Custom Plastic Extrusion Screw Factory & High-Performance Products

The extrusion screw is the functional heart of the polymer processing system. Its design directly dictates shear rate, thermal uniformity, residence time distribution, and volumetric output. Developing custom single and twin screws requires balancing polymer melt behaviors under intense pressure and heat. At BAOD Extrusion, our design methodology relies on simulating shear dynamics and thermal transition patterns across three primary zones: the feed zone, the transition (compression) zone, and the metering zone.

1. The Three-Zone Screw Geometry: Function and Optimization

To process plastics with different viscosities (like LDPE, HDPE, PP, PVC, and UHMWPE), the screw must guide the material through a progressive transition from solid pellets to a uniform, bubble-free melt:

• Feed Zone: Featuring a constant deep channel depth, this zone conveys solid polymer pellets from the feed throat into the barrel. Efficient solid conveying relies on high friction against the barrel wall and low friction on the screw surface. Cooling the feed zone prevents premature melting and bridging.
• Transition (Compression) Zone: The channel depth decreases progressively, forcing the solid bed to compress, melt, and release trapped air. This compression is achieved using tapered root diameters or variable pitch designs. High compression ratios (up to 3.5:1) are critical for high-viscosity materials like PVC, while lower ratios protect shear-sensitive polymers from degradation.
• Metering Zone: Featuring a shallow, constant channel depth, this zone homogenizes the polymer melt temperature and pumps the extrudate at a uniform pressure through the die head. Uniform pressure is essential to minimize wall-thickness variation in precision applications like medical micro-tubing.

2. Advanced Barrier Screw Geometry and Shear Control

For high-throughput applications, conventional three-zone screws can struggle to melt polymer solids completely without overheating the melt. Barrier screws address this by introducing a secondary flight within the transition zone, separating the solid bed from the melted pool. This design ensures that melt is continuously swept away, improving heat transfer efficiency and output rates while keeping melt temperatures stable. Mixing sections, such as Maddock shear mixers or pineapple distributive mixers, are positioned downstream in the metering zone to break up any remaining gels and ensure uniform color dispersion.

Premium Metallurgy and Surface Treatments for Wear Prevention

Extruder screws are subjected to abrasive wear from polymer additives (like calcium carbonate, glass fibers, or flame retardants) and corrosive wear from degradation byproducts (such as hydrochloric acid from PVC). Without proper surface treatment, screw clearances will widen, causing pressure drops and increased material backflow. BAOD Extrusion employs three main wear-mitigation strategies:

• Nitrided Steels (38CrMoAlA): Our standard alloy is subjected to a deep gas nitriding process, creating a surface hardness of HV950-1050 with a diffusion depth of 0.5 to 0.8 mm. This provides excellent wear resistance for standard, non-abrasive polymers.
• Bimetallic Alloys (PTA Welded): For abrasive and corrosive environments, we apply high-performance alloys to the flight tips via Plasma Transferred Arc (PTA) welding. We use nickel-based alloys for corrosive applications and tungsten carbide composites for high-abrasion resistance.
• Sintered Tool Steels: For high-wear applications, like processing medical-grade fluoropolymers or highly filled composite plastics, we machine the screw from solid, high-alloy PM steels. This ensures uniform hardness across the entire cross-section of the screw.