The chemical milling of medical parts often gets overshadowed by the high-tech aura of more recent methods, such as wire EDM and laser cutting. However, the 50-year old chemical milling process has been developed to the point where many designers are unaware of what it can do.

For instance, engineers at Tech-Etch Inc., Plymouth, Mass.(tech-etch.com) recently received a drawing for a board-level shield. The drawing defined the part along with a secondary printing operation for part numbers and positioning marks. Engineers at the chemical-milling company recognized that the information could be etched onto the part at no additional cost, and suggested doing so. The client liked the idea. Adopting it let them cut time, a step, and cost off their production cycle.

Etching with photos

Chemical milling and photo etching are the same process. In a nutshell, a photograph of a flattened part masks material in a chemical bath that dissolves or removes unneeded metal. “Photo etching produces intricate metal components with close tolerances that are difficult to duplicate by other methods,” says Tech-Etch president George Keeler. “It works well for components such as filters, flat springs, fuel-cell plates, heat sinks, electron grids, fluidic circuit plates, reticles, and haptics,” he says.

Photoetching is a mass or batch process, so many small parts are made on a sheet, like postage stamps. They are separated by shearing a row of tabs. Laser and wire EDM, on the other hand, are one-at-a-time processes. Thus, costs are lower for photoetching. Moreover, photoetching leaves no edge burrs or localized heat effects as do laser or wire EDM.

Recent improvements to the process let it etch non-metals such as polyimide films. Features can be as small as 0.002 in. on thin material. Partial etching or chemical milling of bend lines can precisely locate forming guides. And newer chemistries can etch corrosion-resistant aerospace and biocompatible metals such as tungsten, Nitinol, niobium, and titanium. A few of the advantages of chemical milling and what designers should know about it include:

Design flexibility. This means engineers are not locked into expensive tooling. “This is especially true when compared to stamping,” says Keeler. Design changes to stamped parts require modifying a hard tool. Wire EDM, laser, and photoetching just need a simple CAD change to reflect the new part.

Photo etching eliminates expensive hard tooling. “Tooling for a typical photoetched part is only $200 to $300 versus $10,000 to $25,000 for stamping tools and dies,” says Keeler. “Even when conventional dies are slated for production runs, photo etching is often used for prototypes due to its cost and time advantage,” he adds. Keeler points out that wire EDM and lasers have the same quick-change ability. “Etching can make several prototype samples of a redesign quickly since it is a mass process.”

Shorter lead times. Tooling is simply a CAD drawing transferred to film, so tooling time is measured in hours. Prototype quantities are available in one to five days.

Chemical milling works with a wide range of thin materials. Most alloys can be photoetched without concern for edge effects common to lasers, EDM, and stamping.

Etching a different image from both sides make it possible to produce cone shaped holes, bend lines, channels, and pockets.

Post processing

No single manufacturing technique is likely to produce a complete product. So combining etching and forming with laser cutting generates thick parts with grooves. This combination of manufacturing technologies also needs no special hard tooling, and it's repeatable and accurate. Plating materials include gold, nickel, electroless nickel, copper, tin, and tin-lead plating, plus solder hot oil reflow.

Keeler says his company has photoetched precision polyimide haptics for intraocular lenses used in cataract surgery and other corrective implants. Polyimide offers flexibility similar to polypropylene and PMMA, and has greater tensile strength and better shape memory. Polyimides are safe for implants, so additional medical applications can come from its shape-retention spring.

Specialty materials have characteristics useful in implants, springs, cathodes, blades, and stents. “Photoetching and precision forming dies can manufacture prototype quantities in large production runs without the high cost and long lead times of hard tools,” says Keeler

More than flat parts

Electrochemical micromachining differs from photoetching in that it uses an electric current to direct metal away from where it's not wanted. One proprietary method sharpens a range of medical needles and tissue cutters. It also creates smooth surfaces and intricate shapes in thin-wall tubing. What designers should know about this process include:

Any size tube can be sharpened.

There are no grind or tool marks to act as stress concentrators or hiding places for bacteria. “Taking off the top layer by electro-polishing removes induced stresses and surface imperfections,” says John O'Brien, Vice President of Point Technologies Inc., Boulder, Colo.
(pointtech.com).

The process works well with small parts. “Everything in medical is getting smaller,” says O'Brien. For example, feature position and locations are controlled by phototools and are within 0.0001 in. Material thickness can be as small as 0.0005 in. “Electrochemical pointing or etching makes features at submicron scales, which is nano in nature,” he adds.

The company manufactures many small-diameter items including needles in several point styles with 0.010 to 0.095-in. shank diameters. Custom-point styles from submicron points to precision radius tips can also be formed. All items are polished and passivated. “Wire diameters down to 0.001 in. can be pointed or radiused.”

The sharpening method also works on tissue cutters such as biopsy punches and aortic cutters for procedures such as coronary-artery-bypass grafting. It also deburrs and polishes hypodermic needles. Micro-biopsy punches and microtrephines from tubing 0.004 to 0.175 in. ODs are made by selective tube reduction. The technique does not deform the ID, and any size tube and wall can be machined without stress or compressing the walls.

“Tube IDs can also be electrochemically micromachined, removing draw lines and oxide particles impregnated during drawing,” says Clive James, Director of Business Development at Point Technologies. “The process provides a clean and conforming surface that allows further tube drawing to reduce waste and extend tool life,” he adds. “This is especially true for Nitinol.”

Opposite to electrochemical micromachining is material deposition or plating. Entire parts and accurate discrete lengths can be plated. The process allows adding biocompatible metals to critical medical components.

In addition, the company's electrochemical-sharpening puts sub-micron sharp edges on any size tubing. Prototypes are available with materials from 0.0025-in. ID or 0.00425-in. OD and larger. O'Brien says specialty tubing comes in many shapes, sizes, and materials with polished IDs and chamfered or tapered ends. Special materials include super-elastic Nitinol tubing, MP35N, and tungsten.

A short guide to etchable materials

The following materials are a few of those that etch well and have useful properties in medical devices.

Tungsten and molybdenum are difficult-to-etch refractory materials used for high-temperature, corrosion-resistant applications.

Titanium is strong, light weight, and highly resistant to corrosion. Its strength is comparable to 304 stainless steel and it is used for human implants.

Elgiloy is highly corrosion-resistant material with high fatigue strength. It is used for implants.

Polyimide film has good physical, chemical, and electrical properties over a wide temperature range. Its electrical and chemical resistance are excellent even at unusually high temperatures.

Niobium (or Columbium) is a light weight refractive material with excellent high temperature corrosion resistance. It is ductile and easily formed.

Nitinol, a shape memory alloy, has an unusual characteristic that lets it return to a predetermined shape after deformation. The material has excellent biocompatibility, good spring characteristics, and high-corrosion resistance.

Designing initially flat parts

Thickness is key to defining dimensions and tolerances in a selected material. The generic part shows details and limitations for spaces, holes, slots, and other dimensions.

Hole and slot diameters cannot be less than the metal thickness. The table Holes and slots show a more exact relationship.

The smallest inside-corner radius is proportional to metal thickness. On occasion, smaller or sharper radii are possible.

An outside corner radius should be at least 0.75 of the material thickness.

A finger or web cannot be smaller than metal thickness. The table Guidelines for webs and fingers provides a more exact relationship.

“Generally, tolerances are a result of the stability of the photo-tooling material,” says Keeler. Practical tolerances attainable from industry-standard Mylar tooling are listed in Center-to-center tolerances. Alternate tooling materials allow variations from the table figures.

How to chemically machine a part

Chemical milling, according to Fotofab Inc., Chicago (fotofab.com), starts with an drawing or sketch that precisely defines a part. CAD and laser plotting generate an exact image of the part on a set of photographic films, called a phototool. Depending on part size, the phototool may contain from one to several thousand exact images of the part. The phototool photographically transfers part images to a sheet of clean, flat metal coated on both sides with a photosensitive, etchant-resistant polymer, or photoresist. The result is a sheet of metal covered with photoresist over the metal that will become the parts. The sheet is sprayed with a heated etching solution that dissolves metal not covered by the photoresist. The masked parts remain.