Patent GB-747862-A (GB)
Title: Improvements in or relating to injection moulding machines
Inventor: GASPAR EMERIC
IPC: B29C45/82
Filed: 1953-01-26
Abstract: 747.862. Injection-moulding machines. PROJECTILE & ENGINEERING CO., Ltd. Jan. 19. 1954 [Jan. 26, 1953], No. 2241/53. Class 87(2) In the hydraulic supply line 19 to the cylinder 20 that moves the injection piston 21 there is located a relief valve 24. When the pressure in the cylinder 20 reaches a predetermined maximum the relief valve 24 opens to a drain 25 and a pressure switch 77 sets in operation a time switch 76. This time switch operating through an electromagnetically-conrolled valve 55 holds the predetermined maximum pressure in the cylinder space 22 for a time adequate to ensure an initial set of the material of the mould, and then allows the pressure in that space to fall from the predetermined maximum to a predetermined minimum, where it is held before the pressure is cut off and the mould opened. As shown, a main pump 11 having a large output and a holding pump 12 having a small output feed the cylinder 20 through a valve 15, injection-pistonadvancing line 18. At the point A the line 19 communicates with valve 24, from which extends the drain 25 having therein a restricting valve 26 through which pressure leaks. Between the valves 24 and 26 is disposed the electrical pressure switch 77 the contacts of which are closed by pressure in drain 25. The valve 24 has a seatingn 29 on which bears a plunger 31 which is urged to the closed position shown by a spring 32. Through a piston 30 that carries plunger 31 extends a leak passage 33 which leads to another leak passage 40 and a needle-valve seating 39 on which bears a needle-valve 38 urged to the closed position by a spring 41. The magnitude of the thrust 'exerted on the needle-valve by the spring depends upon the position of a plunger 47 carried by a piston 46 working in a cylinder 45 having an advancing line 157 57 and a retracting line 59, 58. Operation:-(1) The parts being in the position shown, push-buttons 75, 85 operate pilot-valve 74 to move valve 15 to direct pressure through line 19 whereby (a) the injection piston . 21 is advanced and (b) pressure is applied to the right hand side of piston 46, via line 56, valve 55 and lines 57, 157, to advance plunger 47 and thereby compress spring 41. The advance of this plunger, and therefore the degree to which the spring is compressed, is limited by the position of a stop 51 on a rod 48 extending rearwardly from piston 46. Pressure builds up in line 19 and main pump 11 unloads, leaving holding pump 12 in action. The pressure in line 19 passes through leak passage 33 so that the pressure on both sides of piston 30 is the same and the valve plunger 31 does not move at this stage. (2) The pressure passing through leak passages 33, 40 rises and opens needlevalve 38 against its spring, fluid escapes past the needle valve and through passages 42, 43 to drain 25, the pressure in space 28 over piston 30 drops, piston 30 rises with plunger 31, pressure passes into drain 25 and 'switch 77 is closed. whereupon time switch 76 is started. (3) When the time switch is started it reverses valve 55 to direct pressure through lines 57, 157 to advance plunger 47 to close the needle-valve, whereupon valve 31 closes and the maximum injection pressure is maintained on piston 21 as long as the time switch is running. (4) Switch 76 times out, whereupon valve 55 is reversed to direct pressure through lines 58, 59 to retract plunger 47: retraction of this plunger is retarded by a restrictor valve 60 in exhaust line 61 (a non-return valve 62 being provided in line 57). As plunger 47 retracts'the pressure of the spring 41 on the needle-valve gradually decreases, whereby the valve 31 is allowed to open gradually and the pressure on the injection piston is thereby gradually decreased. When the plunger reaches the limit of its rearward movement, which is determined by a stop 50, the injection pressure ceases to fall and is maintained at this pressure for any predetermined period. By altering the position of the stops 50, 51 the lower and upper limits of pressure are altered. By altering the duration of operation of the time switch the time during which maximum pressure is held is altered. By altering the restriction valve 26 the rate of fall in pressure is modified.