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topic 42644

Pinhole problems in powder coating


A discussion started in 2006 but continuing through 2018

2006

Q. Hello everyone,

I'm a trainee in a powder coating company and I'm having some problems with pinholes in my paints... I'm already using benzoin, but it seems it isn't working. I'm using 0,7% of benzoin and I'm coating steel plates.

Someone have any idea of other product I could use to reduce the pinholes?

Thanks a lot.

Alexandre Meira
Powder Coating Researcher - Belo Horizonte, Minas Gerais, Brazil


simultaneous 2006

A. Alexandre, pinholes in powder coating are caused by airborne contamination, something landing on the freshly painted part before it goes into the cure oven. The first thing I would do is look for someone using spray paint near the paint line during production.

Sheldon Taylor
Sheldon Taylor
   supply chain electronics
Wake Forest, North Carolina



2006

A. Hi Alexandre,

Benzoin acts like a "solvent" in a powder coating enabling the surface of the coating to remain open long enough to let the volatiles escape. The normal dosage is 0.5% on resin, so you may be putting too much into the powder, which could cause a similar effect. However, there are many causes of pinholes and you have not supplied sufficient information to give a definitive answer. You need to let us know what the full formulation is, i.e., Pigment Volume concentration, resin system, etc.

Hope this helps,

Drew Devlin
- North Lincolnshire


simultaneous

High Performance Powder Coating
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2006

A. Sheldon,

The top layer of the coating curing before any volatiles within the coating or substrate have been expelled causes pinholes. This means that the volatiles have to break the surface thus causing the pinhole effect. This situation can arise in a number of ways, porous material, moisture in your air and poor formulating even coating the object too thickly. What you talk about is airborne contamination, which, dependent on what it is, can give a similar effect to pinholes but it is not. The key is to let the volatiles escape before the top layer cures. Therefore it is a pretreatment, application or a formulation issue, not a contamination issue. Contamination is more likely to result in craters rather than pinholes.

Regards,

Drew Devlin
- North Lincolnshire


2006

Q. Actually, I think the problem is the client wants a high thickness paint... something about 150 microns.

When I paint some plates near 120 microns no pinhole is detected, but when I get 150 microns, lots of pinholes appears.

Someone here knows how can I get this thickness without pinholes?

Thanks a lot.

Alexandre Meira [returning]
- Belo Horizonte, MG, Brazil


simultaneous 2006

A. Something else that can cause pinholes besides airborne contamination is cross contamination. If you use the same hopper for multiple types of paint, the hopper needs to be very, very clean. Same with hoses, pumps, guns and gun tips, paint booth etc...particularly epoxy to polyurethane changes. I've never used(or heard of) benzoin, Drew, I'd have to advise Alexandre to pre-bake his parts and put in a new air dryer if substrate contamination or outgassing were the suspected culprits here :-) It's also a possibility that the paint he is using isn't designed for the type of film build he needs.

Sheldon Taylor
Sheldon Taylor
   supply chain electronics
Wake Forest, North Carolina


2006

A. Alexandre,

Modern powder coatings are not designed to be sprayed at 150 microns. The industry standard is 50 - 70 microns. Spraying at this thickness gives no advantage to the protection of the substrate. In fact it can be detrimental, for the reasons you describe, pinholes. There are other considerations to be taken into account when spraying at this thickness, back ionisation (can give the appearance of pinholes), poor flow and poor mechanical properties. I would recommend you advise your client to coat at no more than 100 microns. If he refuses you need to explore ways of delaying the cure of the top layer of the coating, which will lead to an extended cure schedule for the entire powder, costing the customer more in energy costs.

Hope this helps,

Drew Devlin
- North Lincolnshire


2006

Q. Drew Devlin,
Thats right Drew, but what you could tell me about high thickness pinholes? what I mean is that I need a 150 microns layer, and when I paint my plate in a 120 microns no pinhole is detected, But when I get 150 microns a lot of pinhole rise.

is there anything I could do to get no pinholes and yet reach this high thickness?

thanks,

Alexandre Meira [returning]
- Belo Horizonte, SP, BRAZIL


2006

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.

Terry Hickling
Birmingham, United Kingdom


2006

thumbs up signThanks a lot =)

Alexandre Meira [returning]
- MG, Belo Horizonte


June 4, 2015

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 be do this (it will burn your paint), but if you have temperature controlled heaters you can do it.

jude peiris
Elsteel pvt ltd - colombo, sri lanka


June 23, 2015

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.

Trevor Govender
- Durban South Africa



November 8, 2017

Q. Hi everyone,

I have the similar problem as Alexander but the difference is that powder coating is not only the source of pin holes. We are thinking that our die cast material is also a factor in pinholes. My question is "Which contributes the most number of pinhole defects between die cast materials and powder coating process and how can we reduce this defect by giving action on both factors?"

42644

Bredix Duman
- Rosario, Cavite, Philippines


November 26, 2017

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, always less than 1,000,000 Ohms for safety, <500,000 ohms (or less preferred) for maximum transfer efficiency.

Rudy Gyuka
- Hamilton, Ontario, Canada



September 22, 2018

Q. Hi Everyone.
I am quality manager from an agricultural equipment manufacturer. Last two months we are facing problem of pinholes.
Kindly suggest me the sharp solution.
Thank you.

Bipin kumar kesarwani
- punjab india


September 2018

A. Hi Bipin. If you don'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 it.

Such problems are often complex, and the readers can only help you in one of two ways: either by referring you to all the possible causes as discussed in these many threads, or by you providing sufficient data that the readers can narrow it down and significantly shorten the list for you.

Please start with what is the substrate (hardened mid carbon steel? are any 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? Thanks!

Regards,

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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