The principle of operation can be described as follows:
The product is fed into the mill pneumatically. Size reduction takes place by the impact of material par¬ticles on the rotating grinding sur¬faces and on the fixed liner.
The cooling, conveying and classifying air drawn through the mill by the downstream fan entrains the metered feed material which then passes the blades of the shroud ring.
The air/product mixture is uni¬formly distributed by the blades to the rotating classifier. Due to two opposing forces and the different masses of the material particles the separation into coarse and fine frac¬tion is achieved in the classifying section. Those of high mass are thrown away from the wheel by centrifugal force. The rejected particles return to the milling zone. The fine material is carried with the air through the classifier to the outlet connection - cyclone.
The fine grinding process follows the hot melt compounding. After leaving the extruder die, cooling and squeezing between the chill rolls, additional cooling on the cooling band to a brittle, easily breakable continuous strip and flaking by a kibble unit, the material is usually collected in movable containers and transported to the grinding system –Air Classification Mill.
Powder coating is formulated in a way that the glass transition temperature of the system is at least 40 °C to ensure storage stability of the coating. If the grinding of the material is performed at room temperature by appropriate removal of the heat developed during the grinding process, the material is subjected to the mechanical forces action below its glass transition temperature. Polyester, epoxy or acrylic resins used for the manufacture of thermosetting powder coatings usually have molecular weights between 3000 and 6000. With such a molecular weight, below, their glass transition temperature, these binders are brittle materials with a low modulus which allows easy non-problematic grinding. High molecular weight thermoplastic binders, however, are extremely tough and resilient. Cryogenic grinding at low temperatures is the only method for reducing the size of the particles of the thermoplastic powder coatings.