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The
EXTRUDER
TIMES
Published by
CONSULTANC Y SERV I CES , I ND I A
How is the starve-feed intake capacity of an extruder determined?
- Nikhil Shah, Baroda
Dear Mr. Nikhil,
During starve feeding, the intake capacity of the extruder depends on the Screw RPM,
the Geometry of the intake element such as shape, free volume and lead, and the bulk
density of the input material. Furthermore, the efficiency of conveying depends on the
degree of fill and the type of input material (frictional coefficient). It may be seen with
a glass extruder that at low degrees of fill the conveying efficiency is close to 100% for
most type of material.
The Formula for calculating 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 a Cube of the Screw Diameter (Do).
While feeding Talc that has low frictional coefficient (about 0.2), the conveying efficiency
is generally about 20% for typical intake elements such as regular conveying elements
(RSEs) and SK type elements (SKEs) with the degree-of-fill is less than 0.2. At higher
degrees-of-fill, the conveying efficiency decreases sharply. This is the reason, that in a
starve-fed 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%.
While feeding most other polymer material with medium frictional coefficient (about
0.4), the conveying efficiency is 30% at even higher degrees of fill about 0.4. With
Calcite (frictional coefficient 0.74), the conveying efficiency is the highest (about 50%) 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) such as the OMEGA* the capacity is
increased as a result of increase in Free Area. Certain special intake elements such as SFV
(patent pending), provide increased efficiencies at higher fill improving the capacity of
the extruder with most input material. With a 50-50 blend of Talc (5µ mean) and LLDPE
Polymer Pellets, an increase of 300% was noted.
* OMEGA Series of Co-Rotating Twin-Screw Extruders from STEER Engineering
- Babu Padmanabhan
on behalf of ET expert panel.
For further queries on the subject you may write to: drbabu@steerworld.com.
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