There’s more than one way to skin a cat. Likewise, in medical device manufacturing, there’s more than one type of equipment for machining parts and devices. While traditional computer numerical control (CNC) machining is versatile and precise, nontraditional technologies, including laser and abrasive waterjet machining, also offer a host of advantages.
A noncontact processing method, laser machining can process a range of different materials and promote material integrity. And, based on the use of water, abrasive waterjet machining helps materials keep cool and prevents damage to workpieces.
However, the devil’s ultimately in the details; the best choice of equipment depends on the application in question and the type of material from which it is fashioned.
Machine by Numbers
CNC machines take a piece of raw material and machine it into the final dimensions of shape, finish, and size, so it replicates whatever the design engineer has created, says Ted Driggs, project manager at Okuma America Corp. (Charlotte, NC). The design itself is usually created using computer-aided design (CAD), which generates data that instruct the machine how the cutting tool should traverse a piece of raw material to create a medical device component. “The reason for using a CNC machine is that we can replicate the same part time after time with little or no human intervention,” Driggs adds.
|An acetabular stem in various stages of processing was machined using Okuma CNC equipment.
Different types of CNC machines—including milling machines, lathes, and grinders—perform different functions. For example, in hip-replacement components, the acetabular stem can be machined using a vertical machining center or a multitasking machine. Depending on the component, the appropriate equipment may be able to perform turning and milling at the same time.
Real estate is another consideration for medical device manufacturers concerned about saving space on the shop floor. “The flexibility to perform multiple tasks with minimal human intervention is the chief consideration that comes into play when a medical device manufacturer selects machining equipment,” Driggs says. This task is accomplished using computers. Going back to the early 1950s, CNC machines were connected to huge external computers. But today, they incorporate onboard computers and PC-based controls that enable the machine to interact with the outside world.
“In the world of conventional machining—the world inhabited by CNC—a series of cutting tools removes metal by making small filings or chips—otherwise known as swarf—according to a design,” Driggs explains. “While waterjet technology can also produce the same design, this method has both advantages and disadvantages.
On the positive side, waterjet machining forgoes the use of rotational or cutting tools, instead relying on the force of pressurized water combined with an abrasive substance to create a liquid saw. On the negative side, it is difficult to control cutting depth using waterjet technology, Driggs says.
“In contrast, when making 3-D cuts, a CNC machine can hold depth in all three dimensions by means of its cutting path,” he adds.
Let There Be Laser Light
Like waterjet technology, laser machining also involves flow—the flow of light through a series of lenses. Laser machining is used in medical device manufacturing to trim workpieces, cut holes in metal, and process other structures in medical device parts. It also enables users to control depth by adjusting the speed and concentration of the light.
|A stent processed using athermal laser machining does not produce a heat-affected zone because the technology relies on the use of lasers with very short pulses.
Despite its advantages, laser light can create a heat-affected zone in the workpiece, which Driggs says, causes excitation of the molecules in the material, occasionally causing them to lose their bonds with other molecules and potentially resulting in material weaknesses.
Sascha Weiler, program manager for microprocessing at Trumpf (Farmington, CT), also notes that some laser technologies don’t create heat-affected zones. “There are two ways to use lasers to separate materials. The state-of-the-art laser technology known as fusion cutting, a technology used exclusively to process metals, fashions workpieces by melting the metal. However, another laser machining technique used for cutting material uses ultrafast picosecond or femtosecond pulsed lasers. Featuring a very high peak pulse, this technology does not melt the material but directly vaporizes it. Resulting surface finishes can measure below one micron.”
Using ultrafast pulsed-laser technology does not result in heat input, which is particularly advantageous for processing nonmetals such as polymers. “Customers use our ultrafast pulsed-laser technology to cut bioresorbable stents composed of a special hygroscopic polymer that is not only very sensitive to water but also to heat,” Weiler says. “There is no other way to economically machine this type of material than to use an ultrafast pulsed laser.”
A major advantage of laser machining is that it does not physically make contact with the workpiece, in contrast to CNC machining. “Because it is a noncontact technology, laser machining does not cause material wear,” Weiler says. “In CNC machining, the tool is subject to wear and must be replaced periodically.” And in waterjet technology, water contamination can take place, causing hygroscopic materials to dissolve. However, this problem can be avoided if the abrasive cryogenic jet incorporates liquid nitrogen as the working fluid, which substantially increases complexity and operating costs.
Two trends expected in the medical device industry in the next several years will impact the laser machining field. One trend is the shift toward alternative materials for use in stents. In such applications, manufacturers are beginning to use polymers and composites in place of metals because of the functional advantages they offer.
Miniaturization, the second trend, will continue in medical device manufacturing for years to come, Weiler predicts. However, miniaturization raises machining issues. As device features get smaller and materials become thinner, the heat effect associated with laser processing is a concern. In response, athermal, or cold, laser processing is coming into vogue. Using lasers with short pulses in the pico- or femtosecond range, athermal laser machining processes materials without leaving a heat-affected zone.
“Combined with the use of new materials, including both metal and nonmetal foils, miniaturization will render many of today’s state-of-the art tools obsolete,” Weiler says. “Thus, the industry will have to turn to alternative methods such as athermal processing to deal with different classes of materials. As an alternative method, athermal processing can be used to process any type of material, making it a good candidate for satisfying the future demands of the medical device manufacturing sector.”
Oh, the Water
For machining medical devices and components, abrasive waterjet technology offers several advantages over other methods, says Peter Liu, senior scientist at Omax Corp. (Kent, WA). A versatile machining technique, it can process most materials from the macro to the micro scale, including both reflective and nonreflective metals and such nonmetals as composites, plastics, and ceramics. It can also process laminates. Unlike laser machining, waterjet processing is a cold-cutting process that doesn’t generate a heat-affected zone. In addition, a single waterjet machine can perform an array of processing functions, including cutting/parting, drilling, turning, milling, grooving, and surface preparation without the use of special tooling, as does CNC machining.
|A 0.4-mm-thick titanium bone graft tube with 120 pierces was machined using abrasive waterjet technology.
“For most applications, abrasive waterjet machining offers ample precision and high performance,” Liu says. “For example, the surface finish of an edge cut using very fine abrasives can be as small as several microns.” In contrast, ultrafast lasers can achieve surface finishes in the submicron range, according to Weiler.
Waterjets can also be used as net-shaping tools to produce precise parts from difficult-to-machine materials, such as stainless steel. “CNCs usually cut steel parts in an annealed state, and then the parts are heat treated to achieve the desired hardness,” Liu observes. “However, this method distorts the finished part.” In contrast, waterjets can cut parts in the hardened state, preventing the distortion that results when they are thermally treated after they are formed. Conventional CNC tooling can suffer from wear when it processes abrasive materials such as titanium and hardened steel, which can eventually burn workpiece surfaces. Thus, when used with CNC machines, waterjet equipment can help prolong the life of CNC tools.
An advantage of the latest machining technologies is they can be used for making orthopedic or prosthetic devices tailored to a patient’s specific size and shape. Like other systems based on computer numerical control, waterjets can design and finish such parts in minutes or hours, depending on their complexity. Future hospitals, rehabilitation centers, and medical facilities in remote locations and battlefields may have on-site waterjet manufacturing equipment to produce specialized orthopedic devices on the spot.
“Thirty years ago, waterjets were only good for rough cutting operations,” Liu notes. “Today, waterjet cutting systems, together with high-precision motion control hardware and 3-D kinematic mechanisms, are fully capable of meeting most of the demands of the medical device industry. Once manufacturers recognize this capability, they will adopt waterjets for use in R&D, prototyping, and production applications.”
Bob Michaels is senior technical editor at UBM Canon.