Meaning
Reduction in the ability of a polymer melt to resist breaking or sagging under tension. Melt strength degradation often results from thermal shearing or excessive regrind levels, leading to unstable parisons in blow moulding. It stops being a concern once the material has cooled below its glass transition or melting temperature.
Chains Break
Molecules break down during the heating process when the screw speed or barrel temperature is set too high. This shortening of the chains reduces the number of entanglements that provide the melt with its elastic properties. Flow speeds.
Lower viscosity results from this change, making the fluid behave more like a liquid than a rubbery solid.
Parts Failure
Production with weakened material shows visible signs of failure such as uneven wall thickness or holes. In extrusion coating, the melt may fail to bridge the gap between the die and the substrate. Tension drops.
Operators often see the material necking or swaying, which indicates the internal structure is no longer sufficient for the line speed. High levels of regrind accelerate this problem because the polymer has already undergone multiple heat cycles.
Testing Changes
Testing the melt flow index provides a basic way to monitor these changes.
