Q: I am a production engineer, and I have a few questions regarding the curing process with powder coat. We are currently having issues with leaving touch marks on the parts from where they contact our cart. It leaves a black line on the part, but we have been able to remove it with rubbing alcohol.
Our current oven setup is turning parts out of the oven at roughly 300-350°F, with only 3-4 minutes of cooling before they are placed onto our carts. I believe that our parts have not had the proper amount of time to cool down and harden before they are placed on the carts. Most of our carts are lined with rubber edging, nylon or plastic tubing to keep from scratching the metal cart structure.
My first question is, what is the general amount of time a part should rest before being handled or worked with? Additionally, is there a material or handling method that you might recommend for handling them with our current setup?
A: Your observations are good. The powder coating surface may be a little too soft to be contacted with the cart liners. Powder coatings, being thermosetting materials, harden or crosslink when exposed to the proper curing conditions (heat and time). This thermosetting phenomenon causes the polymer and curing agent to chemically react, thereby creating a harder, more durable film. The hardness of this film will vary depending on its temperature. Most thermosetting powder coatings reach a glass transition temperature (aka Tg) of somewhere between 185-250°F. Tg is the temperature at which the coating goes from a hard, glassy state to a softer rubbery condition.
Therefore, it is important to avoid handling a powder-coated part if it is above the coating’s Tg. Most polyester and hybrid-type powders have post-cure Tgs in the 185-200°F range. I suggest that you allow a part to cool to room temperature and see if you still observe marking. If that is the case, then you may need to change the composition of your cart liners. Alternately, you can switch to a powder coating that has a higher Tg. One other thing to check is whether or not the powder is completely cured. Solvent rub testing is a simple means to ascertain degree of cure.
Another option is to request a lower curing powder coating from your supplier. With a lower curing product, you can lower the temperature of your oven so the parts will exit at a lower temperature and take less time to cool.
It is critical to accurately measure the temperature of the powder coating surface (you probably have already thought of this). A non-contact IR thermometer is a good way to do this.