[M_D]

The parts have to be degreased somehow, we use a heated cleaner for that since we do a relatively high volume of parts and most of them are too complex to successfuly clean by hand.

The etch removes the outer surface of aluminum (just like stripping previously anodized parts does), and in doing so gets rid of the oxidized surface (assuming the surface is oxidized) which results in a better anodized layer. Etching isn't always needed. For parts that have mechanically de-oxidized (machining, polishing, sanding, etc.), skipping the etch gives good results in most instances. Etching tends to dull the parts, depending on the etch time, etch temperature, and concentration. It can be done and still leave a decent glossy finish, but it still dulls the parts somewhat.

We vibratory tumble most, but not all of our parts. It does leave the parts fairly clean if the water and detergent are kept fresh, but for parts that are deburred this way we find it essential to etch them well. If we don't, the colors aren't as even and vibrant, because vibratory (or rotary) tumbling tends to oxidize the surface quickly. Plastic media does not seem to oxidize the surface as bad as ceramic.