Basic HTML Version
The Two popular methods of feeding an extruder are starve-feeding and force-feeding.
During force-feeding, a reserve of material is maintained in the hopper of the extruder and
material is “positively displaced” or “forced” into the extruder. Starve-feeding is the condition
when an extruder is fed at a rate less than the capacity of the screw. The hopper remains
empty and functions as a conduit to avoid material from spilling. Starve feeding is the more
popular method for feeding due to several advantages that will be discussed later.
The modular design of the extruder screw assembly provides an option for a wide selection
2
of screw elements for configuring in the ‘intake zone’. The popular twin-screw elements are
Single Flight Element (SFE), Schubkanten Element (SKE), Special Schubkanten Element
(SSKE), Normal Right-hand Screw element (RSE), Deep flight SK element (DSK), and the
newly invented Single Flight ‘V’ shaped (SFV) element.
Zone # 1 The INTAKE ZONE
TABLE OF INTAKE ZONE ELEMENTS
CHARACTERISTICS
POTENTIAL USE
ELEMENTS
CONVEYING
EFFICIENCY
FREE
VOLUME
Highest Medium Medium All Types
Low Medium Medium Pellets
Low Highest Medium Tri-lobed Force-fed Extruders
High High High Powders, Mix of Powders
and Granules
Low Highest Medium Bi-lobed Force-fed Extruders
TENDENCY TO
BREAKUP AND
COMPACT
GEOMETRY &
PROFILES
NOMENCLATURE
Single Flight
‘V’ Element
Right Handed
Screw Element
Deep Flight
Schubkanten
Special
Schubkanten
Element
Single Flight
Elements
Schubkanten
Element
Medium Low Low Alloys & Blends with different
melt characterization
Free Volume is the free space for material
available in an extruder. This is obtained by
removing the space occupied by screw
elements inside the 8 shaped barrel.
Conveying Efficiency is the ability of the
element to move the material forward in the
extruder. It is 100%, if all the material moves
forward at the completion of each turn of
rotation.
Compaction is the removal of air entrapped in
the material. For example, knocking or
vibrating a vessel containing a powder
compacts the material (reduces the volume)
while stirring does not. Breaking up results in
reduction in particle size or changes the
particle morphology.
THE
HEART OF A
TWIN SCREW EXTRUDER IS
THE EXTRUDER TIMES / ISSUE 05 / JANUARY-MARCH 2008
-3-
Starve-feeding is the condition
when an extruder is fed at a rate
less than the capacity of the screw.