Patent GB-1514140-A (GB)
Title: Plastics injection moulding machine
IPC: B29C45/00
Filed: 1976-03-25
Abstract: 1514140 Automatic control systems ROBERT BOSCH GmbH 25 March 1976 [29 March 1975] 12095/76 Heading G3N [Also in Division B5] In an injection moulding machine having a hydraulic cylinder 11 for actuating a plasticizing and injection screw 12, a device for controlling the pressure in the cylinder in accordance with a signal from a desired value generator 24, and a device for controlling the pressure in the mould cavity 13, means are provided for automatically changing from control of the pressure in the mould cavity (during the filling phase) to control of the pressure in the cylinder (during the after-pressure phase as the injected material solidifies). In Fig. 1 a desired value PFS from generator 24 and the actual value PFI of the mould pressure from the generator 17 are fed to operational amplifier 21 which produces an error signal AF. The signal AF is fed through a bi-polar limiter 27 to an operational amplifier 31 which also receives the signal from the generator 33 for the cylinder pressure. The output from the amplifier 31 controls the servovalve 36 of the hydraulic supply to the cylinder 11. A signal B from an adjustable generator 39 is fed with the desired value signal PFS to operational amplifiers 43, 48 to produce upper and lower control values for the limiter 27. During the mould filling phase the value B is set high, allowing the signal from the mould cavity to predominate. The signal B is reduced stepwise or gradually so that the range of values passed by the limiter 27 is reduced and the effect of the mould pressure signal becomes less significant. Thus as the material solidifies in the mould and the signal from the pressure transducer 15 in the mould falls, the large error signal produced does not cause excessively high pressures to be applied to the screw. Control is then effectively in response to the pressure sensed by transducer 33 in the cylinder 11. In an alternative arrangement Fig. 4 (not shown) the cylinder pressure signal (PHI) and the desired value signal (P S ) are fed to an operational amplifier (31) to obtain an error signal which is applied to operational amplifiers 43, 48 along with signal B to produce the control values of the limiter 27 and thereby effect the changeover to cylinder pressure control as the signal B is reduced and the mould pressure signal fades. In Fig. 2 a single desired value generator 61 or two separate desired value generators (not shown) are connected to operational amplifiers 64, 79 to which the actual value generators 86, 76 for cylinder and mould pressures respectively are also connected. The error signals produced are fed from amplifiers 64, 79 through translators 67, 84 and a summing point 69 to control the servovalve 72 of the cylinder 73. Pressure in the mould 75 and in the line 80 of the cylinder 73, and the desired value signals are fed to an auxiliary con-  troller 89 which modifies the amplification of translators 67 and 84 in response to the effects of changes in the condition of the moulding material on the mould and cylinder pressures. Thus the amplification of the cylinder pressure signal is initially zero and is increased to unity while the amplification of the mould pressure signal is reduced from unity to zero. Another arrangement Fig. 3, (not shown) utilizing only one desired value generator 61 which is used to produce error signals for the cylinder and mould pressures which are summed and applied to the servovalve 72. The error signal for the cylinder pressure is passed through a threshold limiting translator (101) which renders the cylinder pressure control ineffective during the filling phase, and the error signal for the mould pressure is passed through a bi-polar limiter which reduces the effect of mould pressure as this fades in the after-pressure phase.