High-frequency proximity heating for injection molding applications

High-frequency proximity heating was used to rapidly heat injection molds. The principle is based on the proximity effect between a pair of mold inserts facing each other with a small gap and forming a high-frequency electric loop. Because of the proximity effect, the high-frequency current will flow at the inner surfaces of the facing pair, thus selectively heating the mold surface. With this method, the electrical insulation layer beneath the mold surface can be eliminated, resulting in a mold insert made of a single metal. A mold with a cavity of 25 X 50 mm(2) was constructed with careful design on its electrical, structural, and thermal performance. Air pockets with reinforcing ribs were embedded right beneath the mold surface for enhancing the heating performance. The resulting mold cavity can be rapidly heated from room temperature to about 240 degrees C in 5 s with an apparent heating power of 93 W/cm(2). The new mold heating method was applied to thin-wall molding and micromolding, and in all testing cases, short cycle times less than a minute were achieved.