Method and apparatus for forming an expandable foam pellet having a hard outer shell by underwater pelletizing, and expandable foam pellets formed thereby
A pre-expanded hard shell thermoplastic foam pellet is made by controlling the temperature of the melt exiting a die plate in an underfluid pelletizer, and by controlling the temperature and pressures of the cooling fluid as the pellet flows from the cutting chamber through the slurry line toward a centrifugal dryer. The process and apparatus used for controlling the above parameters is described in conjunction with making the pellets. The pellets thus formed may have a generally spherical shape, or they may have odd, irregular shapes with foam hemorrhages protruding therefrom, depending on the conditions during pre-expansion.
1. An extrusion process for producing pre-expanded foam pellets having a hard outer shell, the process comprising:
metering a blowing agent into a polymer melt;
forcing the polymer 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 at a temperature and pressure effective to produce one or more foam pellets having a hard outer shell and a pre-expanded foam center.
2. The process of claim 1 , wherein the blowing agent comprises a propane, butane, pentane, neopentane, isopentane, hexane, CO 2 , or combinations thereof.
3. The process of claim 1 , wherein the blowing agent is added and mixed at a rate of between about 1 and about 7 percent weight.
4. The process of claim 1 , wherein cutting the extruded melt further comprises varying the programmed rotation speed of the cutter blades.
5. 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.
6. The process of claim 5 , further comprising drying the one or more foam pellets.
7. The process of claim 6 , further comprising further expanding the defluidized foam pellets via one or more of a heat and a steam treatment.
8. A pellet formed by the process of claim 1 .
9. A pellet formed by the process of claim 5 .
10. The pellet of claim 8 , comprising a hard outer shell, and a pre-expanded foam center, wherein the hard outer shell encapsulates the pre-expanded foam center.
11. The pellet of claim 10 , wherein the pre-expanded foam center is partially expanded such that it can be further expanded.
12. The pellet of claim 10 , wherein the hard outer shell is generally spherical.
13. The pellet of claim 10 , further comprising a hemorrhaged portion.
14. The pellet of claim 10 , wherein the pre-expanded foam center has a hollow center region.