Twin Screw Extrusion Flashcards
(31 cards)
What are the main challenged with single screw machines?
- Excessive wall slip or degradation of plastics
(PVC) - Inefficient pumping / low output / machine down
time - Poor mixing characteristics
What is the melt transport ‘pumping’ mechanism (single screw)?
‘screw viscosity pump’
Melt flow: Relative motion….screw / barrel
Adhesion to each metal surface, hence shear flowSolids: Flow due to frictional differences
(material / screw & material / barrel)
Screw design: continuous helix channel;
– output (Q) is sensitive to pressure (ΔP)
What are the 2 main types of twin screw extrusions and how does their mesh change?
the same (co-rotating) or
opposite (counter-rotating)
directions and
may be meshing like:
fully intermeshing,
partially intermeshing, or
non-intermeshing.
What are the 2 main twin screw extrusions (TSEs) designs?
- High-speed, energy input TSEs that melt the polymer early and are designed as mass transfer devices:
Co-rotating intermeshing
Counter-rotating intermeshing
Counter-rotating non-intermeshing (or tangential) - Low-speed late fusion TSEs that are designed to mix shear sensitive formulations (i.e. Rigid PVC) and pump at high pressures:
Parallel Counter-rotating intermeshing
Conical Counter-rotating intermeshing
What are the benefits of either intermeshing or counterrotating twin screws?
- The co-rotating intermeshing mode is the best for most
compounding applications. - The low speed, late fusion counterrotating intermeshing twin Extrusion dominates the market for the extrusion of uPVC profiles.
What are the main differences between twin screw and single screw extrusion?
Single-screw extruder: Material is transported mainly by drag—solid pellets move via friction with the barrel, and molten polymer moves via viscous drag.
Twin-screw extruder (intermeshing): Transport is partly positive displacement, depending on how tightly one screw’s flight closes the channel of the other screw.
What is the main flow mechanism of the counter rotating screw?
Counter-rotating screws form mirror images of each other; when fully meshed the flight of one screw seals off and isolates the material in the opposite screw. Materia moves down the channel and recirculates in closed C-shaped sections.
Through C- cavities (intermeshing, shift along extrusion axis from screw helix, cavity volume and based on cavity width and depth).
Can be subject to leakage flows (e.g. flight, calendar, tetrahedron and side leaks).
It is important to note that closely intermeshing, counter-rotating twin-screw extruders can pump materials in a C-shaped, non-drag flow chambers, making this device the only extruder (except a ram extruder) that can function as a positive displacement pump. Single-screw and co-rotating TSEs both pump the melt via drag-flow.
(1) Cannot use continuous shear flow theory
(2) Discontinuous flow
(3) Precise & controllable thermal & shear histories
(4) Little effect of head pressure
(5) Mixing only in specific zones
ref eq pg 17-18
How is the Counterrotating intermeshing, low speed, late fusion (LSLF) TSEs characterized?
a gentle melting effect and
narrow residence time distribution at low screw rpms (less than 50 rpms) combined with
high-pressure pumping capabilities.
This combination is particularly useful when processing a thermally sensitive PVC, or similar formulations.
Screws are one-piece, and are often cored for liquid cooling, which is a preferred design feature for Rigid PVC (RPVC) and similar temperature sensitive processes.
Barrels can be one-piece or modular.
what key features affecting mixing and pressure in twin screw extrusion systems?
Calender gap: Enhances extensional and shear mixing but limits screw RPM and L/D ratio due to deflection.
Gear mixers/blister rings: Provide distributive and planar shear mixing.
Multi-start screws (under vents): Increase melt surface area for better devolatilization.
Positive displacement elements: Ensure high-pressure die discharge
How does a counter rotating conical TSE design vary and what is the benefit of this?
Screw diameter decreases from feed to discharge.
Large feed zone handles low bulk density powders well.
Adjustable radial clearance allows control over mixing and processing.
What are the leakage flow in twin screw extruders a function of ?
Screw geometry; rotation direction; wear; temp
the total flow is found from eg on pg 19
the flight gap (with flow Qf) between the barrel and the screw flight;
the tetrahedron gap (Qt) between the flight walls;
the calender gap (Qn) between the flight of one screw and the bottom of the other
screw channel;
the side gap (Qs) between the flanks of the two screws flight;
the channel gap (Qc) down-channel flow in the screw channel.
How does the flow analysis vary between a twin screw and a single screw?
Gradient dQ/dΔP is negative, but small
More efficient than a single screw machine
Q α N
For the same pressure drop the volumetric output decreases more significantly on a single screw than a twin screw.
Therefore a counter rotating twin screw is
More flexible in design
Cannot use continuous shear flow theory
Discontinuous flow
Precise & controllable thermal & shear histories
Little effect of head pressure
Mixing only in specific zones
Why is PVC difficult/ different than other polymers when processing with counterrotating twin screw?
PVC is thermally unstable, has a high melt viscosity and is made of powder grains.
* Magnitude of viscosity
- high viscosity in comparison with other linear polymers
* Continuum or particulate flow ?
- particulate: flow units that deform individually under applied stress
* Melting endotherm
- broad melting endotherm, 110-200 o
C; imperfect crystallites
* Thermo-mechanical history and thermal instability
- temperature, time & stress
* Complex formulations
- wide range of additives (stabiliser, lubricant, pigment, filler, process aid, impact
modifier…..)
* Wall slip, gelation, fusion
How does material flow in co-rotating twin-screw extruders?
Material is pushed out of one screw channel by the flight of the other, circulating in a figure-eight pattern—promoting intense mixing.
Are co-rotating screws positive displacement pumps?
No, even fully intermeshing co-rotating screws cannot be considered true positive displacement pumps. It is the opposite for counter rotating screws.
What does “longitudinally open” mean in co-rotating extruders?
It means there’s a continuous open path from the inlet to the outlet through the extruder
What are co-rotating twin-screw extruders mainly used for?
Widely used in polymer compounding and reactive extrusion due to their efficient mixing and process control
What is the benefit of modular construction in co-rotating TSEs?
Allows customization of barrel and screw configurations for feeding, devolatilization, and mixing—suited to various polymer systems
How does feeding differ between single and twin-screw extruders?
Single-screw: Flood-fed, output tied to screw speed.
Twin-screw: Starve-fed using controlled feeders, allowing independent control of screw speed and output
Why are co-rotating TSEs ideal for reactive extrusion?
Offer controlled residence time, uniform mixing, and adjustable thermal/mechanical stress via screw speed.
What are the three main types of screw elements in co-rotating TSEs?
Conveying (right-handed), reverse (left-handed), and kneading elements—each creates distinct flow and mixing effects.
What do right-handed (conveying) screw elements do?
They drag material forward via a positive helix angle; higher angles increase conveying capacity
What is the function of left-handed elements in TSEs?
Dependent on the helix angle: They restrict flow, create filled flow zones, enhance heat transfer efficiency, and generate more complex flow patterns
How are feed rate and screw RPM used in TSEs?
They are independently controlled to optimize compounding and devolatilization
