Functional characteristics of exhaust screw
Published:
2022-10-13 11:53
In the extrusion process, the gas to be discharged from the raw material includes three parts: the air brought in between the raw material particles; Moisture adsorbed on granular materials; Gases or liquids contained in raw materials, such as residual monomers, low molecular volatiles and water, etc.
In the extrusion process, the gas to be discharged from the raw material includes three parts: the air brought in between the raw material particles; Moisture adsorbed on granular materials; Gases or liquids contained in raw materials, such as residual monomers, low molecular volatiles and water, etc. If these gases cannot be discharged, in addition to the physical and mechanical properties, chemical properties and electrical properties of the products will be reduced, the surface or temporal part of the products will also have pores, bubbles, scars, dimness and other defects, which seriously affect the appearance of the products, such as the gap will affect the dielectric strength of the cable; Bubbles will make the monofilament unable to stretch; The hard pipe containing water will reduce the pressure borne by the hard pipe; Bubbles and scabs in the plate will cause waste products in vacuum forming. It is generally believed that the content of these ingredients in raw materials before extrusion shall not be greater than 0.2%, and in some cases, for example, when polyester film is drawn, it shall be at least less than 0.02%.
On an ordinary screw, the air brought in by raw materials and the moisture absorbed can escape from the feeding port when the plastic is extruded, or be removed by drying before being added to the feeding port. This process is called pre drying. However, pre drying requires additional drying equipment and consumes considerable power and labor, so the cost will inevitably rise, and the removal effect of some monomers and some high boiling point solvents is not good enough. For some transparent products, the pre drying process increases the chance of raw material pollution. In addition, with the development of extrusion, for plastics (such as polyethylene) with low exhaust requirements in the past, as the screw speed increases, the air entrained by raw materials cannot escape from the feed opening in time, which will also affect the quality of products.
Practice shows that the effect of exhaust extruder is superior. Table 11-1 lists the comparison of exhaust effect between extruder and exhaust extruder.
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