Apparatus and Method for Injection Molding at Low Constant Pressure

Abstract

A low constant pressure injection molding machine forms molded parts by injecting molten thermoplastic material into a mold cavity at low substantially constant pressures of 6,000 psi and less. As a result, the low constant pressure injection molding machine includes a mold formed of easily machineable material that is less costly and faster to manufacture than typical injection molds.

1 . An injection molding apparatus comprising:
 a melt holder for pressurizing molten plastic prior to injection into a mold having a plurality of mold cavities;   a sensor in dynamic communication with the melt holder for sensing a characteristic of the molten plastic; and   a controller in communication with the sensor, the controller receiving a signal from the sensor, the signal being indicative of a melt pressure of the molten plastic entering at least one mold cavity in the plurality of mold cavities, the controller further being in communication with an injection element, the injection element applying a force to the molten plastic to advance the molten plastic from the melt holder into the mold, wherein the controller controls the injection element to maintain a substantially constant melt pressure entering the at least one mold cavity of less than 6000 psi, and   the mold has an average thermal conductivity of more than 30 BTU/HR FT ° F. and the mold has at least one of   a milling machining index of greater than 100%,   a drilling machining index of greater than 100%, and   a wire EDM machining index of greater than 100%.   
     
     
         2 . The injection molding apparatus of  claim 1 , wherein the at least one mold cavity in the plurality of mold cavities is a thin-walled mold cavity having an L/T>100. 
     
     
         3 . The injection molding apparatus of  claim 1 , further comprising an artificially balanced molten plastic feed system. 
     
     
         4 . The injection molding apparatus of  claim 1 , wherein sensor generates an electrical signal. 
     
     
         5 . The injection molding apparatus of  claim 1 , wherein the sensor generates a mechanical signal. 
     
     
         6 . The injection molding apparatus of  claim 1 , wherein the sensor generates a hydraulic signal. 
     
     
         7 . The injection molding apparatus of  claim 1 , wherein the sensor generates a pneumatic signal. 
     
     
         8 . The injection molding apparatus of  claim 1 , further comprising a mold frame and a mold base, wherein at least one of the mold frame and the mold base is made from a material having a surface hardness of greater than 165 BHN and less than 260 BHN. 
     
     
         9 . The injection molding apparatus of  claim 1 , further comprising a hot runner feed system constructed of a material having a thermal conductivity that is substantially equal to the average thermal conductivity of the mold. 
     
     
         10 . The injection molding apparatus of  claim 9 , wherein the hot runner feed system is directly connected to a gate that is fluidly connected with the at least one mold cavity. 
     
     
         11 . The injection molding apparatus of  claim 1 , wherein the mold has a sinker EDM machining index of greater than 200%. 
     
     
         12 . The injection molding apparatus of  claim 11 , wherein the mold has a sinker EDM machining index of between 200% and 1000%. 
     
     
         13 . The injection molding apparatus of  claim 1 , wherein the mold comprises at least four mold cavities. 
     
     
         14 . An injection molding apparatus comprising:
 a mold having at least one mold cavity, the mold having
 an average thermal conductivity more than 30 BTU/HR FT ° F., and 
 a sinker EDM machining index of greater than 200%; 
   a melt holder for pressurizing molten plastic prior to injection into the mold;   a sensor in communication with the melt holder for sensing a characteristic of the molten plastic; and   a controller in communication with the pressure sensor, the controller receiving a signal from the sensor, the signal being indicative of a melt pressure of the molten plastic entering the mold cavity, the controller further being in communication with an injection element, the injection element applying a force to the molten plastic to advance the molten plastic from the melt holder into the mold,   wherein the controller controls the injection element to maintain a melt pressure of the molten plastic entering the mold cavity of less than 6000 psi.   
     
     
         15 . The injection molding apparatus of  claim 14  wherein the controller controls the injection element to maintain a substantially constant melt pressure. 
     
     
         16 . An injection molding apparatus comprising:
 a mold having at least one mold cavity, the mold having an average thermal conductivity more than 30 BTU/HR FT ° F., and the mold having at least one of
 a milling machining index of greater than 100%, 
 a drilling machining index of greater than 100%, and 
 a wire EDM machining index of greater than 100%; 
   a melt holder for pressurizing molten plastic prior to injection into the mold;   a sensor in communication with the melt holder for sensing a characteristic of the molten plastic indicative of a melt pressure of the molten plastic in a nozzle of the melt holder, just prior to entering the mold; and   a controller in communication with the sensor, the controller receiving a signal from the pressure sensor, the controller further being in communication with an injection element, the injection element applying a force to the molten plastic to advance the molten plastic from the melt holder into the mold,   wherein the controller controls the injection element to maintain a melt pressure of less than 6000 psi.   
     
     
         17 . The injection molding apparatus of  claim 16 , wherein the controller controls the injection element to maintain a substantially constant melt pressure. 
     
     
         18 . An injection molding apparatus comprising:
 a mold having at least one mold cavity, the mold having an average thermal conductivity more than 30 BTU/HR FT ° F., and at least one of
 a thin-walled mold cavity (e.g., L/T>100), 
 at least four mold cavities, and 
 an artificially balanced molten plastic feed system; 
   a melt holder for pressurizing molten plastic prior to injection into the mold;   a sensor in communication with the melt holder for sensing a characteristic of the molten plastic indicative of a melt pressure of the molten plastic in a nozzle of the melt holder just before entering the mold; and   a controller in communication with the sensor, the controller receiving a signal from the sensor, the controller further being in communication with an injection element, the injection element applying a force to the molten plastic to advance the molten plastic from the melt holder into the mold,   wherein the controller controls the injection element to maintain a melt pressure of less than 6000 psi.   
     
     
         19 . The injection molding apparatus of  claim 18 , wherein the controller controls the injection element to maintain a substantially constant melt pressure. 
     
     
         20 . An injection molding apparatus comprising:
 a mold having at least one mold cavity, the mold being formed from a material having a surface hardness less than 30 Rc and the mold having an average thermal conductivity of more than 30 BTU/HR FT ° F.;   a melt holder for pressurizing molten plastic prior to injecting the molten plastic into the mold;   a guided ejection system; and   a controller that controls an injection element in the melt holder so that the injection element advances molten plastic into the at least one mold cavity at a melt pressure of less than 6000 psi.