Mold for injection molding of plastics using thin film electric heater

A mold for injection molding of plastic material, said mold having a mold cavity defined by a wall surface, and further comprising:

• a thin film electric resistor formed directly on at least part of said wall surface;
• said thin film electric resistor having terminals to supply thereto electric power to thereby heat said wall surface;
• said thin film electric resistor having a surface resistivity r of a value defined by:
• ##EQU2## wherein L is a length between said terminals as measured along said thin film electric resistor, D is length of said thin film electric resistor in a direction perpendicular to the direction between said terminals, V.sub.a is a maximum value of allowable voltage applied between said terminals, I.sub.a is a maximum value of allowable current flowing between said terminals, and W.sub.r is an input power per unit area of said thin film electric resistor determined to obtain a desired rate of increase of temperature of said wall surface.