Patent GB-1360576-A (GB)
Title: Injection moulding process control
IPC: B29C45/76
Filed: 1972-01-17
Abstract: 1360576 Automatic control of viscosity PENNWALT CORP 17 Jan 1972 2173/72 Heading G3R [Also in Division B5] In an injection moulding process in which a series of plastics articles is moulded by injecting a series of shots of plasticized material into a mould using a hydraulically reciprocated ram, the pressure on the ram is built up to a primary pressure which is maintained for a pre-set time to effect substantial filling of the mould and is then lowered to secondary pressure, the time from attainment of the primary pressure to the complete filling of the mould (which is an indication of the viscosity of the plasticized material) is sensed during one stroke and compared with a predetermined time, and any discrepency is compensated in the next injection stroke by varying (1) the heat supplied to the material, (2) the time for which the constant primary pressure is applied and (3) the value of the primary pressure. The heat supplied to the material is varied by adjusting the heating of the barrel, the rotational speed of the screw or the back pressure. In Fig. 1 an axially reciprocable screw 16 rotates in a barrel 12 heated by elements 14. Closure of mould 32 is effected by pressurizing fluid in lines 46 and 48, which also actuates switch 80 to start a timer 78. The injection ram is actuated by fluid from line 56 fed to lines 24, 26 through 4-way valve 54. Pressure in line 56 is controlled by a primary relief valve 52 set at a high value, 2000 lb/in<SP>2</SP> a primary remote relief valve 70 set at the predetermined value, 1200 lb/in<SP>2</SP>, and a secondary relief valve 72 set at a lower value, 500 lb/in<SP>2</SP>, to provide follow-up pressure at the end of the primary injection pressure. A bleed-in valve 52 permits fluid to reach a small control line 63 and thus when the pressure in line 56 reaches 1200 lb/in<SP>2</SP> the valves 52 and 70 pass fluid to maintain this pressure and to actuate pressure switch PS which initiates the electrical timing circuit (not shown). After a pre-set time the timer 78 energizes a solenoid G to actuate valve 76 and open the drain of valve 72 to reduce the pressure to 500 lb/in<SP>2</SP>. When the mould is substantially completely filled a limit switch LS1 is tripped and signals the end of the measured time period to the timing circuit. This time is compared with a predetermined value by a solid state logic circuit, see below, and high and low differences are used to close switches in the circuit of Fig. 3 to alter the voltage applied to a heat controller for the elements 14. If the sensed time is incorrect one or more switches 3CR, 4CR and 6CR or 9CR, 10CR, and 12CR are operated to insert resistors into the heat control circuit to provide a bucking voltage or an additional voltage, respectively, to that of the thermocouple T/C in the barrel 12. Before another stroke in which time measurement is made, several unmeasured strokes may be made to allow conditions to settle. If the sensing device indicates the need for more than three adjustments consecutively in the same direction an alarm is given. The circuit may be used to effect discrete changes in injection pressure or in the pre-set primary pressure period. The solid state logic circuit, Figs. 2A, B and C (not shown) is based on AND-NOT gates. If the sensed time is less than that of timer T1, a signal is emitted from limit switch LS1 via gates 115A and 88A before the latter is blocked by inhibition of gate 85A by a signal from T1 through gate 87A, whilst if the time is greater than that of both timers T1 and T2 then the latter will have sent its signal through gates 87B and 88B before 87B can be inhibited by a pulse from switch LS1. In the first case a "subtract heat" circuit (gates 93A, 93C) emits signals which reach reed gates 111A, 111B and 112A of Fig. 2C (not shown), to close switches CR9, CR10 and CR12 as previously described; whilst in the second case on "add heat" circuit gates (93B, 93D) signals to gates 103A, 103B and 99B of Fig. 2B (not shown) to close CR3, CR6 or 6R9. The number of injection strokes to be made between time measurements is determined by setting switch 75 Fig. 2A (not shown), to set up the associated sampling circuit. When all three CR switches of either kind have been successively operated, "sentry" gates are released and the alarm Q3 is operated.