Method and apparatus for preparing thermoplastic resin foam

A method for preparing a thermoplastic foam board having a thickness greater than 10 mm by extruding molten thermoplastic resin having uniformly distributed therein a foaming agent for said resin, the method comprising the continuous steps of:

• (a) introducing an extruded mass of said molten thermoplastic resin from an extrusion zone into a cooling zone;
• (1) the cooling zone comprising an elongated barrel having a rotating member positioned axially therein in spaced relationship to the barrel so as to provide a passage for the extruded mass between an outer surface of the rotating member and an inner surface of the barrel,
• (2) the temperature of the cooling zone being controlled by fluid heat exchange, and
• (3) the extruded mass being under pressure in the extrusion zone and in the cooling zone;
• (b) passing the pressurized extruded mass through said passage while simultaneously:
• (1) intimately mixing the mass by means of projections extending radially from the outer surface of the rotating member into the passage and reaching a close proximity to the inner surface of the barrel and
• (2) substantially lowering the temperature of the mass, the cooled mass being maintained in its molten flowable state;
• (c) dividing the cooled mass into a plurality of separate, non-parallel streams in a zig-zag mixer under substantially adiabatic conditions;
• (d) separating and recombining at least a portion of said streams in said zig-zag mixer under substantially adiabatic conditions to form a combined homogeneous mass having a substantially uniform temperature;
• and
• (e) extruding the homogeneous mass into a lower pressure zone to form a substantially uniform thermoplastic foam.