Medical Device Link.

Originally Published EMDM January/February 2003

ENGINEERING INSIGHT

Supplier of heat-shrink tubing and catheter products hails machine's consistency and repeatability

During catheter assembly, thermal bonding equipment developed by Beahm Designs applies heat directly to the shrink tubing while using cooling air to protect the balloon.

Radio-frequency die bonding is a proven process for bonding urethane balloons to catheter shafts, but it comes with a significant price tag. Seeking a cost-effective alternative, a supplier of polyester heat-shrink tubing and related products and services investigated several options. All of those except one—a thermal bonding process using a split-die machine—involved unacceptable trade-offs, according to Mike Barbere, assembly engineering manager at Advanced Polymers Inc. (API; Salem, NH, USA).

API produces a range of thin-wall heat-shrink tubing as well as high-pressure angioplasty and elastomeric balloons, custom catheter assemblies, and custom extruded tubing. The company has developed a proprietary process to manufacture thin-wall polyester heat-shrink tubing that exhibits high tensile strength, even with walls as thin as 0.0002 in., and a smooth glasslike finish. Because of the wall thinness, rapid shrinkage and heat transfer to the parts being fused may occur during processing. In addition to the uniform compression that results in the tube's radial direction, the tubing's longitudinal shrinkage prompts linear compression of adjacent tubes.

In its search for effective alternative bonding methods, API first looked at adhesives, but they "only worked at very low pressure," says Barbere. "Although we got adhesion, the neck of the balloon would stretch as the pressure grew." The peel force was too great, preventing the adhesive from holding the neck in place.

The company had tried using thermal bonding with its polyester heat-shrink tubing in the past, but the process lacked repeatability. "The bonds were glass smooth," recalls Barbere, "and they were incredibly strong." However, it was not a simple process to direct and control the hot air, leading to unpredictable results.

"Sometimes the balloon, which is sensitive to heat, would get distorted. It took a great deal of skill and luck to produce an acceptable part each time," says Barbere. Beahm Designs (Los Gatos, CA, USA), a developer of catheter-manufacturing equipment, felt that it had removed the alchemy from the operation with its Split Die Thermal Bonder. Company president Brian Beahm persuaded Barbere to take it on a test run.

The Split Die Thermal Bonder applies uniform heat and compression in a range of thermal bonding and modification processes. At the beginning of the cycle, the hinged dies are heated to a constant temperature that is controlled by a thermocouple feedback. The tubing and balloon are assembled over a PTFE mandrel, and the polyester shrink tubing is positioned over the bond area. The die halves are then closed around the shrink tubing, applying uniform heat to the component for a predetermined period of time.

Heating the bonding area by direct contact with the shrink tubing while using cooling air to protect the balloon enables the user to apply a constant compression force around the melted balloon neck and the shaft. This causes the materials to flow together into a single polymer structure and produce a smooth glasslike tapered finish at the bond area. After the process is complete, the shrink tubing is removed and discarded.

Equipment setup proved to be remarkably easy, according to Barbere. "Once we made the first jaw modifications to accommodate the balloon necks, it was a matter of one day's worth of engineering work to establish parameters and evaluate the results. In just a few attempts, we had our first bonds," says Barbere. Evaluations were conducted by dissecting the bond areas and burst testing the balloons. The results, adds Barbere, were as good as the company's best efforts using other processes.

API has also begun to use the machine with its polyester heat-shrink tubing to make butt joints in single-lumen tubes. The device attaches soft-tip materials to catheter shafts and tapers tubing ends in a single operation.

With a temperature range from ambient to 525°F, the Split Die Thermal Bonder can process bond widths from 0.062 to 0.5 in. and tubing with diameters up to 0.5 in. Custom dies that can accommodate a range of other widths and diameters are available upon request.

API produces its polyester heat-shrink tubing in diameters ranging from 0.008 to 1 in. with wall thicknesses from 0.0002 to 0.004 in. Polyurethane and other balloons are available in custom and stock sizes.

The Engineering Insight column profiles the use of a technology to improve the effectiveness or safety of a medical device or to optimize its production. If you would like to submit a case study for consideration for this column, please contact the editor by fax, +1 310 4454299, or via e-mail, norbert.sparrow@cancom.com.

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