What are the requirements for impurity content in single-screw plastic recycling machines?
Publish Time: 2025-12-08
In the plastic recycling industry, single-screw plastic recycling machines are widely used in the melting, extrusion, and granulation processes of waste plastics due to their simple structure, convenient operation, and low maintenance costs. However, despite their good plasticizing capabilities, this equipment has relatively strict requirements for the purity of the feed material—especially the impurity content. This requirement not only affects the quality of the final recycled pellets but also directly impacts the operational stability and service life of the equipment.First, "impurities" include visible non-plastic components such as metal fragments, stones, sand, paper scraps, and textile fibers, as well as mixed plastics of different types or colors. If these impurities enter the granulator directly without effective pretreatment, they will cause a series of problems during the high-temperature melting process. For example, metal foreign objects may become stuck between the screw and the barrel, causing severe wear and even scratching the inner wall, compromising the equipment's sealing and shearing efficiency; while sand or glass particles act as abrasives, accelerating the wear of key components during screw rotation and shortening the overall machine lifespan.Second, the presence of impurities significantly affects the physical properties and appearance quality of the recycled plastic. Different types of plastics have different melting points, rheological properties, and thermal stability. When high-density polyethylene (HDPE) is mixed with polyvinyl chloride (PVC), or polypropylene (PP) is mixed with polystyrene (PS), under uniform processing temperatures, some components may not completely melt, while others may overheat and decompose, producing black spots, bubbles, or charred residues. This not only results in uneven particle color and a rough surface but also weakens its mechanical strength, making it difficult for recycled materials to meet the standards for downstream products.Furthermore, "hidden impurities" such as moisture, oil, or residual chemicals cannot be ignored. Waste plastics with excessive moisture rapidly vaporize in the high-temperature barrel, creating steam pressure that can cause melt bursting, unstable discharge, or even splashing from the die, posing safety hazards. Packaging films containing oil or adhesive, if not thoroughly cleaned, will produce smoke and odors during melting and form carbon deposits on the screw surface, affecting heat transfer efficiency, increasing energy consumption, and contaminating the finished product.Therefore, single-screw granulators typically require a comprehensive pre-treatment process at the front end. An ideal recycling process should include multiple stages such as crushing, washing, dewatering, air separation, magnetic separation, and even manual sorting to remove various impurities to the maximum extent, ensuring that the material entering the main unit is a single type of clean, dry plastic fragments or granules. While some high-end models are equipped with exhaust ports to release small amounts of volatiles or use wear-resistant alloy screws to handle mild contamination, these designs cannot replace source purification. After all, the single-screw structure itself lacks the strong self-cleaning and mixing/dispersing capabilities of a twin-screw design, making it more reliant on the purity of the raw materials.It is worth noting that the requirements for impurity control vary depending on the application scenario. If the recycled material is only used for low-value-added products (such as pallets and traffic cones), the standards can be appropriately relaxed; however, if the target is food-grade packaging, film, or engineering plastics, then a near-stringent sorting and purification process must be implemented.In conclusion, while the single-screw plastic recycling machine is an important tool for recycled plastic production, its efficient and stable operation depends on the effective control of the impurity content of the feed material. Only through scientific front-end processing and strict raw material management can the equipment performance be fully utilized to produce high-quality recycled pellets, truly realizing the economic and environmental value of resource recycling.
