During the extrusion process, the gases that need to be removed from the raw material consist of three parts: air entrained between the raw material particles; moisture adsorbed onto the pellets or powder; and gases or liquids contained within the raw material itself, such as residual monomers, low-molecular volatiles, and moisture. If these gases are not expelled, the physical, mechanical, chemical, and electrical properties of the product will deteriorate. Additionally, defects such as porosity, bubbles, pockmarks, and surface dullness will appear on the surface or inside the product, severely impairing its appearance and performance. For instance, voids can affect the dielectric strength of cables; bubbles can prevent monofilament drawing; moisture in rigid pipes reduces their pressure resistance; and bubbles or blemishes in sheets can lead to rejects during vacuum forming. It is generally accepted that the content of these components in the raw material prior to extrusion must not exceed 0.2%, and in certain cases—such as PET film stretching—it should be less than 0.02%.
In a conventional screw, the air entrained by the raw material and the adsorbed moisture can escape from the feed throat as the plastic is compacted, or they can be removed by drying before entering the feed throat, a process known as pre-drying. However, pre-drying requires additional drying equipment and consumes considerable electrical energy and labor, inevitably increasing costs. Moreover, the removal efficiency for certain monomers and high-boiling-point solvents is often inadequate. For some transparent products, the pre-drying process frequently increases the risk of raw material contamination. Furthermore, with the development of high-speed extrusion, plastics that previously had low venting requirements (such as polyethylene) may experience quality issues because the entrained air does not have enough time to escape from the feed opening due to the increased screw speed.
Practical experience demonstrates that the performance of vented extruders is comparatively superior.
