Post Extrusion Cooling of Multilayer Polymer Sheet on Chilled Rolls

To meet the stringent emission and environmental regulations, polymer sheet with as many as six layers is required for thermoformed fuel tanks used in the automotive industry. The 10 mm to 12 mm thick sheet is made using an extrusion process. Typically each polymer layer has its own hopper, screw, barrel, nozzle and extruder. During manufacturing, polymer layers are fused together within the tool and the multilayer polymer sheet leaves the extruder in a viscoelastic state. The multilayer sheet is initially cooled by passing it over chilled rolls. The final cooling of the sheet is done via natural convection to the ambient air while the sheet is transported to the cutting station at the end of the line. The cooling of the sheet on the chilled and polished rolls has a direct influence on the quality of the sheet as well as the scrap. The conductive heat transfer is primarily responsible for the cooling while the sheet is on the chilled roll. It is desired to have the smooth sheet exterior surfaces with constant thickness of each polymer layer in the cross section. Due to the numerous materials in the cross section, and associated variability of material properties, it becomes a challenging task to meet these requirements. This paper discusses the problem of the smoothness of the exterior surfaces of the extruded sheet. The “dimpled” or “orange peel” surface finish is observed to be linked to the cooling of the sheet on the chilled rolls. Experimental data and simulation results are presented that relate the formation of dimples to the local cooling rate. The variability of thermal contact resistance between the sheet surface and the chill rolls also appear to be another variable that contributes to the dimpled surface.