Powder Coating Problem: Structure Formation Sometimes Fails

? Dear Ambitabh,

You might need to explain this question a little further. What do you mean by "structure formation"?

You mentioned the colour of your powder, but not the type (Polyester, Epoxy, plastic, nylon, other).

Can you describe the symptoms you don't like to see, and then the complete process route you use?

A. Try heating the part before you paint it, the paint will melt to the part and solve some of you coverage issues. This method takes practice because you can overpaint very easily and get runs and drips. It sounds like you might be using dirty hooks resulting in a bad grounding situation, the hooks can't be used more than twice(once is even better) before cleaning.

A. Hi Mr. Amitabh,
It's hard to tell who you are addressing. If you have a bad ground the powder will not stick to large areas of this part, you will get moon shaped areas that have very little powder on them. I think what Mr. Crowley is saying is that "structure formation" is not a term typically used in describing a powder coat condition, so we are basically all just guessing on how to answer your question.

A. Dear Amitabh,

I wonder if by "structure formation" you mean the crosslinking of short chain polymers to long chain that happens in curing of powders like polyesters.
It might be helpful if you can describe the symptoms that you see? And what is it that you hope to see that you don't?

A. Amitabh, Are you using an oven designed for powder coating? Does it have a lot of air flow? If you have an oven that was built for wet paint, it is essentially a hot box, it will not work very well with the curing of the powder and you could get patches on a large flat surface that will not cross-link and cure. The uncured patches of powder will be shiny.

A. It sounds like you are using a textured finish powder.

If that's right, then you should not get this problem you describe. It sounds to me like a powder supply fault - mixed textured and untextured.

A. Dear sir,
Have you tried powder coat on panel without phosphating and just cleaning with nc thinner by cloth.
After doing this trial tell us about the results.

Tanuj Tyagi

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Ed. note: A picture is worth a thousand words! We already stand at 14 postings and over a thousand words, and we're still struggling because people aren't understanding or agreeing what "structure" means :-)
Please send a photo of the electrical panel with the "structure" and an arrow where "structure has failed to form" to mooney@finishing.com for posting here.

A. Dear Mr. Amitabh,

I under stood your problem that sometimes you are not getting the proper structure pattern which you want. Kindly mention if when you are getting this defect have you checked the DFT over that defective patch and unaffected patch. please inform us.

A. I think the problem of patches found in the coating is not due to thickness problems but it could be due to the improper mixing of the structuring additive. This seems to be more of a manufacturing problem and not related to the coating application process.

I would suggest that you try using powder of a different manufacturer and see if you can solve the issue.

Please let me know if the issue still persists.

Thanks and regards,

A. Hi,
My response is that, having already tried a different powder supplier, checked earthing and for oil in the airline you found the problem was still present. We presume that you made sure that an even film weight of powder is being applied 90-100 microns (texture powder requires greater film weight than a standard powder). Therefore, the problem must be something quite simple. Supply your operatives with cotton gloves - your explanation of the size of the defect does point to a handling problem. Let me know if this is possible or not.

Terry

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Ed. note: I'm just the curator/moderator but Terry's posting seems right on the mark -- possible fingerprint oils manifesting themselves worst when there's a good phosphatization layer to absorb them :-)

A. You have a SURFACE PROBLEM

PREP
Clean parts, strongly, with acetone [on eBay or Amazon] or alcohol and don't touch them anymore. Then coat.

IMPURITIES FREE
Check for weld, braze, masking removals.

That's it!

A. Try the following steps to get better structure results:

a] Maintain a good coating thickness i.e., 80 to 100 microns

b] The recovered powder should be mixed with at least 70% fresh powder. This is also important to maintain shade stability. [Since RAL 7032 is a comparatively lighter shade]

c] After taking these two precautions, you may go ahead and find different suppliers for your powders until you get the desired result.

A. Hello Mr. Amitabhkumar,

Is your problem solved? If not, please try a simple procedure:

1. Take two ms panels and one Tin panel

2. Clean one of ms and one tin panel with kerosene or N.C.thinner; see that water film does not break to ensure 100% degreasing; dry.

3. Take second MS panel -- Clean and degrease, Rinse, De-rust, rinse, Activate, Zinc phosphate (Zinc phosphate - low coating weight compact silky grain pattern), rinse, passivate and dry thoroughly. Check degreasing is 100% and the phosphate coating is not powdery or coarse. If you do not have Zinc phosphate, wipe the ms panels with three-in-one solution and wipe dry.

4. Now powder coat evenly above 80 microns with three to four cross coats only degreased MS and tin panels and phosphated ms panel with the problematic structure (texture) powder, and bake it with already heated oven to complete cure at 200 °C 10 -15 minutes.

5. Repeat the process similarly with a standard suppliers powder, and compare the results.

Please inform me your results, if there is problem with the powder it will be shown on the only degreased ms and tin panels; if ok, then powder is fine; if problem seen on degreased phosphated ms panels, conclusion is that something is wrong with phosphating parameters, chemical contamination.

Q. Dear Amitabh,

We have exactly the same issue coming in our shop.
We have patches appearing on structure(texture) finish panels after curing. we are using structure powder (Siemens Grey, RAL IS 631/7032 powder). Patch appears like oil mark where structure has not opened and it is smooth on patch. Powder is coated with proper thickness (70 to 100 microns).
Coating Guns are Nordson make Vantage manual Guns.
Re-coating and curing would make patch disappear from the panel.

We tried following
- We have re-made the degreasing bath (we use hot degreasing) thinking patches are because of oil residues but it is not helping
- zinc phosphating is happening properly and nail test is showing good phosphate coating
- chemical concentrations in Degreasing and phosphating are as recommended by manufacturer
- pH values of water is also in range
- If we clean the panel with 3-in-1 solution then this is not happening

Not having any clue as to what is causing the problem. Any pointers towards solution would be very helpful.

Kindly share if you had already overcome this issue

Regards
c.vishwa

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Ed. note: We have determined from previous entires on this thread, plus further reading, that what the Indian writers are describing as "structured", is faitly close to what British & American readers would describe as "textured" or "wrinkled".

A. The question is a little vague.
I assume there is a significant difference in appearance of various areas of textured or wrinkle finish (structured) powder coating?
The most obvious cause would be that a vibratory feeder rather than an aeration feeder system is in use.
Special effect powders require constant re-mixing thus aeration feeder is mandatory.
Hope this helps.
Bill

A. Okay I'll guess at this too! I will assume your discussing textures, etc. First let's agree that you need to have proper pretreatment because if the substrate still has debris on it or it has oils on it, the texture will probably be compromised and/or fail to be cosmetically acceptable as well as fail in performance.
Next, you need to have a viable powder to use. It needs to be powder that is not out of date because out of date powders tend to have many issues, especially textures.
Make sure the texture is being readily refilled with virgin powder at a rate that will continue the cosmetics and performance you require. Make sure your powder supplier tells you if they make ANY changes to the powder or its manufacture. Like any company, they do this because they regularly change their supplies and even though they may say the change won't change your product, I have found they may. Make sure that the substrate is subjected to the proper stoving every time. It's just important to make sure you control the entire process so you don't run into issues. That way, if something does go wrong, it will be easier for you to troubleshoot it.

A. Hi Kuldeep.

I think we need more words. One piece is 55 microns and the next is 150 microns, or (more likely) the coating thickness varies over a single part? Does it vary on edges compared to the middle of flat expanses, or is it lower in concave recesses, or are there blobs of thick powder in random locations? How is the powder applied, robotically or by various operators? There are no other problems -- no pinholes, discoloration, orange peel? You probably need to tell us exactly what you have found in your investigation if you want help in taking the investigation further. Good luck.

Regards,

A. Hi Rajagopalan. All questions are welcome, but not all questions are likely to get replies :-(
We'll see what happens though :-)

Luck & Regards,