Mechanism and Countermeasures for Black Dot Defects

Preventing Black Dots with Purging Pellets.

This catalog, "Mechanism and Countermeasures for Black Dot Defects," comprehensively addresses the critical challenge of black dot defects in plastic molding, outlining their causes, underlying mechanisms, and effective strategies for prevention and reduction. The primary causes of black dot defects are categorized into three areas: foreign matter contamination during material feeding (e.g., from hoppers or dryers), carbonization of resin within the screw, and adhesion to the mold. Carbonization in the screw is a significant issue, resulting from thermal decomposition and cross-linking reactions of resin in contact with the cylinder wall during continuous molding, oxidation of residual resin exposed to air after molding, or prolonged stagnation of easily carbonizable resins at elevated temperatures. The defect mechanism involves abnormal heating of the heating cylinder, which degrades the stable protective film, initiating resin carbonization. These carbonized particles grow, detach due to resin flow, and then new carbonized matter forms on the exposed metal surfaces where the protective film is absent, perpetuating a cycle of black dot generation. Effective countermeasures include disassembly and cleaning, which removes carbonized material but temporarily compromises the stable protective film, potentially leading to initial black dot issues until stability is restored. Alternatively, purging with specialized agents is recommended. Various purging agent types exist, such as foaming, surfactant, and glass-filled types. While glass-filled agents are highly effective for removing stubborn carbon, they pose a risk of screw wear and require careful handling. The catalog highlights the recent emergence of new purging agents from numerous manufacturers, suggesting that exploring these innovative solutions, such as the "ecomaru Purging Pellet" featured on the cover, is essential for overcoming persistent black dot challenges and improving manufacturing quality. Manufacturers are encouraged to actively gather information and experiment with these advanced purging technologies to achieve better product quality and operational efficiency.