Pretreatment for Aluminum Castings

Question:

We paint aluminum sand castings for an exterior application. We want to switch to powder but we are torn between a spray washer and an immersion system. Some of our parts can get up to 24 inches wide x 15 ft long. Its not the size that causes the confusion. We hear different opinions on which of the two, spray or immersion, does the best cleaning and pretreating. D. R.

Answer:

In general, I prefer the spray pretreatment systems. They have several advantages over the immersion systems. One advantage is that cleaning is more thorough because of the impingement of the chemical solutions on the product. Another advantage, because of this impingement, dwell times in each pretreatment stage are shorter. A further advantage, is the reduction in the amounts of chemical solution in use at any given time for spray systems. This may be important when processing large products providing the spray can reach all surfaces. Otherwise, it could be a disadvantage.

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There are other disadvantages as well. One disadvantage when using a spray system is a greater heat loss. One other disadvantage is more maintenance due to more complicated equipment. These disadvantages are out-weighed by the aforementioned advantages.

It's important to remember that the aluminum sand castings, as well as any other castings, must be degassed and dewatered before powder coating. Powder coatings are less forgiving of entrapped gasses and moisture.

Follow Up:

When powder coating aluminum components, folks need to remember that some powders require elevated temperatures for cure. These temperatures can approach or even surpass those used in heat treating the part in question during the coating process. Of course such a situation completely negates the temper that the part had prior to the powder coating process. Please be attentive to this. Thanks and keep up the good dissemination of knowledge. 

Answer:

You raise a very important issue that I should have passed on to the readers. My company learned this aluminum temperature tolerance problem many years ago (when I had a real job). In the late 1950s, we were powder coating aluminum electrical conductors that had previously been insulated by hand taping and varnishing. Fortunately, our metallurgists advised us we were using preheat and curing temperatures very near the critical point for aluminum.

The message that I want to convey is: “Before heating aluminum or any other substrate for industrial processing, determine its upper temperature limit and don’t overheat.”