Extruder Screw Design Concepts to Improve Process Efficiencies

Transcription

1 Extruder Screw Design Concepts to Improve Process Efficiencies

2 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 Section

3 Feeding Elements New Choices

4 Erdminger Bi-Lobe Attributes 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.

5 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.

6 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

7 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

8 Single Lobe Shovel Attributes 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

9 Bi-Lobe Shovel Attributes 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

10 Tri-Lobe Shovel Attributes 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 changeover of dis similar materials Recommended applications Low density feed applications Feed limited processes Irregular materials such as flake or fiber

11 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

12 Trial Results 40mm Omega TSE 50% Talc/LLDPE powder Screw Speed Power Output Available Power Specific Energy Power Utilization (RPM) (kw) (kg/h) (kw) (kw h/kg) (%) Table 1: With SFV screws in the intake zone of the extruder Screw Speed Power Output Available Power Specific Energy Power Utilization (RPM) (kw) (kg/h) (kw) (kw h/kg) (%) Table 2: With Schubkanten type screws in the intake zone of the extruder Screw Speed Power Output Available Power Specific Energy Power Utilization (RPM) (kw) (kg/h) (kw) (kw h/kg) (%) Table 3: With Erdmenger-type screws in the intake zone of the extruder Shovel elements increased throughput over 100% which was limited by power availability rather than feeding capacity From Special Intake Elements to Overcome Feed Limitations in Co-Rotating Twin Screw Extruders, Dr. Babu Padmanabahn, et. al

13 Boron Nitride feeding : (100 rpm results) RSE/SKE : 1 2 oz/min TFV: 3 lb/min RFV: 6 lb/min SFV: 9 lb/min

14 Mixing Elements Task Focused

15 Bi-Lobe Kneading Blocks Attributes 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

16 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.

17 Fractional Kneading Blocks

18 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

19 Fractional Mixing Elements Three Lobe: Low shear intensity with excellent mixing uniformity 4Lobe: Excellent elongation, wetting, and kneading with good mixing uniformity Five Lobe: High shear intensity with good mixing uniformity

20 Fractional Mixing Elements

21 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

22 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

23 Dynamic Mixing Elements

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