The mixing action in an extruder can be described as the operation of plasticizing and melting polymers, adding other polymers, fillers, and additives, and obtaining a uniform compound. The states that occur during this mixing fall into two forms: dispersive mixing and distributive mixing. There is a dispersive action that breaks fillers and polymer gels into fine particles, and a distributive action that stirs the compound to make its composition uniform. When these two actions are balanced and carried out properly, a uniform compound containing different polymers and fillers can be obtained.

Dispersion and Distribution

Dispersive Action

Dispersive action breaks the compound into fine particles by applying forces such as shear and elongation. Through this action, the particle size within the compound is reduced. For example, strong dispersive action is required to break apart fillers with strong cohesive force or substances that tend to agglomerate.

In the process of mixing polymers, a method that finely separates liquids that do not mix well with the matrix resin, or solid particles that have agglomerated, is called “dispersive mixing.” In this case, the influence of internally generated stress plays a more important role than internal strain. In other words, to efficiently distribute clusters of solid particles or liquid droplets within the matrix, controlling the flow during mixing is essential.

In the production of polymer blends and compounds, dispersive mixing is indispensable for droplet breakup, deagglomeration of fillers, and fiber separation. Dispersive mixing is the process of refining different materials and distributing them uniformly, which improves the homogeneity and performance of the product.

Distributive Action

First, distributive action is the action of stirring the compound to suppress variation in composition and physical properties. Distributive action involves elongation, splitting, and rearrangement, and the entire system flows in a complex manner. The flow that promotes distributive action can be described as a process of dividing and recombining streams. The positions of different materials and fillers within the fluid are exchanged, and through the accumulation of these divisions and recombinations, distributive action progresses.

When two viscous liquids are mixed, a state in which 1 component is distributed as droplets or layers within the other is called “distributive mixing.” At this stage, by applying strain, the interfaces between the components expand, the droplets and layers become smaller and thinner, and the mixture reaches a more uniform state.

Distributive mixing becomes especially important for fillers such as glass fiber and carbon fiber, which must retain their shape to perform their function. It is essential to distribute these fillers evenly within the material, and distributive mixing is effective for that purpose.

Shear and Elongation

Shear flow and elongational flow are important concepts that describe the different modes of deformation when a fluid or material is subjected to force. They receive particular attention when dealing with viscous fluids and non Newtonian fluids such as polymers. The flow of a material is broadly classified into two types: shear flow field and elongational flow field. The figure below compares the images of each flow field.

Shear Flow

Shear flow refers to the flow that arises when different layers of a body move parallel to one another. Adjacent layers move at different velocities, and shear stress develops between them. It is a sliding deformation of the body. In shear flow, different layers of a fluid or material move at different velocities. As such, the velocity distribution in shear flow has a characteristic velocity gradient.

Elongational Flow

Elongational flow is a flow that arises when a fluid is stretched. The material extends in one direction or is pulled in multiple directions, producing a flow form that involves volumetric change. Unlike shear flow, the deformation in this flow concentrates mainly along the direction of stretching. Because the fluid or material is being stretched, the entire material spreads in one direction. The velocity distribution shows a uniform variation in velocity.

Simple Shear and Simple Uniaxial Elongation

The figure above shows simple shear, which appears in a shear flow field, and simple uniaxial elongation, which appears in an elongational flow field. In elongational flow, there are not only uniaxial modes but also planar and biaxial modes of stretching. Each flow mode produces different dispersion efficiency.

Flow Behavior at the Tip Clearance

The flow of material inside an extruder can also be divided into “shear flow” and “elongational flow.” In general, shear flow is considered to be the main contributor in a twin screw extruder. Elongational flow, however, plays a very important role in dispersive mixing as well, so both must be taken into account, not shear flow alone.

Shear flow and elongational flow at the tip clearance, the narrowest gap between the screw and barrel or between the two screws, are critical factors. As material passes through the tip clearance, high shear stress is repeatedly applied, and dispersive mixing proceeds. For distributive mixing, elements with complex geometries are sometimes used to generate complex flow fields. These stir the compound and promote uniformity of composition.

*For more details, please also refer to the explanation page “Optimization of Tip Clearance in Twin Screw Extruders.”

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