finishing.com -- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing

HomeFAQsBooksHelpWantedAdvertiseForum
topic 42644

Pinhole problems in powder coating



A discussion started in 2006 & continuing through 2017

(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 (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



This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices


©1995-2017 finishing.com     -    Privacy Policy
How Google uses data when you visit this site.