High-frequency dielectric heating

Preheating is often used to reduce the molding cycle. It can aid in providing even heat through the material and can cause a more rapid rise in heat than occurs in the mold cavity. A warm surface plate, infrared lamps, hot-air oven, or screw/barrel preheater can accomplish preheating. The best and quickest method is high-frequency (dielectric) heating. 

P.L. Jones, High frequency dielectric heating in paper making. Drying Technology, 4(2) 1986. 

Welding at High Frequency Dielectric heating is accomplished with electrodes that are connected to a generator of voltage at high frequency. This method is very efficient but is restricted to polymers that exhibit a high electrical loss factor like PVC and other polar polymers. 

Special drying methods, such as superheated steam, solvent, vacuum, infrared radiation, and high frequency dielectric and microwave heating, are occasionally employed when accelerated drying is desired and the species being dried can withstand severe conditions without damage. None of these methods is of significant commercial importance. 

PVCB is brown in color, obtained by reacting acetylene with carbazole. The plastic has excellent electrical properties and good heat and chemical resistance. Use includes high frequency dielectrics, impregnant for paper capacitors, and photoconductive plastics. 

The term s tan <5 is called the loss factor, and is not the same as the dissipation factor. Materials with a high s tan S are suitable for high-frequency-field heating, i.e., they can be welded in a high-frequency field. These materials are not suitable, on the other hand, as insulating materials for high-frequency conductors. Nonpolar plastics such as poly(ethylene), poly(styrene), poly(iso-butylene), etc., have low dielectric constants ( 2-3) and dielectric loss factors (tan (5 = 10 " to 8 x As insulating materials they are of consi-... 

High-Frequency Dielectric (Radio Frequency) Heating... 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

Thermal effects (dielectric heating) can result from dipolar polarization as a consequence of dipole-dipole interactions of polar molecules with the electromagnetic field. They originate in dissipation of energy as heat, as an outcome of agitation and intermolecular friction of molecules when dipoles change their mutual orientation at each alternation of the electric field at a very high frequency (v = 2450 MHz) [10, 11] (Scheme 3.1). 

PVCs are good insulators even in a wet environment, with fair dielectric resistivities and rigidities, and rather high loss factors. Resistivity decreases when the temperature rises. PVCs heat up under high-frequency current and microwaves. 

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