High Measurement Rates. With profiling, frequent measurements or even measurements of entire samples can be taken rapidly. For example, one guidewire manufacturer has reported that manual measurement of only critical features required 10 minutes of operator time per wire, whereas a profiling system can provide a profile of the entire wire in less than 30 seconds. By adopting profiling technology, manufacturers can achieve adequate inspection plans even for processes that require high sampling rates.

Improved Measurement Capability. When manual measurement methods are used, obtaining a satisfactory measurement capability—which is indicated by a repeatability and reproducibility (R&R) of less than 10% of the tolerance range—is always challenging. Furthermore, maintaining that R&R level as operators and inspectors move to other positions or leave the company requires constant training of new personnel. By eliminating operator-induced variability, a profiling system improves R&R and minimizes the impact of employee turnover on measurement capability.

Process Control and Capability. Closed-loop machine and process adjustment can be achieved by integrating a profiling system into a manufacturing line such as centerless grinding or extrusion. The operator is no longer part of the control loop and adjustments occur quickly and automatically. In addition, because of the improved measurement capability of a profiling system versus manual methods, the process adjustments will reflect true process parameters rather than measurement variations. On production lines where an automatic control loop is too difficult to implement, manual adjustment of the process will become more efficient with a profiling system because the measurements on which the adjustments are based will occur much faster. When either closed-loop process control or more-efficient operator adjustment is combined with the improved measurement capability provided by a profiling system, process capability (as defined by R&R) will also improve.

Data Retention. Profiling data are easily stored for production documentation purposes and are also readily accessible for SCADA applications, statistical process control, trending, and product and process validations.

Faster Startups. On production lines where each shift begins with an operator performing an iterative process of measuring a part and then adjusting the process, startup times can be reduced by using profiling. For example, in bubble/taper tubing extrusion, a significant amount of product goes into a waste bin while the operator cuts samples, makes measurements, and adjusts the process; cuts other samples to determine the effect of the adjustment; and so on, with each measurement requiring several minutes to complete. In contrast, with an on-line profiling system, the operator will receive real-time data on each sample so he or she can quickly determine the effect of small process adjustments and initiate production runs more rapidly.

Expanded Product Capabilities. In the example of the bubble/taper tubing, the reduction in startup times and the improvement in measurement capability resulting from the use of an on-line profiling system offer the possibility of using extrusion to produce more-complex parts with tighter tolerances, such as balloon catheters.

More-Efficient Process Validations. Because it collects greater amounts of more-accurate data than manual measurement techniques do, profiling can simplify and accelerate process validations.

Improved FDA Compliance. The higher sampling rates, improved measurement and process capabilities, better data retention, and more-efficient process validations that are achieved with profiling all facilitate a manufacturer's compliance with the quality system regulation and other FDA requirements.

Cost Savings. All of the factors above also contribute to reduced manufacturing costs. For example, a guidewire manufacturer achieved payback on its investment in profiling systems within six months because its gains in production efficiency reduced its need to add more grinding equipment and associated operators. In another case, an extruder of bubble/taper tubing that had required a staff of five inspectors to perform measurements on one extrusion line eliminated manual inspections by installing an on-line profiling system with sorting capability, thereby cutting labor costs while increasing overall productivity.

GUIDEWIRE MANUFACTURING

Medical guidewires are manufactured by grinding tapers on wire using a centerless grinding machine such as that shown in Figure 3. Typically, these tapers range from 0.014 in. at the proximal end of the wire to as small as 0.001 in. at the distal end. In the grinding process, the wire is fed between two grinding wheels with the smaller, regulating wheel tilted approximately 2° from its horizontal plane. This geometry induces a feed-through effect whereby the component is pulled through the machine by virtue of the grinding action performed by the wheels. The pull or feed rate is subject to many variables, including coolant quality, grinding-wheel diameter and sharpness, material hardness, and temperature. As a result, guidewire dimensions vary from part to part, and throughout a run. In one approach to reducing this variability, sensors are used to detect the motion and speed of the guidewire as it is being ground. Based on these data, the gap between the grinding wheels is continuously adjusted to produce a more-consistent feed rate and, therefore, more-consistent parts.

Figure 2. Typical laser profile of a tapered sample.

The guidewire manufacturing process can also be enhanced by the use of profiling systems. Manual measurement of guidewires is time-consuming because there are often multiple diameters, tapers, and transition points to be measured. Using a profiling system, a guidewire is simply loaded onto a motorized stage that passes through the laser gauge. With a high-speed gauge, the wire's diameter can be measured automatically every 0.001 in. along its entire length. As the diagram in Figure 4 indicates, one profiling system can serve several grinding machines, with the measurement results being fed back to the individual machines for process adjustments.

Figure 3. Centerless grinder for producing guidewires (Glebar Company, Inc.; Franklin Lakes, NJ).

Alternatively, a profiling system can be integrated with a single grinder either via a network or in-line. In such applications, as finished wires are being withdrawn from the grinding zone, they pass through the laser gauge. The resulting data are then used to make automatic grinder adjustments, thus compensating for any drift that may occur during a production cycle.

Figure 4. One profiling system can serve several grinders.

BUBBLE/TAPER TUBING EXTRUSION

In any medical tubing extrusion operation, variables that are difficult to control, such as variations in raw materials, will cause part dimensions to drift over time.¹ For tubing with a constant diameter, correction for this drift can be accomplished by using an on-line noncontact laser gauge that measures the tubing diameter continuously and provides closed-loop adjustment of the extruder's puller speed and internal air pressure. On-line ultrasonic gauges can also be used, although this technology has not yet been widely adopted.² Ultrasonic systems can automatically inspect and sort 100% of extruded tubing for wall thickness as well as outer diameter (OD). Closed-loop machine adjustment can also be implemented for a fully automated capability.

These standard measurement and control-loop systems cannot be used on bubble/taper tubing, however, because the diameter is always changing (see the sidebar, below). Thus, the operators are forced to obtain samples periodically, measure their critical features manually, and use the results to make machine and process adjustments. This manual procedure has been used successfully with the relatively simple extruded bubble/taper tubing used in products such as IV sets, which have wide specification tolerances, but the tubing used in balloon catheters must meet tighter tolerances than have been achievable on extrusion lines with manual inspection. An over-the-wire balloon catheter, for example, must have a precise OD at its distal end that matches the inner diameter (ID) of the balloon. The catheter shaft must also progress over its entire length to a larger proximal-end OD that precisely matches the ID of the manifold to which it is bonded. This diameter change on balloon catheters has typically been accomplished by assembling tubing sections with progressively smaller diameters, or by stretching or "necking" the tubing in a heated die. A key reason these expensive secondary processes have not been replaced by bubble/taper tubing extrusion has been the lack of a suitable on-line gauging technology capable of providing real-time profile visualization and closed-loop process adjustment.

On-line profiling technology now exists that addresses these gauging limitations. In systems designed for use with extruded bubble/taper tubing, a laser gauge feeds diameter data to a PC, an encoder signal provides position information, and the acquisition software generates a graphical display of the part profile. The software can also compare the part dimensions to a stored template and supply a signal to activate a sorting or marking mechanism. In addition, out-of-specification dimensions are flagged for the operator, and the system can provide closed-loop control of the extruder's puller and air pressure controller to compensate for dimensional drift.

CONCLUSION

Profiling technology can offer significant benefits to producers of cylindrically symmetrical components. Non-constant-diameter components that previously could only be measured by slow manual methods can now be measured rapidly, automatically, and accurately. Automated closed-loop control of manufacturing processes is also possible, resulting in more-stable production with higher yields and increased product quality. With the adoption of profiling systems, new product configurations become possible, process validations can be performed rapidly, and the product development cycle can be accelerated, contributing to faster product introduction and increased profits for medical device manufacturers.

ACKNOWLEDGMENTS

The authors would like to thank John Banyan of Glebar Company, Inc.; Jim Klapperich of RDN Manufacturing Company, Inc.; and Mike Nielsen of TSI Inc. for their assistance and input to this article.

REFERENCES

1. Hans W Kramer, Process Considerations in the Extrusion of Microbore Tubing, Medical Device & Diagnostic Industry 21, no. 1 (1999): 108–115.

2. Scott H Taylor, Advances in OD/ID Systems Benefit Medical Tubing Extrusion, Medical Device & Diagnostic Industry 18, no. 9 (1996): 102–109.

Jon Gamble is president of Advanced Controlled Electromechanics Inc. (Minneapolis), which supplies standard and custom manufacturing, measurement, and data collection equipment to the medical device industry. Jim Kimmet is a consultant to Advanced Controlled Electromechanics Inc. who specializes in medical tubing extrusion.

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