Pinhole problems in powder coating

A. Hi,
You do not state the type of resin system you are having a problem with -- I presume it is polyester. The new environmentally and operator friendly polyesters are based on a cross linker called "Primid" that produces water vapour as a by-product during the baking stage - the old TGIC did not. Therefore it may not be due to any other forces except this if it is polyester. So try shooting hot or even two coats - the first coat only being partially cured.

A. The pin holes come from several reasons --
1. water in your air line
2. oil in your air line
3. oil on your work piece surface
4. powder quality of your suppler (about 30% of problem come with this cause; they do not agree with us but I know from my 6 years experience.

If you want to increase to about 150 micron coating thickness, repaint again from about 75 micron layers.

But I think you can get about 150 micron thickness in one coat: try to increase the cure time about 5-9 minutes.
Please note-- if you have IR heaters you cannot do this (it will burn your paint), but if you have temperature controlled heaters you can do it.

A. Hi
I've been in the industry for about 10 years now and from what you say about film thickness does not make sense at all. If you have pin holes, it should be present irrespective of film thickness.

If you are correct on what you are saying and it only happens when you are trying to achieve 150 microns, or above 120 microns, then you are trying to apply the single coat of 150 microns in one shot, i.e., you are forcing the powder onto the product. This change causes a defect called BACK IONIZATION.
If you want to coat at around 150 microns, try reducing your Kv and microamps. You will be surprised at the quality finish of the product by just playing around with electrostatics.

A. Hi Bredix,

To isolate if it is your Zinc Casting 'outgassing' which is causing the problem; after your usual pre-treatment heat the part to a higher temperature than the temperature used to cure the powder coating.

If the problem is still there, you may need to further investigate contaminants not being cleaned from the substrate, or Air-borne contaminants, residual contaminants in the application equipment from Non-compatible chemistries of Powder and/or possibly the powder itself.

It does not appear to be back-ionization in your picture, but rather a seed of 'dirt'. You can always reduce the KV & uA on your spray equipment to verify.
Check your part grounding with a Megger [on eBay or Amazon] , always less than 1,000,000 Ohms for safety, <500,000 ohms (or less preferred) for maximum transfer efficiency.

A. Hi Bipin. If you didn't find good clues in this thread, please search the site for "pinholes powder coating" and you can read at least 7 more threads on the subject and start getting a good feel for the possible causes.

But such problems are complex, and aside from listing the almost countless possible general causes, the readers can help you if you provide sufficient data that they can narrow it down to shorten the list for you.

Please start with what is the substrate (hardened mid carbon steel? are some of the parts castings?). Does the steel have galvanizing or other coating before the powder coating? What is the general pretreatment method? Is there any pattern to the pinholes? Is the powder a polyester or what ... and how thick? If things were fine until 2 months ago, something obviously changed, and maybe the readers can help you figure out what changed if you give them some hints. Thanks!

Regards,

A. Hi cousin Nirmal. Realistically I think you should retain a powder coating consultant to spend some time on-site in your plant and solve this problem for you -- it would surely be much cheaper than changing everything which rarely anything because the best it can do is reset every parameter to a coarse adjustment which is usually unsatisfactory.

But taking it from the beginning, what pretreatment are you doing or trying to do to the galvanized surface? Zinc phosphate? Thanks.

Luck & Regards,

A. Hi Vaseem. You posted this question on thread 53135, which was a thread about how to do zinc phosphating with insufficient room & tankage, but we didn't want that thread to wander into troubleshooting of powder coating. So I'm not sure if you suspect that you didn't get the metal clean & pretreated well enough ... but simply observing for waterbreak should give a pretty good idea whether the parts are clean before powder coating.

Does your shop have a policy forbidding the use of silicone? It is impossible to remove, and an oiliness problem that suddenly developed on a previously well-controlled process might well be an indication of the onset of their use by the maintenance department.

Regards,

? Dear Mr. Thiago Oliveira,

What is the thickness spec recommended by the powder manufacturer?

A. Hi Anka. Anodizing is not generally the right pretreatment for powder coating. While it is possible to anodize and then powder coat, adjustments to the anodizing process are probably necessary, and there can be problems with it regardless.

In short, it's not generally suggested practice, so it would be very helpful for you to tell us what the product is, how it is used, why you are anodizing it instead of chromate conversion coating it per MIL-DTL-5541 [on DLA] for example, and whether it is the anodizing or powder coating that is "above 10 microns" thick. Thanks!

Regards,

A. Hi again, Anka. Sorry, I'm unfamiliar with the process of anodizing and then powder coating cast aluminum brake calipers; searching the site for "powder coating on anodized" will lead you to several discussions on the topic though. And hopefully another reader will be familiar with this technology and be able to advise you what special steps, if any, in the anodizing or powder coating are advisable to achieve your needs.

Sorry, we had to delete the brand of automobile from your posting because we don't know if they publicly share their process steps :-)

Regards,

A. We did a three year research project into the pinhole in powdercoating issue. We uncovered at least 13 major and many more minor contributing factors.
The most commonly occurring one was actually water. Unbelievably, water can be present in substrates. We mostly coat galvanized steel, and tiny amounts of water manage to be present inside the galvanized coating which has microscopic voids. We'd thought of that steel being at 450 °C during the galv process and thought it could never have water in it, but it gets introduced later, either from rain, from pretreatment immersions, or from the atmosphere. We found the same effect, but less of it, in aluminium, and even bare steel. Drying prior to spraying helps a lot. Previously this has been thought of as "degassing", and it sort of is, but I think it more useful to think of it as "drying", because water is the most common culprit. "Degassing" sounds like we're removing some gas (and I suppose water at high temp is a gas), but this distracts from the means of prevention rather than cure. Galvanized steel stored outside that gets wet with rain will surely "gas" and pinhole. It's water.

On ungalvanized steel, various markers that contain volatile solvents were identified. And even removal of the marking didn't totally cure this issue. The standard (ubiquitous) "Sharpie" pen can cause this pinholing.

Driving out the gasses before spraying (and subsequent cooling before spraying) almost always solved this issue, but it requires about 200 °C for some time dependent on material thickness. 15 mins for 1-3 mm thick, longer for thicker (we coat to 50 mm thick, sometimes more).
Hope that might help you find your cause.

A. Re Humidity
We didn't find this to be much of an issue. Typically humidity can cause water to be present (in some form) in the uncured powder layer. This seemed to be rarely a problem, as the moisture was driven out during curing. The problem was moisture in the substrate itself. This only starts to evolve when the substrate gets to a high enough temp, and that only happens in the latter part of the curing, as the substrate temp increases.

A. Pin holes are the sign of contamination--
1 The parts may be contaminated with a grease or any chemical.
2. Make sure your powder is not contaminated.
3. Check if your oven has dirt.
Check your part with magnifying glass so you can see the problem.
Don't be afraid to clean it with lacquer because mostly for contamination of oil or anything your lacquer will br excellent.