There seems to be a general assumption that all wires have circular cross sections. While this is quite understandable—electrical wiring is almost always circular, steel wires are almost always circular, in fact, almost all of the wire in the world is circular—it is incorrect. Before we proceed, let’s start by clarifying what wire is, versus other nitinol products.
Wire is a continuously produced material manufactured by drawing (pulling) the material through a die. This is different from bar, which is rolled, or extrusions, which are pushed through a die. Now, if you think of wire drawing as a variation of extrusion, then it makes sense that a wide variety of cross sections are possible.
The most common non-traditional cross section is rectangular (or square). In medical, rectangular wires are frequently used to make bone staples. In springs, a face wound spring (the wide edge of the wire is wrapped on the mandrel) generates higher forces for the same outside diameter, with wider wires generating proportionately higher forces. At the same time, an edge wound spring (the narrow edge of the wire is wound on the mandrel) has increased lateral stiffness—which is excellent for freestanding springs.
Other cross sections that are occasionally requested include triangular and elliptical wires. These wires are quite rare and very challenging to conduct engineering with, so they’re usually used by companies with large teams of nitinol engineers.
One cross section that we make from time to time is the star cross section (shown above). The number of points in the star can be as many as desired, but the general shape remains the same. The benefit of this cross section of wire is that it has much better heat transfer. With a radically increased surface area, not only is the rate of heat exchange radically increased but the time to heat the center of the wire is radially reduced because the distance from the edge is radically reduced. Since cooling is often a driving problem in nitinol applications, using a non-traditional cross section like this can often make the difference between meeting missing them altogether.
If you’re considering a non-traditional cross section like this, it’s also important to note that it’s usually cheaper to simply draw a spool of wire than it is to grind thirty meters as a sample.
For more information, contact us at NITINOL@KelloggsResearchLabs.com