Special 3D printer filaments are available with blowing agents mixed in. These blowing agents cause the filament to foam as it is extruded, causing the resulting structure to have a much lower density.
How do these blowing agents survive the manufacturing process? Since the filaments are spun out of almost-molten plastic, would that not trigger the agent? Or is the agent's reaction temperature able to be tuned so narrowly that it survives the filament extrusion process but not the printer melting process?