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High five: Improvements to simplify catheter and stent production

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Bob Bessemer, Conair Group, says improvements in downstream extrusion control make catheter and stent production more repeatable and reliable than ever.

When it comes to production of medical plastic tubing for catheters and stents, predictable control of every factor in the extrusion process is essential. The increasingly sophisticated uses of this micro-bore tubing demand that processors exercise the highest possible control over all key characteristics – quality, precision, repeatability, traceability, cleanliness, and more.

Processors must do so while facing significant production challenges: micro-bore tubing sizes (less than 0.060” in diameter) for heart and brain catheters; multi-lumen structures for diagnostic, interventional, and therapeutic uses; increasingly complex bump profiles; and ever tighter tolerances for factors including ID, OD, concentricity, and elongation/burst strength.

Years of development efforts by extrusion equipment makers like Conair have resulted in a steady stream of improvements that make production of medical plastic catheters and stents more predictable and controllable than ever before. At the same time, these improvements can help make off-line work, like QA, cutting, and finishing steps, a thing of the past. 

Next-level extrusion process control

Medical plastics processors know that repeatable extrusion starts well upstream with the fundamentals of material selection, blending, plastication, and die temperature and pressure control. However, it is more recent and less-well-known downstream improvements that open the door to the next level of extrusion process control for medical grade tubing.  These improvements involve:

Let’s review these improvements one-by-one.

Vacuum sizing

Instead of free extrusion, which relies on air pressure within the tube, vacuum sizing offers a more dependable way to control extrusion profile and roundness, including the size and shape of individual lumen. Creating a controllable vacuum sizing process for small-diameter, single- or multi-lumen medical tubing required equipment makers, including Conair, to scale down previous vacuum sizing/cooling equipment, reduce water turbulence and provide finer, more precise vacuum control.  The result, for Conair, was the MedVac line of vacuum sizing/cooling tanks, part of its larger MedLine offering of cleanroom-ready auxiliary equipment. 

MedVac vacuum/cooling tanks enable differential pressure to be applied in two different ways, based on application requirements, giving processors more options for controlling microbore tubing production.  The first way is to apply vacuum within the calibration die itself, which draws the profile wall of the extruded tube outward and shapes it against the surface of the tooling. The second way is to draw vacuum over the entire length of the cooling tank, then allow the increased differential pressure within the extruded tube to gently expand it outward, shaping the tube against sizing rollers as it is pulled through the tank. This approach also prevents cooling water from drooling out the orifices through which the tube enters and exits the tank.

Precise, temperature-controlled cooling

Water-based cooling transfers heat out of extruded tubing and enables it to solidify. However, controlling the rate at which cooling happens is essential to tubing quality.  The latest vacuum cooling tanks enable precise control over water temperatures (to ±1 degree F) and flow rates so that optimal rates of heat transfer are maintained.  Such precise temperature control preserves material properties and ensures consistency in tube sizing and quality.  

The latest vacuum/cooling equipment also gives processors greater control over where cooling is applied. For example, the calibration die and extruded tube can be fully immersed in cooling water or the die can be equipped with a cooling water jacket. Ultimately, the selection of sizing and cooling methods depends on the characteristics of the polymer and the extruded tube. Both can have a positive impact on tubing quality: the differential pressure environment available in vacuum sizing/cooling tanks prevents negative effects on tube concentricity and surface finish, while precise temperatures ensure the most predictable control of tube sizing.  

For applications in environmentally regulated spaces (clean rooms and white rooms, for example), water-cooled condensers are the preferred choice for removing process heat from cooling water used in vacuum cooling/sizing tanks since they operate without air-circulation fans that can blow contaminants within the space.

Digital OD measurement and monitoring

Improved OD monitoring is another factor in the performance of the latest medical tube extrusion equipment.  For example, during the cooling/sizing process in a MedLine cooling/sizing tank, an ultrasonic gauge can be used to monitor wall thickness and ‘hot’ OD, while an OD laser gauge, positioned downstream from the tank, generates final ‘cold’ OD dimensional data. Together, the information from these gauges can be used to help adjust puller speed and vacuum level within the tank, both of which are essential to maintaining consistency in critical tube dimensions. A bit further downstream, one additional monitoring tool, an automated vision system, can inspect finished tubing for gels and surface imperfections, then automatically identify, cut out, and remove ‘bad’ sections of tubing.

Servo-driven pullers and planetary cutters

The addition of servo drives, with servo-rated low-backlash reduction gears and closed-loop feedback control, have dramatically improved the precision and controllability of the pullers that draw tubing through the extrusion line. Specifically, servo controls enable pullers to respond more precisely to process feedback, enabling them to regulate speed and position in very fine increments. The added precision also improves the dimensional accuracy of the cutting process. For example, Conair’s MedLine puller/cutter can maintain cut-to-length tolerances of ±0.015 inch or better on even small microbore tubing (down to 0.008 inch) when used with specialized input devices.

The introduction of micro planetary cutters has also improved the quality and consistency of the cutting process. Planetary cutters utilize a spinning blade to make gentle, lathe-like cuts on even brittle, shatter-prone tubing materials without causing tube deformation or generating particulates—things that more conventional fly-knife cutters cannot do.  

Combining the inputs of in-line gauges and vision systems with servo pullers and software-based control systems makes new levels of extrusion automation possible.  Producers can create specialised programs (or ‘recipes’) for repeatable, automated production of value-added tubing features. For instance, taper or bubble tubing requires repeated adjustment of OD during extrusion so that one end of each cut-to-length section is larger in diameter than the other. Today’s software and control features make production of these complex features simpler and far more repeatable than ever before.

Cleanroom-ready equipment designs

None of these process innovations can help if you can’t employ them, right away, in a medical production environment like a cleanroom or white room. For that reason, Conair developed MedLine, a complete offering of over 40 models and 170 variations of cleanroom-ready auxiliaries. This range includes not only downstream extrusion equipment but also dryers, feeders and material-conveying systems, temperature-control equipment and more. All MedLine equipment is compliant with ISO and FDA regulations, with basic QC, PM, and calibration documentation provided for simpler, cleaner implementation. MedLine equipment comes preconfigured, factory calibrated, and documented. In addition, it is built using materials suitable for cleanroom environments, ie, to help reduce the particulate load and make equipment easier to clean.

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