Stainless steel tubing in medical devices can be formed, cast, and swaged. But to get a part that does the best job, pay a bit more attention to these design details.

Transition angles: When forming a tube with rotary swaging, the optimal tube transition angle is seven degrees per side. The swager works like a set of dies rotating and clapping together. If the angle is larger than seven degrees, the rotating dies spit out the tube. As the angle increases, the force trying to spit the tube increases. For straight line forming, the magic number is 15 degrees. An angle less than 15 degrees generates what is called a locking taper. The taper and tube essentially don't want to come apart after forming the tube. They want to stick together because the angle is so shallow.

Transition radii: In both rotary swaging and straight-line forming, the larger the transition radius, the more economical it is to hold close tolerance on the diameter, and the better the resulting finish. A good rule of thumb is not to have any radius smaller than one wall thickness.

Changing tube diameter: For maximum economy, the percent change in tube OD should equal the percent elongation. For example, if elongation properties of a tube are 35%, the tube can be reduced or expanded 35%, usually without having to add an annealing step.

Wall thickness: A good rule of thumb says if there is no attempt to control wall thickness then

t = [0.8(D/d - 1) + 1] T

where D = original tube diameter (in.); d = formed tube diameter (in.); T = original wall thickness(in.); and t = formed wall thickness (in.)

Strength of form: A swaged taper is strong and resists bending. A form elongated in the bending-force direction increases its strength. Beads, grooves, and ribs increase the hoop strength of a round tube.

Strength of grain flow: Chipless forming, as in swaging, does not decrease the part's strength by cutting across grains. Rather, it causes the grain to flow with the configuration of the part, thereby maintaining or increasing the part's strength.

Eliminate assemblies: It's often possible to combine several formed tubular parts into a single-piece formed tube. This eliminates the need for adhesives or welding, which create heat distortion and finishing problems.

Weldless assemblies: Weldless metal-to-metal assemblies may include parts that are mechanically joined such as a cast tube that is inserted and locked into place. Weldless assemblies have more resistance to vibration fatigue, no heat distortion, and can join dissimilar materials. Weld-free assemblies means heat sensitive parts can be joined and there are no expansion-coefficient problems.

Choosing plastic tubing

Plastic tubing manufacturer NewAge Industries, Southampton, Pa., offers two updated brochures to help specifiers and buyers of flexible tubing and hose select the right product for their application. One brochure, “The Top 12 Tubing and Hose Buying Errors” points out criteria often overlooked when making decisions. This includes suction and discharge requirements, ingredient compatibility, and knowing not only the temperature of what's running through the tubing but the temperature of the surrounding environment.

Another fact sheet, the company's Products & Services brochure, gives an overview of the manufacturer's tubing, hose, fittings, clamps, and accessories. Products are listed by material and style, and include unreinforced, braid or wire reinforced, overbraided, and coiled. The brochure details sizes stocked as well as the many custom options, including heat-formed shapes, hose assemblies, and thermal bonding, available. Request the brochures at www.newageindustries.com/inforeq.asp