressure system for gas assisted moulding - Google Patents

Abstract

Apparatus to control a gas supply for gas assisted moulding apparatus including a master gas pressure control means (18) connected in series to at least two parallel outlets (19,20) for connection to a gas injection position in a moulding apparatus. At least one of the gas outlets (19,20) includes a further gas pressure control means (21,22,23). Each gas pressure control means may be provided by a stop valve and/or a flow control valve. Each gas pressure control means may be controlled by a programmable control means.

Description

EQUALISED IN-MOULD PRESSURE SYSTEM FOR GAS ASSISTED MOULDING The present invention relates to a gas supply system for use in gas assisted injection moulding.

Gas assisted injection moulding is a well established process for the moulding of plastics materials.

In one known method gas is injected into the flow of plastic at the moulding machine nozzle. In another existing method gas is injected directly into the mould, either directly into the moulding article cavity or into a feed runner up-stream of the cavity feed gate. For some mouldings it may be advantageous to inject gas at multiple positions including both in mould and in machine nozzle injection.

For the production of larger or more complex mouldings it is usually desirable that gas is injected at more than one position within a mould cavity or mould. This allows gas channels to be formed in and different gas pressures to be applied at different positions within the moulded article or into different plastic runner feed positions, during the moulding cycle.

Where multiple gas injection positions are employed it is desirable that the pressure of gas at each of these injection positions is controlled individually both in terms of the timing of gas injection and the gas pressure applied at each position.

In an existing system control of gas pressure and timing of gas delivery at different injection positions is achieved by individual, independently controlled modules of pressure regulating valves and ancillary equipment under the control of a microprocessor or computer control system.

It is also desirable when gas is injected at multiple positions that it is supplied on a predetermined pressure/time programme and that injection from the multiple positions is phased. This method of operation of moulding apparatus is described in GB 93913379.9.

Where existing gas pressure control systems for gas assisted injection moulding with multiple gas injection positions are used, problems and inconsistent moulding quality can result from different gas pressures being applied in the same mould cavity or in different cavities in the same moulding simultaneously. In some situations this can result in the transfer or displacement of plastic from one position to another or from one mould cavity to another. Further, in some cases plastic can be forced from a high pressure position to a low pressure position, which may include a thicker gas channel.

This latter situation can lead to blockages of gas channels and the formation of thick solid sections in the lower pressure gas channels, which can then cause gas entrapment which leads to bursting of the moulding after ejection from the mould tool. Further, and for the same reason, 'fingering' can occur. This is the term referring to the penetration of gas into general wall sections leading to occasional isolation of gas bubbles with consequent bursting, and to moulding surface defects.

It is difficult to use existing equipment to apply precisely equal pressure to more than one position in a moulding cavity or to more than one cavity at once, whilst still maintaining independent control of pressure build up in each cavity.

It is an object of the present invention to overcome or at least minimise the above mentioned problems with existing gas pressure control equipment for use in gas assisted injection moulding.

According to the present invention there is provided apparatus to control a gas supply for gas assisted moulding apparatus comprising a master gas pressure control means connected in series to at least two parallel outlets at least one of which includes a further gas pressure control means.

Preferably each gas outlet includes a further gas pressure control means, and each control means enables control of the commencement of gas flow.

The gas pressure control means may comprise gas control circuits as employed in existing pressure control apparatus for gas assisted injection moulding. At least one and preferably all of the gas pressure control means preferably comprises a stop valve and/or a flow control valve or equivalent, to enable the gas pressure control means to both stop or start the flow of gas and to select a desired flow rate.

The commencement of gas flow and/or the rate of flow control may be operable under electric or electronic control.

In operation the master gas pressure control means is preferably connected to a gas supply and controls the pressure of gas to each of the gas outlets and further pressure control means, in series. The master gas pressure control means may be connected to the gas outlets or further gas pressure control means by a manifold.

Each gas outlet is preferably connected to a gas injection position in a moulding apparatus where a gas outlet includes a further gas pressure control means this controls the flow of gas to the gas injection position. As many gas outlets and further gas pressure control means as required may be connected to a single master gas pressure control means, any of the further gas pressure control means may supply gas to still further gas pressure control means.

Preferably each gas pressure control means is controlled by a control means, preferably a programmable control means. The control means is preferably arranged to provide timed operation of the individual gas pressure control means. The equipment for the timed opening of the gas pressure control means may comprise electronically operated solenoid valves or compressed air control stop valves. The equipment for the control of gas flow may comprise hand-operated flow control valves or remote control valves preset to a selectable programme.

Where the further pressure control means supply gas to various gas injection positions in a moulding apparatus, then they may be used to stagger the injection of gas at these positions and/or to control the rate of flow of gas at these positions. As each further gas pressure control means derives its gas supply from a master control means the maximum gas pressure that can be supplied by each further pressure control means is restricted by the master pressure control means. This ensures that the maximum pressure supplied to any one gas injection position in a moulding apparatus and hence, to any cavity in a moulded article is the same. The maximum gas pressure may be set by adjustment of the master gas pressure control means. This combination overcomes the problems associated with existing independent gas pressure control apparatus.

Further, when it is desired to reduce the gas pressure supplied to the moulding apparatus this may be effected by adjusting the master pressure control means to enable the gas pressure supplied to each gas injection position to be reduced simultaneously and thereby avoiding pressure differences within the moulded article which occur when conventional independent gas pressure control apparatus is employed.

In order that the invention may be more clearly understood there are now described embodiments thereof by way of example with reference to the accompanying drawings in which: Figure 1A illustrates a typical prior art moulding gas pressure time cycle; Figure 1 B illustrates a typical prior art moulding gas pressure/time cycle with two independently controlled gas pressure master circuits; Figure 2 illustrates schematically a gas distribution manifold in diagrammatic form, with a common master circuit feeding into two sub-circuits, one gas circuit is fitted with a solenoid stop valve; Figure 3 illustrates schematically a gas distribution manifold with a common master circuit feeding two sub-circuits, each including a manually operated flow control valve; Figure 4 illustrates schematically a gas distribution manifold fed by a common master circuit feeding into two sub-circuits, both fitted with a flow control valve and one fitted with a solenoid control stop valve; Figure 5 shows a pressure/time diagram which illustrates the effect of delaying the commencement of gas injection by one circuit (SC2) in relation to another circuit (SC1), both circuits are controlled by a common master circuit, and one circuit (SC2) is fitted with a solenoid control stop valve; Figure 6 illustrates the effect of reducing the flow rate of gas circuit (SC2) in relation to circuit (SCi) by manually adjusted flow control valves; and Figure 7 illustrates a combination of delayed gas injection commencement and reduced gas flow rate of sub-circuit (SC2) in relation to circuit (SC1).

Referring to Figures 1A and 1B there are illustrated typical moulding cycle gas pressure/time graphs for a moulding with one and two cavities respectively where gas is provided by prior art gas pressure control apparatus.

Pressure is plotted on the vertical axis 1 and time on the horizontal axis 2.

Referring to Figure 1 B the two lines plotted on the axes represent the gas pressure in two different cavities in a moulded article during a moulding process. The two lines, labelled C1 and C2, represent cavities 1 and 2 respectively. Similarly C1 and C2 may represent the injection of gas into one mould cavity, from two injection positions.

The application of different gas pressures to cavities 1 and 2, or at two positions in the same cavity, is illustrated in Figure 1 B. The pressure in cavity 1 initially rises to a maximum value and is maintained at the value for a period of time represented by a flat portion 3 in the plot. The pressure is subsequently reduced via two pressure stages represented at portions 4 and 5 of the plot.

Comparing the plot for cavity 1 with that for cavity 2, it is apparent that the pressure time cycle for cavity 2 is delayed with respect to that for cavity 1 and also that the three decreasing pressure stages, represented by flat portions 6,7 and 8 of the plot, are at different pressures to those in cavity 1 at 3,4 and 5 on the plot.

These pressure differences can cause problems and inconsistent moulding quality can result.

In some situations this can result in the transfer or displacement of plastic from one position to another or from one mould cavity to another. Further, in some cases plastic can be forced from a high pressure position to a low pressure position, which may include a thicker gas channel. This latter situation can lead to blockages of gas channels and the formation of thick solid sections in the lower pressure gas channels, which can then cause gas entrapment which leads to bursting of the moulding after ejection from the mould tool.

Further, and for the same reason 'fingering' can occur. This is the term referring to the penetration of gas into general wall sections leading to occasional isolation of gas bubbles with consequent bursting, and to moulding surface defects.

Referring to Figure 2 there is illustrated one embodiment of the present invention which comprises a master gas pressure control circuit 9 which feeds to sub-circuits 10 and ii Sub-control circuit 11 includes a solenoid stop valve 12. In this embodiment overall control of gas pressure is governed by the master control circuit 9. Also, the commencement of gas flow from sub-circuit 11 may be further controlled by solenoid valve 1 2. This embodiment allows gas to be directed initially only to one injection position with flow to a second injection position being delayed as required.

Referring to Figure 3 there is illustrated an alternative embodiment.

Master control circuit 13 feeds to sub-circuits 14 and 15. Each sub-circuit includes a manually operated flow control valve 16 and 17 respectively. The manual flow control valve allows the flow of gas from either of the sub-circuits to be adjusted in relation to each other. In this embodiment overall control of gas pressure is governed by the master control circuit 1 3. Subsequent gas flow from either of the two sub-circuits 16 and 1 7 may be controlled independently, but the pressure in each sub-circuit will be equalised during the cycle, normally during the high pressure hold phase.

Referring to Figure 4 there is illustrated a still further embodiment which includes a master control circuit 18 and two sub-control circuits 19 and 20.

Sub-control circuit 19 includes a manual flow control valve 21 and sub-control circuit 20 includes a manual flow control valve 22 and a solenoid valve 23.

Overall control of gas flow in this embodiment is effected by master control circuit 18. Flow of gas from each of the sub-circuits 19 and 20 can be regulated by use of manual flow control valves 21 and 22 respectively. In addition, the commencement of gas flow from sub-circuit 20 may be controlled by solenoid valve 23. The use of a solenoid valve enables the commencement of gas flow to be controlled electrically, and preferably timed by the same control means but with individual control of the pressure and timing of the master control circuit.

Referring to Figures 5 to 7 there are illustrated a number of possible pressure time diagrams that can be achieved with gas flow control apparatus of the present invention having two gas outlets. In each diagram the vertical axis represents gas pressure within a cavity and the horizontal axis time. Again, on each diagram there is a solid line. Starting at the left hand side of the diagram the solid line represents a gas pressure in a first cavity (SC1) in a moulded article, supplied from a gas outlet 1 and the dotted line represents the pressure in a second cavity (SC2) in a moulded article supplied by gas outlet 2.

In each diagram the solid line is labelled SC1 and the dotted line SC2.

Referring to Figure 5 the diagram illustrates how the pressure in SC1 can be increased independently of that in SC2. Subsequently, the pressure in SC2 is released by the opening of the solenoid valve and is then maintained at exactly the same pressure as that in SC1. This gas flow control technique could be achieved by any of the above described embodiments.

Referring to Figure 6 the pressure in SC1 is increased initially more rapidly than the pressure in SC2. Subsequently, the pressures in SC1 and SC2 are equalised and remain equal thereafter. In order to achieve this pressure time characteristic it is necessary to be able to control the rate of flow of gas from the two sub-gas flow control means. This pressure time characteristic could be achieved by the embodiments illustrated in Figures 4 and 5 above.

Referring to Figure 7, the pressure in SC1 is increased initially whereas the pressure in SC2 is unaltered. After a certain time, the pressure in SC2 is increased at a lower rate than the original increase in pressure in SC1. When the pressure in SC2 is equal to that in SC1, the pressures remain equal thereafter. The pressure time characteristics illustrated in Figure 7 could be achieved by the embodiments illustrated in either of Figures 4 or 5 above.

The advantages of the above embodiments over existing gas flow control systems are that they enable the pressures supplied to different gas injection positions to be exactly equal and thereby prevent problems of moulded article distortion which can result from differential injection pressures. Moreover, the present invention enables the gas pressures supplied to different injection positions to be increased at different times and at different rates.

The equalisation of pressures throughout a cavity and/or mould ensure that the gas in one gas flow control circuit is not dominating an adjacent gas circuit and therefore influencing the flow of plastic in a mould. The equalisation of pressures is achieved simply and without the complication of adjacent differently programmed gas control circuits. The space required by equipment of the present invention, including distribution manifold and one or more stop valves or flow control valves, is much reduced compared with existing pressure and time control systems. The gas flow control equipment can therefore be located at a convenient position adjacent to a mould or moulding machine. The increased simplicity of the apparatus also reduces the cost of the apparatus compared with existing gas flow control devices.

The methods described of controlling two sub-circuits relative to each other can also be applied to any number of sub-circuits dependent only on the required number of gas outlet positions in one mould cavity or any number of cavities in a mould.

The above embodiments are described by way of example only and many variations are possible without departing from the invention.

Claims (14)

1. Apparatus to control a gas supply for gas assisted moulding apparatus comprising a master gas pressure control means connected in series to at least tvvo parallel outlets at least one of which includes a further gas pressure control means.

2. Apparatus as claimed in claim 1, wherein each outlet includes a further gas pressure control means.

3. Apparatus as claimed in either claim 1 or claim 2, wherein at least one of the gas pressure control means comprises a stop valve.

4. Apparatus as claimed in any of claims 1 to 3, wherein at least one of the gas pressure control means comprises a solenoid valve.

5. Apparatus as claimed in any of claims 1 to 3, wherein at least one of the gas control means comprises a compressed air controlled stop valve.

6. Apparatus as claimed in any preceding claim, wherein at least one of the gas pressure control means comprises a flow control valve.

7. Apparatus as claimed in claim 6, wherein at least one of the gas pressure control means comprises a manual flow control valve.

8. Apparatus as claimed in any preceding claim, wherein any of the further gas pressure control means are connected to still further gas pressure control means.

9. Apparatus as claimed in any preceding claim, wherein the master gas pressure control means is connected to the outlets by a manifold.

10. Apparatus as claimed in any preceding claim, wherein each gas outlet is connected to a gas injection position in a moulding apparatus.

11. Apparatus as claimed in any preceding claim, wherein each gas pressure control means is controlled by a control means.

1 2. Apparatus according to claim 11, wherein the control means comprises a programmable control means.

13. Apparatus according to either claim 11 or 12, wherein said control means is arranged to provide timed operation of the gas pressure control means.

14. Apparatus to control a gas supply for gas assisted moulding apparatus substantially as herein described with reference to any of Figs.2 to 4.