New Method Improves Polymer Mixing in Screw Extrusion

In plastic extrusion processes, uniform mixing stands as a pivotal factor determining product quality. This becomes particularly crucial when incorporating low-percentage color masterbatches, where backmixing—the axial counter-flow blending of materials—plays an essential role. The process requires masterbatch particles to undergo a dramatic size reduction from millimeter to micrometer scale within the extruder, presenting extraordinary mixing challenges.

Extrusion mixing aims to achieve homogeneous distribution of components, ensuring consistent product characteristics like color and mechanical properties. When adding low-concentration masterbatches, initial particle separation may exceed 100 mm. To achieve uniform coloration, final striation thickness must reach micrometer scale—a reduction of five orders of magnitude that demands exceptional mixing capability.

Traditional analysis focuses on cross-sectional mixing (within the screw channel cross-section), governed by Couette shear rate:

γ = πDN/H

Where D = barrel diameter, N = screw speed, H = channel depth. Typical shear rates (50-100 s⁻¹) combined with 20-second residence times yield 1,000-2,000 total shear strain units—sufficient for three-order striation reduction but often inadequate for visual uniformity.

Axial mixing (backmixing), conversely, is pressure-driven flow along the extruder axis. Understanding this mechanism proves vital for optimizing screw design.

For power-law fluids (τ = m(γ')ⁿ), dimensionless velocity φ=v/vmax relates to dimensionless coordinate ξ=2y/H as:

φ = 1 - |ξ|^((n+1)/n)

Newtonian fluids (n=1) exhibit parabolic velocity profiles with zero shear at centerline—creating mixing dead zones. As n decreases (shear-thinning behavior), profiles approach plug flow, expanding low-shear regions and complicating backmixing.

RTD analysis reveals how material residence times vary within the extruder. For power-law pressure flow between parallel plates:

v(y) = v_max * [1 - (2|y|/H)^((n+1)/n)]

The external RTD function f(t)dt derives from velocity distribution, showing that increased shear-thinning (lower n) narrows RTD—reducing backmixing efficiency. Pinto-Tadmor's single-screw RTD model for Newtonian fluids:

F(θ) = 1 - (1 - θ)²(1 + 0.35θ + 0.135θ²)

demonstrates how screw geometry further restricts RTD versus parallel-plate scenarios, emphasizing backmixing challenges.

Key issues arise from near-zero axial shear in screw channel centers. Effective solutions include:

• Mixing pins/slots: Disrupt flow patterns to redistribute material
• Inside-out mixers: Offset flights actively transfer core material to channel peripheries
• CRD mixers: Specialized designs promoting radial-axial material exchange
• Particle size reduction: Smaller feedstock decreases initial striation thickness
• Liquid colorants: Reduce initial striation but may affect barrel lubrication

Backmixing remains extrusion's most demanding mixing task due to inherently low axial shear, particularly in channel centers and with shear-thinning materials. Achieving five-order striation reduction requires either advanced mixing devices (like inside-out or CRD mixers) or reduced initial striation through feedstock modifications. Future innovations may combine geometric optimization with advanced material handling techniques to overcome these persistent challenges.