I have a customer that is trying to process a wrinkle powder in a catalytic IR oven with poor results. Our difficulties include a blotchy finish and inconsistent gloss across the face of the coated part. I would like to know of any general parameters that you may suggest as well as any other pointers on exactly what special conditions wrinkle powders require to flow and cure properly. C.H.
Wrinkle powders are more difficult to apply than smooth powders and require closer process control. For instance, some process parameters that should be controlled closely are film thickness, part cure temperature, bring-up time and batch-to-batch consistency of the powder coating. We will discuss these process control parameters one at a time.
Film Thickness. Wrinkle powders will develop their surface texture at a given film thickness range. If the coating is too thick or too thin, then the wrinkle texture will be different or nonexistent. Therefore, you must control the film thickness during application of the powder within the tolerance specified by the coating manufacturer to ensure acceptable results. Maintaining film thickness control entails proper gun settings, environmental conditions of the spray area, part ground and part temperature at time of coating. To a lesser extent booth airflow can also affect film thickness control.
Part Cure Temperature. Wrinkle powders need to be cured at the part metal temperatures and dwell times specified by the powder coating supplier. Temperatures or dwell times outside these parameters will give poor results. Using a catalytic IR system may exacerbate this problem by heating different surfaces of complex part shapes at different temperatures and different dwell times. Therefore, the oven design can be contributing to your consistency problems. The powder supplier and the oven manufacturer should review this situation.
Bring-Up Time. This process variable describes the time it takes to “bring-up” a part to the given cure temperature. Wrinkle powders sometimes require that a part be brought up to cure temperature rapidly to ensure that the flow characteristics of the formulation be allowed to form the desired surface texture. The powder supplier can check this by running an oven profile to determine the bring-up time of a coated part. A coated part is especially important when checking an IR oven profile.
Batch-to-Batch Consistency. Wrinkle powder coating powders are very sophisticated formulations. Sometimes the powder formulator changes the formulation to improve coating characteristics or overcome raw material supply problems. These changes, no matter how slight, can have a dramatic effect on the batch-to-batch consistency of the powder coating. Checking incoming powder coating materials under ideal and consistent process parameters will determine if the powder coating is good for use in your system.