Analysis of co-rotating twin-screw extruders by finite element method

Co-Rotating twin-screw extruders are widely used in the plastic compounding industry because of the high efficiency of mixing. Quantitative methods for designing screws for such extruders grossly lags behind that of single-screw extruders and is mainly an art with "rules-of-thumb".

Our research objective was to model and simulate polymer rheological responses in twin-screw extruders by using finite element numerical methods. FIDAP, a finite element code, was used in this work. The Phan-Thien / Tanner (PTT) equation, a rheological constitutive equation, was applied. The convective term in this equation was approximated by the Generalized Newtonian equation. The PTT equation was then solved by the NewtonRaphson method. The stresses were treated as pseudo-body forces. The overall iterative scheme used was a successive substitution algorithm. Density, specific heat and heat conductivity were assumed to be constant and independent of temperature. Heat transfer was included in the calculations. It was assumed that the screws rotate at low speeds, so that a pseudo steady-state assumption could be imposed. Four rotation positions were chosen to represent the whole cycle of rotation.

The effect of perturbations of the parameters in the PTT equation on viscosity was studied. Shear viscosity was observed to decrease with an increase in the relaxation time (λ), the material parameter (ε) and the non affine motion parameter (ξ) in the PTT equation. The vectors representing the first principal stress difference were calculated. These vectors indicate the magnitude and direction of the forces applied on the fluid. Thus, the dispersive mixing of the polymer in the twin-screw extruder can be understood better.