Quality Control of Powder Coating
Quality Control Of Powder Coating
Quality control in the finishing industry requires attention to more than just coating. In fact, the majority of problems occur for reasons other than coating faults. To assure quality where coating may be a factor, statistical process control (SPC) can be a useful tool.
SPC involves measuring the powder coating process using statistical methods and improving it to reduce variation at desired process levels. SPC can also help determine the difference between typical variation inherent in the process and special causes of variation that can be detected and eliminated.
A good initial step is to create a process flow diagram of the system. Be sure to go out on the shop floor and observe how the process is actually performed instead of relying totally on how supervisors and process engineers think it performs.
Reading the key control characteristics (KCCs) at each step of the process can then be derived from the flow chart. These key control characteristics are variables that are most important and can be monitored using SPC charts.
A typical list of key variables to monitor may include:
- Dry film;
- Oven cure;
- Powder flow rate of virgin and reclaim;
- Particle size;
- Atomizing air;
- Transfer efficiency.
Since SPC is a data-driven, analytical process, the numbers themselves must be reliable, with as little variation as possible. The more variance in a reading, the wider the SPC control chart limits are for that variable and the less sensitive it becomes to changes in the process.
Formal experiments reveal the capability of your measurement system for the parameter of interest. These include tests such as gage R&R studies and short term machine capability studies. Readily available is literature on how these studies are performed.
A quality assurance/ quality control of powder coating system using SPC enables the powder coating user to be proactive in preventing defects. It allows decisions to be based upon data rather than on subjective opinions. By using SPC to monitor and improve critical components in the coating process, the quality of the final product will consistently improve, lowering total cost.
AVOIDING AND CORRECTING QUALITY VARIATIONS
Close attention to a few critical areas will avoid, or at least minimize, a multitude of quality variations with a powder finishing system. Careful attention should be given to having a clean, dry, compressed air supply, clean-sieved reclaim powder, good ground to parts and equipment, humidity-controlled spray booth air, and regular inspection and replacement of wear parts. The powder coating equipment should be installed and operated as recommended by the equipment supplier’s manual. Follow the recommendations on your powder coating material data sheets. Have a good preventive maintenance program and stringent housekeeping practices.