in a method of compression molding thermoplastic material , a compression gap is set between half - molds of a mold having fixed and movable platens for respective attachment of the half - molds . after setting the compression gap , the mold is closed and the platens are locked by moving locking elements into respective receiving elements . injection of plasticized material then commences into a cavity formed between the half - molds when the mold is closed . the mold then opens as a result of pressure generated by the injected plasticized material until a distance between the half - molds corresponds to the compression gap . subsequently , the mold undergoes a closing movement through application of a clamping force , thereby compressing the plasticized material .