Why blow holes suck (or why suck holes blow)
And now that I got your attention (and probably attracted a few confused weirdos — sorry guys), let’s talk about Pinholes, a common issue with epoxy powder coatings, what we know about it and how we can eliminate it.
Pinhole definition and types
Pinholes are those little annoying pop-corny marks that you might see on varistors, capacitors or any type of epoxy coating powders and resins. Porous materials (particularly very porous tantalum) are most susceptible to pinholes, but even less porous surfaces can also be affected.
There are two types of pinholes that can be observed when using epoxy coating powders:
- “Blow” holes
- “Suck” holes
Those aptly named defects are caused by air bubbles that are trapped under the surface of the epoxy.
Blow holes are caused by air bubbles that expand when heated but get trapped as they try to escape through the coating powder.
Suck holes are caused by air bubbles that contract when cooler than the surrounding area, drawing with them the epoxy inward.
Please note that in our case there is no discussion on pinholes caused by solvents since all our products are 100% solids and therefore have no solvent related pinhole defects.
Troubleshooting Pinhole Causes
To resolve defects caused by pinholes, we address the ways in which the bubbles are caused and can be avoided. There are three major probable causes and those are either the Equipment, the Process or the Materials.
It can happen that the heating element is not hot enough or the fluidized bed is not uniform and causes excessive bubbles.
Solution: The two obvious steps are to turn the temperature up on the heating element and also adjust the fluidized bed to be more uniform. Still not enough? Maybe you need to add more powder (with finer particles) to even out the flow.
There is no “catch all” recommendation for processes. All of them differ and there are just too many things that can go wrong (or right if you are one of those pesky, glass full, people). But we can still point out the most common risk factors.
Speed. Are you too fast or even too slow on the trigger between heating and dipping? That can be cause for concern. Try adjusting the time window between heating and dipping to achieve the best results.
Heat. This is not lazy c/p advice. Heat is incredibly important so just like with speed, the heating element can be inadequate and might be too hot or even too cold for its own good. Try adjusting the temperature up/down until you observe the ideal results.
Contamination. Substrates can potentially be contaminated by previous process steps or even environmental causes. Make sure that your substrate is contamination free before coating.
And that’s where CAPLINQ comes in. Some solid advice when picking the right materials for your application is to look into the following:
- Coating powders should have an “optimum” stick point temperature to ensure air escapes prior to gel
- Coating powders should have a low melting temperature of solids to be sure powder does not block air passage
- Melt viscosity should be low to allow air to pass through before the epoxy is cured
- Epoxy should have “self-healing” flow properties that allow gaps to flow and be closed once pinholes appear
Having issues identifying a product that works well for you? Then discuss with your CAPLINQ representative to make sure you have selected the correct product for your application. We have a large variety of epoxy coating powders to choose from and we are collaborating with world leading epoxy coating powder manufacturers.
Can’t find a suitable product? Our Research and Development team is open to create a product adjusted to your needs. Contact us for more information.