Injection molding machine for thermoplastic resin

An injection molding machine for a thermoplastic resin comprising:

• upper and lower molds which fit together to complete a mold closure, at least one of the molds being a movable mold,
• a clamping hydraulic circuit including a direction switching valve which is switched to a neutral position upon completion of said mold closure,
• a clamping cylinder comprising a forward oil supply port which is connected with a drain when said switching valve is switched to said neutral position so that the clamping hydraulic circuit is set to a substantially pressure-free state,
• an injection unit from which a molten resin is injected into a mold cavity defined between the upper and lower molds thereby providing a resin pressure retracting the movable mold,
• a nozzle provided between said injection unit and said mold cavity and having a nozzle port opening to said mold cavity, said nozzle port being closed simultaneously with completion of injection of a predetermined amount of the molten resin into the mold cavity,
• a control means to open said nozzle at a mold closure complete signal, to infect molten resin into said mold cavity from an injection cylinder of said injection unit that stores said molten resin, and also to charge said molten resin into said injection unit for the next process step upon completion of nozzle closure by said injection unit,
• wherein the switching valve is switched from the neutral position into another position so as to generate a high mold clamping force and conduct molding upon completion of said injection of said predetermined amount of molten resin.