Masking Techniques

Question:

We are looking for a better way of masking off areas on a number of machined parts. Frankly, we are not getting the definition and accuracy that we want. When we get the masked and anodized parts back in our assembly plant, we often have what I can only describe as "feathering" on the masked-off areas. The anodizing appears to encroach significantly on these masked-off areas. Is there any way to stop this? We are thinking that one possible solution might be to machine a small groove around the area to be masked in order to get a cleaner line. Can you give us some tips regarding this situation? N.M.

Answer:

Masking is a difficult task, and no matter how hard one tries, the masking is subject to leakage sometimes. It's a matter of finding the right products for the application and using the imagination to dream up new and better ways of doing the job. There is no such thing as going too far with new ideas for masking. I agree with you that masking a "line" on a flat surface is nearly impossible. Machining a "groove" of some sort in the right areas may give you better definition at the masking line.

I can't tell you how to mask your specific parts, but I can, perhaps, remind you of some different techniques to use. Tape is always an important tool. It works well in certain situations, as you know, and there are tapes made of everything from vinyl to aluminum to lead. There are various paint-on and spray-on products that work well under certain circumstances. There are companies out there that produce a variety of these types of maskants for nearly every application. My company used spray-on materials in combination with other techniques a decade ago on parts for the space station with some success. It helps to apply a chromate conversion coating before applying adhesive maskants. The chem film helps the maskants, whether tape, paint-on or spray-on, to hold tighter to the surface of the part. Masks can also be machined out of materials such as UHMW (Ultra-High Molecular Weight polyethylene) and used with various rubber-like gaskets in a bolt-on fixture. This can work well sometimes in masking flat surfaces, large bores and other configurations. Also, silicone sealant (RTV) is a good product to use to fill any possible voids in spray-on and brush-on maskants. Don't forget that anodizing itself can sometimes serve as a maskant for some areas of your parts. If you would like some specific supplier information you may contact me through PF online.