PURPOSE: To contrive the reduction of scraps through the dispersion in the dimensions and weight of molded article and consequently high productivity and perform stable precision molding by a method wherein dwell pressure is controlled so as to lose the changes in the specific volumes of resin in a mold on the basis of the compressibility, which represents the relation between the specific volume of molded article and the pressure of resin in the mold, and the thermal expansibility, which represents the specific volume of the molded article and the temperature of the resin in the mold.

CONSTITUTION: In the controlling method of dwell, when injection starting signal 19a is outputted from a sequence controlling unit 19 to a dwell controlling unit 15 and a processing unit 14, the dwell controlling unit 15 advances a screw 2A so as to start injection. Further, the processing unit 15 measures fixed mold temperature TWa and movable mold temperature TWb and the resin temperature at a nozzle part TR at the proper time of filling stage so as to calculate the amounts of change ΔTW and ΔTR from the reference values of the mold temperature TW and of the resin temperature at the nozzle part TR, in order to calculate the amount of change of the resin pressure at the nozzle part, which brings the change of the specific volume of resin in a mold to zero, ΔPL=(C1'ΔTR+C2'ΔTW)C3'. On the other hand, when the resin pressure at the nozzle part PL reaches the pre-set dwell changeover pressure P, the amount of change of the resin pressure at the nozzle part ΔPL is outputted from the processing unit 14 so as to output the value PL, which is the sum of the reference value of the resin pressure at the nozzle part PL and the amount of change ΔPL, as pressure command signal 15b to the amplifier 22 of a servo valve.