世纪螺杆组合介绍

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
Screw Design: Pitch
• Screw pitch
Pitch ≈ volumetric displacement capacity Vary screw pitch to influence degree-of-fill Vary screw pitch to influence residence time
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
• Single-flight elements
Large flight width minimizes leakage flow Lower free volume than double-flight elements Highest pumping efficiency Feeding, pumping
Axial distance for one revolution of screw flight Expressed as absolute distance (mm), ratio to screw diameter (D) or flight helix angle (degrees) Free volume = constant !
Screw Design: Number of 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
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
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
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
FLOW DIRECTION
P1 P2 P2 P1
Pressure causes some material to flow through clearance in reverse pitch screws (material flowing through clearance experiences high shear; temperature; dispersion)
• 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
• 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
• 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
Crosswise open channels Axially open channels
Screw Design – Conveying Elements
• Design variables for conveying screw elements
Direction of conveying Pitch Number of screw flights
Strategy for Screw Design – Step 2
Next Webinar presentation:
Identify optimum screw element type for each function: Feeding Melting Mixing Venting Pressurization
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
Intermeshing Co-rotating Geometry
Cross sectional view for two-flight conveying screw elements Cross sectional view for two-flight kneading elements
Self-wiping profile is achieved with constant cross section
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: 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: Conveying Direction
• Reverse pitch screw elements
Convey in opposite direction Create 100% degree-of-fill upstream Back-pressure component (melt seal) Supportable pressure proportional to back-up length Characterized by pitch, number of flights, length
• Reverse conveying
Material flows away from the extruder discharge Use opposite hand from forward conveying
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Screw Design: Definition
Pitch Pitch Pitch
• Pitch
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: 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)
Intermeshing Co-rotating Geometry
Crosswise open (kneading elements) versus closed (conveying elements)
Crosswise closed channels Axially open channels
Kneading Elements
• Characterization of kneading elements