Powdercoating Wheels

[crazy steve]

I know some members have done this, I'm curious as to how you dealt with the bearings and rear wheel 'shock cushions'. My bearings are fine (the later wheels I'm using came off a well-cared for, very low miles bike), so destroying them in removal seems wasteful. Has anybody tried coating some wheels without removing the bearings, and if so, how did it work out? The same thing goes for the rubber drive cushions in the rear wheel; did you leave these in? The manual makes no mention of a removal process for these (and they're not even listed as a separate part, so I'm assuming these are not meant to ever be replaced), so were these left in? The rear wheel is machined in such a way that the only choice for removal will be a drift/punch, so some damage to the holes/bushings would have to be expected.

The coating I'm using has a 'flow-out' temp of 400 degrees, with the 'cure' temp being 350. Some greases will take that, some won't, so that's why I'm asking....

'78E original owner

[Guy_b_g]

There is some controversy here, some say it's ok ro powdercoat, some not. They do make a special lower temp powder for such applications though. They also should never be sandblasted (other media such as walnut shell should be used).

As to the bearings and cush drive I'd remove and replace them but with low temp powder you might get away with it.

Here is a discussion from an engineers forum that may be informative: http://www.eng-tips.com/viewthread.cfm?qid=151053

bleucamaro (Aerospace)
30 Mar 06 15:24
Swall, When I was in the powder coat industry, the typical aluminum cleaning process was as follows: remove any grease with solvent based cleaner (MEK), remove any coatings with media blasting (thin components may recieve alternative method), outgas cast aluminum at 225°F for 45min, degrease with solvent or vapor degrease, then coat.

NickE, our oven control used a single thermostat, the oven was a convection type with furnace box on top and a large blower. heat surveys typically showed a maximum deviation of 10°F from nominal. As far as hot spotting the metal, we'd let heavier materials soak longer (up to an hour as stated in my first post) as the cure time is typically only 15 minutes once material has reached cure temp.

I guess I phrased my question wrong before, if an A356 casting is already at the T6 condition would an hour at 400°F age it even partially to another more brittle condition? which condition would this be? My ASM spec only shows it going to the T6 aging.

Also, how would 400°F for 1 hour affect forged 6061-T6? Would it cause any grain restructuring? My understanding is that strain relieving occurs pretty close to the anneal temperatures, but then again, I'm not exactly a materials wiz.


On a semi side note: I found that BBS uses both a powder coat primer and clear coat for their cast wheels.
CoryPad (Materials)
30 Mar 06 16:01
bleucamaro,

ASM Handbook Volume 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials has the following information:

356-T6: aging at 305 to 315 °F for 2 to 5 hours
Yield stress = 185 MPa
Ultimate stress = 262 MPa
Fracture strain = 0.05

356-T7: aging at 435 to 445 °F for 7 to 9 hours
Yield stress = 165 MPa
Ultimate stress = 220 MPa
Fracture strain = 0.06

Based on these data, a 400 °F treatment for less than 1 hour shouldn't reduce the fatigue strength significantly

Don't forget to outgas!

[Guy_b_g]

http://www.estcp.org/Technology/WP-0614-FS.cfm

Low temperature cure powder coatings were developed through scientific analysis of powder coating chemistries and the discovery that an acid/epoxy reaction could facilitate a low temperature cure of powder coating. These studies led to the down selection of an acid functional polyester resin with a triglycidylisocyanurate crosslinker. Coating material development involved optimizing parameters such as catalyst, resin type, corrosion inhibitor, flow additive, pigment concentration, powder particle size, and the ratio of resin to crosslinker. The resultant coating developed under SERDP project WP-1268 is a VOC/HAP-free alternative to solvent-based coatings, and it features performance properties equal to or better than those of coating systems governed by commonly used specifications such as MIL-PRF-23377, MIL-P-53022, MIL-PRF-85285, and MIL-PRF-22750. The coating can be applied directly to the component substrate, eliminating the need for primers. This low temperature cure powder coating is fully curable at 250°F and can replace other powder coatings, which require temperatures between 280°F and 400°F. Decreasing the curing temperature enables temperature-sensitive substrates, such as tempered aluminum, to be powder coated safely, without compromising their structural integrity and performance. Although all powder coatings offer superior barrier protection against corrosion, the powder coating being examined also contains a corrosion inhibitor, barium metaborate, to further improve corrosion protection in the event of coating damage.

[crazy steve]

...Don't forget to outgas!

These wheels have nearly zero corrosion on them, but trust me, I've learned my lesson about cast aluminum parts already....

'78E original owner