Meaning
Unplasticized polyvinyl chloride polymer containing stabilizers and processing aids provides high stiffness, chemical resistance and flame retardancy. In extrusion and injection moulding, rigid polyvinyl chloride requires precise temperature control due to its high melt viscosity and susceptibility to thermal degradation.
Thermal Stability
Degradation of the polymer melt can occur rapidly if the processing temperature exceeds the safe operating window. When processing rigid polyvinyl chloride, thermal degradation releases corrosive hydrochloric acid gas, which damages the tooling and degrades the mechanical properties of the part. This risk requires the use of specialized corrosion-resistant steels for the mould and screw assemblies.
Moulds are often plated or treated to prevent pitting, which would otherwise ruin the surface finish of the finished profile or part.
Melt Viscosity
High flow resistance makes filling complex or thin-walled moulds difficult. Because rigid polyvinyl chloride has a very high melt viscosity, moulders must use high injection pressures and slow injection speeds to prevent shear heating. This processing limitation often restricts the design of parts to thicker walls and simpler geometries.
Processing Additive
Formulation chemistry is critical to enabling the melt processing of this challenging material. Compounding rigid polyvinyl chloride requires external lubricants and heat stabilizers to prevent degradation during the moulding run. Adjusting these additive levels is necessary to balance flow behavior with the impact strength required for the finished part.
