INJECTION MOULDING OF PLASTICS ASSISTED BY PHASED GAS INJECTION

Abstract: Gas assisted injection moulding of plastic materials offers opportunities for overcoming some of the problems of conventional injection moulding, in some cases for improving the quality of moulded articles, and in others for widening the scope of design of moulded products. This invention comprises a method of improving the control of delivery of gas, normally nitrogen, to the mould tool by more precisely time phasing the pressures required within the gas, and also for controlling the rate of increasing and reducing gas pressures within the mould production time cycle. The invention can be applied to multi cavity moulds and larger complex mouldings which benefit from injecting gas in more than one position. This invention includes the use of linear displacement transducers as alternatives to time controllers.

Description: TITLE t INJECTION MOULDING OF PLASΗCS ASSISTED BY PHASED GAS INJECTION Field Oflnvention The invention falls within the scope of plastic injection moulding, and in particular gas assisted or gas injection moulding. Background In recent years there have been several developments of methods of injecting a fluid, normally in gaseous form, into the molten plastic during the injection moulding process as the plastic enters or after it has entered an injection moulding tool cavity. After injection into the flow of plastic the gas follows the path of least resistance which is in the middle of the plastic flow where the plastic is hottest and least viscous, and thereby forms continuous channels or hollow sections within the plastic moulding. The objectives of assisting the moulding process and/or improving the quality of mouldings by the injection of gas into the molten plastic flow can include one or more of the following :- reduction of weight by replacing plastic material with hollow gas channels. reduction in process cooling time by reducing the wall section of the molten plastic. reduction in moulded-in stress thereby reducing the tendency for mouldings to distort after moulding. reduction or elimination of surface sink marks as a result of the internal pressure exerted by the gas on the plastic. the possibility of combining thick and thin sections within a moulding design including the inclusion of webs, pillars and other design features which are not always feasible in conventional injection moulding design. more uniform pressurisation of the moulding as the plastic cools and solidifies thereby creating a more homogeneous solidification of the plastic. improving the flow path of the plastic into the mould cavity or cavities by the addition of thicker flow channels within the mouldings which become hollow sections after the flow of plastic is completed. creating thicker sections such as handles at positions remote from the feed gate or point of plastic entry into the mould cavity or cavities. For gas assisted moulding to be effective it is necessary to accurately control the injection of gas into the molten plastic with particular reference to the timing of the gas injection relative to the timing of the injection of the plastic by the moulding machine into the mould cavity or cavities. It is necessary to control the pressure within the gas also in relation to the pressure in the plastic within the mould or at the point at which the gas is injected into the plastic. In some versions of gas assisted moulding the gas is injected into the moulding machine nozzle as the plastic passes through the nozzle prior to its injection into the mould. In other processes the gas is injected into the mould, either directly into the moulded article cavity or cavities or into the plastic feed runners upstream of the point at which the plastic enters the moulded article cavity or cavities. In some processes the gas is injected into the plastic in the mould in a hot runner or hot chamber or hot or cold sprue, all of which are upstream of the feed runners within the mould and the moulded article cavity or cavities. During the development of gas assisted moulding, in the last decade or even longer, there have been a number of different methods and processes for which patents have been applied for and granted. Broadly, published and other prior art falls into two groups, the first one covering methods of injecting gas into the plastic at the point of gas injection, and the second group concerning methods of delivery of gas under control to either the moulding machine or the mould. For the moulding process to be successful it is necessary that both the method of delivering the gas and the method of injecting the gas into the plastic are compatible and are both accurately controlled. In all the above variations of methods and positions of gas entry into the molten plastic, it is important that the timing of the commencement of gas injection is accurate, consistent, and is related to the injection of the plastic and the position of the flow of plastic during the filling of the mould cavity or cavities. When the gas is injected into the mould cavity or cavities or feed runners within the mould, it is important that the injection of gas starts only after the points at which the gas enters the mould are covered with plastic as it flows into the mould. If the gas injection commences before the plastic is covering the point of gas entry, the mould cavity will be pre-pressurised with gas and it is unlikely that the gas will enter the plastic which is the objective of this method of moulding. Also if the gas is injected into the molten plastic too near the leading flow of the plastic, the gas may burst through the leading edge of the plastic flow thereby releasing pressure of the gas within the plastic and consequently losing control of the pressurisation of the plastic. Conversely if the injection of gas is delayed too long after the flow of plastic has passed the points of gas injection, the proportion of gas to plastic within the mould cavities will be reduced and the hollow section within the plastic will not occur or spread near to the extremities of the plastic flow within the mould. Consequently it is important for the successful operation of the process to precisely control the timing of the commencement of gas injection into the plastic flow in relation to the position of the flow at the time of gas injection. It has been normal practice to affect this timing by the use of a timer or timers to control the commencement of gas injection after a time interval commencing with the plastic injection of the moulding machine. However if for any reason, including variations in viscosity and temperature of the plastic, and hydraulic driving pressure of the moulding machine, the speed of injection may vary, and consequently the flow of plastic within the mould cavity will vary at the time of gas injection. It is therefore desirable, and the process will be more precisely controlled, if the commencement of the injection of gas is activated when a pre-deteπnined amount of plastic has flowed into the mould. This can be more precisely indicated by the discharge of plastic from the moulding machine injection cylinder during the forward movement of the screw/plunger within the cylinder and therefore indicated by the position of the screw/plunger in relation to the stationary cylinder. As one embodiment of this invention the precise position or different positions of the screw/plunger in relation to the cylinder can be pre-deteπnined and indicated by the use of a linear displacement transducer positioned between the screw or plunger and the machine's cylinder. This Invention This invention covers a method and equipment for the delivery of gas to one or more positions in an injection mould tool from which gas can be injected into the molten plastic during the moulding process. The method of delivery is based on the accurate and variable pressure control within accurately time controlled phases of the gas delivery. Unlike other known processes the invention seeks to clearly define the phases for control of pressure and timing. Some processes are based on the injection of a pre¬ determined mass or volume of gas by discharging from a pre-filled vessel of variable volume. Other processes are based on discharging a controlled variable mass of gas from a pre-charged vessel of fixed volume, and the gas mass is pre-deteπnined by charging the vessel at a controlled pressure thereby determining the mass of gas in the vessel. The method of this invention is based on the fact that gas, unlike liquids, is compressible and that the control of the volume or mass of gas for injection moulding is irrelevant It is an assumption of this invention that it is therefore more important to control the pressure of the gas and to have available throughout the moulding cycle an amount of gas well in excess of the amount required during one moulding cycle and at a pressure above the maximum pressure required during the cycle. As a result it is then possible to exert sufficient pressure according to a phased pre-deteπnined programme at any time during the production cycle, to control the rate of pressure build-up in the moulding cavity to the maximum required during the mould filling phase, to maintain the high pressure whilst the plastic is completing the filling of the mould cavity or cavities, and then to control the rate of pressure reduction, to what is refeπed to as a "hold-on pressure" during solidification of the plastic material, and then a further reduction in pressure to approximately atmospheric pressure before the mould opens and the moulded article is ejected from the mould. The method makes it easier and more effective in controlling individually timed pressure phases to a number of different gas injection points within a mould. This enables each cavity of a multi cavity mould to be separately controlled. It is also more suitable for use with larger more complex mouldings which benefit from multi gas injection positions. The timing of gas injection can be phased and pressures applied can be individually and differently profiled to suit the requirements of each area of the moulding supplied by a gas injection position. This enables the commencement of gas injection to individual positions to be sequential and to follow the flow of plastic over and past each gas injection position. This can be achieved without the provision of separate gas volume controlled chambers. This invention requires only timed pressure control valves for each feed position when separate control is required for each position. Alternatively the commencement of gas injection to individual positions within the mould may be timed to occur simultaneously. In all cases the exhaust of gas may be via a common gas exit circuit, which may be fed to a gas recovery unit or to atmosphere. In order that the invention may be more clearly understood, embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 A gas circuit diagram illustrating the creation and control of gas pressure and its delivery to the injection mould tool 1.4. Nitrogen gas is supplied to the equipment from a gas bottle 1.2, the gas is increased in pressure with a booster pump identified in block 13. The gas is fed to a storage vessel 1.1 and then controlled in pressure and timed phases to a single line by a series of valves indicated in block 13. Block 1.6 illustrates P LC. and digital read¬ out and control apparatus. Figure 2 Graphical representation of the phased gas pressure during the total moulding time cycle (7 phases). Figure 3 Graphical representation of the phased gas pressure during the moulding time cycle (5 phases). Figure 4 Graphical representation of the phased gas pressure during the moulding time cycle and the sequentially controlled gas pressure increased to four different and separate gas injection points. Figure 5 General arrangement drawing showing an injection moulding machine screw and cylinder and the addition of a linear displacement transducer 5.1 to indicate the position of the screw/piston 52 in relation to the cylinder 5.4. The invention includes a method of pre-charging the pressure vessel (1.1 in Figure 1) with gas, which is preferably nitrogen, from a gas source 1.2 via a booster pump and equipment 1.3 to a pre-determined pressure above the maximum pressure required at the mould point of gas injection 1.4. The phased gas pressure control is achieved with a series of pressure relief valves and control circuit illustrated in block 1.5. The timed control of the gas circuit is achieved by a Programmed Logic Control (PLC) with digital control and display, 1.6. The Phased Pressure Control is achieved by multi-function closed loop multi-valve operation and control circuit illustrated in block 1.5 (Figure 1) . By consistently sampling the applied pressure in the gas output circuit a 3-term algorithm microprocessor calculates the requirement for gas on a pre-determined programme. Algorithm controls the outputs which modulate the valves to give the required pressure at a particular moment in time according to the pre-determined programme. The sampling factor is typically 1 in 1,000, while the rapid response achieved by a scan of 2.5 mu/s per 1,000 of instructions and in an output switching frequency of 100 times per second. As an embodiment of this invention the commencement of gas injection at the point of gas entry (at the end of Phase 1) may be more accurately controlled in relation to the flow of plastic into the mould by the use of a linear displacement transducer 5.1 placed between the plastic injection screw 52 or plunger gearbox housing 53 and the injection moulding machine plastic cylinder 5.4 (Figure 5). Thereby the precise position of the screw in relation to the cylinder may be signalled to the gas injection unit which will initiate the commencement of gas injection. Further this may be repeated for a number of different pre-determined positions of the screw or plunger if sequential gas injection times are required at different gas injection entry points (Figure 4 Phase 1/A, 1/B and 1/C). Further if required, the linear displacement transducer may also be used to signal the commencement of other phases of the gas pressure sequence instead of using the conventional time controllers. In another embodiment of the invention the maximum pressure in the gas within the mould may be reduced at a controlled rate comparatively slowly and continuously over a time controlled period (Phase 5) until atmospheric pressure is reached (Figure 3, 33), thereby circumventing the normal hold-on pressure and avoiding the sudden drops of pressure which in some moulded products may cause movement within the plastic wnilst it is cooling and solidifying which could result in imperfections in the moulded article. Typical Example of Phased Pressure Injection In preparation for the moulding cycle a high pressure vessel of a volume well in excess of the amount of gas required by the moulding and feed pipe circuit is pre-charged with nitrogen gas at a pressure above the maximum pressure required. The moulding cycle comprises the following phases (refer to Figure 2) :- Phase 1 (2.1) Commencement of plastic injection into the mould thereby covering the gas injection position or positions with plastic whilst the plastic flows into the mould cavity or cavities. It is prefeπed that the completion of Phase 1 is signalled by the use of a linear displacement transducer as described above. Phase 2 (22) Line gas pressure to the mould is increased to a pressure 2.21 which is below the pressure of the plastic at the point of gas injection, thereby delaying the injection of the gas into the plastic Phase 3 (23) The gas pressure is increased at the gas injection point at a controlled rate of increase to a pressure above the pressure within the plastic to enable the gas to penetrate the outer skin of the plastic and to flow into the molten plastic within the mould cavity, and the gas pressure continues to increase at a controlled rate to a maximum pressure 231 to assist the filling of the mould cavity or cavities with plastic and gas. A typical maximum pressure required may be 5000 p.s.i. (330 bar). In some mouldings, particularly thick sections, the maximum pressure may be significantly lower, and in other long flow path mouldings it may be higher in order to assist the filling of the mould cavity or cavities. Phase 4 (2.4) Maintenance of the maximum pressure during final filling of the mould cavity or cavities with plastic and gas. Phase 5 (23) Following the total filling of the mould cavity or cavities with plastic and gas, the pressure within the gas is relieved or reduced at a controlled rate to a "hold-on pressure" 231. Typical hold-on pressure is 500 p.si. (35 bar). Phase 6 (2.6) Maintenance of the hold-on pressure during cooling and solidification of the plastic enabling the gas pressure to be exerted on the plastic so that the plastic remains in contact with the mould cavity surfaces thereby more effectively replicating the cavity surface shape and texture. This pressure is maintained until the material has totally solidified. During this phase the gas expands to compensate for volumetric shrinkage of the plastic whilst the plastic cools. Phase 7 (2.7) Relief of pressure in the mould cavities to atmospheric pressure 2.71 or just above in order that there should be no differential pressure within the cavities when the mould opens and the moulded articles are ejected from the mould cavities. Alternatively Phase 5 (33) may be extended to include Phases 6 and 7 by slowing the rate of gas pressure reduction to a continuous relief of pressure over a controlled and pre-determined time 33 (refer to Figure 3). Alternatively Phase 1 may be applied to two More more gas injection positions individually controlled by the use of time controllers or signals from a liner displacement transducer as described above and illustrated in Figure 4, PH 1A, PH IB and PH 1C. It is an important requirement of a preferred method that there should be sufficient storage of gas in a pre-charged vessel at a pressure of above the maximum pressure required at the point of injection within the moulding cycle, and so that the pressure may be held at the maximum pressure for a sufficient period of time to enable the mould cavity to be completely filled with plastic and gas. Following filling of the mould cavity and commencement of the solidification of the plastic, the maximum pressure is reduced to a hold-on pressure during which solidification of the plastic is completed before the moulded article is removed by ejection from the mould. It will be appreciated that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims: CLAIMS 1. A method of gas assisted injection moulding comprising the steps of supplying gas to one or more positions in an injection mould tool during the mould cycle characterised in that the mould cycle is divided into phases, the commencement and finish of each phase are controlled and the gas pressure during each phase is separately controlled in order that gas may be injected into the molten plastic within the mould cavity or material feed runners under phased pressure control. 2. A method as claimed in claim 1, in which a high pressure gas vessel of a fixed volume is filled with gas at a pressure above the maximum pressure required during the moulding production time cycle at the point of gas injection to the mould. 3. A method as claimed in claim 2, in which the gas pressure vessel is of a sufficient volume to deliver gas at the maximum pressure required and in volumes sufficient to fill the moulded articles and pressure delivery lines without the necessity for further charging during the moulding time cycle. 4. A method according to claims 1,2 or 3 in which the moulding production cycle comprises the following phases:- Phase 1 Commencement of the injection of plastic into the mould cavity or cavities. Phase 2 Release of gas to lines connecting the mould gas injection points to give a pressure below the pressure of the plastic at the point of gas injection, thereby preventing the gas being injected into the plastic. Phase 3 Increasing the gas pressure at a variably controlled rate of increase to a maximum pressure above the pressure of the plastic at the point of entry of the gas into the plastic, thereby enabling the gas to be injected into the moulten plastic. Phase 4 Maintenance of the maximum gas pressure for a time controlled period during the completion of the filling of the mould cavity or cavities with plastic and gas. Phase 5 Reduction of pressure at a controlled rate of reduction to a "hold-on pressure". Phase 6 Maintenance of the hold-on pressure during the cooling and solidification of the plastic in the mould. Phase 7 Reduction of gas pressure to approximately atmospheric pressure before opening of the mould and ejection of the moulded article or articles. 5. A method as claimed in claim 1, 2 or 3, in which the maximum pressure is reduced gradually and continuously until atmospheric pressure is reached without an intermediate "hold-on" pressure and over a controlled time period. 6. A method as claimed in claims 1,2,3,4 or 5, in which gas is delivered to one or more gas injection points within an injection moulding tool and each gas point is supplied with individually controlled gas pressure and timing according to the phase delivery of gas. 7. A method as claimed in claim 6, in which gas is delivered to a plurality of moulded article cavities within a mould so that the phased gas pressure control may be applied to each cavity and according to a separately controlled programme. 8. A method as claimed in claim 6, in which gas is delivered to a plurality of gas injection points in one moulding cavity so that the phased gas pressure control may be applied to each gas injection point and according to a separately controlled programme for each. 9. A method as claimed in any preceding claim, in which one or more phases are activated by the use of a linear displacement transducer which will accurately signal the predetermined but variable position of the moulding machine screw or plunger in relation to the material cylinder, thereby indicating the precise volume of plastic injected into the mould and the position of flow in relation to the gas injection points within the mould. 10. A method as claimed in claim 4, in which the commencement of phases 1,2,3 and 4 are activated by the use of a linear displacement transducer which will accurately signal the predetermined but variable position of the moulding machine screw or plunger in relation to the material cylinder, thereby indicating the precise volume of plastic injected into the mould and the position of flow in relation to the gas injection points within the mould. 11. A method as claimed in claim 9 or 10, in which the commencement of gas injection is signalled and controlled by a linear displacement transducer to two or more gas injection points within the mould, thereby achieving the sequential commencement of the gas injection at different gas injection points within the mould. 12. A method as claimed in any preceding claim in which the gas pressure is controlled by a microprocessor which operates in accordance with a 3-term algorithm constantly sampling gas pressure and calculating gas requirements on a predetermined programme. 13. Apparatus for the gas assisted injection moulding of plastic comprising a mould defining a mould cavity, means for supplying plastic to the mould cavity, means for supplying gas to one or more positions in the cavity or in runners leading to the cavity characterised by means for controlling the moulding cycle in phases, the means being operative to control the commencement and finish of each phase and to control the pressure of the gas during each phase. 14. Apparatus as claimed in claim 13, in which the means for controlling comprises a gas control circuit incorporating a series of pressure relief valves. 15. Apparatus as claimed in claim 14, in which the means for controlling comprises a time control comprising a programmed logic control. 16. Apparatus as claimed in claim 14, in which the programmed logic control is digital and comprises a display. 17. Apparatus as claimed in claim 13, in which the means for supplying plastic comprises a plunger/cylinder arrangement. 18. Apparatus as claimed in claim 17, in which a linear displacement transducer is associated with the plunger/cylinder arrangement to enable the precise position of the plunger in the cylinder to be determined and employed to initiate gas injection. 19. Apparatus as claimed in any of claims 13 to 19 in which the means for controlling comprises a microprocessor which operates in accordance with a 3-term algorithm constantly sampling gas pressure and calculating gas requirements on a predetermined programme. 20. An injection moulding made by the method as claimed in any of claims 1 to 11 or the apparatus of claims 12 to 17.

Inventors: PEARSON TERENCE COLWYN, PEARSON, TERENCE, COLWYN
Applicants: GAS INJECTION LTD, PEARSON TERENCE COLWYN, GAS INJECTION LIMITED
Application year: 1993
Main classification: B29C45
IPC codes: B29C45