Meaning
Accumulation of oxidized, charcoal-like resin deposits on the internal surfaces of an injection cylinder or hot runner system occurs when thermoplastic material remains exposed to elevated processing temperatures for an extended duration. This carbonized polymer degradation occurs primarily during production interruptions or when excessive residence time allows the resin to exceed its thermal limit.
Thermal Accumulation
Melt temperature settings that exceed the datasheet recommendations of the supplier accelerate the chemical breakdown of the resin. While a virgin resin possesses uniform molecular weight and stable thermal limits, regrind material often exhibits reduced resistance to heat and undergoes carbonized polymer degradation more rapidly. Machine operators can reduce the cylinder temperature during planned downtime to prevent this localized overheating.
Defect Mechanism
Solidified black flecks of degraded material break away from the screw flights or manifold channel walls and enter the melt stream. Once carried into the mould cavity, these dark particles create visual defects that cause the immediate rejection of thin-walled parts. While a datasheet value for thermal stability assumes a static laboratory environment, actual production runs subject the material to high shear forces that further drive the formation of these carbonized inclusions.
This shear heating can raise the effective temperature of the melt far beyond the control sensor readings, making the process highly sensitive to screw rotation speed.
Operational Consequence
Purging the injection unit represents the standard method to remove these degraded deposits from the melt channel. This cleaning process requires specialized high-viscosity purge compounds and consumes productive press time. Severe cases of this degradation may require the maintenance team to pull and manually scrape the screw, which increases tooling costs and extends equipment downtime.
