Large cells (hot line) in foam board.

[jozefiak]

Working on extruding a foam board using Ultem resin (1000) a polyetherimide. Making a board 1.5"thick 48"wide. getting large cells in the center of the board. Most likely the center continues to expand (hot) while the outer cooles faster. This is causing low compression properties. Is there an additive available that controlls cell size. Perhaps something endothermic to help cool the center?