Microblasting: Expanding Options

Originally Published MDDI April 2003

COVER STORY: MACHINING

Uses for modern microblasting technology in medical device manufacturing are increasing as devices become smaller. 

by Garry Slemp and Colin Weightman

The defining term in microblasting is micro—very small. The technology is used to clean, texture, deburr, or otherwise process very small parts and hard-to-reach areas with extreme accuracy. 

The equipment propels a very fine dry-abrasive powder mixed with clean, dry, compressed air. The air is forced through nozzles with openings as small as 0.018 to 0.060 in., and the abrasive blast is pinpointed on the surface to be processed. Processing is always done in some type of enclosed work chamber. Because abrasive powders will clump if exposed to moisture, and because the process creates an abrasive dust, all microblasting systems must integrate an air dryer—or a dry-gas propellant such as nitrogen—and dust collection as a part of the system. 

This very basic description of the microblasting process and equipment is the framework on which all the latest in new design developments are based.

New Equipment Expands Applications

As medical devices have become smaller and more intricate, older methods of surface processing are becoming obsolete. Abrasive tumblers and grit blasting operations are less effective for processing miniature parts. Hand tools such as files, rotary tools, and knives are slow and prone to damaging parts. Microblasting has always addressed the small-sized product, and today new equipment developments take this technology into higher-production areas. This makes it widely applicable to low-cost and disposable-parts processing. Single-station and semiautomated capabilities also add a great deal of flexibility. 

New developments in microblasting technology include the addition of electronics and the redesign of the blasting systems for enhanced durability and reduced maintenance. A wider size range and the enhanced purity of abrasive powders have also increased the number of applications.

Equipment. More-powerful blasters with larger powder-storage tanks have made it viable to process larger areas and deliver abrasive through multinozzle arrays for multiple small-part abrading. In addition, many high-power systems have blasting-operation sensors whose data are exportable to external controllers, opening the door to automated requirements and production-level processing.

Abrasive Media. The sizing and purity of the media have tightened, giving more-controllable results. In the past, some abrasive powders were only available in 50–300-µm sizing. Today, they may be classified at 17.5, 25, or 50 µm. This smaller size means that it is possible to use smaller nozzles to pinpoint the abrasive pattern. The fine abrasives quickly remove oxide layers and contamination from part surfaces, and impart an even, matte finish.

The most common abrasives include aluminum oxide and silicon carbide. These are aggressive, cutting abrasives; crushed glass, glass bead, walnut shell, and plastic media are less aggressive; sodium bicarbonate is the softest abrasive available. Peening abrasives such as glass beads are also available.

The best way to illustrate the wide range of modern microblasting in the medical device arena is to present specific applications. These range from manual to semiautomated systems, and cover the full spectrum of abrasive powders.

Tubing Applications

Cannula and Microtube Deburring. These raw tubes are made primarily from stainless steel or other metal alloys and are the basis for subdural insertion devices such as hypodermic needles. Microblasting technology is used to remove very fine burrs so that there are no sharp projections on the outside edge of the tube tip. Heel burrs are also removed. These are generated on the inside diameter of the tube point. This application benefits from the larger power and capacity of modern blasters because it is a volume process.
 
Cannulae are commonly arrayed either flat, with the tubes aligned next to each other, or in other mass configurations. They are typically processed in very high volumes. A blasting system will often use a multiple-nozzle array at a 90º angle to blast the tubes. Most tube-deburring applications use a glass bead abrasive. This media is available in 35- and 50-µm sizes. Glass beads will quickly remove fine burrs generated in the grinding process without altering the exacting tube tolerances.

Cannula Housing Texturing. When manufacturing hypodermic needles, a housing, hub, or component is often overmolded or bonded to the tube. This requires a textured surface on the circumference of the tube at the point of attachment. This selective texturing alters the surface to provide good adhesion between the tube and the molded or attached components. By using 50-µm aluminum oxide, microblasting provides a good surface finish without being too aggressive. Again, the volume and uniformity of this application suit it for automated systems with multiple nozzles.

Indication Band Texturing. In addition to texturing for component attachment, many production houses also use microblasting technology integrated with custom holding fixtures to texture a number of small, evenly spaced bands on the circumference of each tube. These provide the medical professional with a visual indication of how far the device has been inserted. A secondary benefit of the process occurs after a device has been inserted: the texturing permits medical personnel to easily locate the device when using electronic imaging systems. This type of banding is typically accomplished using 25- or 50-µm aluminum oxide media, often with spindle-type fixturing on the blasting equipment.

Medical Electronics Applications

Medical electronic devices, particularly implants and internal probes, have coatings that must be processed very carefully. This is typically a manual or semimanual blasting application because of the variety of device configurations. A common application is processing the coated wires or leads that are attached to an implantable defibrillator. A silicone covering on the defibrillator leads must be removed in selected areas. The microblasting process uses a sodium bicarbonate material to accomplish this without damaging the underlying coil.

Implant Applications

When orthopedic implants are manufactured, their surfaces may be too smooth for proper tissue adhesion. For this reason, they are often processed with abrasive media to roughen the surface. On some larger parts, a grit blast cabinet may be used. For the more-intricate areas, however, the precision and control of microblasting is advantageous.

Microblasting benefits prosthetic hip assemblies, ball-joint assemblies, bone screws, dental implants, and similar devices in two ways: it removes undesirable surface features that may reduce the device's acceptance by the body, and, by abrading the implant surface, it produces a finish that is conducive to tissue growth. When tissue adheres to the implant, it prevents undesirable movement.

Self-expanding stents are implantable devices typically made of nitinol. They range from very small outside diameters of 0.100 to 0.120 in. to 1.5 in. The shape-memory properties of nitinol make it ideal for this application. The manufacturing process starts with cutting an intricate pattern into a tube of nitinol. The shape of the pattern will dictate the expansion properties of the material. 

To obtain the exacting dimensions, the tube is cut using a laser process. This process will leave an oxide layer on the surface of the stent and remelt on the sides of the struts, as the laser beam becomes more diffuse. 

Microblasting can remove both the oxide layer and the remelt. This requires a precise process: too much abrasion will weaken the joints and cause premature device failure. The process is often controlled by measuring the amount of weight, in thousandths of a gram, that is removed from the stent.

The properties of laser processing cause most remelt to occur on the inner diameter of the stent. Often, the most effective means of removing this is to use an extended right-angle nozzle that blasts from the inside of the stent outward. The normal exit point of the nozzle is sealed off and then a slot or hole is formed in the nozzle tube at a right angle to the nozzle axis. This allows the abrasive to strike the inner diameter of the stent. In most circumstances, the stent is rotating as the nozzle traverses back and forth within it.

For this application, 17.5-µm aluminum oxide has become fairly standard because of its sharp cutting ability. It is able to reach all the nooks and crannies of the stent. Another medium commonly used on stents is silicon carbide, which is more aggressive. Abrasive selection is dependent upon the amount of residue or burrs to be removed.

User Customization. Customized blasting systems are common in the proprietary area of stent design and manufacture. With primary blasting components, the medical device company will typically use a nozzle array with automated or semiautomated fixturing customized to meet its specific design requirements. The basics of abrasive type, blast pressures, and nozzle sizes are the constants in the system. Advanced blasters with few internal parts and integrated electronics make in-house customization easier to do than in the past.

Medical-Part Injection Mold Applications

Processing New Molds. Molds and micromolds are made of tool steel, and are typically created using an electrical-discharge machining (EDM) or laser process to cut mold cavities into the steel material. Such molds are used to manufacture a variety of medical parts. Some create polymer implantable devices that have small, delicate enclosures for electronics, such as miniature cochlear implants. Others may be used to mold a range of medical disposables. Micromolds for electronics and implant assemblies can be as small as 1¼8 by 1¼16 in., with very delicate internal geometries. The parts produced can have a cross section as small as 0.010 in.—about the thickness of a business card. Whether a standard small molded part or a micromolded part is being created, retaining the exact mold geometry is essential. 

The EDM process uses graphite electrodes to create the mold cavities. Graphite residue is sometimes left in the cavity during this process, and must be removed. With a soft abrasive, the microblasting technique easily removes residue without damaging the cavity surface. 

When a laser is used to cut the pattern of the device into the steel, it leaves remelt, commonly known as laser slag. This must be removed without compromising the tolerances within the mold. When the laser slag builds up, it case-hardens, becoming more brittle than the tool steel itself. The abrasive process used to remove remelt is much more aggressive than that used with EDM. This type of mold is usually blasted using aluminum oxide. This is a harsh abrasive, but the precision and control of the blasting process prevent damage to the cavity surface; the unwanted material is quickly removed.

Cleanup of new molds is typically a manual abrasive process, sometimes requiring a magnifying device. The latest high-performance nozzles, typically in the range of 0.018 to 0.046 in. ID, are beneficial in such cases. 

Cleaning Production Molds. Molding-material residue builds up in mold cavities over a period of time. This is easily removed using the microblasting process. Also, some molded medical disposables actually contain abrasive materials—glass-filled nylon or mineral-filled polymer materials, for example. These are produced in multiple-cavity molds to create components and disposables in vast quantities. The product material is aggressive, and it tends to alter the surfaces of the mold cavities. 

Frequently, molds will start out with a slightly textured finish, what is sometimes called an EDM finish. As parts are molded continuously, the textured cavity surfaces become smoother, making it difficult to eject the parts from the tooling. A smooth surface tends to adhere to a part, whereas a textured surface aids in ejecting the part.

It is important to be able to retexture this interior surface without changing any of the actual dimensions of the mold; otherwise, the subsequent product would be ruined. In this case, the pinpoint pattern of the latest nozzles combined with one of a wide variety of micron-sized abrasives can resurface the inside of a mold without changing dimensions. Glass beads, crushed glass, and sodium bicarbonate are the most common abrasives used. When cleaning a mold that has been treated with a titanium nitride coating, walnut shell or plastic media can be used to remove contamination without damaging the underlying coating.

Wire and Catheter Applications

There are two areas where microblasting is used to process wires and catheters: to texture wires in preparation for the application of a coating, and to remove coatings from catheters in preparation for bonding or welding.

To prepare metal alloy wires to accept a coating, the process involves roughening the wire surface. Typically, the coating is either a PTFE or polyurethane.

In preparation for bonding, it is advisable to abrade the surface for secure attachment. Since most polymer material used in catheters is extruded, it often has a high lubricity. When attempting to bond a balloon or other component to such a material, or to a metal tube, the surface must be roughened very lightly to promote bond adhesion. This is normally a manual operation, often incorporating simple part-holding fixtures and using a soft abrasive like sodium bicarbonate. The goal is to provide sufficient texturing without damaging the catheter tube.

Catheters are commonly inserted with guidewires. Removing a coating from a guidewire that is only 0.005–0.020 in. in diameter is a very delicate task. Guidewires differ from tubes in that they are usually much finer and can have a variety of configurations. They can be made from solid wire with a fair degree of rigidity, yet still be flexible. Some examples include springs coiled at the ends. Coatings are often applied to allow easier insertion into arteries, veins, or other subdural areas. It's fairly common to see guidewires coated with a Teflon or PTFE material. 

This coating material must be removed to expose the bare wire before bonding or attaching other components. This typically means abrading selected areas along the length of the coated wire, using multiple-nozzle microblasting configurations for good coverage. Sodium bicarbonate, glass beads, or crushed glass are commonly used.

Surgical Instrument Applications 

Scissors, saws, knives, scalpels, hemostats, etc.—such tools are becoming too expensive to throw away after one use. They are typically refurbished and reused several times, using irradiation and autoclaving to sterilize them. This has created an expanding industry of what might be referred to as medical device job shops. 

These companies specialize in refurbishing medical and surgical tools. Many use microblasting technology because it can get into tight crevices, such as hinge spaces, to remove built-up debris and residue. A soft or peening abrasive is typically used. Sodium bicarbonate is the first choice because its particle structure promotes efficient cleaning, yet it is soft enough to do this without damaging the actual structure of the tool or instrument. Glass-bead abrasives are used to restore the original finish to the instrument because their round-particle form does not cut, but pummels the surface, providing peening action for a like-new finish.

Conclusion 

Microabrasive blasting is the right choice for many medical manufacturing operations because it is cost-effective, versatile, and environmentally friendly. As production requirements grow, the process can be easily expanded to accommodate the increase. 

In some cases, if the process or design limits the amount of recast present, electropolishing alone can remove surface contaminants. Also, where product manufacture does not need a highly precise deburring, finishing, or other surface processing, basic batch finishing can be accomplished using conventional cabinet or broadcast blasting. Much depends on the type of medical device and how it will be used. 

Microblasting technology suits applications that require high precision for parts or features that are extremely small and must be processed without introducing anything that will slough off. Microblasting equipment can remove material in fractions of grams. This precise control, even in array processing, is what the new forms of microblasting equipment and media offer. It is only one step in the extensive process of medical device manufacturing, but it is a step to consider when the end result must be a pristine device that is suitable for insertion or implantation, or whenever there is a need to give longer life to the molds and tools that create these medical parts and devices. 

Copyright ©2003 Medical Device & Diagnostic Industry