Draw-back valve assembly for an injection molding apparatus

Claim 1

In an injection molding apparatus including a barrel for holding molten plastic material, a first passage connected at one end with said barrel for receiving hot plastic material from said barrel, a second passage communicating with an opposite end of said first passage for receiving molten with the first passage for supplying said material to a mold, a cylinder, a third passage connected at one end with said cylinder and connected at another end with said second passage, a piston in said cylinder, movable by pressure means in one direction, and movable by pressure means in another direction to exert a suction action in said cylinder, said third passage and said second passage for drawing molten material in said second passage back into said third passage and said cylinder, and a valve means in said first passage for opening and closing said first passage, the improvement which comprises means for controlling the pressure in said cylinder for at least one predetermined postion of said piston in the cylinder, comprising a positioner connected to said piston and movable in relation to said cylinder in a linear path, a linear position potentiometer disposed adjacent said linear path of said positioner, a comparator electrically connected to said potentiometer, a servo-valve electrically connected to said comparator and pneumatically connected to said cylinder, and a voltage source connected to said potentiometer and said comparator, said comparator having at least one variable resistor electrically connected at one end thereof to said voltage source and at another end thereof to an amplifier, said amplifier being electrically connected to said servo-valve.