Much of the proprietary science being applied to medical manufacturing today involves industrial processes that are already quite well known. Where high-tech aspects come into play are with the complex shapes being generated, the almost microscopic details being formed, and the precise accuracies being held. Examples include EDM, now applied on nano scales and laser systems that photomachine thin-film plastics, laminates, and ceramics.

Agie EDM micromachining is alive and well

Micromachining by electrical discharge machining (EDM) is well on its way to producing marketable products for the medical industry. Machining by nanotechnology is expected to soon follow. Dr. Ivano Beltrami, Head of Research at Agie SA in Losone, Switzerland, notes that EDM can be used to manufacture tiny parts with precision in the sub-µm range. Potential applications in the medical field include the machining of workpieces as well as the production of dies. Medical applications could include micro-optics, medical implants, surgical tools, especially for ophthalmic surgery, and tools for manipulating living cells.

It is possible to produce small and thin structural features because EDM imposes negligible mechanical load on the workpiece. Furthermore, materials such as tungsten carbide, hardened stainless steel, and any other conductive material are easily machined. Another feature of EDM is it produces microstructures in large quantities by means of batch processing.

Current EDM machines are based on conventional mechanical “rolling and gliding,” but engineers at Agie feel that a new generation of equipment will be based on articulated kinematic structures operating without friction or backlash. Even with today's technology, roughness obtained in micro EDM using fine wire is 0.05 µm, and in die sinking it is 0.1 µm. The impulse length controlled by the spark generator and process controller, key to smooth surfaces and high accuracy, is below 200 nanoseconds.

In initial feasibility studies, EDM produced tiny parts and geometric features for the wristwatch industry with high precision. These were mostly gears and small holes. The process has been successful in producing 2D and 3D geometries both in prototyping and small-lot production, as well as in producing tooling for mass production. A large range of structures can be machined without extensive tooling or preparation time.

Setting up the machining system and creating microelectrodes are two challenges in machining microstructures by EDM. For example, to obtain radii below 0.05 mm requires electrodes with a diameter less than that value. And drilling holes with a diameter of 20 µm requires an electrode with a diameter of 13 to 15 µm, depending upon the material of the workpiece material. Electrodes of this size cannot be bought commercially. Instead, a commercial blank electrode with an outer diameter of 0.2 mm is reduced in diameter using die-sinking EDM.

Hole diameters as small as 2 microns can be machined in stable processes with reproducible precision and quality. The aspect ratio of micro holes with a diameter of about 25 µm is routinely in the range of 15 to 18, with an absolute diameter tolerance of 5 µm. But the use of these filigree electrodes is not limited to micro hole drilling. Complex 3D structures are also obtainable. For example, micro EDM has proved to be a good method for producing micro prototype parts.

Photomachining Getting acquainted with lasers

Despite what you have seen in James Bond movies, lasers have uses far beyond their application as weapons by criminals trying to control the world. Above all, the man at the laser shop may be your “go-to guy” when you need something exceptional in the way of a manufacturing process for medical devices.

Even people in the laser business will admit that manufacturing parts by laser is not a cheap or easy process. On the other hand, lasers cut, machine, and mark in ways that no other process can. As a rule of thumb, expect manufacturing costs from lasers to be more costly than conventional processes, especially when the part calls for high precision.

“I am always leery when a customer comes to me and says he already has a working process, but is trying to cut costs,” says Ronald Schaeffer, CEO of Photomachining Inc. “I much prefer to work in applications where lasers bring added value rather than cost reduction, although the two need not be mutually exclusive.”

“In the early days of material processing by lasers, they were the biggest headache, and I spent many years getting over initial difficulties in the field,” says Schaeffer. “I had to deal with people who said they tried lasers in the past, and they did not work. Therefore they no longer considered lasers viable.”

“I don't hear that as much anymore, but lasers still carry a sort of stigma for many people who think of them as mysterious and unknown. I prefer to sell a solution to a problem that happens to use lasers, rather than making the mistake of selling the laser itself.”

Schaeffer points out that there are many different kinds of lasers all designed to address different applications. It is important to use the correct laser for the job. “Our company recognizes this, so we have many different types of lasers in-house,” he points out.

Comparing Processes: Lasers compete at least tangentially with a number of conventional manufacturing processes. They compete, for example, with mechanical drilling in the production of small holes. There are mechanical drills that can generate holes down to about 2 mils in diameter. However, as a general rule, small drills are expensive and are not durable. So laser drilling becomes affordable at about the level of 10-mil diameters.

Above this level, mechanical bits work well and are economical. Below this level, lasers become more economical, and show even greater advantage as features get smaller. Mechanical drilling can produce high aspect ratios as well as taper-free holes, but the downside is tool wear and possible damage from contact with the workpiece.

Chemical etching, because it is a batch process, is a good way to make a multitude of holes cheaply. For instance, to etch either one hole or 100,000 holes in a workpiece basically takes the same amount of time. On the other hand, lasers have to do work sequentially, so drilling 100,000 holes takes 100,000 times longer than drilling one.

But a drawback of chemical etching is it requires caustic fluids that have to be disposed of properly. Minimum feature size is on the order of 100 microns, although some shops can go smaller. On the other hand, there is a concern about aspect ratios, which generally are limited to about 1:1 or maybe 1.5:1. Therefore, if it is necessary to etch a 50-micron hole, the workpiece can only be about 50 micron thick, or thinner. And there will always be taper and perhaps undercutting.

EDM is another competitive process to lasers and a good way to make parts accurately and at low cost. But the workpiece must be conductive, and that eliminates a lot of potential applications in the medical field.

There are hundreds of laser job shops in the U. S., but most of these are involved in conventional metalworking, cutting, drilling, welding, and so forth. It is said that there are few involved in what is termed high-precision work. These are applications where the feature size is almost always less than 1mm, and usually much less, and where material thickness is likewise less than 1 mm, and usually much less. Also, feature resolution in high-precision work is normally on the order of a few mils or less, and positional tolerances are on the order of fractions of a mil.

Types of Lasers: There are three major types: CO₂ lasers, Nd:YAG variety, in which the frequency is either fundamental, doubled, tripled or quadrupled, and UV versions. CO₂ lasers provide the deepest penetration depth and fastest material removal, but they allow the least control. The resulting process is thermal, and the beam spot size is limited to 75 to 100 microns. YAG lasers with fundamental or doubled frequency allow fast material removal, but have undesirable thermal effects. YAG lasers at the fundamental frequency operate in the infrared. UV lasers, which include excimer and well as tripled and quadrupled YAG, are the slowest but provide the best quality. UV types eliminate or minimize thermal effects because cuts are made by breaking chemical bonds. UV provides the most shallow penetration and clean ablation.

In general, CO₂ lasers are the least expensive to own and operate and also easiest to operate. The expense and processing difficulty increase with moves toward and into the UV. CO₂ lasers are also efficient and robust.

Schaeffer suggests a checklist that begins by asking if the job can be done by any other method than lasers. If so, the benefits of using a laser must be reviewed carefully because they may not be justified. If it is decided that a laser is to be used, a CO₂ laser should be the first one considered. If suitable, it should be used because it will run the job with least cost and highest speed.

If a CO₂ laser is not acceptable, consider an Nd:YAG laser. If this type is not suitable, then consider an excimer laser. It is the laser of last resort because it is the most costly and difficult to operate.

Insofar as costs are concerned, a rule of thumb is that processing with a YAG laser is two to five times more expensive than with a CO₂ laser, and an excimer is 10 to 50 times more expensive than using a CO₂ laser.

One advantage of the excimer laser is that parts can be masked. So if a part needs a lot of closely spaced exceedingly small holes, the excimer might be more cost-effective.

The major uses of UV lasers in medical applications are the machining of drug-delivery orifices in plastic parts, use with thin-film plastics, micromachining, and the machining of laminates and ceramics.