When mixing plastic solids (e.g., polymeric masses, dough-like materials) in processing, which operation most directly facilitates uniform mixing and dispersion of fillers or pigments?

Introduction / Context:
Plastic and polymer processing frequently requires mixing solid or highly viscous masses with additives such as pigments, stabilisers, or fillers. The operation chosen must generate distributive and (when needed) mild dispersive mixing in a high-viscosity regime.

Given Data / Assumptions:

• Material behaves as a plastic solid or very viscous paste.
• Goal: uniform incorporation of additives without degrading the base polymer.
• Equipment examples: sigma-blade mixers, internal (Banbury-type) mixers, twin-roller mills.

Concept / Approach:
Kneading applies strong shear and elongational deformation while continuously folding the mass. This promotes distributive mixing (spatial uniformity) and controlled dispersive mixing (breaking agglomerates). Mastication is more specific to raw rubber softening; high-speed propellers are ineffective in laminar, very viscous regimes.

Step-by-Step Solution:

Verification / Alternative check:
Industrial practice uses sigma or Banbury mixers for PVC plastisols, masterbatches, and thermoplastic compounds, confirming kneading as the standard choice.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing kneading with high-speed shear mixing; attempting propeller mixing in a paste, leading to overheating and poor uniformity.

Final Answer:
Kneading (using sigma or Banbury-type mixers)