Time independent cycle control for plastic injection molding machines

Apparatus for controlling a cyclically operated screw-injection plastic molding machine having a screw reciprocably and rotatably mounted within a barrel, means for feeding raw plastic material into said barrel, means for rotating said screw to plasticize and advance said plastic material toward a sprue located at one end of said barrel, said sprue communicating with a mold having a cavity of predetermined form, means for clamping the mold together, whereby the mold cavity assumes said predetermined form, means for forcing said screw toward said sprue whereby said plasticized plastic material which has accumulated in said barrel is injected through the sprue into the mold cavity to form a molded plastic part, means for applying pressure to the screw to hold the plastic in the cavity until the plastic solidifies, and means for unclamping said mold whereby said molded article may be removed therefrom and said molding machine may be recyclably operated, the improvement comprising:

• a pressure transducer for generating mold cavity pressure signals,
• a first comparator means for comparing mold cavity pressure signals with a first predetermined value representative of the pressure in the mold cavity during injection which insures that the plastic fills the mold cavity, said comparator means generating a first coincidence signal,
• first pressure control means for changing the pressure applied to the screw from an injection pressure to a holding pressure in response to said first coincidence signal,
• a second comparator means for comparing said cavity pressure signals with a second predetermined value representative of the pressure in the mold cavity after injection to insure that the plastic has sufficiently solidified so that the holding pressure on the screw may be reduced and for generating a second coincidence signal,
• second pressure control means for reducing the pressure applied to the screw from said holding pressure in response to said second coincidence signal,
• a third comparator means for comparing said cavity pressure signals with a third predetermined value representative of the pressure in the mold cavity after injection at which the molded plastic is sufficiently solidified to be ejected from the mold and for generating a third coincidence signal,
• a temperature transducer for generating signals representing part temperatures in the mold cavity,
• a fourth comparator means for comparing generated temperature signals with a predetermined value representative of the temperature at which the molded plastic can be ejected from the mold and for generating a fourth coincidence signal,
• means for unclamping the mold and ejecting the molded part,
• and gate means for holding the first occurring of the third and fourth coincidence signals until the last occurring of the third and fourth coincidence signals whereby the mold is unclamped and the part is ejected only after the cavity pressure and the cavity temperature have both achieved predetermined values.