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Wall-slipping materials
The content is currently being processed and will be made available shortly. Please be patient. When considering the flow processes in rheometers but also in practical modules such as dies, plasticating units and between gaps, the flowing melt is presumed to be wall adhering. In polymer melting this is true for the majority of the materials. However, there are some materials, among which the ultrahigh molecular polyethylene of high density (HDPE) and the economically important polyvinylchloride (PVC, frequently provided with outward lubricants for a gentle processing), which are wall slipping under certain conditions.
In order to calculate the plastisicating process with the simulation programs REX and PSI, the option “wall slipping” has been added in the software. For this, critical shear stress values must be entered in the rheological material data as explained in the Chapter Rheological Material Data.
When choosing the menu item Wall slipping, REX/PSI considers that wall slipping occurs above the critical wall shear stress. For this, the shear stresses at the barrel wall and the root surface are evaluated and limited to a value whose maximum is equal to the critical shear stress at the wall.
$$τ_ {WH,S} > τ_{krit} ⇒ τ_S = τ_{krit}$$
This limitation affects the flow profile in the screw channel and decreases the local pressure gradient. Thus, the pressure build-up capacity decreases but also the pressure consumption of overridden screw sections.
Platzhalter Abb. 9.36: Druckverlauf mit und ohne Wandgleiten