Optimal Extrusion Die Design for Production of Polymer Tensile Test Specimen

One of the processes to form plastic objects is extrusion. A process that consists of forcing a melt of thermoplastic material to flow through a given section to obtain a long and continuous product. In the extrusion process, the material enters in the form of pellets or granular powder. Most of these pellets are standard materials with known mechanical and thermal properties; however, at the Processing Laboratory of Plastics at ESPOL, new materials are processed, thus there is the need to perform tests for determining their mechanical and physical properties.  For this reason, the design and construction of an extrusion die to produce plastic slabs that later will be cut into tensile test specimens is necessary. This work aims to present and describe the design of a new extrusion die with uniform exit flow. Computational Fluid Dynamics (CFD) is used during the optimization process to achieve the minimum pressure drop across the die, low energy consumption, high output and excellent properties of the extruded specimens.

A computational model based on the finite volume method is used to simulate the polymer flow across the die. A three-dimensional incompressible viscous flow is considered and the Cross model is employed for taking into account the non-Newtonian behavior of the flow. Moreover, the CFD model is validated with experimental results from Vlachopoulos and Scott (1985). The working fluid is low-density polyethylene (LDPE) at 190 ^oC. Material flow velocity fields, tangential stresses, and pressure contours were obtained for different polymer flow conditions to achieve the final shape and length of the die and the shape of its internal cavity. The final design includes, in the beginning, a conical reduction area and then it gradually transitions into the rectangular cross-section of the slab. This second zone is an elongated flow restriction channel of smaller cross-section, where the material is supposed to experience gradually-decreasing pressures along the length of the elongated flow restriction channel and finally converging the polymer into a melt laminate at the exit. This final design was optimized for a mass flow rate of 7.89x10^-3 kg/s and atmospheric pressure at the exit. This operating condition is obtained for a screw rotational speed of 40 rpm. The pressure drop across the die is 1.4 MPa.

Optimal design of the internal die cavity was achieved, in which the pressure gradient could be reduced, by widening the cross-section area at a specific location. This section increase is included to decrease the flow average velocity and thus increase the local static pressure in this section to promote a uniform pressure gradient along the flow direction without sacrificing volumetric flow rate in the extrusion die. Also, a coat-hanger transverse section was included at the transition between this widening section and the final shape of the slab, which promotes uniform velocity flow at the exit. Finally, the die was manufactured in two identical parts due to the complexity of the die shape. Each part was CNC machined to form the internal cavity, which were then joined using bolts.