I had a set of wheels powder coated for a vehicle restoration about five years ago. The wheels were sandblasted and immediately delivered to the powder coaters. The vehicle has been outside much of this time but has been driven less than five miles since then. Yesterday, while washing the vehicle (project resuming), I noticed that two of the wheels have a cracked powder coating and some rust stains extending from the cracks. The wheels are standard stamped steel wheels found on cars since the beginning of time. What could be the cause?
--Chris M., Cookeville, TN
There are a couple of possibilities for this type of failure. First of all, it’s important to know where the cracking occurred. Cracking happens for a few reasons. If the powder coating was applied too thick, you can experience stress cracking around lugnut holes due to torque when installing the wheels. Another cause for cracking can be due to undercured powder. You mention that these are “standard stamped steel wheels,” which by their nature are heavy heat sinks. Excessively thick, undercured powder has a high potential to crack. If your powder coater didn’t get the wheels up to the required temperature for enough time, then the powder is probably undercured. Standard powders typically should see 15 minutes at 375°F to achieve acceptable cure. (The 15 minutes are after the wheels reach 375°F metal temperature. Steel wheels can take up to 45 minutes to get up to temperature in a small oven.)
Here’s what I would do to diagnose the problem. Check the film thickness of the coating with a magnetic film gauge. (You might have to borrow one from a local powder coating shop.) Reasonable film thickness is 2.0 to 3.5 mils. Greater than 4.0 mils can become brittle. Check also the solvent resistance of the coating. This is a simple measure of cure. Take a strong solvent such as MEK, acetone or even gasoline and rub a non-appearance spot (back of the wheel?) with a solvent-soaked shop rag. A very small amount of the baked powder may come off onto the rag. If the powder surface is easily softened and removed, then the coating was severely undercured and therefore prone to crack. If the coating is undercured, then your powder coater owes you a recoat at no charge.
My company has created a revolutionary weighted exercise, weight loss and therapy vest. We have been using a tin plating for our lead weights for the prototype phase, but now that we are preparing for production, we want to find a coating and a coating method that will encapsulate the weights in a thin, durable and possibly flexible coating. The weights will be in direct contact with the end user when inserting and removing and in contact with sweat and body acids while working out. We really need to know what the best coating and coating method should be in order to serve our customers best and keep them safe.
--Cosmo Raines, Hyper Wear, LLC
This sounds like a good application for thermoplastic powder. I contacted Tim Brinner at Innotek LLC (a manufacturer of thermoplastic powder coatings) to get his opinion, and he agrees:
“If I’m grasping the application correctly, the requirements are:
- A functional coating (as opposed to a paint)
- Flexibility over hardness
- A thicker coating to absorb the shock if dropped on a corner of the weight
- Fairly good abrasion resistance
- Suitable for extended human contact
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