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How to increasing the production capacity of twin-screw extruders by optimizing screw combinations
2024-02-23
2024-02-23
In most mixing and compounding processes, the production capacity of twin-screw extruders is limited by:
Power size
What is the feeding amount
We cannot easily change the internal volume of the extruder, but we can increase the mechanical power and conveying capacity of the input extruder by using optimized screw combinations to achieve the goal of increasing production.
We can try to increase the production capacity of twin screw extruders by optimizing the screw combination:
a. Improving volumetric feeding capacity - premixing
b. Improving melting efficiency
c. Improving volumetric feeding capacity - side feeding
d. Improving extrusion efficiency
Improving volumetric feeding capacity - premixing
The influencing factors of feeding high filling powder to the cylinder:
● Machine capacity indicators
✔ Screw element diameter, screw element size to diameter ratio
✔ speed
✔ Screw combination design
✔ Feed density
When the main feed is fed with most of the powder, the small lead conveying threaded components here are filled with material.
Improving melting efficiency
The screw speed remains unchanged, while the production increases, the melting efficiency decreases;
The kneading block must be able to raise the material temperature to the melting temperature range;
Extruder wear can lead to a decrease in melting efficiency:
✔ Maximum screw speed
✔ Limitations on the length of the process flow
✔ Capacity limitations
When the feeding amount increases, the position where the material reaches the molten state will move towards the machine head direction; This displacement will shorten the plasticization time and reduce the quality of the blend.
An increase in screw speed and machine wear can also cause the melt position to move towards the head direction.
After designing three threaded components in the melting zone, the simulation graph shows an increase in the melt temperature before side feeding.
Optimizing feeding - side feeding
Side feeding with a large amount of powder or short fibers
● Feeding capacity indicators:
✔ Extruder/side feeding screw diameter, inner to outer diameter ratio
✔ Extruder/side feeding screw speed
✔ Screw combination design
✔ Feed material density
✔ Melt quality
Similar to the main feed pre mixing system, after side feeding, a decrease in pitch will limit the internal volume of the equipment.
Optimizing screw combinations
The material at the side feeding front end must reach 100% melting state; Incomplete melting state will lead to mixing and side feeding exhaust problems.
The screw combination in the side feeding area requires the use of large lead conveying threaded elements (≈ 2D) until the mixing area is reached.
Optimize mixing:
● Dispersion and distribution:
✔ Types of threaded components with mixed effects
✔ Pinch blocks, toothed threaded components, and others
✔ Sequence and quantity of threaded components
✔ Position of threaded components (related to feeding port)
✔ Stay time in the mixed area
Under the given operating conditions, the 1D simulation provides a quantified value of the mixed mass. The design effect of different screw combinations can be evaluated based on the increase and decrease of the mixture index value.
Optimizing screw combinations:
Under given operating conditions, 1D simulation provides a quantified value of pressure. The pressure index value can be maximized (such as improving the dispersibility of the filling material) or minimized (such as reducing fiber breakage)
Optimize material extrusion:
Insufficient process length for pressure building:
● Extrusion index
✔ Screw diameter, screw inner to outer diameter ratio
✔ Compression extrusion length
✔ Screw speed
✔ Screw combination design
✔ Melt density
Screw Combination Design - Head Pressure ▼
An increase in head pressure can lead to an increase in the length of the pressure break (such screen changer blockage ).
The longer the pressure building section, the worse the devolatilization effect (vacuum section residence time)
The longer the compression section, the higher the melt pressure, and the significantly higher the melt temperature. The reason is that the material will slide through the gap between the threaded blades and the barrel.
If a single head screw element is used in the compression section, the compression length will be shortened. The lifespan of the replacement screen increases, the melt temperature decreases, and the vacuum section stabilizes.
Increase feeding amount:
✔ The thread lead of the conveying section does not decrease
✔ Lead of conveying threaded components ≈ 2D
✔ Single headed threaded components and other specially designed threaded components can be used.
Improve melting effect:
✔ Restricted by process length
✔ Use 3 helmet pinch blocks for better results
✔ But using other specially designed threaded components
The optimization of screw combinations in each stage (feeding, melting, vacuum, pressure extrusion, etc.) can improve productivity, improve mixture quality, and yield.
By optimizing the screw combination, different materials can be produced on the same extruder (reducing conversion time) at the cost of limited production capacity.
In recent years, the selection of threaded component types has been diversified, and the performance of high-speed and high torque machines can be ensured by trying different components.