a bottom insert for a mold cavity for the production of moldings of plastic material . the mold cavity has a main body with a hot side which has a recess for receiving molten plastic material under pressure and a cold side having a recess , the inside contour of which is intended to come into contact with the molding to be produced and to shape the outside contour thereof . a gate is provided that connects the recess of the hot side to the recess of the cold side and is intended to introduce molten plastic material from the hot side into the cold side . in accordance with the invention there is provided a device for thermal insulation within the bottom insert between the hot side and the cold side of the bottom insert to permit more rapid cooling of the cold side by restricting heat flow from the hot side .