Meaning
Chemical degradation of polymer chains occurs when the processing temperature exceeds the thermal limits of the resin or its additive packages. Preventing thermal breakdown requires careful control of extruder barrel temperatures and melt residence times. This degradative process breaks the polymer backbone, leading to a loss of physical properties in the molded part.
Molecular Consequence
Chain scission and cross-linking are the primary molecular reactions that take place when a polymer overheats. During thermal breakdown, the reduction in molecular weight causes a sharp decrease in melt viscosity, which can lead to flashing or drooling in injection molding. This changes the mechanical performance of the resin, leaving the finished products brittle and prone to cracking under minor stress.
Visual Defect
Splay marks, black specks, and severe yellowing are common visual signs that a material has degraded inside the heating cylinder. The thermal breakdown of additives like colorants or flame retardants releases gaseous byproducts that become trapped as bubbles in the melt, forming silver streaks on the part surface. Operators must purge the barrel immediately after a prolonged stop to remove this degraded material before restarting production.
If these degraded parts slip through quality control, they can lead to complete batch rejections and expensive recalls by the customer.
Economic Loss
Wasted material and machine downtime are the immediate financial results of letting resin degrade in the processing equipment. Recovering from thermal breakdown requires cleaning the screw and die, which can take several hours and consumes expensive purging compounds. Moulders must balance processing speeds and temperatures to maximize output while keeping the melt safely below the degradation point.
