Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Blow Molding Grade
Meaning
Thermoplastic resins engineered with high melt strength prevent parison sag during the formation of hollow containers. A typical blow molding grade balances the requirement for stiffness in the finished part with the need for easy flow through the extrusion head. These materials often exhibit strain hardening to ensure that the parison remains stable as it hangs under its own weight before the mould closes.
Melt Strength
Resistance to deformation under gravity is the primary requirement for large part production. In a blow molding grade, high molecular weight components provide the chain entanglements necessary to support the weight of the molten tube. Without sufficient melt strength, the parison thins at the top and pools at the bottom, leading to uneven wall distribution and part failure.
Moulders measure this property using a haul-off test where a filament is pulled until it breaks. Higher values indicate that the material can sustain its own weight for longer periods during the extrusion phase. This characteristic is vital for industrial drums where parison length can exceed two meters.
Physical Property
Impact resistance and environmental stress crack resistance define the longevity of the moulded container. These properties in a blow molding grade are determined by the density and the molecular weight distribution of the resin. Higher density increases stiffness and chemical resistance but may reduce the ability of the part to withstand sudden drops.
Economic Selection
Sourcing managers compare the cycle time against the material cost when choosing between virgin and regrind blends. Because blow molding grade resins can be reprocessed several times, most moulders incorporate a percentage of flash and scrap back into the production stream. This practice reduces waste while maintaining the mechanical integrity of the final bottle or drum.