As users of Epoxy Coating Powders may understand, there are really two processes involved when applying the Epoxy Coating Powders to passive components such as varistors and capacitors: the application process and the curing process. The goal of this article is to highlight the two distinct processes and to help the user understand how best to optimize the parameters of each during the test phase to get good looking varistors and capacitors after coating and to avoid air bubbles, discoloration or other common defects.
The Epoxy Coating Powder Application Process:
The application process starts from the point the parts are prepared and ends when the parts have been coated, but before post mold curing. It is important to note here that our newest development powders (be sure to contact us to ask about our latest Halogen-Free Epoxy Coating Powders) use our latest technology, and as such are less sensitive than the original DK17-0925 Epoxy Coating Powders to contaminations on the surface of the part. When the parts were originally tested, they were not perfectly clean, and the ones that they were applied directly with the DK17-0925 did not give the nice finish we were used to seeing. This could have been for several reasons, including that the epoxy was prevented from curing properly by the impurities on the part, and so never cured properly from the start. Our new chemistry is much more forgiving and thus gave a much cleaner result without any extra cleaning steps.
The second thing that happens during the application process, is that the material “gels” onto the part. Gelling means that the material goes from a solid to a liquid (when it melts and sticks to the part) and then to a solid again. During the gelling phase, the epoxy is maybe 70 — 80% cured, and the final material properties of the powder are not yet defined. What is important during this phase is that the material goes liquid long and slow enough to allow any trapped air to escape — while it is still liquid. If the material gels too quickly, then the air bubbles may appear and then get “frozen” in the epoxy because it cures too quickly before allowing the air to escape and the air entrapment scars to heal.
What is important during this phase then is that the epoxy coating powder is applied immediately after the preheating. If the part starts to cool then this could attract the air and moisture that then could cause the bubbles during dipping. To answer a common question, “Is it the same if we reduce the temperature and increase time?”, this approach is worth testing. The goal is not to CURE the part during this phase, only to get it properly gelled. The LOOK of the part should not change at all during post curing, this should all be fixed after the gelling phase. During the testing phase you can and should play with the time and temperature parameters to see which combination gives you the best looking part AFTER the application process but BEFORE the curing process. The parameters that we can provide to you are great starting points, but use them only as guidelines if you find something that works better, please use it and let us know.
The Epoxy Coating Powder Curing Process:
The application process gets the epoxy coating powder to look how it should, but the curing process ensures that the epoxy coating powder behaves as it should. It is this post mold curing that brings the curing percentage from 80% up to 99.9% cured and this in turn determines the epoxy coating powder’s final properties such as hardness, CTE, modulus and glass transition temperature (Tg). During this stage, the important parameters are how long and at what temperature the material has been exposed. Smaller parts come to temperature much more quickly than bigger parts, and only with a thermocouple can you see exactly how much time this takes. The general rule of thumb though is that every 10C hotter reduces the cure time by half and every 10C cooler, doubles the required cure time.
This said, once the material has reached 99.9% cured, any more time in the oven is just wasting time. It doesn’t hurt the material, but it doesn’t help it any more either. Some materials also require a MINIMUM temperature to obtain its properties. In the case of our new products, these might be as low as 125C, and if the part is small enough, then one hour is also sufficient. If users want to cure it at 140C, this should also be no problem.
Our new epoxy coating powder is a different chemistry from the original DK17-0925 epoxy coating powder, and so can and does have different curing parameters. It is an improved product which in addition to being halogen-free uses our latest technology for epoxy coating powders. We have a good understanding of how and why it was developed, but in the test phase, users should always experiment to see how to get the best behaviour. So please experiment and play and feedback what you learn so that together we can feel confident that we are offering you the best material for your application.