Medical tubing: Getting a grip on precision

In many ways, these are troubled times for North American extruders. The recession kicked the feet out from under the housing market, and among the fallen are a good many extruders of composite and plastic decking and siding.

But following the old adage that when a door closes, a window opens, there’s probably never been a better time to be involved in the extruded medical products market. With an increasing trend today toward minimally invasive surgery, examination by miniaturized optics, or microsurgery using laser techniques – courtesy of ageing baby boomers – the demand for medical tubing continues to grow.

Interested processors should enter the market with their eyes fixed firmly on the potential hurdles. First, ever-tighter tolerances being set on tube dimensions make the field a tough one to break into.

Second, with the prices of medical plastic compounds heading ever higher, extrusion shops that want to remain in the black can’t afford to waste material on parts that don’t measure up. Unlike most extruded applications, quality – in the form of dimensional stability, tight tolerances and reduced variability – often trumps output in the world of medical parts.

Third, stringent quality demands on manufacturers courtesy of the health care industry make medical tubing, more so than almost any other form of plastics processing, a zero-sum game. Either do it to spec or kiss your manufacturing contract goodbye.

The good news? The equipment suppliers are stepping up to deliver new technologies designed to help processors hit higher levels of accuracy in tube extrusion the first time, every time.

EXCELLENT EXTRUSION

When it comes to extruding medical tubing, there’s no chicken-or-egg type question about what comes first in order of importance; while all of the various parts of the extrusion process need to be operated under optimized and integrated conditions, the design of the extruder is an obvious make-or-break component for achieving precision tube extrusion. Especially since medical-grade PVCs and other materials often contain low levels of plasticizer and stabilizer, your grandfather’s all-purpose extruder probably won’t cut it nowadays. This field requires custom-made equipment.

American Kuhne has just unveiled its Ultra R/S extruder line, its most recent development for the medical market. According to Steve Mason, sales manager, the extruder’s design includes a proprietary, hinged roller-feed assembly that’s easily opened by loosening two swing bolts, providing access for common service and maintenance. The scraper blade is also easily accessed for adjustments or removal. The casting of the machines is nickel-plated, and the hinged arm and hardened feed liner are stainless steel for abrasion and corrosion resistance. A second hinge opens to ease removal of the feed roll and bearings for service. Feed roll bearings feature special seals to minimize silicone contamination; spare feed rolls and bearings can be provided for quick rotation during material/color changes.

Each American Kuhne extruder, Mason continued, includes a Steward Barrier Screw designed specifically for the application. “The screws are designed to maximize pressure stability for the tightest possible dimensional tolerances, ideally plus or minus one per cent or better,” he said. “Also, they minimize melt temperature variation, ideally plus or minus one degree or less.”

The company, which can probably stand as a good bellwether for the growing demand for medical tubing, reports that more than half of its orders from the medical market are for complete turnkey tubing systems.

TOOLING FOR TUBING

A crucial component of the overall extrusion process for medical tube production is the die. Different sets of tooling are often used, with varying pin and bush dimensions to match the rheological characteristics of the chosen polymer at the defined output rate and line speed.

As with the extruder itself, bespoke equipment is crucial to performing competitively in the world of ever-tighter tolerances.

Guill Tool & Engineering has been involved seriously in the manufacture of extrusion tooling for medical applications for over 10 years. Unlike other areas of its business, however, demand for tubing has held steady over the past year. “This demand is, in fact, the only thing that’s remained steady during the recession,” said Bill Conley, the company’s sales manager for extrusion.

To further penetrate the market, Guill Tool recently made available its new Series 819 EZ MediFlow extrusion die, capable of producing tubing finer than a single human hair, Conley said, with minimum tolerances of 0.002 inches. “The new head and inline dies of the MediFlow system are specifically engineered for the tiniest applications required by today’s medical applications – including the latest minimally invasive arthroscopic surgical procedures,” Conley said.

The tooling includes a spiral design for upstream thermal balancing to ensure an equal flow resulting in close-tolerances, thin-walls and multilayered products, Conley continued, as well as optimized deflectors for minimum chamber residence time.

While the growth of medical tubing applications is an obvious piece of good news for the plastics industry, it’s not without a significant challenge or two for tooling suppliers such as Guill Tool. “Materials for current medical extrusion applications have become very complicated, involving PEEK polymers, urethanes, engineered plastics and even biopolymers in some instances,” Conley explained. “As a result, tooling providers have to be very careful of the viscosity of the material in multilayer medical tubing applications, and make sure that one layer doesn’t have too high a head pressure, which could distort all of the layers. For this reason, everything that our company designs is done with advanced flow analysis software, which allows us to calculate for fluid flow, pressure drop and heat transfer for all types of materials.”

HANDLING HAUL-OFF

“Processors can extrude a perfect tube, but if isn’t cooled properly or if the puller or cutter is out of alignment, the finished part risks winding up worthless,” Conley continued.

Here, then, is a last piece of the puzzle, but no less important than any other: the downstream equipment. The wrong downstream unit will quickly become the weak link in any medical tubing line by undermining product stability during haul-off.

With the precise requirements of the medical tubing market in mind, CDS – Custom Downstream Systems recently launched their medical high-precision vacuum sizing tank. According to company vice president Antonio Pecora, the unit provides efficient filter water cooling while maintaining an enhanced heat transfer rate that stabilizes the tubing. “The cutting head sits on both X and Y axes, making it adjustable for running on different settings on the belt,” he explained. “Operators can also move the cutting head out of the way completely and still have the option of using the haul-off.”

The stainless steel retractable water reservoirs allow for easy cleaning access, Pecora continued, reducing maintenance and downtime and increasing overall line productivity – the magic words in today’s economic climate.

Conair is another equipment supplier with a recent offering for medical tubing extruders. The company’s MedLine puller/cutter uses servo-drive technology, inline gauges and advanced microprocessors to eliminate speed variation, and c
ontrol and validate the all-important parameters of dimensional stability and tolerances that can make or break a tubing production run.

According to Dave Czarnik, senior tooling/process engineer with Conair, the MedLine unit periodically adjusts the outside and inside diameters to expand the tube at a point where – to take one example – a catheter would be attached to other medical equipment. “Servos also govern the gap between puller rollers to minimize slippage that can affect speed and dimensions,” he said. “The cutter portion of the MedLine can terminate the tubing at exactly the right place to yield a nearly finished catheter tube.”

PERFECT PROCESS

If you haven’t guessed by now, the key process parameters in medical tube production are dimensional stability and tolerances on all dimensions. Hitting these heights can be even harder for extruders than it sounds given the near microscopic tolerances of today’s cutting edge intravenous and cardiovascular tubes.

“The secret to successful medical tube extrusion comes down to consistent process control and a mastery of the tremendous number of variables at play,” said Guill Tool’s Bill Conley. “More than with almost any other plastic part, there has to be a perfect union between the tooling supplier, the equipment manufacturer and the processor.”