GB765967A - Improvements in or relating to means and method for compression moulding of plastics - Google Patents

Abstract

765,967. Moulding fibre-reinforced resin. ZENITH PLASTICS CO. Sept. 3, 1953 [May 1, 1953], No. 24377/53. Class 87(2). [Also in Group XXIX] A compression mould is heated to a predetermined resin-curing temperature and lined with fibrous reinforcement, viscid thermosetting resin is poured into the mould over the reinforcement, the mould is closed in an initial rapid closing movement with a force less than that required for final moulding and then in a slower final move. ment with a force equal to the required final moulding pressure and in a time interval substantially corresponding to but not extending beyond the commencement of gellation of the resin. An object of the invention, which has special relationship to deep drawn articles, is to avoid tearing or cracking of the preform. The upper, male die 32 is mounted on a movable platen 29 hydraulically operated by a piston 28 in a cylinder 26 ; the lower, female die 40 is mounted on a piston 36 of large area which fits in a bore 35 in a fixed platen 23. The dies 32, 40 are heated by conventional means, not shown. The movable platen 29 is guided on rods 24. A pre-form of fibre-glass reinforcement is laid in the lower die, and thermosetting polyester resin is poured into the female die cavity. Power for the hydraulic operation of the machine is supplied by a motor 50 which drives a medium pressure pump 51, Fig. 3, and a high pressure pump 53, a motor (not shown), which drives a comparatively low pressure pump 54, and a motor 120, Fig. 4, which drives a pump 122 which delivers hydraulic fluid, e.g. oil, at medium pressure. A four-way valve 57 is set to connect the pump 51, through check valves 59, 62, with the cylinder 26, and also the pump 53 through check valves 90, 62. To assist the two pumps 51, 53 in depressing the piston 28 the pump 54 also supplies oil, at low pressure and high volume, to the cylinder 26 through a check valve 89. Oil escapes from beneath the piston 28, as it is lowered, through a valve 92 and the four-way valve 57 to the storage tank 84. The valve 92 is partially closed by a cam 94 on an adjustable rod 96 fixed to the descending platen 29, so causing deceleration of the platen before it is stopped by collars 38 adjustably fixed on the rods 24. When the piston 28 is unable to move further downwards the oil pressure in the cylinder 26 rises and the check valve 89 therefore closes, at which the oil pressure from the pump 54 opens a relief valve 102 and the oil from that pump returns through a pipe 103 to the storage tank. The pressure continues to rise until the check valve 59 closes, under the higher pressure from the pump 53, the oil from the pump 51 then opening a relief valve 104 through which it returns to the tank. The high pressure pump 53 continues to supply oil to the cylinder 26 till a relief valve 82 is opened and the oil returns to the tank. This hydraulic system reduces the time of building up the final pressure above the upper die, but in order for there to be adequate separation of the dies the capacity of the cylinder 26 requires to be large enough for the travel of the piston, and the compressibility of the oil therein introduces a delay factor. To quicken the pressure build-up the piston 36 is forced upwards by hydraulic pressure initiated by a finger 113 on the rod 96 and a finger 112 on a rod 114 engaging microswitches 111, 110 mounted on the guide rods 24. The micro-switch 110 closes a solenoid valve 135 and opens a solenoid valve 143, whereupon the pump 122 sends oil through a pipe 145 and or a by-pass 146, including a volume control valve 147, into the bore 35 beneath the piston 36 ; simultaneously the micro-switch 111 closes solenoid valve 154 between the bore 35 and a storage tank 123. The final die pressure thus builds up very quickly owing to the large area of the piston 36. After the dies have remained closed for the time required to complete polymerization of the resin the four-way valve 57 is operated to cause oil from the pumps 51, 53 to pass through a pipe 91 to the underside of the piston 28 and to connect the cylinder 26 above the piston through pipes 64, 95 with the storage tank 84. The platen 29 therefore rises, the fingers 112, 113 disengage their microswitches 110, 111, resulting in the stoppage of the supply of oil to the bore 35 and the opening of the valve 154, through which oil is allowed to return to the tank 123 as the lower, die 40 descends under its own weight.