Varistor and Capacitor Processing Parameters with Epoxy Coating Powder

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:

Poor Results with Epoxy Coating PowdersThe 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.

For more infor­ma­tion about Epoxy Coat­ing Pow­ders or any oth­er of our prod­ucts, vis­it us or con­tact us for more details. Please click here for the list of tech­ni­cal datasheets.

About Chris Perabo

Chris is an energetic and enthusiastic engineer and entrepreneur. He is always interested in taking highly technical subjects and distilling these to their essence so that even the layman can understand. He loves to get into the technical details of an issue and then understand how it can be useful for specific customers and applications. Chris is currently the Director of Business Development at CAPLINQ.

3 thoughts on “Varistor and Capacitor Processing Parameters with Epoxy Coating Powder

  1. One com­ment from the chemist was that for varis­tors or oth­er ther­mal shock sen­si­tive devices it’s best if the part goes direct­ly from dip­ping to post cure with no cool down. If it must cool down then it’s best if it’s not gelled at this point because the coat­ing will crack on cool down. If there is still flow left when it returns to a cure oven it will remelt and form a con­tin­u­ous coat­ing again.

  2. Thanks for shar­ing your thoughts. I real­ly appre­ci­ate your efforts and I am wait­ing for your next write ups thank you
    once again.

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