Steady-state extrusion force

Fig. 3 Steady-state extrusion force as a function of the length-to-radius ratio of the die for microcrystalline cellulose-lactose-water (5 5 6) at constant die diameter (1.5 mm) and extrusion rate (20cm/min).

Harrison et al. (27,31) obtained force-displacement profile during extrusion of microcrystalline cellulose (MCC) only formulations using a ram extruder and resolved it into three stages, as seen in Figure 9 compression, steady state, and forced flow. Based on surface smoothness and cohesive strength, a predominant steady state region was found necessary... 

The change from steady state to forced flow is caused by the movement of fluid under pressure. Extrusion in a ram extruder is continuous, and this phenomenon is... 

When rubber is deformed during processing operations such as extrusion or calendaring, its inherent elasticity will cause some subsequent recovery. A fundamental material constant characterizing this tendency is, defined in Eqs. (6.12) and (6.14). Elastic recovery is also reflected in die swell, D, defined as the ratio of the flnal and initial dimensions perpendicular to the flow. Die swell is governed by the normal force, although predictions of D for an arbitrary deformation are not possible. However, for steady-state flow there are approximate relations between D and the first (primary) normal force (Tanner, 1970),... 

Screw speed and barrel temperature are used as forcing functions to disturb the extrusion systems. Therefore, the motor torque, die pressure, and material temperature shift from initial steady states to new equilibrium. 

Figure 4 A force-displacement profile for a microcrystalline cellulose-lactose-water mixture showing the three stages of extrusion on a ram extruder Compression, steady-state flow, and forced flow (ram speed, 4mm/sec die diameter, 1.5 mm L/R ratio, 12). (From Ref. 31.)...

Steady state was possible with the MCC and MCC lactose samples but not with lactose alone. As can be seen with the MCC samples, the duration of the compression stage was water level dependent with no effect seen on the steady state stage. Additional studies indicated the effect of ram speed (extrusion speed) and die LjR ratio. An increase in ram speed increased duration of the steady-state stage with no effect on the compression stage. The LjR ratio had no effect on either compression or steady state. Wet mass composition, therefore, influenced the ability to achieve steady state while the water level and ram speed influenced duration. Higher water levels decreased the force to produce steady-state flow but increased... 

Pipe haul-off units form a key part of the extrusion process since they have the function of exerting a steady, uniform pull back through the cooling and calibration zones to supplement the extrusion pressure at the die head. The haul-off unit effectively creates a tensile stress in the solid material that balances the hydrostatic and viscous forces in the liquid state of the extrudate and overcomes the resistance of external friction. Haul-off units may be belt driven or multi-pad caterpillar systems. Large diameter pipes require multiple caterpillar units and thin wall pipe also require more units to avoid distortion. 

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