Basic HTML Version
-5-
Output
g/min
Output
in kg/h
2
5
10
25
50
75
100
125
150
175
200
33.3
83.3
166.7
416.7
833.3
1250.0
1666.7
2083.3
2500.0
2916.7
3333.3
Mean
Residence
Time (s)
607.5
243.0
121.5
48.6
24.3
16.2
12.2
9.7
8.1
6.9
6.1
Table 1: Mean Residence time in a
25mm 40 L/D (Do/Di = 1.71)
extruder with 75% degree of fill.
Figure 3: Process Zone in a Co-rotating twin-screw extruder
Conclusion:
The co-rotating twin-screw extruder is a versatile device for material preparation. The
earliest generation extruder had mean residence time of 300 seconds During the course of
the last 50 years, the residence time has been reduced to half its original value every ten
years or so. The current generation extruders have the capability to reduce the time to less
than 10 seconds.
A good understanding of the forces experienced by the material inside the extruder helps in
precise control over the process to achieve the exact results that are required from the
process. Understanding the physics and visualization of the process allows for better
utilization of the co-rotating twin-screw technology. A comprehensive mathematical model
will provide the required clarity and is being pursued with great Vigor.
Table 1 shows the range of residence time possible in a co-rotating twin-screw extruder with
a higher flight depth Outer Diameter/Inner Diameter (Do/Di = 1.71) and high torque
capacity (Specific Torque = 17.0 Nm/cm3). The residence time in the extruder is
independent of the speed of the extruder. It is possible to change the energy input and the
type of work done by varying the time spent in various zones as detailed in Figure 3. Some
interesting applications become possible by reducing residence time to less than 10
seconds.
The challenges are to engineer small extruders to have high torque ability, high intake
capacity and high venting capacity. Such engineering activity improves the process
capability and each step along the way has raised the possibility of new applications.
References:
1. Booy M. L (1978), Polymer Engineering and
Science, Sep 1978.
2. Erdmenger R (1954), U.S. Patent 2,670,188
Feb. 23, 1954.
3. Englich, Sascha (2007), “Influence of the Screw
Geometry..”, Master’s Thesis, U. Chemnitz
4. Michler G. H (1998), Kunststoff –
Micromechanik; Hanser Fachbuch 1998.
5. Padmanabhan (2004), U. S. Patent 6.783,270,
Aug 31, 2004.
6. Padmanabhan et al (2005), “Effect of Element
Geometry…”, ANTEC 2005.
7. Tadmor et. al. (1979), Principles of Polymer
Processing, John Wiley, N.Y.
THE EXTRUDER TIMES / ISSUE 08 / JANUARY-MARCH 2009
This article is contributed by:
Dr. Babu Padmanabhan is the Founder and
Managing Director of STEER Engineering Private
Limited. He has done PhD in Mechanical
Engineering from Virginia Polytechnic Institute
and State University, Blacksburg, VA.
See: www.drbabupadmanabhan.com
E N H A N C I N G
Capabilities
in a Twin-Screw Extruder
MELTING
AND
MIXING