Low Frequency Shear Rheology Metrics for Assessing Extrudate Swell in Bimodal Pipe Resins
Low frequency storage modulus G prime below 0.1 rad/s isolates high molecular weight elastic recovery to predict bimodal pipe swell and prevent over-extrusion.
Parison Expansion
Meaning
Controlled radial growth of a softened thermoplastic tube occurs when gas pressure pushes the material outward against the inner surfaces of a closed mould. Parison expansion happens during blow moulding as the extrudate encounters the cavity wall. This process defines the final distribution of resin thickness across the product geometry.
Proper management of the pressure profile ensures the structural integrity of the moulded part.
Process Mechanics
Uniform wall distribution depends on the initial weight profile of the tube and the rate at which gas inflates the molten resin. Air entry speed dictates how quickly the surface stretches before contact with the cooler mould interface. Operators adjust the timing of the blow sequence to prevent thinning in deep draw regions.
High viscosity resins require higher pressures to overcome resistance during the inflation phase.
Resin Requirements
Melt strength remains the primary material property determining stability during the inflation stage. Polymers lacking sufficient melt strength suffer from excessive sag or tearing before the internal pressure reaches the target magnitude. Regrind batches often exhibit lower elasticity than virgin resin which changes the stretching behavior under identical process conditions.
Consistent feedstock quality prevents the formation of weak sections or inconsistent wall density in the final geometry.
Cost Implications
Excessive expansion leads to localized thinning that forces designers to increase the total grammage of the part to meet minimum safety standards. Scrap rates rise when uneven inflation prevents full capture of the mould details or causes burn marks from trapped air. Efficient cooling cycles follow effective pressure management because intimate contact with the mould facilitates faster heat transfer.
Optimizing the blow ratio relative to the cooling rate maximizes machine throughput.