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in injection moulding , the function of a non-return valve is to stop the flow of melted resin back into he screw channels while the screw is moving forward during injection . Valves are manufactured from steels or alloys that are best suited to the processing environment in which they will be used various tool steels that are heat-treated and nitride , nickel alloys , chrome plating and special coatings are all options from which to choose .
valves may be of a special design . However , styles most commonly used are the three piece free-flow , three piece locking , four piece valve plus side or front discharge ball check types.