Figure 1 shows the steps of the ENTEK manufacturing process. Precipitated silica is combined with ultrahigh molecular weight polyethylene (UHMWPE), process oil and various minor ingredients (e.g., antioxidant, carbon black) to form a mixture that is extruded at elevated temperature through a die to form an oil-filled sheet. The oil-filled sheet is calendered to a controlled thickness with the desired rib pattern. Next, the majority of the process oil is extracted with an organic solvent. The sheet is then passed through a dryer and hot air oven to remove the solvent and leave behind a porous structure. Finally, the sheet is slit at multiple positions to form rolls of microporous polyethylene separators that have the appropriate profile for customers’ battery designs. The term “profile” refers to the width, backweb thickness, rib height, and shoulder design of the separator.