The formula for finding the intake capacity is:
Capacity = Free area x lead
x screw speed x bulk density
x conveying efficiency x degree-of-fill
Since both 'free area' and 'lead' are related to the screw diameter (Do), it can be seen that the capacity of an extruder varies as the cube of the screw diameter (Do).
With an intake zone of 80 length requiring a compression of 20 bar at melting, while feeding talc that has low frictional coefficient (about 0.2), the conveying efficiency is generally about 20 per cent for typical intake elements.
These elements include regular conveying elements (RSEs) and SK type elements (SKEs) where the degree-of-fill is less than 0.2. With regular elements, at higher degrees-of-fill, the conveying efficiency decreases sharply. This is the reason, that in a starve-feed extruder, the hopper is never allowed to flood or fill-up. While processing low bulk density material, it can be practically seen that a flooded hopper has a lower feeding capacity than an empty one. In other words, once the intake zone is filled up (degree-of-fill = 1.0), conveying efficiency drops to less than 4 per cent.)
While feeding most other polymer material with medium frictional coefficient (about 0.4),
the conveying efficiency is 30 per cent, even at higher degrees of fill up to 0.4. With calcite (frictional coefficient 0.74), the conveying efficiency is the highest (about 50 per cent) at a similar fill ratio. With these materials, increase in degree-of-fill does not decrease conveying efficiency significantly. Therefore, highest capacity is achieved when the flights are fully filled. This is the reason why a flooded hopper always creates a torque overload with most polymers.
With a deeper flighted extruder (Do/Di = 1.71) the capacity increases. This is due to increase in free area and improvements in conveying efficiency. Do is the outer diameter of the screw, and Di is the inner or root diameter of the screw. Apart from the advantage of greater intake capacity, deeper flights lead to increased efficiencies and reduced shear rates in a twin-screw extruder. With higher screw speeds, this feature is advantageous for increasing the volumetric capacity of the 'extruder, while maintaining the mixing rates.
