Stress Distribution in Solid Polymer Compacts

During the extrusion process, polymer solids are conveyed forward, heated, and compacted under pressure. For some powder resins, the compaction process can increase the density of the solids by a factor of between 2 to 3. For all extrusion solids conveying models, this pressure (or stress) is assumed to be isotropic in the plane perpendicular to the flight. The data presented here, however, indicate that stresses in the solid bed are anisotropic. This anisotropy will affect the solids conveying forces at the metal surfaces of the extruder and hence affect the accuracy of the current published models. A new compaction cell was developed to measure the bulk density, coefficient of storage friction, the lateral stress ratio, and the coefficient of static friction at the wall. The lateral stress ratio is defined as the stress in the secondary direction divided by the stress in the primary direction.