Patent GB-572748-A (GB)
Title: Improvements in or relating to the moulding of plastics
IPC: B29C43/00
Filed: 1944-01-27
Abstract: 572,748. Electrically-heated toggle presses. JOHNSON, J. Jan. 27, 1944, No. 1581. [Class 39 (iii)] [Also in Groups V and XXXVII] A cold preformed lens blank is inserted between opposed dies, the dies are brought gently into contact with the blank, and the electric heat supply to the dies is switched on. When the dies have reached the moulding temperature the following operations occur automatically.- (a) The main heating current is cut off, but a subsidiary heating current remains to maintain the dies (still in contact with the blank) at the moulding temperature for a predetermined time, at the end of which (b) the subsidiary heating current is cut off and water-cooled blocks press the dies towards each other to mould the blank. The dies, and hence the blank, are thus cooled as the blank is pressed. Spring pressure may be maintained on the blank as it cools. When the dies have been cooled to a certain temperature, (c) the pressure of the dies on the blank is released. Figs. 4, 5 and 7 show a toggle press arrangement in which the blank 12 is pressed between dies 13, 14 disposed in sockets 15, 17 having electrical heating elements 16, 18. A thermocouple 27 and a moulding-space surround 19 are disposed within the lower socket. The sockets are normally spaced from watercooled blocks 20, 23 by springe 51, 53. The upper block is reciprocated by an electric motor 47 via gearing 47a, 47b, 47d, 47c, bell crank lever 43a, and toggle 43. Operation:-(a) The block 23 is lowered, by finger-switch control of the motor, until the upper die just touches the blank. (b) The motor circuit is switched from manual control to automatic control, and he operator presses a button to close switch 61a and energise relay 62. This closes the circuit of the primary of transformer 63 (through contacts 62a) which then supplies heating current to the socket heating units 16, 18. (c) When the socket 15 reaches a certain temperature, thermo-couple 27 therein sends a current to potentiometer 87 to close a switch therein, energise relay 73 and de-energise relay 62 (by opening contacts 72). The main heating current from 63 is thus cut off, but a shunt resistance circuit 75, 99 supplies enough current to the transformer primary to maintain the temperature previously attained. (d) This " maintenance heating " continues until a time switch 74 (previously set by energisation of relay 73) energises coil 100 which closes switch 101 and energises relay 76 which opens contacts 99 (to cut out the maintenance heating) and energises (through 103) "Down" motor relay 68, whereupon the upper die descends until a limit switch 83 opens to break the circuit of relay 68. (e) When the moulded blank has cooled to a certain temperature, thermocouple 27 operates another switch in potentiometer 87 to energise relay 80 which (by opening contacts 79) de-energises relay 76 which breaks the circuit for the downstroke relay 68 and makes the circuit for the "Up" relay 67, whereupon the inner die rises until a limit switch 84 breaks the circuit of relay 67. A similar circuit is described with reference to Fig. 6 (not shown). In a modified arrangement (Figs. 1 and 2, not shown) thermocouples are disposed in both sockets and are so linked with potentiometers and switches as to (a) cut off the heating current to the lower socket when the latter reaches a certain temperature and (b) equalise the temperatures of the two sockets. Specification 556,743 is referred to.