Twin-screw extrusion feeding

As the screw turns, it conveys the pellets from the feed zone towards the melting zone. A combination of external heating and mechanical work melts the polymer as it is transported towards the metering zone. The metering zone pumps a uniform stream of molten polymer to a forming device, such as a profile die. Other types of extruders that employ two or more screws are commonly used for compounding polymer blends. The principles of twin screw extrusion will be discussed in Chapter 12. 

Molten formulation also presents the advantage of preventing the use of solvents, which helps reduce the environmental impact, increase dispersion and reduce costs. The mixing and reactions can be performed by twin-screw extrusion, for example, with a rotation speed of 30 rpm and a temperature profile of 80, 85 and 90°C from the feed hopper to the screw. 

Muller M, Krause B, Kretzschmar B and Potschke P (2011) Influence of feeding conditions in twin-screw extrusion of PP/MWCNT composites on electrical and mechanical properties. Compos Sci Technol 71 1535-1542. 

Extrusion alone cannot reduce the antigenicity of soybeans. Soy allergies are becoming more common as more people consume soy products. Twin screw extrusion with forward feed screw elements reduced soy meal antigenicity as barrel temperature was increased from 70 to 134°C (Ohishi et ai, 1994). The addition of kneading disc elements toward the die end of the screws produced comparable reductions at only 66°C. Feed rate and screw speed did not affect antigenicity reduction. 

Unlike single-screw extrusion, numerous materials, both solid and liquid, are fed downstream in parallel twin-screw extrusion. This provides maximum extrusion flexibility by allowing materials to be introduced into the melt at different stages or locations along the extruder barrel. Examples of downstream feeding advantages are ... 

Feed rates With twin screw extrusion, much attention is paid to controlling the feed rate of the plastic to the machine, so as to minimize disruptions in the continuous stream of molten plastic to the die. With single screw extrusion and molding, gravity typically provides adequate control of the feeding process. 

Figure 12 Influence of the amount of water on the extrusion forces (gravity feed extruder) and power consumption (twin screw extruder) on extrusion of MCC. , gravity feed extruder (N) , twin screw extruder (W). Source From Ref. 37.

A gravity feed extruder with L/D = 2 recorded (28) higher forces, compared to power generated from a twin-screw extruder with L/D = 0.9, upon extrusion of mixtures of MCC with either lactose or DCP. The extrudate from the basket extruder was denser with a smoother surface than that from the screw extruder. Similarly, material extruded through a screen with L/D = 4 in a basket extruder was denser than that with L/D = 2 (37). However, a smaller L/D ratio is preferred in an axial extruder. An improper L/D ratio could result in a loosely bound extrudate with large surface defects (37), resulting in a formulation less amenable to pelletization, as seen from Figure 14. Since the mean diameter of the pellet often approximates the diameter of the die used in extrusion, choice of the latter is also dictated by desired pellet size. 

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