|The cold-drawn tubing method such as that used by Veridiam can control tubing dimensions and strengthen 300-series stainless steels.|
Medical device tubing is ubiquitous. Available in a variety of shapes, sizes, and colors, it is commonly made from such materials as thermoplastics and silicone and manufactured using extrusion techniques. But another family of medical device tubing is made from metals, including various types of stainless steel. Either cold drawn or rolled, such tubing can be used to fabricate many different medical devices, from stents and fixtures to surgical cutting instruments and bone screws. Which method is right for you?
The most common method for producing metal device tubing is known as the cold-drawn process. Because it can control tubing dimensions and impart necessary mechanical properties to metals, it is particularly suitable for fabricating critical and demanding medical devices.
The cold-drawn method can achieve tightly controlled inside and outside dimensions while controlling wall thickness variation, explains Patrick Lorenz, vice president of business development at El Cajon, CA–based Veridiam Inc. and a trained metallurgist. Thin tubing manufactured using this technique can feature a small inner diameter down to 0.003 in.—close to the diameter of a human hair—and an outer diameter from needle size to more than 1 in.
In addition, the cold-drawn technique can strengthen 300-series grades of stainless steel through cold work. “To achieve the required strength and hardness in a 300-series stainless steel, you need to complete the tube fabrication process with just the right amount of cold work remaining in the tubing to ensure that the customer’s desired properties and specifications are met,” Lorenz comments. “Because non-cold-worked 300-series stainless steel is relatively soft, it’s not generally useful unless it incorporates cold work to increase its ultimate tensile strength and hardness.”
In the cold-drawn process, a die almost always made from carbide is used to control the shape and dimension of the outer diameter. Generally, there is a matched inside tool—either a long mandrel/rod or a carbide plug—to precisely control the inside diameter and wall thickness. This method controls the flow of material as it is pulled through the die set, reducing and changing the tube’s dimensions—a process that also imparts cold work into the product, according to Lorenz. “The key to production efficiency and to achieving the exact final material properties in this process is to control the cross-section percent reduction of the tubing to minimize the number of draw cycles while not overworking and breaking the material.”
Both seamless and welded tubing can be produced using cold-drawn processing, Lorenz notes. Seamless, nonwelded tubes begin as a 6- to 12-in. solid bars, through which a hole is drilled. In contrast, welded tubes begin from a tightly controlled flat strip of metal that has been cold-rolled to a precise thickness, slit to the required width, and coiled. The coil is fed into a continuous tube mill, where it is formed into a round shape through a series of 10 or more forming rolls. Then, the seam is welded, resulting in a mother, or starting, tube. The weld is then monitored on the tube mill to ensure its integrity, after which the tubing is cut into 15- to 20-ft lengths as it exits the mill, becoming feeder stock for the cold-drawn process.
While drawn-tube processing is the conventional method for reducing the diameter and wall thickness of tubing to achieve the required specifications, a technique patented by Somerset, NJ–based Micro known as rolled-tube processing can also be employed to manufacture medical device tubing. Originally developed by the company to produce a reengineered disposable surgical device, this technique reduces component and subassembly costs and is used to manufacture endoscopic subassemblies.
|Micro fabricates both rolled (left) and seamless and drawn tubing (right).|
“Our rolled-Tube process produces tubing in a progressive stamping die,” remarks Carl Savage, vice present of sales and marketing at Micro. “The progressive stamping die was developed to stamp such features as holes, coiled areas, and trim areas in the early stage of the progression.” Once the features are completed, a rolling process begins in which a series of forms gradually starts rolling the flat stock material into the finished round tube shape. This process results in a longitudinal seam. The diameter of the tube, coupled with the material thickness, determines the rigidity of the tube and its movement, if any. In this process, the gauge of the base material is typically the required wall thickness of the finished product.
In conventional drawn and welded tubing, the weld seam is homogenized and blended into the tubing during successive drawing and annealing operations, according to Savage. “While there will always be a material difference between the seam and the rest of the tubing, the objective of the manufacturing process is to have this area as close to the balance of the tube as possible. Achieving this often requires additional draw/anneal operations. You can’t eliminate the annealing of the weld seam on drawn and welded tubing because it’s part of the process to obtain the required size and dimensional requirements.”
Tubing created using the rolled-tube process can also call for a welded seam, Savage adds. This seam can be continuously welded along the entire length of the tube or spot welded at specific locations. However, unlike cold-drawn tubing, rolled tubing typically does not undergo an annealing process following the weld.
Metal Tubing Challenges
“It is challenging to fabricate very small and straight tubing featuring diameters and wall thicknesses with tolerances less than 0.001 in., Lorenz remarks. This may be necessary if the final product contains components that are designed to mate or press-fit together.” Take morcellators or shavers, for example. Often consisting of a tube within a tube in which one of the tubes spins, such devices have clearances and material properties that are critical to the design performance and use life of the device. In addition, many devices are often assembled using laser welding, which inherently drives tight assembly tolerances because of the precision and very small diameter of the laser-welding beam.
Because of the process steps and high mechanical forces employed during cold drawing, it is also challenging to create defect-free surface finishes, Lorenz adds. Devices must be free of pits and scratches, which can harbor contaminants and become mechanical failure initiators if they are large enough. Above and beyond functional design issues, however, is the concern that surface blemishes are unsightly. “What surgeon in the operating room wants to see a device that isn’t completely uniform and consistent in appearance? Thus, manufacturers of medical device tubing are challenged to produce tubing that is dimensionally precise, has specified material and mechanical properties, is functional and safe, and features pristine surfaces.”
Bob Michaels is senior technical editor of MDDI. Reach him at [email protected].