Single and twin screw extruder structure characteristics and working principle differences
Extrusion molding process is the polymer processing field in the production of the most varieties, the most variation, high productivity, adaptability, wide range of uses, the proportion of the production of the molding process method. Extrusion molding is to make the polymer melt (or viscous fluid) in the extruder screw under the action of extrusion through a certain shape of the mouth die molding, the product is a continuous profile with a constant section shape.
Extrusion molding process is suitable for all polymer materials. Can form almost all thermoplastics, also can be used in thermosetting plastics, but in phenolic and other few thermosetting plastics. Plastic extrusion products are pipe, plate, bar, sheet, film, single wire, cable coating layer, all kinds of profiles and plastics and other materials complex. About 50% of thermoplastic products are now made by extrusion. In addition, the extrusion process is often used for plastic coloring, mixing, plasticizing, granulation and plastic blending modification, based on extrusion molding, with blowing, stretching and other technologies, and developed for extrusion blow molding and extrusion tensile molding hollow blow molding and double axial stretch film and other products. It can be seen that extrusion is the most important method in polymer forming.
Extrusion equipment has screw extruder and plunger extruder two categories, the former is continuous extrusion, the latter is intermittent extrusion, mainly used for high viscosity material molding, such as PTFE, ultra-high molecular weight polyethylene.
Screw extruder can be divided into single screw extruder and multi - screw extruder. Single screw extruder is the most basic extruder in production. In recent years, twin-screw extruders have developed the fastest and been widely used. At present, in the processing of PVC plastic door and window profiles, twin screw extruder has become the main production equipment, single screw extruder will be phased out. But in other polymer extrusion processing, the single screw extruder is still dominant. Both have their own characteristics:
Single screw extruder：
① Simple structure, low price.
② Suitable for polymer plasticizing extrusion, suitable for granule extrusion processing. The shear degradation of polymer is small, but the material stays in the extruder for a long time.
③ Easy to operate, simple process control.
Twin screw extruder:
① Complex structure, high price.
② Good mixing and plasticizing ability, short residence time of materials in extruder, suitable for powder processing.
③ Large output, fast extrusion speed, low energy consumption per unit output.
Single screw extruder is suitable for granule processing. The raw material used is granule after granulation or pulverized granule. Twin-screw extruder is suitable for powder processing, can directly use mixed PVC material, reduce the granulation process, but more waste grinding process.
In recent years, the quality of domestic twin screw extruder has basically reached the level of import twin screw extruder, the price is only 1/3 ~ 1/5 of the import machine. Because the output of twin screw extruder is large, the extrusion speed is fast, generally can reach 2 ~ 4 m/min, suitable for large-scale production of PVC plastic door and window profiles. The single screw extruder is generally used for small auxiliary profile production, extrusion speed is only 1 ~ 2 meters/minute, many PVC profile processing plants have eliminated the single screw extruder, switch to a twin screw extruder multi-cavity production of small auxiliary profile.
