Meaning
Thermal breakdown of a polymer material occurring under variable temperature conditions over time characterizes the thermal instability during typical processing cycles. When evaluating non-isothermal degradation, engineers analyze how the resin molecular weight decreases during the dynamic heating and cooling stages of injection moulding or extrusion. This degradation mode is bounded by the ceiling temperature of the polymer and the duration of its exposure to high temperatures.
It determines the maximum temperature limits that a resin can withstand before losing its physical properties.
Reaction Kinetics
Chemical reaction rates for chain scission accelerate as the polymer melt travels through the heated barrel and nozzle. Unlike constant temperature holds, non-isothermal degradation occurs across a wide temperature spectrum. This requires tracking the cumulative thermal history of the melt to predict molecular weight loss.
Moulding Impact
Molded parts produced from thermally degraded polymer show reduced tensile strength, increased brittleness, or severe discoloration. The onset of non-isothermal degradation can often be recognized by a drop in melt viscosity, which can cause flashing or short shots in the mold cavity. This breakdown is particularly critical when processing regrind material, which has already undergone one or more thermal cycles.
Maintaining a stable process helps prevent the loss of physical properties in the finished product.
Thermal Control
Restricting residence time in the barrel is essential for preventing the polymer from reaching its critical decomposition threshold. Proper barrel zoning and optimized cycle times ensure that the resin is not subjected to conditions that trigger non-isothermal degradation during production interruptions. When the moulding cycle is interrupted, the heating bands must be turned down to protect the polymer melt.
Implementing these control measures ensures consistent part quality and avoids expensive resin scrap.
