Patent GB-673120-A (GB)
Title: Improved method and apparatus for injection moulding, pressure casting or transfer moulding
IPC: B29C45/70
Filed: 1949-10-21
Abstract: 673,120. Casting machines. LESTER ENGINEERING CO. Oct. 21, 1949 [July 16, 1949], No. 27096/49. Class 83 (i). [Also in Groups V and XXIX] The pressure exerted on the metal to force it into the two-part mould is so high that if it were maintained subsequent to the filling of the mould, minute separation and flashing of the mould parts would occur; but when the pressure in the mould has been built up to a value not greater than that able to cause an imperceptible non-flashing separation of the mould parts, the pressure in the mould is reduced by means of a pressuresensitive element or other device which energizes a pressure-reducing device, the reduced pressure being such that the mould-closing mechanism will retain the mould closed without the occurrence of minute separation and flashing. A test run may be made to ascertain the proper setting of the pressure-sensitive or other device. Figs. 1 and 7 show a machine in which the injection piston 9 is hydraulically operated by pressure lines 27, 28, reversing valve 23, pressure line 21 and pump 18. The pressure line 27 which advances the injection piston is connected by line 32 to relief valve 31, which communicates by line 80 with reservoir 17 and by line 33 with solenoid-operated valve 34. Valve 34 communicates with pilot relief valve 36 by way of line 81. Solenoid 35 in valve 34 is energized by operation of a pressure switch 37 exposed to fluid pressure in cylinder 7 or line 27. Operation. Valve 22 is operated (manually or in timed sequence by solenoids, cams, &c.), to close the mould parts 15, 16 and valve 23 is similarly operated to advance injection piston 9. During the advance of the piston the valves 31, 34, 36 are in the positions shown, in which the injecting pressure extends also through line 32, through valve 31 by way of narrow conduit B and wider conduit C, and through line 33 to valve 34, where it is halted. A spring E urges piston A in valve 31 to the left to shut off the relief line 80. The solenoid 35 in valve 34 is de-energized, and passage 83 in solenoid valve member D is connected to the reservoir through line 82a. When the mould has been filled, the pressure in cylinder 7 rises and operates switch 37, thereby energizing solenoid 35 and closing hold-up relay 41. Solenoid 35 being thus energized, its valve member D moves to the left, and passage 84 therein connects lines 33 and 81, whereby the valve member in pilot valve 36 is unseated and pressure in lines 81 and 33 and in chamber C in valve 31 escapes to the reservoir through line 82. The high pressure in line 32 thereupon moves piston A in valve 31 to the right and pressure escapes through line 80, whereby the pressure on the material in the mould is reduced. This reduced pressure is maintained until the injection plunger is retracted, the solenoid 35 being kept energized, despite the reduction of pressure in cylinder 7, by hold-up relay 41. A check valve 30 retains pressure in line 24 to keep the mould closed. When the mould opens, a cam 42 on movable platen 12 opens relay 41 to deenergize the solenoid. The passage C may be connected to a valve that opens at, say, 1000 pounds per square inch, to preclude strain, the valve 36 being set to open at, say, 200-300 p.s.i. Modifications. (1) The pressure-sensitive element may be installed not in the injection circuit but in the mould-closing circuit, Fig. 6 (not shown), in which the switch 37 is connected to the mould-closing cylinder 4 and operates the devices shown in Fig. 1 to reduce the pressure in cylinder 7, the setting of the switch 37 being such that an imperceptible separating movement of the dies 15, 16 increases the pressure in the mould-closing circuit to operate the switch. (2) (Fig. 3). The hold-up relay is eliminated by leading the pressure line P from the injection circuit or mould-closing circuit through a checkvalve 44 to switch 37. When switch 37 is closed it is kept closed by pressure trapped therein. (3) (Fig. 5). A high-pressure pump 61 fills the mould and a low-pressure pump 62 maintains reduced pressure whilst the material is cooling in the mould. Delivery of the pumps is controlled by a solenoid-operated selector valve 64 which is operated by the switch 37 connected to the injection or mould-closing circuit. The hold-up relay 41 may be replaced by a check valve as in Fig. 3. (4) (Fig. 4). Instead of a pressure-sensitive device a cam 53 on an extension 52 of the injection piston is used, this cam operating a switch 54 which energizes a solenoid 55 to shift a by-pass valve installed in the pressure line 57 from pump 47, whereby the pressure is reduced. Specifications 631,641 and 673,054 are referred to.