GB604079A - Improvements in or relating to the manufacture of preforms of synthetic resinous materials - Google Patents

Abstract

604,079. Dielectric heating. BAKELITE, Ltd. Nov. 23, 1945, No. 31658. Convention date, Nov. 25, 1944. [Class 39(iii)] [Also in Group V] A preform of thermosetting synthetic resinous material such as phenol-formaldehyde resin with a long fibre filler, is made by subjecting a charge of the material to pressure and to heat generated by a high frequency electrostatic field, the rate of input of energy to the field and the time of heating of the material being sufficient to soften the material but insufficient to result in a loss of plastic flow. A preforming disc mould, comprising walls 11, top and bottom members 12, 13, a base 14, a top plate 16 and leads 18, 19 terminating in contactors 20, 21 connected to brass, copper or bronze electrodes 23, 24, is filled with a charge of material and then the mould is placed in a press and heat and pressure applied simultaneously. The members 12 and 13 may be secured to the platens of a press. The material may be first brought to a softening temperature and then pressed. The mould is preferably of insulating material such as ceramic, glass, low loss mica-filled phenolic resinbonded material or wood. A high frequency generator may be used operating at about 30 megacycles with a power input to the generator of 1,250 watts, the generator having an efficiency of 50 to 60 per cent. The rate of energy input is then 12 to 15 watts per gram. The minimum energy input is about .6 watt per gram. The heating may be applied for 10 seconds to bring the temperature of the material to 90‹C. The maximum temperature to which the material may be raised without loss of plastic flow is about 140‹C. Specification 572,425 is referred to.