Pretreatment for Aluminum Castings

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Posted on: 9/1/2004

  Question: We paint aluminum sand castings for an exterior application.

 

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.

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.

 


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