TWIN® Knife Manufacturing: SIGMAFORGE® SIGMAFORGE® is one piece precision forging used in manufacturing several ZWILLING J.A. HENCKELS TWIN® knife series including TWIN® Four Star II, TWIN® Select, TWIN® Pro "S", TWIN® Five Star, and Four Star. SIGMAFORGE knives are forged from a single piece of steel and have the following benefits: Perfect geometry High forging precision Higher stability of the blade Less metal fatigue - improved steel structure Improved cutting edge retention and lasting sharpness Friodur® ice hardened - Maximum no stain properties and more blade strength Shown below are examples of SIGMAFORGE knife series; TWIN® Pro "S" and TWIN® Four Star II TWIN® Pro "S"