Chemical milling removes metal by controlled chemical dissolution rather than by cutting or grinding. It is the process of choice for producing complex, contoured, thin-walled structures in aerospace and defense manufacturing — structures that would be impractical or impossible to achieve by mechanical machining. Maskant is the essential enabler of chemical milling: it defines which areas of the part dissolve and which are protected, making selective material removal possible with chemical precision.
The Chemical Milling Process Overview
Chemical milling starts with a part that has been machined, formed, or heat treated to approximately the required shape. The part is cleaned to remove all oils, oxides, and contaminants that would prevent maskant adhesion or interfere with uniform etching. Maskant is then applied over the entire part surface, allowed to cure or dry, and scribed — cut along precise patterns — to define the areas to be chemically removed. The maskant is peeled from the areas to be etched, and the exposed substrate is submerged in a chemical etchant that dissolves the metal at a controlled rate.
At the conclusion of the etch cycle, the remaining maskant protects the un-etched areas. The part is removed from the etchant, rinsed thoroughly, and the remaining maskant is stripped. The result is a part with chemically removed pockets, channels, or tapered sections exactly where the scribing defined the etch boundaries.
How Maskant Protects the Covered Surface
The maskant coating works through physical isolation: it prevents the etchant chemistry from contacting the substrate beneath it. For this protection to be complete, the maskant must:
Form a continuous, pinhole-free film. Any discontinuity in the maskant — a pinhole, a bubble, an inadequately adhered area — exposes the substrate to etchant. Even small pinholes in the maskant allow etchant to attack the substrate locally, creating unwanted pits or depressions in the masked surface. Maskant application technique and the maskant’s film-forming properties determine whether the applied coating is pinhole-free.
Adhere tenaciously to the substrate surface. The etchant in chemical milling is aggressive — sodium hydroxide solutions at elevated temperature for aluminum alloys, or mixed acid solutions for titanium. At the perimeter of the etched areas, the etchant is in contact with the maskant edge. If the maskant’s adhesion to the substrate is inadequate, the etchant can undercut the maskant edge — penetrating laterally under the maskant beyond the scribed line. Undercut reduces the dimensional accuracy of the chemical milling by removing material in the masked zone adjacent to the scribed boundary.
Maintain chemical resistance throughout the etch cycle. The maskant is immersed in the etchant for the duration of the etch — which may be 30 minutes to several hours depending on the depth of material removal required. The maskant polymer must resist chemical attack, swelling, or dissolution by the etchant throughout this exposure without degrading or delaminating.
Maintain adhesion under temperature and agitation. Chemical milling baths are typically heated (40–70°C for aluminum NaOH etching) and agitated to maintain uniform chemistry at the part surface. The maskant must resist thermal softening and remain adhered under the mechanical agitation forces of the bath.
Scribing: Defining the Etch Boundary
Scribing is the precision operation that converts the maskant from a blanket protective coating to a selective mask. A scribe tool — typically a knife blade, stylus, or template-guided cutting instrument — cuts through the maskant along the lines defining the areas to be etched, without cutting into the underlying metal.
Scribing depth control is critical: too shallow and the maskant is not fully cut through, preventing clean peel; too deep and the scribe marks the metal surface, creating stress concentration sites or dimensional irregularities. Scribing is typically performed against templates or under numerical control for production parts.
After scribing, the maskant within the etch area is peeled away by lifting an edge and pulling the cut piece free. The surrounding maskant remains in place, held by its adhesion and the containment of the scribed line. The scribed edge becomes the chemical milling boundary — the line between the etched and masked surfaces.
Undercut and Etch Factor
At the boundary between masked and etched surfaces, chemical milling does not create a perfectly vertical wall. The etchant contacts the exposed metal surface and dissolves it, but some etchant also penetrates laterally under the maskant edge. This lateral penetration creates an undercut — a rounded profile under the maskant edge — that means the actual etched area is slightly larger than the scribed line would suggest.
The ratio of etch depth to undercut is called the etch factor, and it is a characteristic of the etchant, temperature, alloy, and maskant adhesion. For a given process, the etch factor is consistent enough to be accounted for in the scribe pattern: the scribe line is offset inward from the final desired edge by an amount calculated from the expected undercut. Parts designed and scribed to account for undercut achieve accurate final dimensions after chemical milling.
Maskants with higher adhesion produce less undercut, resulting in higher etch factors and more accurate dimensional control. This is one reason that adhesion — not just chemical resistance — is a critical maskant performance parameter for chemical milling.
Email Us to discuss maskant selection for your chemical milling operation.
Maskant Removal After Etching
After the etch cycle is complete, the remaining maskant must be removed without damaging the etched surfaces or leaving residue. For peelable chemical milling maskants, removal is by mechanical peeling — the maskant is flexible enough to peel away in sheets. Chemical stripping in appropriate solvents removes any remaining adhesive residue.
The cleanliness of the stripped surface is critical for subsequent operations — bonding, painting, or NDT inspection. Residual maskant or adhesive on stripped surfaces must be removed completely, as even thin films can interfere with subsequent processing steps.
Maskant Types Used in Chemical Milling
Chemical milling maskants are typically neoprene-based, urethane-based, or vinyl-based materials formulated for the specific etchant chemistry. Neoprene maskants are traditional for aluminum chemical milling in sodium hydroxide; they provide good chemical resistance and adequate etch factor. Urethane and vinyl formulations offer different adhesion and peelability characteristics for specialized applications.
Two-part maskants applied by brush or dip coating are common for large aerospace structures. Spray-applied maskants allow uniform coating of complex geometries. Template-applied peel-and-stick maskant sheets are used for simpler geometries where tape-like application is practical.
Incure’s Chemical Milling Maskant Products
Incure develops peelable maskant formulations for chemical milling and etching operations, with chemical resistance, adhesion, and scribing characteristics optimized for aluminum and specialty metal etching processes.
Contact Our Team to discuss maskant requirements for your chemical milling process and identify Incure products with the chemical resistance and adhesion profile needed for your etchant chemistry and part geometry.
Conclusion
Maskant works in chemical milling by creating a physically and chemically resistant barrier that protects covered areas from etchant while exposed areas dissolve at a controlled rate. Successful chemical milling depends on maskant with adequate adhesion to prevent undercut, continuous film formation to prevent pinhole attack, and chemical resistance to survive the full etch cycle. Scribing precision and maskant adhesion together determine the dimensional accuracy of the final chemically milled part.
Visit www.incurelab.com for more information.