Powder coating on zinc die-castings

A. Hi Mathew. Yes, zinc phosphatizing should reduce blistering, but by how much depends on what the current/alternative pretreatment is. Zinc diecastings, like everything else, require proper pretreatment before powder coating.

Zinc doesn't melt for another couple hundred of degrees C, so melting sounds unlikely, but it is possible that outgassing rather than inadequate pretreatment is the main cause of the difficulties. Based on the blisters all looking to be small and of pretty much the same size, my guess is it's an outgassing problem. There is a good, quick, read at
pfonline.com/articles/outgassing-in-die-cast-parts

Luck & Regards,

A. A few recommendations. Bubbles in powder paint many times is due to outgassing. There are a few ways to mitigate this: prebaking at a higher temperature than the powder cure temp. This can release trapped gasses in the casting.
Use a powder paint with a casting additive that helps keep the powder liquid for longer allowing gas to escape.
In terms of pretreatment in my experience Chromate or zirconium work best. I would not advise phosphate, specifically iron phosphate. Iron Phosphates works best with steel. Some chemical companies claim their iron Phos pretreatments can also treat other metals but in my experience they do not work so well for castings.
A final alternative may be to use a wet paint that does not require baking.

A. My company has plenty of experience in pretreating zinc die castings, and we have found many approaches that work. I do know a number of direct-to-metal painters that use a simple trick to get the powder to pre-cure. They pre-heat the castings in the oven and spray on the powder when the parts are hot.

This doesn't work for everyone, and it sounds as if your customer is doing some serious work on the part after it has been painted. I agree that the epoxy certainly should stand up to the hard knocks. But without proper cleaning and pretreatment there is the chance that the loss of adhesion could be due to surface contamination by handling, mold releases or environmental dirt.

I agree esentially with your assessment that a phosphate pretreatment, particularly an iron phosphate product that has been specifically designed to attack the zinc and re-deposit zinc phosphate on the castings would be the correct treatment. Of course, cleaning with a multi-metal safe cleaner would be the first step.

Sealing with chrome may or may not be necessary. I suggest evaluating parts without a seal first to see if that is enough to eliminate the adhesion failures.

Good Luck.

A. You may be experiencing a sporadic undercure situation with your powder coating. An undercured epoxy powder would have a tendency to chip or flake under the stress of machining. Also excessive film thickness could contribute to this problem.

A. You did not mention the smoothness of the substrate surface.

A small amount of blasting(i.e. shot grit)could possibly create enough surface profile to solve your adhesion problems.

Additionally, I would agree with Mr. Biller concerning some degree of undercure. A faster curing system may be more forgiving especially if the parts have a high mass density.

I wish you success in finding a solution.

A. They should try drilling before they powder coat. This way the powder can fill the drill holes over slightly then they can be re-drilled after powder. Worth a try on a small batch?

A. Dear sir,
Greetings !!!

I would like suggests some solution.the following are probable root causes..
1.Lead / Tin contamination in Zinc casting itself -u ned to confirm at casting or melting

2.Your pretreatment may not be insufficient -improve cleaning at all water rinses.need to conduct trials by providing extra rinses manually.

3.Some De oxidation chemical excessively attack metals resulting porosity hole severely can cause blisters

4.By improving power Flow in oven can reduce blisters.For which you need to ask your powder supplier

let me know details of process it may help you.

A. Do some trials if possible on die-casted parts which have not gone through extensive second operations other than being clipped / broken off.

If you do not see any blistering on these parts, it's worth continuing your troubleshooting (such as pre-cleaning, powder quality, etc.)

If however you experience issues with raw parts then you should work with the supplier of casted parts on the quality of their casts.

Good luck!

A. If the pretreatment is too acid or too alkaline it will dissolve the less porous "skin" from the die castings and expose the unlaying more porous part of the casting. Entrapments in these pores will explode right through the coating upon the high temperature of curing.

If the final two rinses are less than deionized, then the residue (calcium, Magnesium, chlorides, etc) left on the surface will also explode right through the coating upon high curing temperature.

Iron phosphate is not very effective on zinc die cast anyway, a light thin chromate conversion coat would be better.

An extreme, but highly reliable fix, would be to electroplate pure zinc on top of the 12% non-zinc, then apply a thin chromate, then powder coat.

A. Hi, check the cure of the paint first, it the parts pass that test(MEK rub)my suggestions would be to lower the temp on the cleaner(110* max), take out the Luprep and make it a fresh water/overflow tank, lower the concentration in the phosphate (2-2.5%), and make the final rinse a DI.

A. Hi Frank. In the USA it's estimated that 60% of all diecast parts are Zamac 3, which is the most plateable and probably the most powder coatable of the zinc diecasting formulations. So wherever you read about pretreatments for zinc diecastings, they apply to Zamac 3 unless otherwise noted.

So three potential pretreatments are chromating (as mentioned by Bob Probert), which is fine but involves hexavalent chrome, which becomes more problematical each month due to toxicity; or modified iron phosphate as introduced by Craig Burkart; or perhaps best of all zinc phosphate. There are also phosphate-free proprietary conversion coatings.

Chromate tanks operate at room temperature to slightly warm; phosphating is usually proprietary and operates at maybe 120 -140 °F. Good luck.

Regards,