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The working principle of a plastic extruder

Time：

2017-04-26

The extruder’s primary function is to convert solid plastics into a uniform, consistent melt through heating, pressurization, and shear, and then deliver this melt to the next processing stage. Melt production involves processes such as blending color masterbatches and other additives, compounding resins, and regrinding. The finished melt exhibits uniformity in both concentration and temperature. High pressure is applied to extrude viscous polymers. The extruder accomplishes these operations via a barrel equipped with a screw and helical channels. Plastic pellets enter the barrel through a hopper at one end and are conveyed by the screw toward the opposite end. To generate pressure, the depth of the screw’s threads decreases as the distance from the hopper increases.

The extruder’s primary function is to convert solid plastics into a uniform, consistent melt through heating, pressurization, and shear, then deliver the melt to the next processing step. Melt production involves processes such as blending color masterbatches and other additives, compounding resins, and regrinding. The finished melt exhibits uniformity in both concentration and temperature. High pressure is applied to extrude viscous polymers. An extruder accomplishes these tasks via a barrel equipped with a screw and helical flights. Plastic pellets enter the barrel through a hopper at one end and are conveyed by the screw toward the opposite end. To generate pressure, the depth of the screw’s threads decreases as the distance from the hopper increases. External heating, combined with internal heat generated by friction between the plastic and the screw, softens and melts the material. Different polymers and applications often impose distinct design requirements on extruders. Numerous options include variations in die configurations, multiple feed ports, in‑screw mixing devices, melt cooling and heating systems, or even designs without external heat sources (heatless extrusion), as well as adjustments to the clearance between the screw and barrel and the number of screws.

A plastic extruder, in accordance with the material formulation requirements for extrusion molding of plastic products, precisely meters multiple primary and auxiliary materials as specified, then sequentially feeds them into a mixer in the prescribed process order for thorough blending until the constituent ingredients are uniformly combined. The uniformly blended raw material is subsequently cooled and stirred to reduce its temperature before being fed into the extruder’s hopper. From the hopper, the material enters the extruder barrel through the feed port.

Subsequently, the rotation of the screw in the plastic extruder forces the material forward along the barrel. Due to the resistance offered by the filter screen, the porous plate, and the die at the front of the barrel, combined with the gradual reduction in the screw’s flight volume, the frictional resistance opposing the material’s forward movement increases as the material buildup ahead grows—resulting in progressively higher pressure on the material. Meanwhile, the material is subjected to external heating from the barrel wall, as well as internal compression, shear, and mixing actions. Coupled with friction between the material and the barrel and screw surfaces, and with molecular‑level motion and intermolecular friction, the material’s temperature rises gradually within the barrel, causing its physical state to transition from the glassy state to a molten, highly elastic state—ultimately reaching a viscous flow state and achieving complete plasticization. As the screw continues to rotate steadily at a uniform speed, the uniformly plasticized melt is extruded through the die under constant pressure and at a consistent rate, forming the desired product shape. The extrudate is then cooled and set into its final form, cut to specified lengths, or wound into coils, thereby completing the production of extruded plastic products.