Over Curing Vs. Under Curing

Q: We are experiencing some inconsistency with our powder-coated parts after the cure. How can we tell if we are over curing or under curing? 

A. Powder coating manufacturers publish specifications for curing time and temperature (typically as a “curing window” chart in product data sheets). To fully realize all the benefits of powder coating, the powder must be cured at the specified temperature for a specific amount of time. It is important to note the temperature of the part is measured, not the temperature inside the oven.

The most common method of checking the part surface temperature while it is in the oven is with a non-contact infrared thermometer. The advantage of an infrared thermometer is that you don't need to be up close to an object to measure its surface temperature. You can accurately measure items from a safe distance, thereby minimizing the danger of an accidental burn. Once the proper temperature is reached, set a timer as recommended in the powder coating manufacturer data sheet.

Proper curing is critical because it ensures your powder coating finish becomes durable and its color is fully developed. However, if the part is over-cured (too hot or for too long) or under-cured (not hot enough or not long enough), the coating will exhibit defects in color, finish or performance.

Over-curing powder coating may result in cracks and breaks in the finish, forcing you to sandblast the part and start all over. Also the thinner areas of your product will cure faster than thicker areas, so keep an eye out for it to cook just long enough for the thickest part to reach proper temperature and cure.

Under curing powder coating creates issues as well.

If you notice that the powder coating is not smooth (and you are sure you did not apply the powder too thick) then you can add a few more minutes to the curing time. Most powders are durable enough to cure longer but be careful to stay within the curing window and do not over cure.

Under cured powder coatings may be too soft, which will reduce durability. To determine if a coating is under cured, conduct a solvent rub test. The solvent rub test is usually performed using methyl ethyl ketone (MEK) as the solvent. When curing is complete, the coating will be resistant to 50 strokes of cotton soaked in MEK solvent, and will only partially lose its gloss. But if the curing is not complete, the coating will turn soft and will be easily peeled off.