Some silicone-tubing applications require the durometer to vary along the length yet remain unitary in construction. Engineers at Specialty Silicone Fabricators Inc., Paso Robles, Calif., have figured out how to change durometer “on the fly” for extruded tubing.

For example, the durometer of an inserted or implanted tip of a peripherally-inserted central catheter is soft, flexible, and tissue friendly. The proximal end must be more durable and kink-resistant with a greater crush strength. In another application, it may be useful to provide a unitary catheter with radiopaque markings to indicate distance along the catheter. Durometer-switch extrusion can also be used to reduce vibration and it can be a partial switch, that is, it doesn't have to be a complete cross-section durometer change.

The company can work with greatly varying durometers. For example, they are currently working on a 80/50 duro switch. There are two heads and two methods for switching, depending on length in which the switch is to occur and switch speed.

The materials mix at the switch boundary during extrusion, which makes a good bond. The silicone is vulcanized at the same time. Dissimilar durometer materials bond extremely well when cured at the same time especially when the polymer chains are aligned during the extrusion. The sheer on the material makes for a strong joint.