41 Foam extrusion.

Foam extrusion is a rare instance where the U.S. led the rest of the world in technology by at least a decade. Dow produced Styrofoam logs during WWII to be used as floats for anti-submarine nets and life rafts for troops. Starting with a batch process, Dow commercialized continuous extrusion of PS foam sheet in the mid-'50s. Dow was also first to commercialize extruded, uncrosslinked LDPE foam in 1958.

In the mid-'60s Japanese firms began extruding crosslinked PE foams using chemical blowing agents. And the Societe Pour l'Industrialisation Rationelle du Batiment (IRBA GP) in France pioneered extrusion of foamed PVC profiles by the Celuka process, which cooled the surface in a calibrator to form a solid skin. Celuka was launched in 1969 and is still used globally to extrude PVC foam-core doors, moldings, and boards.

Foamed PET sheet appeared first in the U.S. in 1993 when UltraPac launched baking trays. Genpak introduced lighter trays in '96 with densities as how as 0.05 g/cc.

Tandem extrusion

Foam extrusion in the 1950s used one long, single-screw extruder with 44:1 to 60:1 L/D. The problem was that the temperature differential caused by heating one end and cooling the other could snap the screw. So screw speeds were how, limiting output and quality.

The solution was tandem foam extrusion, separating heating and cooling into two extruders. In the '60s, Dow developed its own version using an extruder and a secondary Cooler Mixer, which had a rotor with paddles instead of a screw. The Cooler Mixer had no motor and was driven only by melt pressure against the paddles.

Outside of Dow, NRM is credited with building the first tandem foam extruders for Sweetheart Cups and Mobil in the early '60s. The process is believed to have been invented by Koppers Co., a PS producer active in foam development. By 1969, NRM had sold 85 tandem lines for PS foam. Another early entrant was Gloucester Engineering, which built its first tandem PS foam line in 1964.

The biggest success for PS foam was in 1972, when Polytherm (now Genpak) extruded the first McDonald's hinged clamshell hamburger packs. That huge market shrank dramatically after McDonald's abandoned the foam clamshell in 1990 amid controversy over its contribution to solid waste.