Radio frequency preheating

Radio frequency preheating can also be used to dry molding pellets or other molding materials before molding or incorporating into a mixture. It has also been... 

Retractabie radio frequency preheater (optionai) Three-phase power distribution... 

Radio-Frequency Preheating n (RF preheating) A method of preheating used for thermosetting molding materials to facilitate the molding operation or shorten the molding cycle. The frequencies most commonly used are near 20 or 40 MHz. 

Preheating methods are convection, infrared, radio frequency, and steam. Thermostatically controlled gas or electrically heated ovens are inexpensive methods of heating. The quickest, and possibly the most efficient, method is radio-frequency heating, but it is also the most expensive. Preheaters are located adjacent to the molding press and are manually operated for each cycle. 

In dielectric preheating, however, the specific heat of a molding powder is of more direct concern this knowledge along with the amount of material and time enables one to calculate the required amount of radio-frequency power. For example, to raise the temperature of a 1-kg perform of specific heat 0.35 through 80°C in 1 min requires 1X 0.35 X 80 or 28 kcal/min or about 2 kW. 

The process is similar to the cooking action in the familiar home microwave oven. The preforms are usually placed in a nonmetallic hay that is positioned between two flat plate electrodes in the preheater. By the action of radio frequency (generally 10-100 MHz) the preforms can be brought to a temperature of 130- 150°C in 7-10 s. The preforms are healed from the inside out, so that maximum removal of volatiles occurs with minimal stresses developing in the preform. 

The delivery of heat into the material prior to its entering the die is accomplished via Radio Frequency (RF) preheating, induction heating or by conventional conductive heating of the resin. Desirable characteristics such as increased process rates and the reduction of thermal stresses induced by gradient of temperature and less content of voids can be achieved by the heating methods listed over more conventional approaches (8). 

Press Curing - Press curing includes the molding of articles by compression, transfer, or injection methods. Blocked-in articles which are cured directly between press platens also come under this classification, as well as unblocked slabs. The heat source is generally saturated steam. Electrically heated platens are also used in some installations. Radio frequency waves have been suggested as a means of curing or for preheating blanks to reduce the press cure time. 

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