How to mask & etch aluminum when caustic eats resist as well as aluminum

A. This all depends on how deep you want to etch the aluminum. If it is more than say 0.005", you will have a problem as the lye will eat any aluminum that it comes in contact with, even that material that is under the maskant. For masking you might try the plastic sticky backed shelf paper. You will need to avoid air bubbles and you will need to burnish the edges with a tongue depressor or a wood paint stick or a plastic ruler. All of these need the sharp edge rounded off first. For a liquid maskant, you could use a liquid vinyl that is sold by many hardware stores for dipping plier handles. Take a look at the solvents in it and buy a small can of that solvent. You will need to thin the vinyl to less than 50% to get it thin enough to brush.

There are lots of commercial products sold, but most do not want to sell a tiny can.

A. Negative photoresists are chemically more resilient than positive ones and should withstand an alkaline environment. Some of the older and better established negative resists are based on polyvinyl cinnamate, diallyl phthalates or isoprenoids. The downside of negative resists is that they do not offer the same resolution and are more difficult to strip. However, why do you want to etch aluminium in alkali? Aluminium is an amphoteric metal and will readily dissolve in acid etchants and these can be used with positive resists that are easier to use, easier to strip and more readily available. Furthermore, they offer better image resolution. Etching aluminium in alkali can result in inferior finishes due to uneven etching caused, primarily, by gas evolution. You may, therefore, want to consider the more readily available acid systems.

A. Hi Alice. Although caustic is a powerful etchant for aluminum, it is not the only etchant nor the most appropriate one for all circumstances. So we appended your inquiry to a thread where Trevor Crichton suggests acid etching instead. ferric chloride ⇦this on eBay or Amazon [affil links] can work, and it's also possible to etch with simple mixes like copper sulphate ⇦this on eBay or Amazon [affil links] and table salt (although electricity greatly speeds it). Patience with our search engine will reveal a lot of good information and very good links about etching aluminum; one recommended formula was nitric and phosphoric acid, with the claim that the nitric dissolves the anodizing while the phosphoric dissolves the raw aluminum. Good luck.

Regards,