Principle Of Twin Screw Extruder Degassing
● Operate at an economic throughput
● Keep max. melt temperature as low as possible (degradation, color)
● Minimize the residuals
● Achieve good optical and mechanical properties
● Avoid gels and "black spots".
Degassing processes.
Polymer | Material to be degassed | ||
Type | Initial content | Final content | |
LDPE | Ethylene | 2,000 ppm | < 100 ppm |
LLDPE | Hexane, cyclohexane | 10 - 15 % | < 500 ppm |
3% | < 250 ppm | ||
EVA | Vinylacetate | 5,000 ppm | < 50 ppm |
PS | Styrene | 3,000 – 5,000 ppm | < 300ppm |
ABS | Acrylnitril (AN) | 20 ppm | <4 ppm |
PC | Methylene chloride | 20% | < 50 ppm |
PA | Caprolactame | 8% | < 0,8 % |
SBR, SBS | Hexane, cyclohexane | 15 - 70 % | < 1,000 ppm |
Separation of the gas from the polymer melt by diffusion. The gas, which is incorperated in the melt has to come to the surface of the melt layer.
The diffusion is influenced by:
● residence time ---> long degassing zone
● viscosity of the melt---> low viscosity (= high temperature)
● thickness of the melt layer ---> low fill factor of the screw (40%)
● renewal of the melt surface ---> intensive mixing
● pressure difference ---> vacuum
Form A
• Up turning screw is covered
• Intermeshing area is covered
• Down turning screw is partly covered
Polymer melt, which doesn't stick to screws (polyolefins)
Form B
• Up turning screw is covered
• Intermeshing area is covered
Polymer melt, which tends to stick to screws (PA, PET)
Form C
• Up turning screw partly covered
Low viscous polymer melt (degassing of solvents)
● Feed temperature too low --> Efficiency for rear / back vent low
● Foaming in vent ports --> Limitation in vacuum level
--> Increase of screw speed
 | => |  | => |  |
● Gas velocities --> Throughput, screw speed, screw design
● max. melt temperature --> Screw speed / screw configuration
Degassing.
Guide lines for polymer degassing.
● The polymer has to be completely molten.
● The fillers and reinforcing material have to be wetted completely with melt.
● The melt temperature has to be as high as possible.
● The degassing zone has to be as long as possible.
● The vacuum has to be below the partial pressure of the material to be degassed.
● The fill factor should be below 40%.
● The vent port has to be chosen according the polymer characteristics
