Dry expansion of expandable polymer beads

Abstract

Pre-expansion of expandable polystyrene (EPS) and other expandable polymer beads has long been carried out with heat of condensation from steam, which permeates the beads and deposits water droplets within expanding cells of each bead. Because long conditioning periods (to dry the pre-expanded beads) allow much of the blowing agent (e.g. pentane) to escape, the resultant pre-expanded beads often lack sufficient pentane to permit adequate final expansion when the pre-expanded beads are used to mold coffee cups and other articles. When conditioning periods are too short, excess water droplets often cause variations in bead fusing and even leaks in the molded product. The present invention avoids these long-standing problems by using a heated gas, such as air, to pre-expand the raw EPS beads through conduction only. The resulting pre-expanded beads differ from those pre-expanded with steam because thermal conduction enlarges peripheral cells more than interior cells in each bead, while steam acts by convection to permeate each bead (and condense within each cell), thereby expanding both peripheral and interior cells to substantially the same degree.

Dependencies are inferred from the claim text.