Reduction of Defects

Q: We have been running our powder coating line for a little over a year now. Results are mixed as some days things run well and other days we struggle with excess rejects and rework. Light coating and heavy coating are the most common cause, but we also get a lot of orange peel and small areas with a lot of texture. Our application process is all manual with two operators. We have three standard colors but will do a number of smaller runs and specials. Do you have any general advice for things we should look at for improvement in our yields? 

A: Since I have never seen your operation, I cannot comment on specific issues that are causing the inconsistent film build and appearance issues you describe. A review of basic operating control should help you find some issues and make gradual improvements. Typically, no one thing is the root cause of the kind of defects you describe. Establish and maintain a consistent Standard Operating Procedure for the best results.

It starts with material control. Your supplier(s) work hard to give you uniform and reliable product but occasionally, the grind size or some other factor causes inconsistency that can create application issues. Also, when powder is shipped it is often on a truck with no atmospheric control so exposure to heat and humidity may cause agglomerations inside the container. If you do not already, start storing your powder in an air-conditioned environment to keep it dry. When you put a container into production you should sift it or screen it to remove or break apart any clumps of powder. Allow the material to fluidize for 10 minutes or so before spraying so that you know it is in good condition for fluidization and flow.

Ensure you are using clean contacts on your racks and hooks to maintain earth ground at all times. A lack of ground will make it very hard to control the film thickness, leading to light and heavy coating. Keep the racking in good working order by removing powder build-up, keeping hooks and clips in good repair and hanging everything consistently.

Next, look at your gun set-up. Ensurethe wear parts in the delivery path are clean and not too worn. Blockage or wear in the pick-up tube, hose, air cap or any connector can cause gun surging and spits and also may cause the operator to use too much pressure to get the powder to flow at a sufficient rate for coverage. Do a pre-check of the system before a shift.

Voltage can run near the top of the range as long as the micro-amps are controlled. If the micro-amps run too high, it can cause back-ionization, pin-holes or other appearance issues and make it hard to control film thickness. Most guns have controls that limit amperage and factory presets for recoats and Faraday areas. Use them to keep the amperage controlled.

Operator training and technique are also critical. Ensure the operators have enough time and training to practice good technique and consistent patterns. The manual guns should typically be held around 6" from the surface. If the part has inside recesses, they should be covered first. If the easy-to-coat surfaces are covered first, it can increase the resistance in the inside corners (Faraday areas) and make them hard to cover.

Some practice or trials on parts that give you the most trouble could be very valuable. Run a few practice parts and analyze the outcome to look for problem areas.

I would also recommend that you enclose your application booth and surrounding area with an insulated room and add atmospheric control for temperature and humidity. That level of process control can be a big help with the kinds of issues you describe. If the temperature is kept between 60^ºF and 80^ºF and the humidity is kept between 40% and 60%, it is much easier to establish standard settings for the application equipment.