A method for achieving uniform expansion of a dielectric plate made by injection molding includes the steps of forming core pins having a rhombic cross section at predetermined positions inside a mold for injection-molding the plate, injecting a plasticized dielectric material into the mold wherein the core pins guide the plasticized material flow for properly orienting molecules of the dielectric material, and curing and forming the dielectric plate in which rhombic holes corresponding to the core pins are formed. The properly oriented molecules of the dielectric material and the rhombic holes reduce the difference between thermal expansion coefficients in longitudinal and lateral directions of the plate thereby achieving uniform expansion thereof.