American Leistritz Extruder Corp. ran a carbon dioxide foam profile line at NPE 2003, the first time by a twin-screw extruder supplier at the Chicago show, according to the company.
Leistritz produced a closed-cell foam of a Nova low density polyethylene. A key feature of the system is rapid solubilization of carbon dioxide liquid in the polymer melt.
``Carbon dioxide liquid is far easier to mix in the polymer melt than is the gas,'' said Bill Thiele, company founder and now technical consultant to the Somerville, N.J., firm.
``Over the next decade we will see the advantages of going to environmentally friendly alternatives like carbon dioxide,'' Thiele said when comparing the NPE line to the industry's common use of hydrocarbon blowing.
Thiele said the 50-millimeter, twin-screw line powered by a 150-horsepower, alternating-current motor can make up to 200 pounds per hour of foamed plastic, but it was running at well below that rate at NPE to minimize the amount of scrap foam for disposal.
After the initial melt and mixing section, liquid carbon dioxide is pumped into the polymer melt, where it dissolves and reduces the melt viscosity. A heat exchanger downstream cools the melt and brings the viscosity back up. At the die exit, pressure is relieved and carbon dioxide expands to foam the cooling polymer.