Impact fusion is defined as "the tendency of finely divided powders to combine with other particles in the application equipment during the application process."
A number of factors can relate to impact fusion problems. Impact fusion can be the net result of a combination of these factors within the finishing system.
High delivery pressures can contribute to impact fusion. It may be beneficial to first examine your system's transfer efficiency. This may help explain why the powder pressures are set high. The fact that your system has 12 guns and that the line speed is 12 fpm does not tell the whole story. Line density, gun-to-part distances, air velocity (at the part) and gun spacing are some of the other items that affect delivery pressures.
When designing a powder line, powder equipment manufacturers try to keep powder deliveries in the range of 15 to 25 pounds per hour. If you measure gun output closer to 40 pounds, this could explain some of the problem.
Poor-quality compressed air can also force abnormal air pressures to be used to fluidize, transfer and deliver powder to the target. High moisture may cause the powder to cling to itself and equipment contact surfaces (i.e., hoses, guns and booth cartridges). The compressed-air system needs to be capable of providing air at a dew point not higher than 38°F. It should not contain more than 0.1ppm of oil and should have no particulates greater than 0.3 microns.
A well-designed compressed-air system includes components that will ensure the air meets the above requirements. In addition to the air compressor, a quality air system will include an after cooler, separator, receiver, prefilter, dryer and after filter.
The powder formulation can have a bearing on the impact fusion characteristics. If a coating has been formulated to meet certain appearance and performance standards, there may be a limited number of things a chemist can do to change the impact fusion tendencies of a specific formula.
Finer powder particles may have the tendency to fuse faster because it takes less energy to move it to a fused state. So, virgin powder particle sizes can influence how many powder fines build up in a recovery system. The incoming (virgin) particle size range should be adjusted as high as possible, as long as there is no sacrifice to film smoothness or film thickness limits.
Finally, cleaning the hoses and fittings on a regular basis may help to reduce the problem. If the hoses are very old and there is already a buildup, you may find that the easiest solution is to start with new hoses. Be sure to try and use the type of hose the equipment manufacturer recommends.