Q: We are utilizing two separate grinder/mixers for premixing and chips milling in the powder coating laboratory. Can you explain why using separate grinders is so important to prevent craters or other surface imperfections? Is cleaning only one extruder for premixing and chips milling insufficient to overcome the crater formation?
A: The reason for using separate mixer/grinders for premixing and milling (pulverization of extruded chips) is to avoid contamination of one product into the other. Specifically, contaminating chips with premix will probably result in film defects such as craters and extraneous “bits.” This is because the unprocessed raw materials in the premix are incompatible with the extruded material. For example, some of the additives in the premix (e.g., the flow agent) can cause craters in extruded chips or the final powder. In addition, dry pigment and fillers may not be dispersed (deagglomerated) and could cause “bits” in the final film.
Extrusion (aka compounding) melts and mixes the resinous components in a powder formula, thereby making them compatible. The shear caused by the extrusion process also deagglomerates and disperses the pigments. This provides better color development and consistency.
If you intend to use the same mixer/grinder for both premixing and milling, you must thoroughly clean the equipment between processes, especially going from premixing to milling.