Polymer processing systems including screws

Abstract

Injection molding and blow molding systems that include screws having low L:D ratios are provided. The systems are capable of producing microcellular polymeric materials. In some cases, the systems may be formed by retrofitting conventional polymer processing systems. Retrofitting may involve changing (e.g., machining or replacing) existing components of the conventional system, as well as, adding new components to the system. For example, retrofitting generally involves replacing the conventional polymer processing screw with a new screw designed to satisfy conditions needed for processing microcellular materials. In other cases, the systems may be newly manufactured. The retrofitted or newly manufactured systems of the invention are considerably less expensive than newly manufactured microcellular processing systems having higher L:D ratios.

1 . A polymer processing system that is operable cyclically, comprising:
 a screw mounted within a barrel to define a polymer processing space between the screw and the barrel, the screw having an L:D ratio of less than or equal to about 24:1; and   a source of blowing agent connected to a blowing agent port to permit blowing agent flow from the source to polymeric material within the polymer processing space.   
   
   
       2 . The system of  claim 1 , wherein the system is operable to cyclically inject polymeric material into a mold or to cyclically eject polymeric material from a die. 
   
   
       3 . The system of  claim 1 , comprising at least two blowing agent ports. 
   
   
       4 . The system of  claim 2 , wherein the at least two blowing agent ports are located axially along the barrel relative to each other. 
   
   
       5 . The system of  claim 1 , wherein the blowing agent port is in the barrel. 
   
   
       6 . The system of  claim 1 , wherein the blowing agent port is in the screw. 
   
   
       7 . The system of  claim 3 , comprising a blowing agent passage from the blowing agent source to the blowing agent port that passes through a downstream end of the screw. 
   
   
       8 . The system of  claim 1 , wherein the screw is reciprocatable within the barrel. 
   
   
       9 . The system of  claim 1 , wherein the system is a retrofitted system. 
   
   
       10 . The system of  claim 1 , wherein the screw includes a restriction element positioned upstream of the port, the restriction element being designed to restrict the upstream flow of polymeric material or blowing agent therethrough during at least a portion of an injection or an ejection cycle. 
   
   
       11 . The system of  claim 10 , wherein the restriction element comprises a restriction valve constructed and arranged to permit downstream flow of polymeric material therethrough in an open configuration and to restrict upstream flow of polymeric material therethrough in a closed configuration. 
   
   
       12 . The system of  claim 11 , wherein the restriction valve is a ring check valve. 
   
   
       13 . The system of  claim 11 , wherein the restriction valve is a ball check valve. 
   
   
       14 . The system of  claim 11 , wherein the restriction valve is a piston valve. 
   
   
       15 . The system of  claim 11 , wherein the screw further comprises a tip valve, and the restriction valve closes at the same time as or before the tip valve during operation. 
   
   
       16 . The system of  claim 10 , wherein the restriction element comprises a reverse screw element. 
   
   
       17 . The system of  claim 10 , wherein the restriction element comprises a blister ring. 
   
   
       18 . The system of  claim 1 , wherein the screw includes a tip valve positioned at a downstream end of the screw, the tip valve permitting downstream flow of polymeric material therethrough in an open configuration and restricting upstream flow of polymeric material therethrough in a closed configuration. 
   
   
       19 - 43 . (canceled) 
   
   
       44 . A method of processing polymeric material comprising:
 conveying polymeric material in a polymer processing space defined between a screw and a barrel, the screw having an L:D ratio of less than or equal to about 24:1;   introducing blowing agent into the polymeric material within the polymer processing space to form a mixture of polymeric material and blowing agent; and   injecting the mixture of polymeric material and blowing agent into a mold, or ejecting the mixture of polymeric material from a die.   
   
   
       45 - 47 . (canceled) 
   
   
       48 . A method of retrofitting a system previously used to process polymeric material, the system comprising an extruder including a first screw having a length and a diameter, the system being operable to cyclically inject polymeric material into a mold or to cyclically eject polymeric material from a die to form microcellular polymeric material, the method comprising:
 replacing the first screw with a replacement screw having an L:D ratio of less than about 24:1; and   connecting a source of blowing agent to a port to provide a pathway for blowing agent flow from the source to a polymer processing space defined between the replacement screw and the barrel, when the replacement screw is mounted within the barrel.   
   
   
       49 - 75 . (canceled)