Method and apparatus for forming an expandable foam pellet having a hard outer shell by underwater pelletizing, and expandable foam pellets formed thereby

An extrusion process for producing pre-expanded foam pellets having a hard outer shell, the process comprising:

• feeding a polymer material into a material blending system to form a blended material;
• feeding the blended material into a throat of an extruder to form a polymer melt;
• metering a blowing agent into the melt;
• extruding the melt through an extrusion die and into a cutting chamber comprising a fluid inlet and a pellet slurry outlet and enclosing a cutter hub assembly comprising a plurality of cutter blades;
• cutting the extruded melt by rotating the plurality of cutter blades at a programmed rotation speed to produce one or more pellets;
• directing a flow of fluid in the fluid inlet, through the cutting chamber, and out the pellet slurry outlet to pull the one or more pellets away from the cutting chamber in a pellet slurry;
• and
• cooling the one or more pellets in the pellet slurry effective to produce one or more foam pellets having a hard outer shell and a pre-expanded foam center.