Extruder Screw Design

Concepts to Improve Process

Efficiencies

Evolution of the Modern Twin Screw Extruder

Advantages• Higher screw speeds• Higher torque handling capability• Larger drive motors• Greater throughput• Reduced residence time

Disadvantages• Feeding limitations• Higher peak shear• Higher melt temperatures

The Modern Extruder Has Evolved Much Faster Than the Evolution of the Process SectionThe Modern Extruder Has Evolved Much Faster Than the Evolution of the Process Section

Feeding ElementsNew Choices

Erdminger Bi-LobeAttributes• Proven performance over decades of use• Most readily available 

Advantages• Fully self‐wiping• Good pressure generating capability• No leakage

Disadvantages• Limited forwarding capacity, especially at 

high screw speeds• High radial forces

Recommended applications• High density feed applications (pellets)• Processes where significant changes in color 

or formulation occur frequently• Feeding operations involving liquids, melts, 

gels, ect.

Single Lobe Attributes• Proven performance over decades of use

Advantages• Self wiping• Very good pressure generating capability• More free volume for the feeding of low 

bulk density materials

Disadvantages• Forwarding capacity can diminish as screw 

speeds are raised• High radial forces

Recommended applications• Medium density feed applications• Processes where significant changes in color 

or formulation occur frequently• Feeding operations involving liquids, melts, 

gels, ect.

Radial Forces Affecting Solids Conveying

• The conveying angle of the leading face of the element forces materials outwards, away from the axis of the shafts

• Centrifugal forces created by the spinning shafts enhances the fluidization of low feed materials

• Resulting fluidization reduce bulk density at the feed throat further reducing forwarding capacity

• Due to these characteristics solids forwarding capability significantly diminishes as screw speeds increases

Radial forces generated by typical feeding elements throws material outwards limiting their effectiveness for solids conveying at high screw speeds

Radial forces generated by typical feeding elements throws material outwards limiting their effectiveness for solids conveying at high screw speeds

Schubkanten (SK)Attributes• First significant improvement for the feeding of low bulk 

density materials• Leading edge of flight has a face angle that vertically aligns 

with the shaft

Advantages• Significant improvement in the forwarding capability of low 

bulk density solids

Disadvantages• Not fully self wiping/cleaning• Increases residence time distribution due to leakage• Can reduce forwarding capability of liquids or cohesive 

materials • Not fully self wiping, can result in cross contamination after a 

product change‐over of dis‐similar materials

Recommended applications• Difficult‐to‐feed low bulk density materials

Single Lobe ShovelAttributes• Possesses the good dry solids conveying attributes of a single flight feeding element• Possesses negative angle of leading flight in relation to axis of shaft

Advantages• Significant reduction of radial forces, excellent forwarding of materials at high 

screw speeds• Significant improvements in the forwarding of low bulk density materials• High feeding capacity of lubricious materials• Can be used with both high and low density feed materials• Can transform feed limited processes to torque limited processes

Disadvantages• Poor pressure generating capabilities• Can reduce feeding capability with liquids or cohesive materials• Not fully self wiping, can result in cross contamination after a product change‐over 

of dis‐similar materials• Can reduce forwarding capability of liquids or cohesive materials 

Recommended applications• Low density feed applications• Lubricious feed applications• Irregular shaped feed applications

Bi-Lobe ShovelAttributes• Possess negative angle on leading flights in relation to axis of 

shaft

Advantages• Significant reduction of radial forces, excellent forwarding of 

materials at high screw speeds• Significant improvements in the forwarding of low bulk density 

materials• Can transform feed limited processes to torque limited processes

Disadvantages• Poor pressure generating capabilities• Can reduce feeding capability with liquids or cohesive materials• Not recommended for use with hard pellets• Not fully self wiping, can result in cross contamination after a 

product change‐over of dis‐similar materials

Recommended applications• Low density feed applications• Feed limited processes

Tri-Lobe ShovelAttributes• Improves dry solids conveying with the negative angle of leading flight in 

relation to axis of shaft• Minor flights increases laminar orientation of irregular shaped feed materials 

thus increasing bulk density

Advantages• Significantly improves the transport of flake and fiber shaped materials• Reduction of radial forces, excellent forwarding of materials at high screw 

speeds• Improves in the forwarding of low bulk density materials• Can transform feed limited processes to torque limited processes

Disadvantages• Poor pressure generating capabilities• Can reduce feeding capability with liquids or cohesive materials• Not recommended for use with hard pellets• Not fully self wiping, can result in cross contamination after a product change‐

over of dis‐similar materials

Recommended applications• Low density feed applications• Feed limited processes• Irregular materials such as flake or fiber

“Don’t’s” for Undercut Elements

• Don’t: Use elements for cohesive powders

• Don’t: Use elements for liquid additives

• Don’t: Use elements for additives with adhesive characteristics

• Don’t: Use elements where pressure generation is required

The buildup of material in the non‐wiped region of undercut feeding elements significantly reduces the ability to convey solids

Screw Speed Power Output Available Power Specific Energy Power Utilization(RPM) (kW) (kg/h) (kW) (kW h/kg) (%)

300 10.21 50.00 12.75 0.20 80.08600 22.73 140.00 25.50 0.16 89.14900 35.47 210.00 38.25 0.17 92.73

Screw Speed Power Output Available Power Specific Energy Power Utilization(RPM) (kW) (kg/h) (kW) (kW h/kg) (%)

300 5.51 28.00 12.75 0.20 43.22600 12.15 63.00 25.50 0.19 47.65900 18.55 84.00 38.25 0.22 48.501000 20.79 84.00 42.50 0.25 48.92

Screw Speed Power Output Available Power Specific Energy Power Utilization(RPM) (kW) (kg/h) (kW) (kW h/Kg) (%)

300 6.26 28.00 12.75 0.22 49.10600 11.92 42.00 25.50 0.28 46.25900 19.22 70.00 38.25 0.27 50.251000 21.76 84.00 42.50 0.26 51.20

Table 1: With SFV screws in the intake zone of the extruder

Table 2: With Schubkanten type screws in the intake zone of the extruder

Table 3: With Erdmenger-type screws in the intake zone of the extruder

From “Special Intake Elements to Overcome Feed Limitations in Co-Rotating Twin Screw Extruders”, Dr. Babu Padmanabahn, et. al

Trial Results40mm Omega TSE

50% Talc/LLDPE powder

Shovel elements increased throughput over 100% which was limited by power availability rather than feeding capacity

Boron Nitride feeding :(100 rpm results)

RSE/SKE : 1‐2 oz/minTFV: 3 lb/minRFV: 6 lb/minSFV: 9 lb/min

Mixing ElementsTask Focused

Bi-Lobe Kneading BlocksAttributes• Decades of proven performance• Self wiping• Multiple capabilities, shearing to mixing• Provides all primary sources of mixing energy input, from distribution 

to shearing to wetting• The “Cocktail” of mixing attributes – “You get it all, want it or not”

Advantages• Readily available• Proven performance in most every process application

Disadvantages• You get all of the mixing attributes of the KB’s, wanted or not• Energy input cannot target specific attributes• All forms of energy input, from shearing to elongation, are imparted 

on processed materials

Recommended applications• Has been used for most all mixing energy input for extrusion 

processes for decades

Fractional Kneading Blocks

Attributes• Evolution of prior eccentric KB technology• Fully self‐wiping

Advantages• Significantly reduces shear peaks experienced by material within the mixing 

elements• Significantly improves the uniformity of shear energy that materials experience• More effective dispersion with lower energy input (melt temp)• More efficient dispersion response per unit length (axial)

Disadvantages• Theoretically, low free volume of some KB’s could restrict flow (has not been 

observed)

Recommended applications• Dispersion of pigments and fillers• Shear sensitive dispersion applications• Mixing of materials with a large viscosity differences• Mixing of liquids with polymers, etc.

Fractional Kneading Blocks

Fractional Mixing Elements

Attributes• Novel evolution of eccentric kneading technology• Uniform residence time in a high mixing (elongational) environment

Advantages• Significantly reduces shear peaks experienced by material within the mixing 

elements• Significantly improves the uniformity of mixing energy that materials experience• More effective dispersion/distribution with lower energy input (melt temp)• More efficient dispersion and/or wetting response per unit length

Disadvantages• Low energy kneading properties are not suitable for aggregate dispersions

Recommended applications• Dispersions of typical fillers and reinforcements, particularly those of renewable 

sources which tend to be temperature sensitive• Temperature sensitive applications

Fractional Mixing Elements

Three Lobe: Low shear intensity with excellent mixing uniformity 

4 Lobe: Excellent elongation, wetting, and kneading with good mixing uniformity 

Five Lobe: High shear intensity with good mixing uniformity

Fractional Mixing Elements

Distribution Mixing Elements

Attributes• Represented with TME’s, SME’s, ZME’s, etc.• Provides very effective laminar mixing energy

Advantages• High degree of elongational mixing

Disadvantages• Not fully self wiping• Cross linking or heat/process sensitive plastics will 

create deposits within non‐self wiping regions• Increases residence time distribution vs, typical mixing 

elements

Recommended applications• Dispersion and distribution applications where the 

materials are miscible or fillers have a low surface energy

• Distribution of dispersed fillers/reinforcement

Dynamic Mixing Elements

Attributes• Mixing energy similar to a co‐kneader• Fully self‐wiping

Advantages• Significantly increases elongational mixing, the most 

efficient mixing per energy input• Promotes low residence time distribution• Provides open architecture for devolitolilization or gas 

removal while wetting fillers/reinforcements

Disadvantages• To be determined, new intro to market

Recommended applications• Dispersion and distribution applications where the 

materials are miscible or fillers have a low surface energy

• Pre‐wetting of biomass‐fillers/reinforcements and other sensitive fillers/reinforcement prior to dispersion

Dynamic Mixing Elements

Using task specific feeding and mixing elements can improve the capacity and quality of modern

extruders.