An injection molding apparatus for producing a plastic product having a grain pattern thereon. The apparatus includes a female die having a grain pattern on an inner surface thereof, the female die further provided with a vacuum drawing mechanism for drawing a vacuum therethrough, and a male die having an injection aperture for injecting a thermal formable plastic material therethrough. The male die further includes a burr cutter adapted to slide fit in a gap between the male die and the female die and a guide pin with a spacer provided at four comers of the male die for controlling a distance between the male die and a female die.