“Complex catheters, such as endo-vascular diagnostic and guide catheters, often require multi-layer tubes consisting of an inner polymer liner, stainless-steel braid reinforcement and outer polymer jacket,” Lazas told PlasticsToday. “Mandrels maintain inner lumen dimensions, particularly during the high-tension braiding stage, which would crush the inner tube without support.” If the manufacturing process involves high temperatures, then a metal mandrel will be the only option. Under other circumstances, however, a plastic mandrel—typically made from acetal—may be the optimal choice for a very simple reason: “Since mandrels are a processing aid, and are discarded or recycled, plastic mandrels are preferred due to cost,” says Lazas.
For example, a diagnostic catheter with a Pebax inner liner, braid reinforcement and Pebax outer jacket “is ideally suited for an acetal polymer mandrel since the processing temperatures are not exceptionally high.” By contrast, guide catheters with a PTFE inner liner will need to undergo high-temperature processing, which will call for copper mandrels.
So-called free extrusion—the use of air or pressure to maintain tubing dimensions until the melt has cooled to the desired shape—is preferred by processors, since the process foregoes secondary operations. However, says Lazas, “polymer melt strength can limit the diameters, wall thicknesses and tolerances achievable by free extrusion. This has become a critical issue in cardio-vascular and neuro-vascular catheters. New procedures often require catheters with larger inside diameters for therapeutic devices or implant delivery. Meanwhile, outside diameters are limited by blood vessel anatomy. This results in a demand for tubes with thinner walls and tighter tolerances,” says Lazas, which is facilitated by mandrel-assisted extrusion. Selecting a metal or plastic mandrel for this operation will depend on the size of the finished tube, the processing temperature and economic considerations, he adds.
Acetal polymer mandrels are often a cost-effective choice for extruded tubes with inside diameters of 0.025 to 0.125 inches, according to Lazas. Larger diameter tubes up to 0.500 inches, such as those used in gastro-enterology devices, can be achieved with custom polyolefin mandrels, he adds.
As previously noted, metal mandrels are required in high-temperature extrusion involving materials such as PEEK. “Also, metal mandrels are better suited for producing tubes with very small inside diameters, such as those used for neuro-vascular devices. For example, copper-based mandrels are available in diameters as small as 0.005 inches,” explains Lazas.
Tekni-Plex has more than 30 years of experience manufacturing acetal polymer mandrels and custom polyolefin mandrels used for braid- and coil-reinforced tube manufacturing and mandrel-assisted extrusion. They are supplied on spools for ease of use in manufacturing operations. Tolerance is a critical feature of our mandrels, adds Lazas. 'Tolerance of the mandrel determines the final inside diameter tolerance of the finished tube, and ensures easy removal of the mandrel when the tube is completed.”