Coextrusion heads are widely used to produce multi-layered products such as pipes and vessels in plastics industry. Spiral mandrel dies which are used in coextrusion heads provide good thickness uniformity with a broad range of processing parameters (raw material, throughput, temperature), short residence times (material or colour changes), low pressure drop and good thermal control. In the present study, the effects of operating conditions such as production rate and temperature on the pressure drop through the spiral mandrel die and the occurence of melt fracture are investigated by Computational Fluid Dynamics (CFD) simulations. The temperature dependent viscosity versus shear rate data for grade QB79P (CarmelTech) polypropylene (PP) melt under study are measured by use of an Anton-Paar Physica MCR 301 model rotational rheometer with a 25 mm diameter parallel plate geometry and a capillary rheometer using capillary dies of various lengths with 1 mm diameter hole [11]. The numerical simulations are performed by use of Ansys Fluent, a commercial software [12]. Stress terms in the momentum equations are modeled by Generalized Newtonian Fluid (GNF) Model. For this, Bird-Carreau Model employed as the viscosity model for the polymer melt. In order to avoid exceeding the critical shear stress which causes melt fracture and the value of which is dependent on the processing material, the operating temperature may need to be increased to reduce the viscosity or the flow rate to be decreased. In this case, operating cost increases as a result of energy consumption for heating and the production rate decreases, respectively. Hence, designing of flow channels of the spiral mandrel die by taking into consideration of material property (melt fracture) and extruder limitation (maximum pressure to be supplied) is of critical importance for extrusion with desired production rate at a specific processing temperature.

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