Meaning
Volatile compounds and broken polymer fragments are generated when a resin is exposed to excessive heat or prolonged residence time in the injection moulding barrel. These thermal degradation products alter the polymer’s molecular weight distribution, which reduces the mechanical properties of the finished part and introduces cosmetic defects such as silver streaks or splay. Sourcing high-stability resins or reducing the barrel temperature can mitigate this breakdown of the polymer chain.
The presence of these byproducts indicates that the processing window has been exceeded, necessitating immediate process adjustment.
Degradation Mechanism
Chain scission occurs when the thermal energy exceeds the bond energy of the polymer backbones. In resins like polyvinyl chloride or polyacetal, this breakdown can release corrosive gases such as hydrochloric acid or formaldehyde, which can damage the mould steel and the screw. To prevent this, moulders must minimize the residence time of the melt in the barrel.
Cosmetic Defects
Trapped gases from the degraded material migrate to the flow front and form bubbles that are flattened against the mould wall, appearing as splay. This defect can be misidentified as moisture splay, but it persists even after the resin has been thoroughly dried. Decreasing the screw speed or the back pressure reduces the shear heating that accelerates the formation of these gases.
Sourcing Impact
Recycled resins and regrind contain a higher concentration of these broken chains because they have undergone multiple heating cycles. Using high fractions of regrind can therefore increase the formation of thermal degradation products, which lowers the impact strength of the moulded parts. Sourcing virgin resin is necessary for parts that require high impact resistance.
