What are the functions of a three-roll calender? How should the process be controlled during extruded board production?

The melt extruded from the flat die head should immediately enter the three-roll calender, as shown in the figure below, whose function is: 
① Smooth the slab surface and cool it to set the shape; 
② Exerts a certain pulling effect on the slab, adjusts the speed consistency across different points of the plate to ensure its flatness. 
The three-roll calender typically has the center roll fixed in position, while the upper and lower rolls can move vertically to adjust the roll gap. All three rolls are hollow and equipped with jackets through which steam, oil, or water can be circulated for temperature control. The first and second rolls together apply pressure to the sheet blank, compressing it to the desired thickness and ensuring uniform thickness and a smooth surface. The second roll further calenders the sheet to improve surface finish and cools it to set its shape. The third roll serves both calendering and cooling functions; its surface must be chrome-plated and polished, with a surface roughness Ra ≤ 0.2 μm. During extrusion, the three-roll calender should be positioned as close as possible to the die head, generally within 5–10 cm. The circumferential speed of the calender rolls should have a wide adjustable range, with a typical speed ratio of about 1:20 and a maximum circumferential speed of 2–8 m/s. MPa.
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