[Majin]

In lieu of all the powder coating questions, and for me to understand the entire process better, I have decided to try and answer this (with your help) once and for all.


What is powder coating?

Powder coating is the newest form of surface finishing techniques in use today. It was first used in Australia about 1967. (Hi Steve! )

Powder coating is the technique of applying dry paint to a part. The final cured coating is the same as a 2-pack wet paint. In normal wet painting, (such as house paints), the solids are in suspension in a liquid carrier, which must evaporate before the solid paint coating is produced.

In powder coating, the powdered paint is applied either of the following two techniques:

• The item is lowered into a fluidized (definition: a mass of solid particles that is made to flow like a liquid by injection of water or gas) bed of the powder, which may or may not be electro-statically charged
• The powdered paint is electro-statically charged and sprayed onto the part.

The part is then placed in an oven heated to 160° - 210° C (depending on the paint powder used) and the powder particles melt and coalesce to form a continuous film.

Two main types of powder available to the surface finisher:

• Thermoplastic powders that will remelt when heated
• Thermosetting powders that will not remelt upon reheating.

During the curing process, (in an oven), a chemical cross-linking reaction is triggered at the curing temperature. It's this chemical reaction that gives powder coating many of its desirable properties.

Preparation

Just as in any kind of finishing work, a good coating depends on its preparation. The vast majority of powder coating failures can be traced to a lack of a suitable preparation.

The preparation treatment is different for different materials.

In general, for all applications the preparation treatments are as follows:

Aluminum:

• Clean
• Rinse - oils and/or greases are removed in weak alkali/caustic or neutral detergent solutions
• Etch - used to remove heavy oxides
• Rinse
• Chromate or Phosphate - used to form a conversion coating (serves the purposes nested below) on the aluminum. this film is chemically attached to the aluminum.
  • it presents a surface to the powder which favors adhesion more than the oxides that form very readily on aluminum surfaces
  • it reduces the incidence of under film corrosion, which may occur
• Rinse
• De-mineralized Rinse - used to reduce the presence of chemical salts on the aluminum surface. as you may already know, salts have always been found to cause corrosion in humid conditions.

Steel: (Exterior applications)

• Clean
• Rinse
• Etch
• Rinse
• Grain Refine - used after acid cleaning of steel surfaces and before the zinc phosphate process, otherwise the zinc phosphate coatings produced will be very coarse with low adhesion. The powder coating applied to a coarse phosphate will produce rough coatings (a little like "sandpaper") and possess low adhesion.
• Zinc Phosphate
• Rinse
• Acidulated (made slightly acidic) Rinse

Hot Dipped Galvanized Steel: (Exterior applications) - must have been stored for more than about 4 hours before powder coating

• Clean
• Rinse
• Etch - required to remove the zinc corrosion products which begin to form almost immediately the zinc is removed from the galvanizing kettle.
• Rinse
• Grain Refiner - ensures a fine phosphate is produced.
• Rinse
• Zinc Phosphate
• Acidulated Rinse

How is it done -- electrostatic spray?

The powder is applied with an electrostatic spray gun to a part that is grounded.

Before the powder is sent to the gun it is fluidized. This is done to:

• Separate the individual grains of powder and so improve the electrostatic charge that can be applied to the powder
• Ensure that the powder flows more easily to the gun

Because the powder particles are electro-statically charged, the powder wraps around to the back of the part as it passes by towards the air offtake system. By collecting the powder, which passes by the job, and filtering it, the efficiency of the process can be increased to 95% material usage.

The powder will remain attached to the part as long as some of the electrostatic charge remains on the powder. To obtain the final solid, tough, abrasion resistant coating the powder coated items are placed in an oven and heated to temperatures that range from 160° - 210° C (again, depends on the powder).

Under the influence of heat, a thermosetting powder goes through 4 stages to full cure.

MELT --> FLOW --> GEL --> CURE

The final coating is continuous and will vary from high gloss to flat matte depending on the design of the powder by the supplier.

Powder coating guns

There are at least three types of electrostatic guns in use:

1. Corona charging guns - electric power is used to generate the electrostatic charge. Corona guns are either internal or external charging.
2. Tribo charging guns - the electrostatic charge is generated by friction between the powder and the gun barrel.
3. "Bell" charging guns - the powder is charged by being "flung" from the perimeter of the "bell."

Not all powder is applied using guns. One system makes use of electrostatic tunnels.

How is color introduced?

Color is added to powder coatings during the manufacturing process, (before the powder reaches the powder coater). There is little that can be done to change the color consistently, once the powder leaves the manufacturing plant. Be sure you or the powder coater has enough for your entire job before you let them begin.

Why powder coat?

Powder coating produces a high specification coating which is relatively hard, abrasion resistant (depending on the specification) and tough. Thin powder coatings can be bent, but this is not recommended for exterior applications.

The choice of colors and finishes are almost limitless, IF you have the time (and money ) to have the powder produced by the powder manufacturer.

Powder coatings can be applied over a wide range of thickness.

Care must be exercised when quoting minimum thickness because some powder will not give "coverage" below 60 or even 80 micron. "Coverage" is the ability to cover the color of the metal with the powder.

Color matching is quite acceptable batch to batch.

Installations and maintenance

During installations, the powder coating should be protected from damage due to abrasion and materials of construction such as mortar and brick cleaning chemicals.

Once installed, maintaining the initial appearance of a powder coating is a simple matter. The soot and grime which builds up on surfaces from time to time contains moisture and salts which will adversely affect the powder coating and must be removed. Powder coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The coating should be washed down with soapy water -- use a neutral detergent or my wheel cleaner P21S -- and rinsed off with clean water.

When powder coated items are installed without damage to the powder coating and they are maintained regularly, they should be relatively permanent. The correctly applied coating, although not metallurgically bonded to the metal will not crack, chip or peel as with conventional (liquid) paint.

[nandanrp]

So, was our earlier conclusion regarding forged wheels and powder coating still true?

[Majin]

i was looking to see what temperatures are needed to anneal forged aluminum, if they are in the 320° - 410°F (160° - 210°C) range, then yes, it will weaken them.

i will post findings in here and expect anyone else with knowledge of this to post in here as well.

[wrxser]

Just say no to powdercoating forged wheels.

[SCOOBYTRAP]

I looked into powdercoating my stockers and found alittle information here and there, but I learned alot by calling and talking to some local shops. I would recommend the same, call at least 3 and get a feel for how they do business. What I found and heard, was that I would play it safe and keep the bake process under 300F, 300F max. I wouldn't trust most shops either. Basically most shops will have a certain powder...and it'll cure at or above 400F, they do all their coatings at this temp. 400F is excessive for forgings in my mind. To powdercoat a forged Aluminum wheel properly without effecting the base metal I wouldn't go beyond a 300F cure. Now you can get these powders but they're more expensive- maybe because they're used less or the shop is only interested in smaller batch (for you)? Or the shop will have to buy some huge amount of this special powder and they'll want to pass this cost onto you, only because it'll be the one color YOU want. Having a powder that cures at 300F will also require a longer cure time than 400F powders so it'll tie up a shops oven longer- they don't like this- An entire oven for 4 wheels. Well, I guess it depends on the shop, but most are all about throughput (at least the ones in my area). And that means turning up the temp to get the product out. I guess if you got the $ any shop would do it. I talked to some local powdercoat shops before I ended up painting mine. If you're dead set on powdercoating, check with some shops and make an educated decision.

[SCOOBYTRAP]

I forgot that I eventually did get a reply to this:

Gesendet: Montag, 4. September 2006 23:01
An: Zentrale
Betreff: Powdercoat, Subaru BBS Wheels

I have the stock BBS wheels that come on a '05 Subaru Impreza WRX STi and would like to powdercoat them. I have read alot about people saying how the temperatures involved with powdercoating can harm a forged aluminum wheel. I've heard it is best to powdercoat at temperature not exceeding 300 degrees F as to not harm the forging. I am a mechnical Engineer and know how heat can change material grain structures but not that familiar with Aluminum forgings. If you could provide any suggestions or insight on this subject it would be much appreciated. Really I just want to change the color and have it hold up well. Thank you. -Mike
---------------------------------------------------------

Hello Mike,

we make powdercoat our wheels at temperature 180°C.

Best regards
BBS Kraftfahrzeugtechnik AG

Natascha Zerr
Kundendienst
Tel.: 07836/52-1-277
Fax: 07836/52-4-277
E-Mail: zerr.natascha@bbs-ag.com
www.bbs.com


So there it is

--------------------------------------------------------------------------------

[Majin]

thanks SCOOBYTRAP. seems like the range i posted was correct, but this is a better thread with your post.

[wundej]

Here is a reply that I posted a few times (or similar here and on nabisco).
The only other thing we'd need from BBS is the time the wheels are baked for and the alloy (which can be found other ways). The key is that you don't want to overage the wheels, so regardless of the baking temp you could end up in a situation where they are heated too long and become weaker.


Quote:
<table border="0" cellpadding="6" cellspacing="0" width="100%"> <tbody><tr> <td class="alt2" style="border: 1px inset ;"> Originally Posted by jermdeez
ive heard they are more likely to break on bad potholes.
</td> </tr> </tbody></table>
This topic has been debated over and over again. I don't remember how many times I've replied to posts like this on this or other boards.

1. you need to know the previous mechanical history of the wheel to know where you are starting from. What is the specific material? Has it already been heat treated? Has it been powder coated in the past? How long and what temp was it cured at?

2. then you need to know what parameters your powder coater will be using for the curing process (temp, duration, etc).

3. then you need to know your mechanics of materials. If a metal is material X with heat treat grade Y, and I then subject it to another XX minutes at YY degrees, what will happen to the material properties? What types of impact loads could the wheel handle stock...what types of conditions is the wheel prone to failing? Fatigue, impact, both?

4. TEST TEST TEST. Real world test data is key. In our case it is going to be hard to get this info because we don't want to go and try to bend one of our nice wheels for the sake of this arguement. Of course it helps to simulate on a computer (which most structural engineers with access to FEA software should be able to help you do, it will take some time to setup the models but if they know what they are doing you can get very accurate results).

5. make a conclusion on if it's safe.

6. ?
7. Profit (sorry, had to throw those in there)

Do you trust your powder coater to know all of the above information? I sure as heck wouldn't trust anyone except the original wheel manufaturer.

You run the risk of overaging the metal, which could lower some of its mechanical properties.

That being said, a lot of people run powder coated wheels without any issues at all. Find someone that knows what they are doing, who has good equipment that has good monitoring capabilities and use a low temp curing temp and you should be OK. I was going to PC one set of my wheels but I couldn't find a quality shop in the area for a decent price.

If you have someone doing your wheels who has no clue what he's doing and normally is just PC non structural parts...chances are you could end up with a cracked wheel if you hit a nasty pot hole, which could have also broke a non-pc'd wheel as well.

Sorry if that sounded like a rant, but this has been talked about over and over again with "I heard this, I heard that this is the truth, my dentist said that powder coating wheels is good for them". Unless you know all of the info mentioned above, you can't make a definite satement either way. We can all make educated guesses, just make sure you take all that info into account.

Also,
Austin has a good link there, read through all the BS and believe what you will. There are a lot of people around who know a lot about what they are talking about, but there are also a lot of people who think they know a lot about what they are talking about. The key is to find out who is who.

[Del]

Originally Posted by wrxser

Just say no to powdercoating forged wheels.

Simon (Yo Chang) had his stock BBS's powdercoated and races them with no problems.

[Knuckleball22]

What is the cost for powdercoating 4 stockers?

[Dragicon]

Originally Posted by Knuckleball22

What is the cost for powdercoating 4 stockers?

most places will probably charge around $250 for 4 rims.

[qwksti]

Here is my $.02 on this. ive done a lot of looking and asking around before i had my sets of rims PC. I came to the conclusion that %50 of the poeple said its bad, and %50 of the people said its ok. So at that point i decided to talk to BBS, and the people who do powder coating. I probly called 10 shops in the SO CAL area. BBS said they would not reccomend doing it, only because if they are over baked and for to long they cant say for sure if it would harm the metal. But even the guy i talked to was not sure. Plus what kind of crap wheel would it be if it cant take some heat for a little wile. From what i know the BBS STi wheels are very good wheels. So i take that as if you think the PC place have done them before and know what they are doing then its probly ok. Even baked at 350 for a few hours is not hot enough to change the metal from what i was told. Im sure my brembos put off much hotter temps than that racing at the track all day.

So i figured i will do it and see, besides i wanted a new look and most after market wheels are heavy and cost to much. So i found a place that said they PC wheels for race teams and wheel manufactures. So i gave it a shot. they came out GREAT !! they look sweet and for the last few months i have had no problems with them at all. no cracking, ,no fade, perfect. the PC seems more durrable than paint and wipes clean very easy. plus PC keeps the rim cooler by keeping heat out, so im told. So this could have some advantages at the track/autocross days. I also had a set of PC rims on my old MR2 for years and never had any problems after many track days. And the place that did them was very iffy, and i never even thought twice about it. Lucky maybe,, who knows?

I used a place in OC CA called Specialized powder coating 'specializedcoating.com'

here are pics of the red ones i use daily, and the green ones im selling. this has been my $.02 and i would do it again. ill keep posting updates if i run into problems.



and the green ones im selling

[wundej]

Originally Posted by qwksti

Plus what kind of crap wheel would it be if it cant take some heat for a little wile. From what i know the BBS STi wheels are very good wheels. So i take that as if you think the PC place have done them before and know what they are doing then its probly ok. Even baked at 350 for a few hours is not hot enough to change the metal from what i was told. Im sure my brembos get much hotter than that racing at the track all day.

Plus what kind of crap wheel would it be if it cant take some heat for a little wile.

That is a pretty risky statement, all alloys perform differently under different conditions. Read up on the process of heat treating and over aging metals before making a general statement.

Don't think that just because a place has PC'd wheels before, means that they know what they are doing.

Also, your brembos are a completely different animal. They are not subjected to anywhere near the same amount of loading as a wheel, and the material that was selected for them is something that will stand up to the thermal stresses involved with heat cycling a caliper.

Most people won't have issues, but everyone needs to understand there is a risk. It's going to be hard to get a straight answer from anyone that knows what they are talking about because there are a lot of factors that contribute to the answer (see post above).

Anyway, they look nice and they will definitely be nicer to clean. It's generally a much more durable finish.

[qwksti]

Originally Posted by wundej

Plus what kind of crap wheel would it be if it cant take some heat for a little wile.

That is a pretty risky statement, all alloys perform differently under different conditions. Read up on the process of heat treating and over aging metals before making a general statement.

Don't think that just because a place has PC'd wheels before, means that they know what they are doing.

Also, your brembos are a completely different animal. They are not subjected to anywhere near the same amount of loading as a wheel, and the material that was selected for them is something that will stand up to the thermal stresses involved with heat cycling a caliper.

Most people won't have issues, but everyone needs to understand there is a risk. It's going to be hard to get a straight answer from anyone that knows what they are talking about because there are a lot of factors that contribute to the answer (see post above).

Anyway, they look nice and they will definitely be nicer to clean. It's generally a much more durable finish.

true i agree. i meant with my brembo statement that they give off heat and the wheel it subjected to the heat from the brembos.

[wundej]

Originally Posted by qwksti

true i agree. i meant with my brembo statement that they give off heat and the wheel it subjected to the heat from the brembos.

Got ya, that makes sense.