Poor concentricity is a leading cause of scrap when performing paste extrusion of fine powder PTFE resins into cylindrical shapes. Efforts to achieve consistently good concentricity should begin upon receiving each new shipment of resin. How the material was handled before shipment, during shipment and upon receipt can ultimately affect how the resin extrudes, including the concentricity of the resin during extrusion.
While the extrusion processor cannot control what happens to the resin prior to receipt, there are evaluations that can be done upon receipt of material that will provide clues as to how well the resin is going to extrude. For instance, moisture content of the resin should be quantified BEFORE any mixing with extrusion lubricant is done. Resin that has high moisture content can produce higher extrusion pressures or highly variable extrusion pressures. The variations in extrusion pressure can affect the mechanical alignment of the mandrel and pin and actually push them off center, resulting in poor concentricity of the extruded product.
Other incoming receiving inspection tests can provide even more data on how the resin will perform during extrusion.