Extrude Foamed Wood Composites Without Drying.

A new two-stage extruder system devolatilizes and extrudes wood-filled compounds in one extrusion pass without needing a dryer or a second extruder. The first stage of the system is a preheating and drying section with two short, heated screws and a passive vent. It feeds vertically into a longer twin-screw plasticating and mixing section with three vacuum vents. This is the first extrusion system developed by Coronado Engineering Inc. in Salina, Kan., which typically designs and installs blending and compounding plants. Coronado delivered its first extruder system in May and has five more on order. The first is of 4-in. diam. and makes plastic lumber. Coronado is now designing 5-in. and 6.2-in. versions.

The new system increases the volume of melt exposed to devolatilizing by using screws with deeper roots for longer residence time. The system has 34:1 L/D, including both sections, but devolatilizes as effectively as a 42:1 conventional extruder, Coronado says. It can extrude foamed or unfoamed wood-filled PVC, PP, and PE at rates up to 1200 lb/hr.