Paul Dvorak, Editor
Mention tubing in a medical context and most designers think of extruded polymers. But there is more to medical tubing than plastics. Metal tubes play a significant role in most hand-held medical devices. A conversation with Al Carolonza, director of market research, and Steve Santoro, executive VP, at contract manufacturer MICRO reveals a few manufacturing tricks and capabilities for metal tube that may have eluded medical device designers.
For example, MICRO has a patented rolled-tube technology that is interesting because it starts by stamping a flat sheet of metal. “That allows putting what would be secondary features in the flat blank,” says Carolonza. “Now it’s possible to stamp and roll a finished tube off a power press in one second, versus drawing raw tubing, cleaning it, cutting it to length, and putting in the secondary features afterword. The roll-tube method lends itself to high-volume applications, those that call for 500,000 devices a year. Single-use 5 mm endoscopic instruments are one such device. Overall, between scissors, graspers, dissectors and babcocks (tissue-holding forceps), we produce over 700,000 units a year – and the backbone of that is the rolled tube method. Although this production method is economically viable for high-volume applications, many medical device quantities fall within the 10,000- to 300,000-piece range,” he explains. Recognizing this, the company rounds out its offering with custom-drawn tubing and forming solutions.
Until recently, production equipment was relatively inflexible. A new device feature or an adjustment to an existing feature involved an additional, time-consuming setup. “But working with the latest equipment available, be it laser cutters, Swiss screw machines, or combination, means a change to the design now requires a much simpler programming change, instead of a more expensive tool change. A design adjustment becomes more flexible, especially if there is slight difference in the product.” The production trend, Carolonza adds, is that CNC equipment and laser cutters are used more predominantly than stamping.
To get a designer’s mind moving toward new possibilities and greater design flexibility, consider that tubes need not be round. “Tube can be drawn in a range of profiles,” Santoro says. “Our tube mill can draw many different profiles. Drawing refers to extruding a stock tube through a die to impart a particular cross section. A D-shaped profile is common, but a octagon cross section is another recent shape, and the oddest appeared like a figure 8. Of course, some shapes are more difficult than others, but the capability gives designers flexibility, and because the mill is a dedicated facility, it is easy to develop a profile in a lot less than the six-month lead time other companies may require.”
Santoro also suggests that designers think about the tolerances of the tube they might be considering. He says company equipment and operators are capable of holding ID and ODs to within 0.0005 in. “Normally designers spec ± 0.001 in., but we can tighten that up, especially when a design calls for over-molding with plastic, such as a hand grip, over the tube. The tighter tolerances are necessary to avoid flashing. If the diameter varies too much, the molding operation will cause flash and tooling may need changing to avoid flash. Flash around critical features must be minimized to avoid fit and function issues in the final device assembly.”
A tube’s wall thicknesses depend on the application. “A lot of medical tube is thin-walled, meaning 0.010-in. thick,” says Santoro. “But if the tube surface requires machining, consider a much thicker wall, such as 0.030 or 0.040-in.”
The company recently acquired a Swiss CNC screw machine with laser-cutting capability to improve its tubing production. “It allows laser cutting and machining in one setup. Previously, two machines were needed for turning and laser cutting, and the transfer from one machine to the other increases the likelihood of misaligned features.” The recently added capability also means it’s possible to put features into a thicker-wall tube as well as cut through it. Designers should be aware of value in a single setup, especially when a part will include machined and laser cut features with critical orientations. “That is now possible in one setup, so you get registration capability within 0.0005 in. In the end, it saves cost when you can do things in one setup instead of two or three. You get a better part and in a shorter cycle time.” Santoro adds that although a machine such as the $500,000 laser Swiss cutter has great potential, learning to coax out that potential takes a lot of learning and experience.
Wire Electro Discharge Machining (EDM) brings other capabilities to cutting tube features. “First of all, the machine uses a taught wire carrying high voltage to remove metal in a way that milling cannot. That allows for unusual features, such as window shapes not attainable with laser cutters. This is useful when the angle of the cut in the tube wall must mate with other components. Although wire EDM is considered a relatively slow production process, creative fixturing of multiple stacked parts can streamline the process and in fact make it cost competitive.”