In counterrotation, the surface velocities of the screws in the intermesh region work in the same direction which results
in materials being forced between the screws where extensional mixing occurs, referred to as the calendar gap. Geometric characteristics
inherent with counterrotating screw designs makes it possible to have up to six lobes at the same flight depth as bilobal
co-rotating designs, which results in more mixing events for each screw rotation (see Figures 7 and 8).
Figure 7 (All figures are courtesy of the author.)
LSLF twin-screw extruders.
The LSLF counterrotating, intermeshing twin-screw extruder is designed for gentle melting and mixing combined with a narrow
residence distribution and high-pressure generation capabilities. The screw flights converge in the same direction in the
nip region, causing the gap between the flights to be small and minimizing leakage from one screw channel to the next. Mixing
occurs between the screws as the material experiences shear effects in the calendar gap. This device can be used for shear-
or temperature-sensitive materials that do not require intensive mixing.
Figure 8 (All figures are courtesy of the author.)
Generic twin-screw extrusion related formulas
Shear forces result in mixing, which is the primary function of most HSEI twin-screw extruders. Shear rate describes the velocity
gradient between two surfaces moving at different speeds. For a HSEI twin-screw extruder, this is a function of screw outside
diameter, screw speed, and overflight gap.
The following formula is relevant:
Peak shear rate = (π × D × n)/(h × 60)
in which D is the screw diameter, n is the screw speed in rpm, and h is the overflight clearance. So for a HSEI twin-screw extruder with 27 mm OD screws and an overflight gap of 0.1 mm operating
at 600 rpm, the following applies:
Peak shear rate = (π × 27 × 600)/(0.1 × 60) = 8478/s-1
This equation does not take into account extensional shear, an important component for dispersive mixing but does provide
a usable benchmark for comparison and troubleshooting purposes.
The magnitude of the applied stress that the materials experience is a function of the shear rate and viscosity, and is reflected
by this formula:
Shear stress = Viscosity (Ec) × Shear rate
Barrel temperatures are used to manage the viscosity of the melt, which impacts the mixing quality. Cooling is often used
to raise the viscosity (Ec), which facilitates dispersive mixing in a HSEI twin-screw extruder. A smaller Ec is preferred if distributive mixing is the goal.
Specific energy (SE) is the amount of power that is being input by the motor into each kilogram of material being processed.
This is calculated in two steps:
kw (applied) = kw (motor rating) × % torque × rpm running/max. rpm x 0.97 (gearbox efficiency)