An automated multinozzle system is designed to process heart valves.
Implants, minimally invasive devices, diagnostic systems, and other products that interact with the body often require advanced surface treatment. However, processes such as electrochemical etching and micromechanical machining can be too costly or insufficiently targeted for all applications. Because of these drawbacks, Morton Bowen Inc. (Broomfield, CO; www.mortonbowen.com), a contract manufacturer specializing in the processing of specialty metals, sought a flexible and cost-effective surface treatment alternative. The company turned to microabrasive blasting equipment manufactured by Comco Inc. (Burbank, CA; www.comcoinc.com) as a potential solution.
Blasting through the Basics
Microabrasive blasting can be described as a highly refined and miniaturized version of sandblasting. It involves mixing small amounts of a very fine abrasive powder with a pressurized stream of air that is metered and regulated. The treatment method is designed to clean, texture, cut, and deburr very small parts and to process hard-to-reach areas.
In manual systems, an operator holds a pen-sized hand piece that directs the abrasive-and-air mixture from a nozzle, which can have an opening between 0.018 and 0.060 in. The shape, size, and hardness of the abrasive particles, as well as the air pressure and length of blast time, dictate the effect blasting will have on the material surface. Abrasive media is available in sizes ranging from 10 to 250 µm.
One key benefit of the process is that it can negotiate close-tolerance cleaning and texturing with a high degree of precision without changing the dimensional integrity of the part. The pinpoint accuracy achieved with microabrasive blasting allows an operator to process very small parts without the need for masking.
“Microabrasion is easier to use and less hazardous than many chemical processes,” says Morton Bowen president Clive James. “It makes it very easy to focus the process on the exact area required, which removes some of the limits on what we could previously accomplish.”
Putting the Technology to the Test
Texturing stainless-steel tubing enhances the adhesion of finished device assemblies.
To determine whether microabrasive blasting was a good fit for the company’s needs, Morton Bowen decided to test the process for three of its typical operations: oxide removal, surface texturing for attachment and overmolding, and conditioning for echogenicity.
Following a heat-setting operation, nitinol requires oxide removal to prepare it for the final electropolishing stage. Using 17.5- to 50-µm aluminum oxide at pressures from 40 to 90 psi, the company found that microabrasive blasting assisted in providing initial oxide removal. In turn, the electropolishing process produced a clean surface unmarred by blemishes or cracks.
Morton Bowen also tested 17.5- to 50-µm aluminum oxide abrasive media to see if the blasting process could enhance adhesion for overmolding or adding attachments to stainless-steel devices; air pressure was matched to the specific needs of each product tested. “Blasting on stainless-steel surfaces provided the ability to selectively treat even the smallest area to enhance adhesion of a coating, overmolding, or attachment,” James says.
In the third set of tests, the company employed microabrasive blasting for conditioning echogenicity, which is the ability to return a signal in ultrasound examinations and is the uniform scattering of radio frequencies to better see an implanted device. “This might [require] the roughening of a tip or a section of a stainless-steel needle to make sure it will change the sound-echo waves to show the location of the needle more clearly,” James explains. “In ultrasound diagnostic tools, the only other methods to enhance this application are to apply a coating or dissimilar metal plating or cladding to the instrument.
“[Microabrasion] can be localized to a very specific section: bands, half a diameter, inner or outer surfaces on components, whatever the customer needs,” James adds. “This is very difficult to do when you are dealing with some of the more complex electrochemical systems because you don’t have that finite control.”
Automating the Process
In order to optimize the proven technology for customers, Morton Bowen worked closely with Comco to tailor microabrasion systems to automated and semiautomated applications.
“Our typical customer will start out using a manual station. As the project advances through clinical trials, greater numbers of parts are required,” says Colin Weightman, applications engineer at Comco. “In order to maintain a high level of quality, special fixturing is required. This typically involves a means of holding the nozzle and accurately rotating the part.
“More powerful blasters with increased powder storage tanks have made multinozzle arrays and fixturing very easy for semiautomation,” Weightman continues. “An array of nozzles can be pointed at the fixture targeting critical areas, or the part can rotated at a controlled speed while the operator uses a fixture to sweep the nozzles over the top surface.”
Despite these reassurances, James was still skeptical about automating the process within the clean environment required. Because microabrasive blasting creates dust, work is conducted in a controlled environment that evacuates the particles through a vacuum system. A commercial dryer is also needed to maintain absolutely dry media. His concerns, however, were quickly quelled. “The newest blasters have fewer internal parts and integrated electronics, which make in-house customization easier to do than ever before,” Weightman says. “With new industrial vacuum systems, dust is not a problem except in the most extreme cleanroom applications.”
The teaming of the two companies gave Morton Bowen the edge required to create production processes that met continually changing customer needs. “Now, prototypes are produced using skilled operators and simple fixturing, and the accumulation of data provides the opportunity for a controlled process,” James says. “Once past this stage, we can look at using semiautomation and finally full in-line automation of the blasting process as demands grow.”