These miniature parts, micromolded by Rapidwerks, are used in a vascular device as a high-speed bearing surface
Miniaturization continues to be among the most prominent trends in the medical device industry. Driving this ubiquitous trend are several factors, including the rising popularity of minimally invasive surgical techniques, the shift to portable and handheld diagnostic equipment, and a demand for more-comfortable and less-obtrusive implants. "Every field is driving toward this [trend]--and at a fast pace," observes Scott Herbert, president of Rapidwerks
(Pleasanton, CA; www.rapidwerks.com
), a company dedicated to micromolding.
But as medical devices continue to shrink, conventional manufacturing methods and equipment are often unable to produce the necessary micron-sized parts and submicron feature sizes. In order to fabricate these smaller form factors and minuscule feature sizes, medical device development is increasingly requiring a level of precision currently unrivaled by micromanufacturing equipment and service providers. That's why, through specially developed processes and equipment, micromanufacturing is making the fabrication of miniature parts no big deal.
When the Conventional Just Won't Cut It
One of the most common misconceptions by medical device OEMs, according to suppliers, is that the conventional equipment employed in "macro" manufacturing can also be used for micromanufacturing operations. Manufacturers that attempt such a practice are in for a rude awakening, especially when it comes to micromolding, according to Herbert. "You just can't apply the same principles to injection molding that you apply to micromolding. [People have] tried to micromold on a conventional injection machine and it creates a lot of problems," he adds.
Problems arise because of the extreme precision required to mold miniature parts and features, for which the use of conventional equipment is typically not practical. Micromolding, for example, is often defined by those in the industry as the process of molding components that weigh just fractions of a gram. Donna Bibber, who is active in various facets of micromanufacturing through her positions as the president and CEO of Micro Engineering Solutions LLC
(Charlton City, MA; www.microengineeringsolutions.com
) and a technical partner for microPep
(East Providence, RI; www.micropep.com
), further characterizes a micromolded part as also having a wall thickness between 125 and 250 µm and geometry that is viewed through a microscope. Micromolding can therefore also encompass a standard-size part that requires microsize features.
Although molding microsize parts by conventional means is possible, it's not advised by most micromanufacturing experts. First off, molding dust-speck-sized parts on a standard injection molding machine doesn't allow for a proper fill of the mold cavity, emphasizes Bibber. She attributes this drawback to the substantial pressure loss for the material during processing.
Micromachining tools, like those by Kern, provide the accuracy needed to consistently produce miniature features.
When attempting to mold a microsize part in a standard injection molding machine, the material is also sensitive to time spent sitting idle in the screw, Herbert adds. "Typically, by using a bigger machine, you degrade the material over time while you're waiting for these tiny components to be molded. What that equates to is you may get a molded part, but it may be brittle--and that's a problem," he explains. "In contrast, with microprocessors, they dose and expend every shot, so there's no resonance time for the material." Common problems can also include inconsistent part weights, inconsistent shots, yield issues, and parting line flash.
Micromachining operations should not be performed on standard machining equipment for similar reasons. "Basically, it's like changing a tire with a pair of pliers," muses Gary Zurek, president of Kern Precision Inc.
(Webster, MA; www.kernprecision.com
), a manufacturer of machine tools for various applications, including micromachining. "You'll eventually get the tire off with the pliers, but you'll certainly experience a lot of frustration and the lug nuts won't look too good once you have the tire off."
Ultimately, molding a miniature part with conventional equipment could compromise quality--and that's a risk most manufacturers shouldn't take.
The Right Tools for the Job
Because macromanufacturing equipment may not be up to the task of successfully performing micromanufacturing operations, OEMs need to adapt. To do so, they should seek out specialized equipment designed specifically for these precise processes, or suppliers outfitted with such equipment that can cater to these niche needs.
"Many devices and components require small, precise cutting tools and require a significant amount of accuracy from the machine tool to produce miniature features consistently," notes Zurek.
In the past, Zurek states, OEMs have often turned to electrical discharge machining (EDM) to create miniature features because they believed that milling them was impossible. As micromanufacturing has moved into the mainstream, however, multitasking machines have surfaced that, in some cases, obviate the need for EDM to produce extremely small features. Instead, many features can now be directly machined using micromilling equipment that combines high-speed machining, hard milling, and micromanufacturing capabilities all on one platform.
"In the micromanufacturing world and with the proper machine tool and system, what was thought to be impossible to do is no longer the case," Zurek says. "Most companies think holding ± 1 µm in the z-axis over a lengthy period is impossible to do on a machine with a high-speed spindle due to spindle growth--not so for Kern." The company supplies four models of equipment to the medical device industry that accommodate micromachining needs. Its Evo 25 and 44 models, for instance, can achieve part accuracy of ±2 µm and surface finishes of less than 100-nm Ra.
Equipment and tooling designed specifically for micromachining, such as Kern's, have solved a host of problems in an emerging area for device manufacturers. But their fabrication and design have also created new challenges for micromachining companies. Service providers such as Micro Engineering Solutions face the ongoing challenge of developing an end mill that is not only thinner than a hair, but also needs to be able to endure the intense machining process and produce the desired feature. "The level of accuracy to build tooling to tenths of thousandths of an inch is vastly different than building to thousandths of an inch," Bibber says.
Precision tooling must also undergo a rigorous design and development process for micromolding operations, for which it is essentially the enabling factor. With the tool playing such an integral role in micromolding, some service providers don't leave its manufacture up to chance. Rapidwerks, for one, relies on mold-flow analysis to assist in the verification and validation of high-quality tool designs. By employing the simulation technique, the company is able to minimize the need for potential modifications after the tool has been made. "Mold-flow analysis allows us to see how the material will flow into the tool and how the part will be molded," Herbert explains. "It will show hot spots, sinks, or voids--it helps in the process of making a good tool from the beginning." Getting it right the first time helps to prevent delays downstream, he adds.
A PEEK implant is machined with a 0.001-in. end mill to achieve 0.0005-in. radii.
IMAGE COURTESY OF MES
Micromanufacturing techniques are not only broadening the horizons of medical device design in terms of size, they're also opening doors to the expanded use of high-performance materials. While standard, off-the-shelf materials have maintained a strong presence in micromanufactured devices, high-performance materials such as PEEK have found growing application. Perhaps the most significant impact that expanding micromanufacturing capabilities have had on material selection is that they have allowed OEMs to design with desirable materials that may have previously been deemed cost-prohibitive.
Falling into this category are absorbable polymers. Polylactic acid (PLA) and polyglycolic acid (PGA) are biocompatible, biodegradable polymers that are increasingly being used in a number of medical applications, including orthopedic devices. The materials degrade after they have served their purpose in vivo and are then safely absorbed by the body.
Limiting their use, however, has been their hefty price tag, which has been the main deterrent for many manufacturers. Absorbable polymers can cost between $3000 and $20,000 per pound, which can induce sticker shock in OEMs accustomed to the $1- to $2-per-pound cost associated with typical thermoplastics, according to Bibber. But because micromolding processes require and, subsequently, waste such nominal amounts of material compared with their macro counterparts, absorbable polymers can now be implemented in device designs without blowing a project's budget.
Along with making some materials more affordable, micromolding is also motivating the development of custom metal compounds as its popularity continues to swell. "In terms of metal injection molding (MIM), there are now powders and compounds developed that are much, much smaller in size that allow you to fill places that are 5 µm in size, while in conventional MIM, you can only get to 0.020-in. wall thickness," Bibber notes.
"Every part that we come across is a challenge, and it's not always from a material point of view in terms of pushing the limits of the material, but in trying to mold such thin cross sections," Herbert adds. "Ten years ago, we wouldn't be having this conversation. But today, with the aid of mold flow and the ability to control and meter material in a very precise manner, being able to fill thin-wall sections is something that can be done."
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