Minimizing Carryover in a Phosphate System

Q. We need to reduce carryover in a conveyorized spray phosphate system. The parts are approximately 200 mm diameter by 3,500 mm height with a conveyor speed of 0.75 m/min and parts hanging at 400 mm spacing. The process uses an activation stage with a length of 750 mm, followed by a drain of 1,500 mm. After that, spray phosphating for 1,875 mm then drain for 750 mm with a 750 mm rinse. The distance between the part and spray nozzles is 300 mm. The whirl jet type spray phosphate nozzles are set up at 13.2 ipm.

A. Thank you for the thorough background to your question. First, check if your nozzles have been knocked out of alignment, which has little or nothing to do with the design of the system. This is fairly common and something that can be rectified quickly. Given the proximity of the parts to the nozzles, it is not unusual for that to occur. Next, ensure that the intermediate drainboard flows back to the intended tank.

If you verify these mechanical items and are still experiencing the problem, it could be that the parts are carrying too much between stages. It may be possible to minimize this by increasing the part spacing or slowing the conveyor to minimize the carryover. The last and most difficult solution is to increase the distance between stages to minimize the carryover. This may be the most effective, but also the most difficult to modify because it would involve equipment modification.

Originally published in the August 2015 issue.