Chemical milling is a process that is popular among design and manufacturing engineers from the aerospace, automobile, and electronics industry. This process is a metal alteration process. It uses chemical etchants to erode pre-determined areas of metal sheets and parts. And makes them fit the application's requirement.
People often use the phrases chemical milling and chemical etching interchangeably. And while it synonymously is not entirely incorrect. Some minor differences set the two processes apart.
In this article, we will begin by describing chemical milling and chemical etching. And then we will highlight the small factors that differentiate between the two seemingly similar processes.
Chemical milling is the process of producing markings, incisions, and deep- and shallow-cuts on metal sheets and parts using acidic or alkaline solutions called etchants.
This process produces shallow cavities on the metallic surface of the industrial equipment and devices. It works to remove material and reduce the weight of the parts in weight-sensitive applications like in aircraft.
The milling process has two working components. An etchant and a maskant.
The etchant is an acidic or alkaline solution that chemically reacts with the metal and converts it into metallic salts. These metallic salts then leave the substrate metal and precipitate into the etchant solution.
Maskant is an elastomeric or copolymeric material that covers the metal sheet and protects it from corroding due to the etchant. It ensures that the chemical only mills the required areas and the rest of the metal stays smooth.
Chemical milling finds the most application in the aircraft and aerospace industries. These industries use the milling process to reduce the weight of the part by selectively removing the unnecessary metals and making shallow and precise incisions.
The most common materials that the chemical etching process performs most efficiently on including steel, aluminum, titanium, nickel, copper, and their alloys.
Chemical etching is very much similar to milling in that it also uses an etchant to etch the material off the metal surface. However, there are some differences between the two processes, that we will get to in a moment.
This process also uses some type of etchant-resistive coating to protect the metal from the etchant’s corrosiveness.
The type of film used and the method for scribing through the protective film determine the type of chemical etching. There are two main types of chemical etching, photochemical etching, and mechanical etching.
Photochemical etching uses a polymeric film, called photoresist, UV light to create the acid-resistive mask and an etchant.
On the other hand, mechanical etching makes use of the waxy ground to protect the entire metal sheet from dissolving away with the etchant. It is called mechanical or manual etching because the millers scratch and remove the ground from the metal surface that they have to etch.
Chemical milling and etching are the same in their working principles. Both of these processes use an etchant to etch and chemically remove parts of the substrate to make them fit the design requirements.
However, as we have mentioned above, there are some features distinct to each of these processes. Following are the aspects that set chemical milling and etching apart from one another:
The Process of Chemical Milling
The working process of chemical milling also shares similarities with the etching process. However, the difference that we have highlighted above reflects in the process, as you will see below.
A typical chemical milling process is a five-step procedure that proceeds as follows:
Cleaning is the first and most important step in the milling process. Because if the surfaces are not clean enough, the maskant will not adhere to the metal surface properly resulting in the etchant reaching the areas where it wasn’t supposed to go and creating poor surface finish, irregular etching, and uneven depth of the etched-surface.
Therefore, before the application of maskant, millers clean the metal sheet thoroughly using grease-removing solvents, alkaline cleaners, and de-oxidizing solutions.
After cleaning, the metal sheets are ready for the application of maskants. Millers apply maskants on the surface of the metals to ensure that the etchant only mills the required areas while other areas remain protected.
As far as the type of maskant is concerned, the type of etchant and the properties of the substrate determine the type of maskant that will be most suitable for the process.
The maskant plays a vital role in ensuring the precision of the chemical milling process that it enjoys a good reputation for. Therefore, the maskant must adhere to the metal optimally, not too tight, to present difficulty inscribing and peeling. Or too weak, to let the etchant come in contact with other areas of the metal.
Scribing is the process of removing the maskant from the areas that the millers need to mill. It may be done by hand if the design is not too complex. Or etchers may use a CNC machine in case the design involves fine lines and intricate incisions.
After the metal sheet has been covered with maskant and the design scribed on it comes the process of chemical milling.
The maskant-covered and scribed sheet of metal is dipped into an etchant and allowed to sit for a pre-determined period.
Many factors may determine the depth of the incision and the time required to etch like the strength of the etchant, type of etchant, how long does the metal sit in the etchant, and the temperature of the solution.
During the milling process, it is imperative to keep a check on the depth of the engraving, which is done using an ultrasonic sensor. When the required depth of the engraving is achieved, the millers take the metal out of the etchant and proceed with the next step.
Demasking is the last step of the milling process. In this step, the millers remove the maskant from the metal sheet with their hands or using a scrapper.
The metal sheet is cleaned off the etchant by washing it with cold water. Millers may as well use a de-oxidizing bath to remove the oxides that might have formed during the milling process.
Chemical milling, like etching, is often the preferred process for machining complex designs on parts. Following are only a few of the many reasons chemical milling has earned its edge over other similar processes:
Chemical milling and chemical etching do share striking similarities which is why it is not incorrect to use the terms interchangeably.
However, the fact that chemical milling is an alteration and metal removal process while chemical etching fabricates pre-made metals differentiates milling from etching.
Moreover, chemical milling uses a maskant as an etchant-resistive material while etching employs photo-resist films or ground which is another differentiating factor between the two processes.
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