In order to select the proper screw design for a particular resin (or resins), a basic understanding of screw design variables and their effect on processing is essential.  Excluding the use of multiple flights (as used in barrier screws) and mixing sections, there are five variables that may be utilized in the design of a metering screw that have a bearing on how a solid resin becomes a melt.  In addition to these variables, there are barrier and mixing screw designs that can significantly affect processing results.