Chemical milling, also called photochemical milling or etching, is one of the fastest, most accurate and inexpensive methods of fabricating parts for medical applications. It works on sheet metal that is 0.0005 to 0.125-in thick.

Photochemical machining provides many advantages over stamping. Phototools are significantly less expensive than traditional dies, and they make it easy and economical to quickly incorporate minor design changes. In many cases, prototype parts can be produced in less time that it would take to create a die or other standard metalworking tools.

In addition, phototools do not “wear” like traditional tools, so they last longer and are more accurate in large-volume production. Etched metal parts are burr-free, stress-free, and have no deformation. In addition, photochemical machining does not affect the magnetic properties of the metal being etched.

A wide variety of metals lend themselves to chemical milling including titanium, stainless steel, nickel, copper and copper alloys, brass, Kovar, nickel-iron alloys, beryllium copper, and zirconium. We have been etching titanium and stainless steel components for medical implantables for many years with special equipment that meets demanding applications.

Titanium and stainless steel parts are highly resistant to organic chemicals and many inorganic chemicals, provide excellent corrosion resistance, and are easily cleaned, so they can work well for scalpel knife blades as well as implantables. Other components fabricated by the process include parts for eye surgery, titanium meshes for use in maxillofacial and cranial surgery, and cathodes and anodes for implant batteries. Medical parts are shipped in sealed bags with argon to prevent contamination during transport.

How chemical milling works

Upon receiving sketches or electronic files, engineers use CAD to make a repeated pattern of the part, maximizing the number of parts that will fit on each sheet of raw metal. When necessary, the CAD operator incorporates “tabs” to hold parts together after they are chemically milled. However, parts can be often etched using screens, thereby eliminating a de-tabbing operation.

Because most parts are etched from both sides, the pattern is usually laser photo plotted onto two pieces of film. The film is processed and aligned top to bottom. Once alignment holes are punched for registration, the film is ready to make parts.

The metal sheet gets cleaned with a proprietary cleaner to remove contaminates that might interfere with the adhesion of photoresist, a light-sensitive material. A hot-roll lamination coats the sheet on both sides with a dry-film photoresist. The sheet is placed between the two pieces of film, which makes up the complete phototool. A vacuum prevents or minimizes light getting under the image while the tool is exposed to UV light to cure and harden the photoresist. A conveyor then carries the sheet into a developing solution to wash away uncured areas, leaving bare metal in those areas to be etched. The areas where photoresist remains protect underlying areas from the etching process.

An etching machine sprays chemicals onto the sheet, dissolving the unprotected areas and leaving the part profile. (Ferrous metals are etched in ferric chloride, non-ferrous metals in cupric chloride, and titanium and titanium alloys in a hydrofluoric-based etchant.) Depending on the job, an alkaline stripping solution in a tank or a conveyorized system removes the photoresist. Finishing and forming operations would follow.

Chemical milling poses a few challenges in tooling design. For example, part dimensions might need altering because the photochemical process attacks sidewalls as well. Therefore, all part features incorporate a compensation factor. For example, a hole measuring 0.010 in. in the part may need to measure 0.008 in. in the tool. After etching, the hole will meet size specifications.

Our company was the first in the photochemical machining industry to separate etching acids to maximize etching uniformity and minimize downtime. In addition, we regenerate our acids with chlorine to maintain consistent etching. The constant monitoring and documentation of free acid, temperature, and oxidation reduction potential (ORP) controls the process to ensure repeatability.

Design guide for photochemical machining

Because etching process tends to undercut at the edges of the photoresist pattern on the part surface, part dimensions, tolerances, and configurations are mostly a function of the material and the thickness of the stock being etched. A few guidelines should help expedite the design or specification of chemically machined parts.

Generally, the diameter of a hole, (D), cannot be less than the metal thickness, (T). However, this relationship varies with different metal thicknesses:

Metal thickness, in.	Smallest hole diameter, in.
0.001 to 0.005	0.004 in.
0.005 or over	At least 110% of the metal thickness

A few practical hole sizes in sample thicknesses include:

Metal thickness, in.	Hole diameter, in.	Tolerance, in.
0.0010	0.004	±0.0005
0.0050	0.006	±0.0010
0.0070	0.008	±0.0015
0.0100	0.012	±0.0020
0.0200	0.024	±0.0030

Practical features such as length and width follow the same rules as holes. When in doubt, use 1.2 × T for dimensions and 0.15 × T for tolerance.

The width of metal between holes is not a particular problem. However, when this comprises the remaining surface area in a large field of holes, there are limitations on the metal width between holes:

Metal thickness (T), in.	Space between holes (W), in.
Less than 0.005	At least the metal thickness
0.005 or over	At least 120% of the metal thickness

Other design considerations:

The smallest corner radius (R) should usually be approximately equal to the thickness of the metal.

Outside corners tend to etch more sharply than inside corners. Radii less than the metal thickness are therefore possible. As a general rule, outside radii, (r), should be at least 75% of the metal thickness.

Etchant attacks the material laterally as well as vertically, so material being etched equally from both sides produces a bevel. As a general rule, the bevel is about 10% of the metal thickness.