Why is vacuum needed for plastics extruder degassing?

Various plastic extrusion methods utilise vacuum for degassing the polymer and/or for cooling of the formed product.

Extruder degassing, also known as hot melt degassing, venting or devolatization, is carried out during various extrusion processes to remove any residual moisture, air, monomers, oligomers, solvents, reaction products, decomposed materials as well as to purify plastics in various steps of plastics recycling.

The vacuum levels used for degassing could be anywhere between 1–200 mbar depending on the end product. Effective degassing has multiple effects on products in the plastic industry, including an increase in the free volume, reduction in the residual moisture content, improvements in odour, changes in the visual appearance, alterations in mechanical properties, prevention of bubbling or foaming, and homogenous mixing. Proper degassing can easily be attained by changing the number, location and geometry of the degassing ports. The degassing process is most effective when used over a conveying section after a mixing section on the extruder.

The greatest effect of extruder degassing is seen on hygroscopic polymers like PET, PA, POM, PBT etc. that readily absorb moisture from the atmosphere. Water vapour must be removed from the polymer melt as it will otherwise degrade the quality of the final product.

• Residence time under the vent or vents – Longer is better but not usually possible. Oxygen, shear, time and temperature may contribute to degradation and side reactions.
• Surface area of the melt – Higher is better but is a function of screw geometry and operating parameters.
• Surface renewal – Higher is better. Renewed surfaces come from rolling pools and partially filled screw channels.
• Vacuum level applied to vent zone(s) – it can make a big difference in the degassing efficiency. Low viscosity material may require decreased vacuum levels.