生产过程管理20830瑞特.ppt

DuPont Performance Coatings Page * EXTRUSION PROCESSING if significantly higher temperatures are reached during extrusion, the premix will begin to react and precure will begin precure may lead to bit formation, reduced flow of the final finish and reduced mechanical properties if we cannot achieve a low temperature at the feed zone we should consider investing in a further supply of chilled water melted materials in the feed section of the screws will slowly break away, leading to more bits being formed adding coarse powder reduces the extrusion rate and leads to the increased possibility of bit formation DuPont Performance Coatings Page * EXTRUSION PROCESSING generally, any powder coating should only be extruded a maximum of two times, by the 3rd extrusion bits are being formed. we are generating an ongoing problem by adding coarse powder to our premix coarse powder additions should be ceased wherever possible DuPont Performance Coatings Page * COOLING rapid cooling of the extrudate is required to avoid the possibility of precure the cooling rolls should be supplied with chilled water at all times as a guide, the chip thickness should be 1 to 1.5mm thicker chip will be slower to cool and may begin to precure, it will also be harder to mill the extrudate should be solid and brittle about 1.5 metres from the cooling rolls DuPont Performance Coatings Page * MILLING the milling process generates heat, which is the prime cause of bits cooled air should be supplied to the mills, ideally the air temperature at input should be 10oC maximum even with cooled air, the powder temperature will rise on milling - preferable maximum exit temperature of the powder from the mill is 30oC with 25oC being preferable during the milling process, there is some melting of the chip onto the grinding rotor and classifier experience has shown that the mill should be opened at least every 10 hours of operation and the milling chamber cleaned (rotor and classifier) to prevent the formation o