Patent GB-1361911-A (GB)
Title: Injection moulding process
IPC: B29C45/76
Filed: 1972-02-23
Abstract: 1361911 Digital control of injection moulding process PENNWALT CORP 23 Feb 1972 [1 March 1971] 8371/72 Heading G4H [Also in Division B5] In an injection moulding process in which the injection ram moves over part of its stroke at a constant pressure which is subsequently altered, variation in viscosity of the injected material is compensated by measuring the time required for the ram to advance over a selected part of its stroke, comparing that time with an empirically predetermined time and, during the same injection stroke, changing the time at which the pressure is altered by increments determined by the difference between the measured and predetermined times. Thus if the viscosity falls the constant pressure time is reduced and vice versa so that the delivered shot is substantially the same. An injection moulding apparatus comprises an axially reciprocable screw 16 rotating in a band 12 heated by elements 14. Closure of the mould 32 is effected by pressurizing fluid in lines 46, 48, which also activates switch 80 to start a control timer T. The injection ram is actuated by fluid from a line 56, the pressure in which is controlled by primary relief valves 52 and 70 to a predetermined pressure of 1200 lb/in.<SP>2</SP> When this pressure is reached a pressure switch PS starts clock C, Fig. 2. Before the end of the constant pressure portion of the stroke a limit switch LS1 is tripped to stop the electronic timer, and the measured time is compared with the predetermined time by a solid state logic circuit (see below). Any discrepancy is used to alter the time at which the control timer T opens the drain of a secondary relief valve 72 to reduce the pressure in line 56 to 500 lb/in.<SP>2</SP> The timed interval may begin during the pressure build-up, and the secondary pressure may be zero or greater than the constant primary pressure. Time control system (binary logic circuit, Fig. 3, not shown, and Fig. 2).-The closing of switch PS starts the electronic clock C by triggering a pair of multivibrators to emit 1000 Hz pulses until switch LS1 closes. These pulses are passed to a divider D comprising a 4-decade counter and the counter output (BCD) is registered on one side of a comparator H. The predetermined time interval is manually preset on the other side of the comparator and the difference is signalled to flip-flops which pass either 0À03 or 0À05 second timed signals to a decade counter J which includes a decade converter. The decade counter has a mean position with four working positions on either side, plus further positions for alarm signals for larger time discrepancies. The size of the discrepancy determines which output of the decade counter J will be activated to pass a signal via an inverter to a reed relay K. This has switches to correspond with each position of the counter which light appropriate signal lamps and select resistance increments to be inserted (Figs. 4, 5, not shown) into the timer circuit T to determine pressure change point. When the time discrepancy is so large that an alarm is operated the system must be manually reset.