Consistency in Curing

Q: I run a small powder coating operation and we are having trouble with getting consistent results from our curing cycle. What are some of the things that might contribute to inconsistent quality?

There are a wide range of variables that might contribute to quality issues post-cure. Oven dwell time and air temperature are important factors in getting your parts up to cure temperature. Substrate type, thickness and configuration also play an important role because ultimately it is the temperature of the part itself that is most critical to achieving a proper cure for the powder coating. Keep in mind that in order to achieve the maximum physical properties and performance of the coating, it must be fully cured.

In simple terms, the equation for cure is as follows:

Temperature + Time = Cure.

In more technical terms, cure is the measure of the crosslinked oligomer chains to fully reach double bonds residing in the powder coating matrix following exposure to the curing process.

Paying attention to the cure window is crucial. If parts are not reaching the recommended cure window for the product it can lead to under-cure or over-cure of the powder coating.

Issues associated with under-cure are as follows:

• Poor mechanical property
• Poor chemical property
• Color not fully developed
• Gloss at lower level
• Poor adhesion
• Poor MEK results (Reagent A or B)
• Incorrect appearance (grainy)
• Cracking or grazing especially on clear coat powder

Issues associated with over-cure:

• Color out of specification (for example, a white coating turning yellow)
• Very brittle (poor adhesion)
• Poor mechanical property
• Gloss fluctuation
• Burning or deterioration of the powder coating
• Poor adhesion
• Delamination of the powder coating

When it comes to primer and topcoat, it is always necessary to make sure that the primer is gelled rather than cured. If primer is allowed to fully cure, the following problems may occur:

• Delamination issue
• Inter-coat adhesion issue primer and top-coat

Clear topcoats should be fully cured (not under-cured or over-cured). Always refer to the powder coating manufacturer's product data sheet for the correct curing window, as it may vary from product to product.