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topic 1846

Maximum allowed curing temperature for coated zinc diecastings


(1998)

Dear Sir,

After coating and curing of zinc diecastings at 200 °C we had blistering problems. What is the maximum allowed curing temperature for coated zinc diecastings? Is there an ASTM specification about this subject?

Thanks in advance.

Johan Nelissen
CP - The Netherlands


(1998)

You need to provide more information. What type of coatings are you applying, plating or organic finishes? What type of blisters are you referring to? Are you worried about actual melting of the die casting? Generally, porosity in the die cast is the culprit for most blister problems with die castings, especially if you are breaking through the skin of the casting in a cleaning process, over cleaning. With organic coatings, sometimes the coating will skin over and entrapped air expansion will cause blisters, or solvent cleaning residue may vaporize and cause blisters. Take a razor blade and determine what the cross section of the blister looks like and you will get lots of input from here. Blisters on zinc die castings are well known.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


(1998)

Any porosity in your casting can lead to blisters in a heat cured coating as the material contained within the pores expands or volatilizes during the heat of cure.

Solutions can be one or more of the following:
- Reduce casting porosity
- Pre-heat the casting to cure temperature in order to release the blister forming gases, then let it cool before coating
- Use a coating powder designed to tolerate substrate out-gassing
- Use a coating powder that will cure at a lower temperature

The vast variety of castings, coating materials, and oven conditions makes this an experimental process for you, but once you determine the solution and employ it, your success should continue as long as the mentioned factors remain consistent.

There is no ASTM method that relates directly to your question. Your coating powder supplier should be able to help you find a reasonable solution.

Jeff Hagerlin
paint supplier - Houston, Texas


(1998)

Johan,

The advice provided in the previous responses is good. To answer your original question, zinc diecastings can be heated above 200 °C without fear of melting them.

Die castings can suffer from a number of forms of porosity.

* Blind porosity where a small blind pore penetrates to the surface. "Casting" grade powders together with preheat reduce or eliminate the effect.

* Porosity that goes completely through the casting. "Casting" grade powders together with preheat can reduce or eliminate the effect.

An effective process in these instances is to apply a very thin film of powder and cure that to the gel stage and then recoat to the final thickness and fully cure.

* Hidden porosity that is subsurface and that is not visible on the surface even with a magnifying tool (hence hidden). These are nasty because the end result is a pimple on the surface - no pinholing. The effect of these can be diminished by curing the powder at the bottom end of the temperature range allowed by the powder manufacturer. I have had success curing powder at 165 °C whereas "pimples" stood out at 200 °C.

Mario Pennisi
powder coater painter - Brisbane, Queensland, Australia



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