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Precision Technology As It Applies To Producing Medical Device Components
By Rick Meier, founder of Meier Tool & Engineering Inc.
Lessons learned in ordinary life can often apply to what we are trying to accomplish in our professional lives. Anyone who has attempted a seemingly simple repair job around the house has experienced how a 10-minute job can turn into an hour-long ordeal. In the same way the various precision technologies I will discuss in this article may seem easy to grasp on the surface. But each has its own peculiar complexities and pitfalls in the executional phase.
As in many other industries, medical instruments and medical electronics are becoming increasingly miniaturized for applications such as minimally-invasive surgery, microsurgery, and inplantable procedures. As products go from small to miniature to micro-sized, progressively higher precision is required. Component tolerances reduce from being expressed in +/- hundredths, to thousandths, to ten thousandths of an inch.
It goes without saying that precise dimensional control of critical components is important for proper device performance where life hangs in the balance. Consistent geometry with minimal variability from lot to lot is also important for a trouble-free assembly process. If components from different manufacturers or even the same manufacturer do not mate accurately, the result can be line fallout, line slow-downs, shutdowns, and possibly field failures.
This article will discuss several precision technologies and their overall advantages and trade-offs. There is a lot more to each of these technologies than anyone could cover in the scope of an article such as this. You could spend several years becoming an expert in just one technology. The scope is therefore limited to helping the product development team narrow down the choices. The final decision can only be made by talking with companies who offer the technology and who have become experts in all the details of the executional phase -- experts in making sure a 10 minute job stays a 10 minute job.
The precision technologies I will cover are:
- High Speed CNC Milling
- Electrical Discharge Machining (EDM)
- Metal Injection Molding (MIM)
- Photochemical Etching
- Precision Metal Stamping
- Screw Machining (also called Swiss Screw Machining)
I will start by overviews of each technology, then discuss how various factors help decide which technology is best for your application.
High Speed CNC Milling
Computer Numerically Controlled (CNC) milling is a computer driven process that applies various rotating cutting tools to "sculpt" the metal according to part specifications. With advances in the newly developed ultra high speed spindles, RPMs in excess of 40,000 is now practical. The higher RPM allows smaller cutter diameters and therefore more intricate component detailing. It can achieve very precise tolerances, down to +/- .0002" on small parts. Machine cycle times are longer than multi-cavity molds or metal stamping, but piece part costs are still competitive in low volumes. Its efficiency depends on the complexity of the part and as the number of cutting tool-changes and fixturing operations increases, the component cost increases unfavorably relative to alternatives such as casting, molding, or stamping. CNC milling has high front-end equipment costs, but it can make up for this under certain circumstances, especially low quantities, because tooling is minimal, usually requiring only special cutting tools and part fixturing. On some extremely high volume parts, a dedicated multi-spindle machining center may be tooled to produce just one part number or a family of similar parts at piece costs similar to other highly efficient processes.
Electrical Discharge Machining (EDM)
Wire EDM is an extremely accurate and flexible manufacturing process that's widely used to precisely create the intricate geometry on precision dies, molds, and fixturing for the toolmaking industry. Because EDM uses an electrical arc to erode material, there is actually no cutting force on the work piece. The EDM type most widely used in producing production components for the medical industry is CNC Wire EDM. This type of machining 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 are also capable of "slanting" the wire to produce tapers, cones, etc. using the U and V axis simultaneously and in conjunction with X and Y. This sounds complicated but a skilled programmer makes it easy! The electrode wire diameter can range from .001" to .012" depending on the geometric intricacies of the component. Generally, the more miniaturized the component geometry becomes, the smaller the wire size is required. As the wire diameter decreases, the EDM eroding speed takes longer, and the cost increases accordingly. Because the electrode wire must pass completely through the workpiece, blind-pocketed features are not possible. The technology can be applied to virtually any material as long as it is conductive. Since the cutting forces on the work piece are non-existent, fixturing costs are lower than CNC machining. Because of low start-up costs, the EDM process is very affordable for development and short run volumes, but is usually too slow and costly for high volume programs. Often times it is practical to design a component so that the amount of metal to be removed by EDM is relatively small. Dimensional accuracy for tooling is + - .0001" and on production components is +/- 0.0004".
Metal Injection Molding (MIM)
This process injects liquefied metal into a mold. It can match the complex detailing and tolerances of machining, including different wall thickness within a single component. Its tooling costs, however, are the highest among the precision technologies discussed here, so high volumes are required to justify it. It can fulfill high volume requirements simply by adding more cavities to the mold.
MIM is available in many alloys and is a practical technology for many surgical instrument applications including distal end components. This molding process allows raised or recessed features as well as multiple-thickness or tapered wall sections within a single component. Since it is almost impossible to eliminate parting lines on the outside surface and voids inside the molded material, MIM components can be brittle which could rule them out where high material tensile strength is required. Because the material shrinks during the sintering process, there is variability from batch to batch, and MIM parts may require further machining to meet tight tolerances. The strong suit for MIM is in applications where the component shape is more robust and requires multiple wall thicknesses with surface detail.
Photochemical Etching
The photochemical etching process can be used to effectively create the 2-D profile geometry on thin flat parts. In simplified steps, a phototool (mask) that's 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 flat component 2-D geometry. The rule of thumb for tolerance capability using this technology is "the thinner the better". Feature sizes can be held within + - .001" or better when the material thickness is .002" or less. Tolerances must be increased as material thickness increases. For example, a typical tolerance needed on a hole or feature in .015" material would be + - .003".
Even though piece part costs are relatively high, tooling costs are low, often times only amounting to a couple hundred dollars' phototool charge after your CAD geometry is supplied. Throughout the prototyping phase of a program, photochemical etching can be quicker and less costly then purchasing the “hard" tooling required for other processes. Another advantage is that 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 some flat medical device components. This technology requires the raw material to remain flat during processing and if the designed part geometry requires 3-D bent, embossed, or drawn features, they must be handled in subsequent processing steps.
Precision Metal Stamping
Precision metal stamping refers to metal stamping of close-tolerance parts that are small, miniature, and micro-sized. A continuous strip of sheet metal of the desired width and thickness is automatically fed from a coil into a die. The strip is automatically cut, coined, formed, and/or drawn into the desired part shape through a series of operations that take place at stations within a tool that is referred to as a “progressive die". Part complexity dictates the number of stations needed within the progressive die. The stampings can be joined with other parts in the press by mechanical staking, or by integrating laser or resistance welding technologies. The unique advantage of precision metal stamping is speed and low tooling maintenance costs. Some high-speed precision metal stamping processes run at several hundred cycles per minute, enabling them to produce parts in the millions at competitively low costs per piece. Accordingly, it offers competitive advantages in single use or disposable device markets.
Sometimes lower volume parts with a great deal of complexity can be precision metal stamped more cost effectively than machining. The up front tooling costs require higher volume or high part complexity to render this process cost competitive. Precision metal stamping can be applied to virtually any metal or alloy, including difficult and exotic metals, and precious alloys.
Screw Machining (also called Swiss Screw Machining)
Screw machining is commonly used to produce cylindrical components. The connector industry utilizes screw machines heavily because they produce tight-fitting seamless connectors that minimize adverse effects on the electronic signal. Screw machines have developed into completely automated machines and require no operator intervention aside from set-up, material stocking, product inspection, and insert maintenance. With the advancement of CNC technology instead of mechanical cams, setup times have dropped from around four hours to 30 minutes. However, cycle times are comparatively slower and product costs higher than that of processes with faster cycle times.
Selecting the Right Precision Technology
Selecting the right technology to achieve precision tolerances is governed primarily by the application, the costs, volume, and expected product life. For example, the right choice for a low volume product is not necessarily the right choice as the volume increases. If volume is unknown, it is often best to start with a technology with low front-end costs, even if it has a higher cost per part. The lesson from ordinary life is to stick your toe in the water before taking the plunge. Then, as market acceptance and volume grows, lower cost process may be substituted.
The Application
It is often helpful for product development team members to observe how doctors use the instruments in the operating room. You can view videos, but there is no substitute for being there. Different surgeons have different preferences for the instrument's tactile feel, spring properties, flexibility, and maneuverability, to name a few. By observing these things first hand, a number of ergonomic issues come to light, and the experience might spark ideas to make the procedure easier to perform or safer. Often different models of the same device are manufactured to meet different doctor preferences.
The Costs
Costs of the machinery, tooling, and the component must be considered in choosing the right technology, as well as the component's material, shape, and tolerance requirements. Where there is a high selling price and a low cost of goods, component per-piece costs are less critical. Where performance is critical, erring on the side of quality is the best insurance policy. You also must consider the influence of perception. Sometimes materials or features that are more expensive than necessary can give marketing ammunition to generate more favorable perceptions.
Volume
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 favored over more high volume technologies because of front-end costs in machinery and tooling. As volume in a product line grows, however, more efficient technologies can often replace more costly parts while recouping the tooling investment. The money saved can be dropped to the bottom line, or used to upgrade other product features to yield competitive advantages in the marketplace.
Expected Product Life
Many innovation-intensive market segments are characterized by short life cycles as new advances render products obsolete in short time frames. In such segments, it is often preferable to minimize front-end costs in tooling even if the cost per piece is much higher. As mentioned earlier, however, sometimes a technology requiring tooling is the best way to meet component requirements and tolerances.
Make or Buy?
Companies who are buying large quantities of parts from outside suppliers naturally consider buying the machinery and making the parts in house. Another lesson from ordinary life may apply. In Minnesota, the Land of 10,000 Lakes, the prevailing wisdom is 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. If you partner with a supplier who already has the capital equipment and trained personnel, you get all the parts you need and the outside supplier gets all the headaches. In addition, with increasingly rapid innovation, you never know if today's expensive investment will in a short time become tomorrow's "boat anchor".
The Limitations of Generalizations
I'm sure many of my colleagues in the industry who offer the technologies discussed above will feel I shortchanged them on the advantage side of the equation, and overstated the shortcomings. The fact is, once a company invests a great deal of monetary and human capital in a particular technology, it's amazing what they can make that technology do. That's why I said at the beginning that you should not use this information to make the call, but to narrow down the options. Only after entering into discussions with suppliers will one technology emerge as the clear winner in fulfilling your product's component requirements.
Happy sailing.
Meier Tool & Engineering, Inc. was founded by Rick Meier in 1979, and has been producing precision metal stampings that are small, miniature, and micro-sized for companies whose new product activities require it. Your comments or questions are welcome. Email rick_meier@meiertool.com