Screw Design: Definitions
• Forward conveying
Material flows in the direction of extruder discharge Right hand screws for clockwise rotation, view from die Left hand screws for counter-clockwise rotation, view from die (e.g. Berstorff)
Screw Design: Conveying Direction
Screw channel is still lengthwise open in reverse pitch screw elements
Screw Design: Conveying Direction
Pressure Gradient P2 > P1 P2 > P1
Screw Design: Pitch
• Low screw pitch ≈ 0.25 D – 0.75 D
Lowest volumetric displacement Slowest conveying speed Increases degree-of-fill downstream of medium pitch screws Maximum degree-of-fill without downstream pressure Heat transfer, pumping
• Medium screw pitch ≈ 1D
Intermediate volumetric displacement Moderate conveying speed Increases degree-of-fill downstream of high pitch screws “Compression” after feeding, intermediate melt conveying
Screw Design: Undercut
Normal Profile
Double-Undercut Profile
• Undercut screw elements
Increased free volume (approx. 10-15 percent or more) Improved conveying of free-flowing solids Primary feed zone (not self-wiping)
Screw Design: Number of Flights
• Triple-flight elements
Highest shear relative to single and double flight elements Divides melt flow downstream of double-flight elements Shallow channel depth Melting, dispersive mixing
Strategy for Screw Design – Step 3
Following Webinar presentation:
Develop screw design as sequence of unit operations Optimize screw design for specific application
Left Hand Screw
Right Hand Screw
• Right hand screw
Flights progress upward to the right when viewed vertically
• Left hand screw
Flights progress upward to the left when viewed vertically
Screw Design: Number of Flights
• Double-flight elements
Co-rotating “standard” for free volume and torque Lower shear relative to triple-flighted elements Divides melt flow downstream of single-flight elements Solids feeding, melt conveying, venting, pumping
Strategy for Screw Design – Step 2
Next Webinar presentation:
Identify optimum screw element type for each function: Feeding Melting Mixing Venting Pressurization
Introduction to Screw Design
Adam Dreiblatt Director, Process Technology
Strategy for Screw Design – Step 1
Today’s Webinar presentation:
Identify design variables for conveying screws and kneading elements Describe geometry for mixing within co-rotating intermeshing twin-screw
Intermeshing Co-rotating Geometry
Two-flight screws = 3 channels
Channel 1 Channel 2 1 2 3 Channel 3
(Three flight screws = 5 channels)
Intermeshing Co-rotating Geometry
Screw Design: Numberห้องสมุดไป่ตู้of Flights
Single Flight
Double Flight
Triple Flight
• Number of screw flights (n)
1, 2 and 3 flights “standard” Can combine 1, 2 and 3 flights on common shaft (requires transition geometry or discontinuity) Divides melt flow into multiple [(2 x n)-1] channels
Crosswise open channels Axially open channels
Screw Design – Conveying Elements
• Design variables for conveying screw elements
Direction of conveying Pitch Number of screw flights
Intermeshing Co-rotating Geometry
Crosswise open (kneading elements) versus closed (conveying elements)
Crosswise closed channels Axially open channels
Geometry formed by screw element type: Conveying elements Lengthwise open screw channels Crosswise closed screw channels Kneading elements Lengthwise open screw channels Crosswise open screw channels
Screw Design: Pitch
• High screw pitch ≈ 1.5 D – 2 D
Largest volumetric displacement Fastest conveying speed Lowest degree-of-fill Feeding, venting
Screw Design: Pitch
Screw Design: Pitch
• Screw pitch
Pitch ≈ volumetric displacement capacity Vary screw pitch to influence degree-of-fill Vary screw pitch to influence residence time
Kneading Elements
• Characterization of kneading elements
• Available in discrete lengths
Permits strategic location of mixing and shearing zones with respect to barrel openings and process requirements
Screw Design: Definitions
Screw Design: Number of Flights
• Single-flight elements
Large flight width minimizes leakage flow Lower free volume than double-flight elements Highest pumping efficiency Feeding, pumping
Intermeshing Co-rotating Geometry
Cross sectional view for two-flight conveying screw elements Cross sectional view for two-flight kneading elements