Coating Aluminum

Question:

We are attempting to powder coat a white texture over a black electrocoat on aluminum and mild steel. The mild steel looks very nice and white, but the aluminum pieces turn much darker. They appear to be over-cured. The pieces were painted side by side with the same line speed and oven temperature. What could our problem be? We are also having a hard time getting the wrinkle to be even. What problem could we be having with the aluminum pieces? How can we get the wrinkle to be consistent? D.R.

Answer:

How fast a particular material gets to temperature is directly related to its specific heat and thermal conductivity. Steel has a specific heat of 0.12, and aluminum has a specific heat of 0.23. That means it takes twice as much energy to heat steel than aluminum. Furthermore, the thermal conductivity of steel is 27 vs. 110 for aluminum. That means that for the same heat source aluminum will reach maximum temperature almost four times faster than steel. Of course, all this assumes that the mass is the same between the aluminum parts and the steel parts. Actually, aluminum parts are normally much lighter than steel parts. This makes your situation even worse.

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The color and texture (wrinkle) of powder coatings are directly affected by cure time and temperature. An over-cured part will not have the same color or texture when compared to a properly cured part. Typical powder coatings have an over-bake resistance of up to 100%. That means that the part can be in the oven for twice as long before the color changes and the mechanical properties are affected.

What is happening in your case is the aluminum parts are achieving temperature faster (because of their mass and thermal conductivity properties) than the steel parts. As a result, the cure time (time at cure temperature) is longer for a given dwell time (oven time) than your steel parts. Texture appearance (wrinkle appearance) is affected by the rate (speed) of temperature rise and the time at cure temperature. Furthermore, the aluminum parts are at temperature for a longer period of time, exceeding the over-bake resistance of the powder coating you are using. This makes the color different on the aluminum parts when compared to your steel parts. Both of these situations can be corrected by running your steel parts in a different batch than your aluminum parts. Stick with the temperature and line speed you are using now for your steel parts but run your aluminum parts at a lower temperature. Your powder supplier can profile your oven for each of your parts to determine the optimum settings for your aluminum and steel parts.