Methods for controlling co-injection plastic pressure ratio between individual flow front layers

Abstract

A method and system for co-injection molding of two molten plastic materials that allows monitoring and utilization of injection pressure and optionally melt pressure and/or flow front pressure during an injection run. A controller alters the injection pressure so as to achieve and maintain optimal or desired ratios of injection pressure, and optionally melt pressure and/or flow front pressure, of the two molten plastic materials. This allows for more precise part manufacture, including reducing the thickness of a skin or shell layer compared to a core layer of a molded part.

1 . A system for controlling melt pressure during co-injection molding, the system comprising:
 an injection molding machine comprising a first injection unit, a second injection unit, and a mold forming a mold cavity, wherein:   (i) the first injection unit is adapted to inject a first material into the mold cavity at a first injection pressure, and   (ii) the second injection unit is adapted to inject a second material into the mold cavity at a second injection pressure;   a controller adapted to control operation of the injection molding machine, wherein:   (i) the controller is adapted to adjust the first injection pressure based on comparing a first flow front pressure to a first optimal flow front pressure, and   (ii) the controller is adapted to adjust the second injection pressure based on comparing a second flow front pressure to a second optimal flow front pressure; and   two or more sensors coupled to the injection molding machine and the controller, wherein each of the two or more sensors is adapted to measure one or more variables of the injection molding machine and transmit the measured one or more variables to the controller.   
     
     
         2 . The system of  claim 1 , wherein:
 (i) the first material is a regrind material,   (ii) the second material is a regrind material, or   (iii) both the first material is a regrind material and the second material is a regrind material   
     
     
         3 . The system of  claim 1 , wherein the first optimal flow front pressure has a first value at a first point in time and the first optimal flow front pressure has a second value at a second point in time. 
     
     
         4 . The system of  claim 1 , wherein the second optimal flow front pressure has a first value at a first point in time and the second optimal flow front pressure has a second value at a second point in time. 
     
     
         5 . The system of  claim 1 , wherein the injection molding machine is adapted to:
 start the injection of the first material into the mold cavity;   in response to waiting a predetermined first period of time, start the injection of the second material into the mold cavity;   in response to waiting a predetermined second period of time, cease the injection of the second material into the mold cavity; and   in response to waiting a third predetermined period of time, cease the injection of the first material into the mold cavity.   
     
     
         6 . The system of  claim 1 , wherein the injection molding machine is adapted to:
 start the injection of the first material into the mold cavity;   then, in response to determining a first variable is approximately equal to a first threshold value, start the injection of the second material into the mold cavity;   then, in response to determining a second variable is approximately equal to a second threshold value, cease the injection of the second material into the mold cavity; and   then, in response to determining a third variable is approximately equal to a third threshold value, cease the injection of the first material into the mold cavity.   
     
     
         7 . The system of  claim 6 , wherein the first threshold value is a first flow front position setpoint of the first material, a second flow front position setpoint of the second material, a first cavity melt pressure setpoint of the first material, a second cavity melt pressure setpoint of the second material, a first position of a first screw for injecting the first material, a second position of a second screw for injecting the second material, a first distance traveled by the first screw, or a second distance traveled by the second screw. 
     
     
         8 . The system of  claim 6 , wherein
 the first cavity melt pressure setpoint of the first material may be measured directly or indirectly; and   the second cavity melt pressure setpoint of the second material may be measured directly or indirectly.   
     
     
         9 . The system of  claim 6 , further comprising one of a microwave sensor kit, an x-ray sensor kit, an ultrasonic sensor kits, an air pressure sensor kit, an air temperature sensor kit, or a sub-surface temperature sensor kit. 
     
     
         10 . The system of  claim 6 , wherein the first variable, the second variable, and the third variable are measured by the two or more sensors.