Drilling microholes in plastic medical devices

High-speed injection molding and extruding are used extensively to manufacture disposable, plastic medical devices. These processes can produce a vast array of part shapes and configurations.

Frequently, though, problems arise when specifications call for microholes in molded or extruded components. Either the specific process can’t produce the hole, or it would be uneconomical to do so from the standpoint of yield or equipment costs. In these cases, a secondary process is required.

Image of extruded medical tubing provided courtesy of Microspec Corp., Peterborough, N.H.

It’s often laser drilling.

Lasers—particularly ultraviolet and ultrashort-pulse styles—are chosen because polymers can’t be EDMed, chemical etching won’t yield the necessary aspect ratios, and mechanical drilling leaves burrs and debris and costs too much for holes with diameters of 100µm or smaller.

Laser-drilled products

Liquid- and gas-flow devices used for drug delivery are frequently laser-drilled. They include catheters—extruded, flexible tubes inserted into the body to allow for drainage, localized drug delivery and to provide surgical instruments access to internal organs.

The holes in drainage catheters are larger and produced to less-stringent tolerances than holes in other catheters. They usually are made by mechanical processes. 

Drug-delivery catheters, on the other hand, require holes that are precisely sized and positioned to ensure that the drug reaches just the target area. Holes can be placed longitudinally, spirally along the length or in a number of other different patterns. Also, a catheter sometimes has different-diameter holes laser-drilled into it. 

When laser drilling catheters, care must be taken to avoid compromising the wall opposite the beam-entry wall. A metal mandrel usually is inserted into the catheter before processing to block laser light from damaging the far wall. (In a pinch, a nickel guitar string works well too!)

These catheters, which often incorporate molded plastic components and a guide wire or needle, are generally meant to be used and then removed from the body. Sometimes, other areas of the catheter—the tip, for instance—also are laser-machined.

Angioplasty balloons are another type of extruded device that is laser-drilled. Both UV and ultrashort-pulse lasers can produce holes in balloons, which doctors insert in clogged arteries in order to clear them.

Fluid meters are common injection-molded medical devices. Their drug-delivery holes are laser-drilled because the molding process can’t produce a sufficiently small orifice.

App success factors

The first step in ensuring that a good, clean hole is drilled is choosing the proper beam wavelength. Second, the required hole size must be achievable. UV lasers are the best candidates for most applications.

Figure 1 shows the top view of a 25µm hole drilled with a UV laser in a polycarbonate injection-molded part. Drilling rates of 1,000 parts per 40 minutes were achieved using an excimer laser with a split beam, 8-up delivery system (drilling eight parts simultaneously). However, the process required three people—one to load parts, one to unload parts and a third to inspect them. And, the large number of optics required highly skilled operators.

The process was later transferred to a shorter-wavelength laser with a single-beam delivery. This laser, chosen because longer-wavelength lasers did not produce the desired results, was incorporated into a system designed for the production floor. The system consists of a bowl feeder and tooling that orients and locates parts at a precise point in space, allowing the front surface of a single part to be laser-drilled. Drilling 1,000 parts takes more than 1 hour, but the operation now runs unattended.

Figure 1: A 25µm hole laser-drilled in a polycarbonate injection-molded part. Changes to the processing setup allowed the parts to run unattended. Images courtesy PhotoMachining.

Also, thanks to the addition of a rotary indexer, every part can be inspected after lasing, instead of checking random samples as had been the case with the 8-up system. Testing is done via a calibrated flow meter and air. Air is used instead of a liquid because the parts must remain dry. Air readings are correlated to liquid flow. Since gas flow is much more dependent on temperature, humidity, elevation and other variables, the testing devices are calibrated to a set of “gold” standards that have been measured with liquid and deliver acceptable results.

Because molded and extruded parts often have odd shapes, tooling is usually required to hold parts for laser drilling and to achieve consistent quality. And, frequently, the tooling is specific to the job.

For instance, when laser drilling tiny, precise holes in molded parts, it is important to focus on the front surface. This cannot always be done if the parts are simply nested during processing because of irregularities in the molding process. In these cases, spring-loaded positioners can be used that butt the front surface of the part to be laser-drilled against a known stop that is accurately positioned, relative to the focus of the laser.

Figure 2: V-block jig with a vacuum pump holds catheters being laser-drilled. Image courtesy PhotoMachining.

A simple way to hold round parts is by using a V-block (Figure 2) with a vacuum pump. It can be used to maintain focus while, for instance, drilling holes along a catheter’s length. Holes drilled in the bottom of the V let the vacuum pump firmly secure the catheter during processing.

With simple but clever tooling like the above and application of a laser with the proper wavelength, tight-tolerance, small-diameter holes (< 25µm) can be drilled and placed accurately in extruded and molded medical devices. µ