Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published MDDI July 2005

Precision Technology

As devices become more complex, many manufacturers are turning to precision technologies to meet their requirements. However, not all precision processes are created equal.

Rick Meier

Very little in life is as simple as it seems. Anyone who has attempted a simple repair job around the house has seen how what was planned as a 10-minute job can turn into an hour-long ordeal. In the same way, various precision technologies may seem easy to grasp on the surface, but each has its own particular complexities and pitfalls.

Medical instruments and medical electronics are being increasingly miniaturized for applications such as minimally invasive surgery, microsurgery, and implantable devices. As products have gone from small to micro-sized, progressively higher precision has been required. Component tolerances have been reduced from being expressed in hundredths, to thousandths, to ten-thousandths of an inch.

Precise dimensional control of critical components is important for proper device performance. Also important, to ensure a trouble-free assembly process, is consistent geometry with minimal variability from lot to lot. If components from different manufacturers, or even the same manufacturer, do not align or mate accurately, the result can be assembly line fallout, slowdowns, or shutdowns—and even product field failures.

This article discusses the advantages and trade-offs of several precision technologies. It is important to keep in mind that there is more to each technology than anyone could cover in the space of one article. Therefore, this article's scope is limited to helping product development teams narrow their choices. The final decision can be made only by talking with companies that offer the technology and that are experts in its executional phase. Such experts can make sure a 10-minute job stays a 10-minute job. The precision technologies examined here include the following:

• High-speed computer numerically controlled (CNC) milling.
• Electrical-discharge machining (EDM).
• Metal-injection molding (MIM).
• Photochemical etching.
• Precision metal stamping.
• Screw machining (also called Swiss screw machining).

High-Speed CNC Milling

CNC milling is a computer-driven process that applies various rotating cutting tools to sculpt metal according to part specifications. With advances in ultra-high-speed spindles, speeds in excess of 40,000 rpm are now attainable. The high-speed capability enables smaller cutter diameters and, therefore, more-intricate component detailing. CNC milling can achieve precise tolerances down to ±0.0002 in. on small parts. CNC machine cycle times are longer than those achieved with multicavity molds or metal stamping, but piece-part costs are still competitive in low volumes because of low start-up costs.

CNC's efficiency depends on the complexity of the part. In addition, as the number of cutting-tool changes and fixturing operations increases, the component cost increases unfavorably compared with alternatives like casting, molding, or stamping. CNC milling has high front-end capital equipment costs. A machine with the sophistication to produce precision end results can cost around $250,000. However, it can make up for this high cost under certain circumstances. For example, at low quantities, CNC milling requires only programming, cutting tools, and fixturing, thereby eliminating the need for costly molds or dies associated with other processes. On some high-volume parts with geometry requiring machining, a dedicated multi-spindle machining center may be tooled to produce just one part number or a family of similar parts. Under these circumstances, piece-part costs are closer to other processes.

Electrical-Discharge Machining

Wire electrical-discharge machining (EDM) is an extremely accurate and flexible manufacturing process. EDM is widely used to create the intricate geometry on precision dies, molds, and fixturing for the toolmaking industry. Because EDM uses an electrical arc to erode material, it exerts no cutting force on the workpiece. The type of EDM that is most widely used in producing components for the medical industry is CNC wire EDM. It involves a taut, vertical, electrically charged wire that is programmed to travel laterally through the workpiece along the exact geometry path in the x- and y-axes. Wire EDM machine controls can also slant the wire to produce tapers, cones, etc., using the u- and v- axes simultaneously and in conjunction with the x- and y-axes.

The electrode wire diameter can range from 0.001 to 0.012 in., depending on the geometric intricacies of the component. Generally, the more miniaturized the component geometry becomes, the smaller the required wire size. As the wire diameter decreases, the EDM process takes longer and the cost increases accordingly. Also, because the electrode wire must pass completely through the workpiece, blind-pocketed features are not possible. The technology can be applied to virtually any conductive material.

Since there are no cutting forces on the workpiece, fixturing costs are lower than those for CNC machining. In addition, the EDM process is very affordable for development and short-run volumes because the start-up costs are only a few hundred dollars for programming and setup. However, in this article EDM is the slowest process discussed. EDM is too costly for most high-volume programs. It is often practical to prepare a component using less-expensive processes so that the amount of metal to be removed by EDM is relatively small. Dimensional accuracy is ±0.0001 in. for tooling and ±0.0004 in. for production components.

Metal-Injection Molding

MIM processes inject liquefied metal into a mold. MIM can match the complex detailing and tolerances of machining, including different wall thicknesses within a single component. It is typically used for parts with weights that range from 220 mg to 100 g. The MIM process begins by mixing fine metal particles of approximately 5 µm in size with a binder for flow viscosity. This compound is then heated and injected into a mold to create parts in the unfinished, or green, stage. Parts are degated, then undergo debinding and sintering, where shrinkage of around 20% occurs. Part tolerances of ±0.004 in. per inch of part size are typical without further processing. Because total tooling costs are high, manufacturers typically need order quantities of 10,000 pieces or more to justify this process. Mold costs start at around $20,000 for a single-cavity mold and can reach up to $60,000 for a multicavity mold.

MIM, which can be used with many alloys, is a practical technology for many surgical instrument applications. This molding process enables raised or recessed features, as well as multiple thicknesses or tapered wall sections, within a single component. However, it is almost impossible to eliminate parting lines on the outside surfaces and voids inside the molded material. Therefore, MIM components can be brittle. If high material tensile strength is required, MIM may not be suitable for the application. Also, the material shrinks during the sintering process, creating variability from batch to batch, so MIM-produced parts may require further machining to meet tight tolerances. MIM is a good choice for applications in which the component shape is robust and requires multiple wall thicknesses with surface detail.

Photochemical Etching

The photochemical etching process can effectively create 2-D profile geometry on thin, flat parts. Essentially, a photo-tool, or mask, that is the exact shape of the component's flat blank is applied to metal. Next, the parts are processed with an etchant whereby the waste material is chemically corroded away, leaving the intended 2-D geometry on the flat component. Using this technology, the rule of thumb for tolerance capability is the thinner the better. Feature sizes can be held within ±0.001 in. or smaller when the material thickness is 0.002 in. or less. Tolerances increase as material thicknesses increase. For example, a typical tolerance needed on a hole or feature in 0.015-in. material would be ±0.003 in.

Tooling costs are low, typically a few hundred dollars for programming the phototool, versus thousands of dollars for hard tooling required for other processes. Photochemical etching can be implemented in a matter of days, rather than weeks, for molds and dies, making the process attractive for prototyping and low quantities on flat parts. In addition, the etched part will be free of burrs and residual cutting stresses. These attributes make photochemical etching the preferred technology for disk-drive components, thin lead frames, and certain flat device components. This technology requires the raw material to remain flat during processing. If the designed part geometry requires 3-D bent, embossed, or drawn features, those must be handled in subsequent processing steps.

Precision Metal Stamping

Precision metal stamping refers to metal stamping close-tolerance parts that are small, miniature, or micro-sized. Tolerances of ±0.002 in. are typical for formed features on miniature parts, and higher precision is achievable on cut profiles and hole diameters.

Prototype and short-run quantities up to a few thousand parts can be produced using hand transfer, or stage, tools costing from a few hundred dollars to $10,000. Production tooling costs ranging from $8000 to $50,000 usually require volumes in excess of 10,000 parts, or more-complex part geometry at lower volumes, to render this process cost competitive. Precision metal stamping can be applied to virtually any metal or alloy, including exotic and difficult-to-process metals and precious alloys. During the stamping process, a continuous strip of sheet metal of the desired width and thickness automatically feeds off a coil into a die that is mounted into a press. The die automatically cuts, coins, forms, and draws the strip into the desired part shape through a series of stations within one progressive die.

Piece-part intricacy and complexity dictate the number of stations needed within the progressive die. Sometimes low-volume parts with high complexity can be created more cost-effectively using precision metal stamping than with other precision technologies. This is because additional piece-part complexity is achieved simply by adding stations within the progressive tool while maintaining the same production speeds. And, since parts are produced on a continuous strip, mechanical staking can join stampings with other premade parts in the press. Integrating laser or resistance welding technologies can also join pieces. Parts can either remain on the strip to facilitate ease of assembly or be singulated at the progressive die if discrete pieces are desired.

The advantages of precision metal stamping are high production speeds and tooling longevity. It is common for a die to produce tens of millions of parts, outlasting the life of the product without major refurbishment. Some precision metal-stamping processes run at several hundred cycles per minute, enabling them to produce millions of parts at low per-piece costs. Accordingly, the technique is suitable for single-use or disposable device products.

Swiss Screw Machining

Swiss screw machining is commonly used to produce cylindrical components. The connector industry uses screw machines heavily because these machines produce tight-fitting seamless connectors that minimize adverse effects on the electronic signal. These machines are now completely automated and require minimal operator intervention. Only setup, material stocking, product inspection, and insert maintenance are required. With advances in CNC technology, setup times have dropped from approximately 4 hours to 30 minutes. And with available attachments, cross-hole drilling and machining is more practical. Cycle times are comparatively faster than those of CNC milling and slower than those of precision stamping or MIM.

Selecting the Right Precision Technology

Selecting the right technology is governed primarily by the application, costs, volume, and expected product life. For example, a low-volume product will most likely require technologies different from a high-volume product. If volume is unknown, it is often best to start with a technology with low front-end costs, such as photochemical etching or screw machining, even if it results in a higher cost per part. In other words, stick your toe in the water before taking the plunge. Then, as market acceptance and volume grow, substitute a process that provides lower per-piece costs.

Of course, the cost of the machinery, the tooling, and the component itself must be considered. Equally important are the component's material, shape, and tolerance requirements. Where there is a high selling price for the finished product and a low cost of goods, component per-piece costs are less critical. Where product performance is critical, erring on the side of component quality can be the best insurance policy. But it is also important to consider the influence of perception. Sometimes, more-expensive materials or features, and therefore precision technologies, can give marketing departments ammunition to generate more-favorable perceptions. For example, titanium is sometimes used for its perceived quality, even when stainless steel is adequate for the application.

Volume is also an important consideration. In the medical device market, most product volumes are low because there are only so many hospitals, doctors, and procedures of a certain type. Technologies with high costs per piece are often tolerated to avoid tooling costs required to achieve a lower component cost. For example, a component that costs more than a dollar when processed with CNC or EDM might be produced for less than a quarter using precision metal stamping. The progressive die to achieve this reduction might cost $30,000. As a product line grows in volume, this tooling investment becomes progressively more affordable for manufacturers to convert to the more cost-efficient process. The money saved can be dropped to the bottom line or can be used to upgrade other product features to yield competitive advantages in the marketplace.

Manufacturers should also consider a product's expected life cycle. Many innovation-intensive market segments are characterized by short life cycles as new advances quickly render products obsolete. In such segments, it is often preferable to minimize front-end tooling costs, even if the cost per piece is much higher. However, sometimes using a technology requiring tooling is the best way to meet component requirements and tolerances.

To Outsource or Not to Outsource

Companies that buy large quantities of parts from outside suppliers naturally consider buying machinery and making the parts in-house. Here, a lesson from ordinary life may apply. One idiom says that it's better to be friends with someone who owns a sailboat than to own one yourself. That way, you get to have all of the fun with none of the hassles. The same could be said for precision components. Partnering with a supplier who already has the capital equipment and trained personnel means fewer problems in-house. Partnering with a supplier means the supplier shoulders the burden of purchasing, maintaining, and upgrading the equipment; training employees to keep up with changing technologies; and all the other complexities of producing the components in-house. As a result, you have a more trouble-free operation and get all the components you need.

Conclusion

As medical instruments become increasingly smaller and more complex, they require higher precision during manufacture. Precise dimensional control, reliable assembly processes, and minimal variability are essential for such products. To fulfill these requirements, precision technologies offer many benefits to medical device manufacturers. However, it is important that manufacturers understand the advantages and trade-offs that come with each technology. High-volume parts can sometimes offset high front-end costs. However, it might be better to choose a lower-cost technology for development or short-run volumes. There is much to consider when choosing a precision technology. One of the best ways to begin the process is to talk with companies that are experts in the technologies, and narrow down the choices from there.

Copyright ©2005 Medical Device & Diagnostic Industry