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The detachment behavior of a polymer melt from the hot roll surface/s during roll-driven processes affects the maximum production rates achievable in various processes including calendering, embossing and shear roll milling. Here the continuous shear roll milling process is used to investigate the conditions that lead to the detachment of the polymer melt from the surface of one or both heated rolls. During the shear roll milling process the polymer melt needs to continuously stick to one roll and periodically to stick and then to detach from the other roll (at the nip). Different roll speeds, surface roughness and temperatures are used to facilitate continuous stick to one roll and periodic stick and detachment from the other roll. Shear roll mill process fails to convey the melt in the forward direction under conditions for which the melt detaches from both rolls. The shear roll mill processing of a high density polyethylene revealed the detachment of the melt from both rolls under various combinations of operating parameters. A calendering model enabling the use of different roll diameters and roll velocities for incompressible generalized Newtonian fluids under isothermal and lubrication flow conditions was developed and used to understand the detachment dynamics. The detachment of the melt from both rolls was revealed to occur when the maximum shear stress at the roll surface exceeds the critical wall shear stress at which the onset of macroscopic wall slip is observed in simple shear flow. These findings suggest that the processing window for roll-driven processes is affected by the wall slip characteristics of the polymer melt and that the processing window can be widened using a better understanding of the wall slip behavior of the materials being processed.

Detachment of the Polymer Melt from the Roll Surface: Calendering Analysis and Data from a Shear Roll Extruder