He's president of Welex Inc. (Blue Bell PA) a major manufacturer of extrusion sheet line equipment. He referred to the report on calendering published in the June issue (see p. 26). According to Nissel calendering is "never" used if extrusion can also process a polymer because extrusion is less expensive and more flexible. And he says in a letter to PW the tolerances and sheet quality are the same for both processes. Thermal instability of resin often drives producers of polyvinyl chloride and Barex® modified acrylonitrile toward calendering instead of extrusion. However Nissel says that certain sheet suppliers have created a profitable niche by extruding PVC with qualities that cannot be matched by those produced via calendering. Nissel concludes that the article on calendering "was planted as a last ditch defense of PVC as the clear sheet...and nobody mentioned chlorine or recyclability." Despite the long experience of Frank Nissel PW editors believe that its report was both accurate and informative.