How to Select the Appropriate Barrel and Screw Materials?
When selecting barrel and screw materials based on the specific characteristics of the plastics being processed, the core principle is: "For abrasion, choose bimetallic; for corrosion, choose stainless steel; for general applications, choose nitrided steel; and for extreme conditions, choose powder alloys." You must make a precise match based on whether the material contains glass fibers (determining the requirement for wear resistance) and whether it generates corrosive gases (determining the requirement for corrosion resistance). Strictly avoid a "one-size-fits-all" approach using standard materials; doing so will lead to a drastically reduced equipment lifespan or product contamination.
Drawing upon your background as an equipment maintenance professional, the following are specific selection strategies tailored to different plastic characteristics:
1. High-Abrasion Conditions: Plastics Containing Glass Fibers or Mineral Fillers
When processing reinforced plastics-such as PA (Nylon), PBT, or PC-that have been compounded with glass fibers, carbon fibers, or minerals, the abrasive scraping action of the material against the metal surface is extremely intense.
Pain Point: Standard nitrided steel (e.g., 38CrMoAlA) lacks sufficient hardness and is prone to developing grooves from abrasion. This leads to material stagnation and degradation, and can even result in metal fragments mixing into and contaminating the final product.
Recommended Material: Bimetallic Screw/Barrel.
Process: A layer of tungsten carbide, cobalt-based alloy, or nickel-based alloy-typically 2–3 mm thick-is applied to an alloy steel substrate via centrifugal casting or thermal spraying.
Performance: Surface hardness can reach over HRC60. The wear-resistant lifespan is 3 to 5 times that of standard nitrided steel, enabling it to effectively withstand the long-term abrasive scouring caused by glass fibers.
Applicable Scenarios: Glass-fiber reinforced PA66, flame-retardant PBT, mineral-filled PP, etc.
2. High-Corrosion Conditions: Plastics Containing Chlorine, Fluorine, or Heat-Sensitive Materials
When processing PVC, flame-retardant plastics, fluoroplastics (such as PVDF), or certain medical-grade transparent materials, the acidic gases (e.g., hydrogen chloride) released during high-temperature decomposition can severely corrode metal components.
Pain Point: Standard steel materials are prone to developing rust spots, which can lead to color variations and embrittlement in the finished products. Furthermore, if a chrome plating layer peels off, it accelerates corrosion and contaminates transparent products. Recommended Materials: Stainless steel or corrosion-resistant alloys.
Grades: 9Cr18MoV (martensitic stainless steel) is the preferred choice; after integral quenching, it achieves a hardness of HRC 55–58, offering a balance of corrosion resistance and wear resistance. For high-end applications, 316L stainless steel or titanium alloys are available options.
Advantages: Eliminates iron ion contamination and exhibits strong resistance to acids and alkalis, making it particularly suitable for the processing of transparent products-such as those made from PC or PMMA-that demand exceptionally high color clarity.
Note: For PVC processing, in addition to selecting corrosion-resistant materials, the screw structure should ideally be of a gradual-transition design to minimize shear heat and prevent material decomposition.
