Polymer processing systems including screws

A polymer processing system that is operable cyclically to produce microcellular material, comprising:

• an extruder including only a single screw, the screw being mounted within a barrel to define a polymer processing space therebetween, the screw being rotatable to convey polymeric material in the polymer processing space in a downstream direction to an extruder outlet within the polymer processing space, the screw having an L:D ratio of less than or equal to about 24:1, wherein the screw includes a wiping section having an L:D ratio of between of between about 0.5:1 and about 3:1, and a mixing section having an L:D ratio of between about 2:1 and about 6:1;
• a source of blowing agent connected to a blowing agent port associated with the system to permit blowing agent introduction into the polymeric material within the system;
• an accumulation region downstream of the screw and fluidly connected to the polymer processing space, the accumulation region designed to have a mixture of polymeric material and blowing agent accumulated therein;
• and
• a mold downstream of the extruder outlet,
• wherein the system is designed to move the screw in a downstream direction to inject the mixture of polymeric material and blowing agent from the accumulation region into a cavity of the mold and to form a microcellular material therein and wherein the wiping section includes a screw flight that passes beneath the blowing agent port.