Resin size distribution
The resin is first reduced in size by passing through a hammer mill or similar piece of equipment to give the required particle size.
Raw material blend
The crushed resin is then blended with the other ingredients to produce a homogeneous mixture.
The mix is then fed into the extruder, the barrel of which is maintained at an exact predetermined temperature. The barrel temperature is set so that the resin is only just liquefied. Too high a temperature will give a low melt viscosity, low shear and poor pigment dispersion which in turn produce coatings of low gloss. The feed rate on the hopper and the speed of the screw are then balanced so that the screw is kept fully loaded. By careful adjustment of these three parameters, conditions of high shear and therefore intimate mixing are maintained within the extruder. The molten extrudate passes between cooling rollers and emerges as a thin sheet which is then broken up into chips of about 10-20mm.
The chips must be subjected to a further grind to reduce them to the right size, depending on how the particle will be applied. For this a pin-disc or hammer mill is employed. In modern mills an internal classifier is used to limit the maximum particle size, oversized particles being continuously fed back to the mill.
This is the final operation in the process, irrespective of the method used to produce the powder.
After final sieving, the powder should be stored in tightly closed containers in a dry place so as to prevent possible contamination by environmental fall-out or possible moisture pick-up.
When transporting powder, the powder is contained within a polythene bag, suitably sealed, which in turn is contained within a cardboard box or tinplate drum. The polythene bag should therefore be resealed after withdrawing the required amount of powder. For long term storage powder should be maintained at as low a temperature as consistent within the factory area, preferably below 25oC. Powders should be kept away from moving parts as frictional heat will cause the powder to fuse and possibly cure.