Chemical machining, also known as chemical milling or industrial etching, is a subtractive manufacturing process during which chemicals are engaged to remove or extract unwanted metal from a metal surface. Over the centuries, operators have learned to perfect the process. It creates products free of imperfections and/or undue mental stress. It is also quick and simple to perform, requiring few tools and minimal labor.
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Chemical Machining Process
Typically, chemical machining is performed in five steps: cleaning, masking, scribing, etching, and demasking. The first step, cleaning, is a preparatory step by which contaminants like oil, residue, primer coatings, and grease are removed so that the surface material will not have any issues with accuracy or depth during etching. Manufacturers usually apply a solvent to them or immerse whole parts in deoxidizing solutions or alkaline cleaners to clean surfaces. Next, during masking, manufacturers apply a maskant material to the whole of the surface. When it comes time to etch, any surface area covered in this maskant will not be able to be etched. This is key to accurate etching. For them to work properly, it is important that they adequately adhere to the surface on which they are placed. Maskants are typically applied either via dip masking, during which the part is dipped into an open tank of maskant and then allowed to dry, or flow coating, during which the maskant is cascaded over the part. Sometimes, conductive maskant is applied via electrostatic deposition, during which, as electrical charges are applied to it, the maskant is sprayed onto the surface of the material. After masking is scribing. During scribing, those areas that will be etched are rid of the maskant. If this is done during an industrial operation, manufacturers will likely do so with the help of a template or CNC automation. If the scribing is being performed for decorative purposes, on the other hand, manufacturers are more likely to remove the maskant by hand with a scribing knife or etching needle. Second to last is the step of etching. Manufacturers achieve etching not with instruments but by immersing the piece in a chemical bath. Note that the longer the time a piece spends in a chemical bath, the deeper its etching will be. After etching, a chemically machined part is damasked. This is when both the remaining maskant and the etchant in which the part was just bathed are removed. Typically, this is conducted either simply using cold water or cold water with additives. Sometimes, the part is also put in a deoxidizing bath. Rarely the chemicals are scraped off by hand.
Types of Chemicals for Chemical Machining
For the best results, manufacturers must select the right chemicals for the job. In general, maskant materials must be adequately chemically inert with adhesion levels around 350 g cm-1. At approximate levels around 350 g cm-1, the maskant is neither too weak to stick to the surface nor too strong to be removed during scribing. The maskant must also be compatible with the material upon which it will adhere. Usually, maskants are made in some aspect from isobutylene-isoprene copolymers or neoprene elastomers. The nature of etchants depends heavily upon the type of material they are etching. Standard steel surfaces, for example, are mostly etched by hydrochloric acids or nitric acids; stainless steel surfaces are usually etched by ferric chloride, and mild steels are primarily etched by Nital, which is a mixture of nitric acid and methanol, methylated spirits, or ethanol. Aluminum products are mainly etched by Keller’s reagent or sodium hydroxide, while copper may be etched by a variety of chemicals and chemical combinations, including hydrochloric acid and hydrogen peroxide, ammonia, ammonium persulfate, cupric chloride, ferric chloride, and 25% to 50% nitric acid.
Chemical machining has been around since the nineteenth century, and people have been using organic chemicals and semi-organic chemicals like citric acid, lactic acid, and vinegar for etching metals since as early as 400 BC. To figure out what chemical maskants, etchants, and chemical machining tool combinations might be right for you, consult with an experienced and proven leading chemical machining expert.
Applications of Chemical Machining
Chemical machining offers incredible precision and is often used with applications that require complexity or intricacy that other machining methods simply cannot achieve. It can be utilized either as a primary application or as a finishing application after the initial, less complicated machining of a product has been completed. Industries in which chemical machining is used include aerospace, automotive, electronics, semiconductor fabrication, and any industry in which micro components must be etched.