Apparatus for heating sheet-like thermoplastic preshapes

#The present invention relates to an apparatus for heating sheet-like preshapes, consisting of a thermoplastic, especially of a fiber-reinforced thermoplastic, by means of an infra-red heating apparatus to which the preshapes are fed by a conveyor belt.

#Machine processing of preshapes consisting of a thermoplastic, or of a fiber-reinforced thermoplastic, to give shaped articles is usually carried out by an intermittent process, the time cycle of which is essentially dependent on the dimensions, structure and material of the preshapes. An important component of this cycle is the time which is required to heat the preshapes to the requisite processing temperature. Fitting this time into the production cycle, and varying the energy transfer in accordance with the properties of the preshapes, provide the basis for optimizing and streamlining the process.

#It is known that in general the heating of thermoplastics has to be carried out at relatively low rates of supply of heat, since the low heat conductivity of the thermoplastics, combined with their heat sensitivity, does not allow them to be subjected to severe heat exposure. The use of a great variety of infrared heating equipment for generating the requisite thermal energy is well-known. With these, the preshapes are heated not only by radiation but also by heat conduction and by convection. Depending on the particular application, the heating apparatus is either switched on continuously and travels over the preshape to be heated, or is rigidly mounted and switched on and off for each heating cycle. The intensity of energy release by the infra-red radiator, whether by radiation or by conduction or convection, does not vary with time, except for the switch-on stage, so that different energy uptakes which may be required, due to the nature of the material or size of the preshape, in order to reach a given processing temperature can only be achieved by employing different heating times. This however necessarily produces temperature gradients in the preshape which is to be heated.

#The invention is illustrated below by an Example with reference to the schematic drawing 1 in which

#FIG. 1 shows a schematic diagram of an apparatus according to the invention, and

#FIG. 2 shows a schematic diagram of the apparatus of FIG. 1 which has a plurality of heating zones.

#As shown in FIGS. 1 and 2, the apparatus essentially consists of an infra-red heating unit (1), which is fitted with bright infra-red radiators (7) and is constructed of contiguous individual heating zones (6a-6c), in accordance with the dimensions of the preshape to be heated. The heating apparatus is supplied with energy from the feed unit (2). This unit is a semi-conductor apparatus, made up of controllable rectifiers, ie. thyristors, 20 and with this apparatus it is possible to meter the electric power infinitely variably up to a maximum which depends on the technical data of the rectifiers. Preferably, this metering is achieved by a phase control system, in which periodically generated needle pulses are shifted, in their phase position, relative to the alternating current or three-phase current employed as the feed. By means of these needle pulses, the controllable rectifiers are periodically brought from zero setting to maximum and vice versa, so that, depending on the phase position of the three-phase current or alternating current applied to them, they dose a varying current intensity to the infra-red radiators.

#The control unit (3), which forms a single unit with the microprocessor (3'), serves to generate a periodic electrical vibration, whose time-curve and amplitude corresponds to the variation, as a function of time t, of the optimum heat uptake rate A.theta. of the preshape which is to be heated (FIG. 1). This vibration consists of a branch, rising to the maximum, in the shape of an exponential function, and an adjoining hyperbolic decrease region. It is generated as an electrical voltage U, as a function of time t, having values of from 0 to 10 V, or as an electrical current of from 0 to 20 mA. This control voltage or control current produces the phase shift between the needle pulses and the alternating current or three-phase current applied to the thyristors, and effects the power metering.

#The optimum heat uptake/time function can be determined experimentally for any preshape. Thereafter, it can be stored by means of punched tape, punched cards or the like, and be called up by the microprocessor (3') and reproduced as often as desired. The frequency with which the vibration is generated can be set either by means of the microprocessor itself or by means of the time cycle of the preshape processing operation.