Time saver plastic draw-back valve assembly

Abstract

This invention decreases the time of an injection molding machine cycle by cutting off communication between the mold and the source of material as soon as injection is complete, and accumulating material for the next charge during the cooling part of the cycle. Pressure is maintained on the material in the mold, and material is supplied to compensate cooling shrinkage, from an auxiliary cylinder in which a minor amount of hot material is accumulated. After sufficient cooling, runner material is drawn back by suction of the auxiliary cylinder. A three-way valve cuts off the main material supply and puts the auxiliary cylinder in communication with the mold.

What is claimed is: 
     
       1. An injection molding apparatus including a barrel for holding molten plastic material, a first passage connected at one end with the barrel for receiving hot plastic material from the barrel, a second passage connected with the other end of the first passage for receiving molten material from the first passage for supplying the material to a mold, a cylinder, a third passage connected at one end with the cylinder and connected at its other end with the second passage, a piston in the cylinder, movable in one direction to maintain pressure on any molten material in the cylinder, third passage and second passage, and movable in the other direction to exert a suction action in the cylinder, third passage and second passage for drawing molten material in the second passage back into the third passage and cylinder, and valve means, located between the first and second passages and between the second and third passages, movable into different positions including a first position that closes off the third passage from the second passage and that puts the first passage in communication with the second passage, and a second position that shuts off the first passage from the second passage and that puts the second passage in communication with the third passage.Iadd., said valve means preventing communication between said first and third passages in said two positions.Iaddend.. 
     
     
       2. The injection molding apparatus described in claim 1 characterized by the valve means including a housing, a valve chamber within the housing, all of the passages opening into the valve chamber, and the valve means including also an element in the chamber movable into said first and second positions. 
     
     
       3. The injection molding apparatus described in claim 2 characterized by the valve means being a single three-way valve, the valve chamber being of circular cross-section, and the valve element being a plug that fits into the valve chamber and that is rotatable about the longitudinal axis of the valve element, the passages opening into the valve chamber at angularly spaced locations around the circumference of the chamber, the valve element having passages therein that communicate with the various passages at different angular positions of the valve element, and an actuator for moving the valve element back and forth between said first and second positions. 
     
     
       4. The injection molding apparatus described in claim .[.1.]. .Iadd.10 .Iaddend.characterized by an actuator for the piston including a controller for supplying energy to the piston to urge the piston to move in one direction when the valve means is in .[.said first.]. .Iadd.the second .Iaddend.position to maintain pressure on any molten material in the .[.third passage.]. .Iadd.cylinder .Iaddend.and .[.second passage.]. .Iadd.mold runner .Iaddend. during the cooling portion of a molding cycle .[.and while the valve means is in position that puts the third passage in communication with the second passage.]. whereby the piston forces additional material into the mold to compensate for shrinkage of the material in the mold while cooling, and automatic means associated with the controller for reversing the direction of the energy supply to the actuator.Iadd., .Iaddend.when the valve means is in said second position.Iadd., .Iaddend.to move the piston in a direction to draw back material from .Iadd. the .Iaddend.runner.[.s.]. .Iadd., .Iaddend. .[.with which the molding apparatus is used.]. after a predetermined cooling period. 
     
     
       5. The injection molding apparatus described in claim .[.4.]. .Iadd.10 .Iaddend.characterized by a valve operator for moving the valve means between its different positions including an automatic control that shifts the valve means from its first position at the end of an injection operation of said injection molding apparatus. 
     
     
       6. The injection molding apparatus described in claim .[.5.]. .Iadd.10 .Iaddend.characterized by a helically threaded plasticizer screw in the barrel, pressure means for advancing the screw longitudinally toward the first passage on an injection stroke while the valve means is in its first position, said valve means in its second position shutting off flow of material from the first passage, a motor for rotating the screw in a direction to urge material in the barrel toward the first passage after the valve means has moved into its second position whereby the continued rotation of the screw causes it to back away from said first passage, automatic means for stopping further rotation of the screw after it has backed away far enough to accumulate, between the screw and the first passage, enough material for the next mold charging operation. 
     
     
       7. The injection molding apparatus described in claim 6 characterized by the pressure means for advancing the helically threaded screw being a cylinder-and-piston motor that moves the helically threaded screw longitudinally when the motor for rotating the screw is not energized, and the automatic control for the valve operator being responsive to the movement of the screw toward the first passage. 
     
     
       8. The injection molding apparatus described in claim .[.1.]. .Iadd.10 .Iaddend..[.molten.]. .Iadd.characterized .Iaddend.by apparatus for injecting molten material at high temperature and pressure into the first passage and through the valve means to the second passage when the valve means is in its first position, the valve means shutting off all flow of material through the first passage when the valve means is in its second position, and a valve operator for moving the valve means from first position to second position and vice versa. 
     
     
       9. The injection molding apparatus described in claim 8 characterized by an automatic control responsive to the substantial completion of the filling of the mold for shifting of the valve means from first position to second position. .Iadd. 10. An injection molding apparatus including a barrel for holding molten plastic material, a first passage connected at one end with the barrel for receiving hot plastic material from the barrel, a runner that supplies molten material to a mold cavity, a second passage connected with the other end of the first passage for receiving molten material from the first passage for supplying the material to the mold runner, a plasticizer including a helically threaded screw in the barrel, with a front end of the screw facing the first passage, pressure means for advancing the screw longitudinally toward the first passage to discharge material from the barrel on an injection stroke, a motor for rotating the screw in a direction to urge material in the barrel toward the first end of the screw during rotation of the screw as it backs away from said first passage, during its recovery stroke, automatic means for stopping further rotation of the screw after it has backed away far enough to accumulate, between the screw and the first passage, enough material for the next mold charging operation, means for regulating the plasticizer cycle to be less than that absent such means including a cylinder for communication at one end with the mold runner, a piston in the cylinder, movable in one direction providing means to maintain pressure on any molten material in the cylinder and in the mold runner during shrinkage of the molten material, the piston being movable in the other direction providing means to exert a suction action in the cylinder for drawing molten material in the runner back toward the cylinder, and valve means, located between the first and second passages and movable into different positions including a first position that puts the first passage in communication with the second passage, and a second position that shuts off the first passage from the second passage and provides communication between the cylinder and the second passage, said valve preventing communication between said cylinder and said first passage so that recovery of the plasticizer can begin before shrinkage of the plastic in the mold and before drawback of plastic from the runner.Iaddend.. .Iadd. 11. The injection molding apparatus of claim 1, wherein said valve means prevents communication between said first and third passages in positions other than said two positions of said valve means. .Iaddend..Iadd. 12. The injection molding apparatus of claim 1, wherein the diameter of said cylinder is about equal to the diameter of said third passage. .Iaddend..Iadd. 13. The injection molding apparatus of claim 12, wherein said cylinder diameter is less than the diameter of said barrel. .Iaddend. .Iadd. 14. The injection molding apparatus of claim 1, wherein said piston exerts sufficient suction so as to sever molten material in a runner from a molded part in the mold. .Iaddend..Iadd. 15. The injection molding apparatus of claim 10, wherein the diameter of said cylinder is about equal to the diameter of said second passage. .Iaddend..Iadd. 16. The injection molding apparatus of claim 15, wherein said cylinder diameter is less than the diameter of said barrel..Iaddend.