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What are the opportunities - existing and new?
The twin-screw extruders have made it possible to produce high
quality, specialty plastic compounds used in the most common
products as well as highly engineered products. These products
ultimately provide humans with a higher quality of life and enable us
to reach a higher level of technology. New opportunities for using
twin-screw extruders are constantly being developed. Some of the
latest materials are high performance elastomers, nano composites,
biopolymers / natural fiber composites that utilize renewable
resources, and biodegradable compounds. Due to its high flexibility
to adapt, the twin-screw extruder remains a viable machine for future
processing requirements.
Performance measures for twin-screw extruders.
There are essentially three performance indicators used to quickly
gauge an extruder’s overall performance capability for a given
process or range of applications:
1. VOLUME
2. TORQUE
3. SPEED
The equation for calculating the theoretical centerline distance with
known values for outer diameter and inner diameter:
C = R + R or 0.5 (D + D)
d
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Channel depth can be calculated as follows:
h = R – R or 0.5 (D – D)
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c. Diameter Ratio ( D / D or D / d )
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The ratio between the outer diameter of the screw (D ) and the inner
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diameter (D) of the screw. Larger diameter ratios have more screw
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volume. The centerline distance and diameter ratio set a limit on the
maximum volume of the screw. As the outer diameter (D or D) of the
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screw increases, the inner diameter (D or d) must decrease in order
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to maintain the fully intermeshing, self-wiping attributes of the
Erdmenger design. It is important to note that the inner diameter
determines the maximum diameter of the screw shaft which has a
limiting effect on maximum allowable shaft torque
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d. Number of Flights
1. VOLUME
The channel volume is also dependent upon the number of
Theoretically, higher free volume of the screws results in a higher
intermeshing flights. The Erdmenger geometry requires a tip angle
melt conveying capacity, yielding a higher throughput rate. Free
greater than zero to maintain the closely intermeshing relationship.
volume of the screws can be calculated as follows:
Flight tip angles are also equal regardless of the number of flights.
The free volume of a twin-screw extruder is dependent upon the
mechanical design parameters of the screw geometry.
a. Screw Diameter
Screw diameter typically refers to the Outer diameter (D ) expressed
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in millimeters. Screw elements have both an outer diameter and
inner diameter (D).
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b. Centerline Distance (C )
D
The measured distance between the center of the two output shafts
of the gearbox. The centerline distance affects the maximum
channel depth.
Number of
Flights
Ratio of
diameters (Rd)
1 1< D / D<
∞
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2 1< D / D< 2.41
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3 1< D / D< 1.37
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4 1< D / D< 1.18
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THE EXTRUDER TIMES / ISSUE 09 / APRIL–AUGUST 2009
EQUAL TIP ANGLES