GB572425A - Improvements in or relating to the moulding of thermo-setting plastics - Google Patents

Abstract

572,425. Dielectric heating thermosetting plastic. BAKELITE, Ltd. Oct. 12, 1943, No. 16680. Convention date, Dec. 11, 1942. [Class 39 (iii)] [Also in Groups V and XXXV] A method of moulding a thermo, setting plastic comprises the steps of (a) inducing heat within the plastic mass by subjecting it to the action of a high frequency electrostatic field, energy being supplied to the field at such a rate that a hardening temperature is reached within the mass before a degree of hardening interfering with plastic flow is obtained, (b) shaping the mass in a mould by pressure and (c) heathardening the. shaped mass. Preforms or sheets which are to be pressure-moulded may be heated between plates operating as a condenser and then quickly transferred to a hot mould which may be heated either by H.F. electrostatic field or in the usual manner, A number of preforms may be heated simultaneously by the field, e.g., by being carried between condenser plates by a movable belt, and enclosures may be provided for supplementary conduction or radiation-heating. Alternatively and as shown in Fig. 2, a condenser unit may be built into the mould and moulding powder or a preform, pre-heated by H.F. electrostatic field, is placed in the mould and simultaneously heated and pressed therein. The mould shown comprises top and bottom forces 42, 43 keyed at 47 to a top plate 46, and a chase 41 keyed at 45 to a bottom plate 44. These parts are made of insulating material such as mica-filled ceramic material. The forces are lined with non-ferrous metal (brass,. copper, bronze) sheets 53, 54 to form electrodes. Leads 48, 49 from a highfrequency generator terminate in contacts 50, 51. The mould may be placed between the platens of a press; the plates 44, 46 may be fixed to the lower and upper platens. Fig. 1 shows a circuit for generating high-frequency oscillations, in which plate electrodes 1 are fed from a 220-volt A.C. supply through an auto-transformer 15, power-transformer 17, two half-wave mercury arc rectifier tubes 18, 19, audio-frequency filter choke 33, radio-frequency choke 36, tank coil 6, radio-frequency coil 5 and tuning condensers 2, 3, 4. The operating frequency is determined largely by the sizes of the coil 6 and its associated plate condenser 7, which are connected to the plates of oscillator tubes 8, 9 operating in push-pull. The plate current can be varied over a large range by a small adjustment of the condenser 4. The Specification as open to inspection under Sect. 91 states that the heating of the material may be carried out partly by high-frequency and partly by heating in an oven, to a point below that at which hardening is initiated, heating being continued by means of a H.F. electrostatic field. This subject-matter does not appear in the Specification as accepted.