65/138 Conical Twin Screw Barrel – Pvc & Wpc Extrusion

Executive Summary

• Configuration: The 65/138 conical twin screw and barrel are designed for the high-efficiency extrusion of rigid PVC, wood-plastic composites (WPC), and similar thermoplastics. It uses two intermeshing screws that taper from a 138 mm feed diameter to a 65 mm discharge diameter, creating a strong and natural compression zone.
• Material and Process: The screws and barrel, precision machined from high-quality nitriding alloy steel such as 38CrMoAl, undergo rigorous heat treatments. Standard processes include gas nitriding (0.45–0.7 mm depth at 850–1000 HV hardness) and optional bimetallic alloy casting (2–5 mm wear-resistant layer at 50–65 HRC hardness).
• Performance: This design significantly enhances the efficiency of industrial plastic extrusion lines while maintaining melt stability by providing comprehensive homogenization and high melt pressure at moderate screw speeds.
• Operational Advantages: Key advantages include extended service life and energy-efficient operation when processing abrasive compounds. The self-wiping intermeshing motion ensures a homogeneous melt and prevents material degradation. Applications such as heavy-duty PVC pipe manufacturing, roofing sheets, and WPC flooring profiles make the product extremely reliable for continuous industrial operations.

Design and Materials

The 65/138 barrel features a conical design that tapers from a 138 mm feed end to a 65 mm discharge end, with two counter-rotating screws. This stepped taper consistently increases melt pressure along the barrel.

• Base Structure: The base structure is made from premium alloy steels, primarily 38CrMoAl. This class of nitriding steel provides exceptional core toughness (HB 280-320 after tempering). Both screws and barrel undergo gas-ion nitriding to achieve a highly durable surface layer with a hardness of 850–1000 HV (0.45–0.7 mm depth).
• Surface Coatings: To ensure superior resistance against corrosive and abrasive fillers, a bimetallic wear-resistant layer is typically cast onto the inner wall of the barrel. This alloy layer is 2–5 mm thick and reaches a dual alloy hardness of 50–65 HRC. Alternatively, hard chrome plating (0.025–0.10 mm thickness) can be applied to nitrided steel to resist micro-abrasion from materials like chlorinated PVC.
• Precision Machining: To guarantee smooth plastic flow, surface roughness is maintained at ≤ Ra 0.4 µm. Screw linearity is controlled with a precision of 0.015 mm to ensure rigid alignment during assembly, preventing mechanical friction/interference under high torque conditions.

📊 Technical Specifications Table

Technical Advantages

• High Output & Mixing: The pronounced taper from 138 mm to 65 mm naturally generates pressure, enabling high-capacity extrusion at moderate RPMs. The intermeshing design offers excellent self-cleaning action, resulting in a completely homogeneous melt and higher efficiency compared to similar-sized parallel alternatives.
• Abrasion Resistance: The combination of 38CrMoAl base steel with bimetallic casting (2–5 mm layer) provides maximum protection against highly filled formulations (e.g., high CaCO₃ content in pipes or silica in WPC). This significantly delays wear, extending maintenance life.
• Efficient Heat Management: Low operating speeds reduce shear-induced overheating, which is critical for processing heat-sensitive PVC. Zoned heating and integrated cooling ensure absolute temperature stability in the feed, transition, and metering zones.
• Self-Cleaning Feed Zone: The special taper geometry near the 138 mm feed end actively draws material in and self-cleans, preventing moisture and complex additives from accumulating, thus ensuring highly reliable initial startups.
• Customization: Screw pitch, channel depths, and compression ratios can be highly customized to meet the exact requirements of industrial plastic extrusion lines, whether optimized for roofing sheets or thick-walled pipes.

Applications and Industries

• PVC Pipe and Profile Extrusion: Perfectly suited for industrial plastic extrusion machines producing rigid PVC pressure pipes, corrugated pipes, and architectural profiles. The conical design efficiently processes stabilizers and mineral fillers.
• Roof Sheets and Panels: High efficiency and stable melt pressure guarantee uniform thickness and mechanical strength for extruded PVC roof sheets and corrugated panels.
• Wood-Plastic Composite (WPC): Highly effective for WPC decking and flooring. The strong compounding effect ensures excellent dispersion of wood flour within the PE/PVC matrix, while the bimetallic barrel shows resistance to highly abrasive wood fibers.
• Cable and Pipe Manufacturing: Used for high-volume electrical pipes and continuous cable sheathing processes; offers flawless surfaces and high resistance to compound additives.
• General Thermoplastics: Capable of processing a wide range of polymers, including PE, PP, and ABS, providing superior shear control for consumer and industrial plastic products.

Installation and Compatibility Notes

• Mechanical Compatibility: The 65/138 barrel is mounted on standard counter-rotating conical twin screw extruder gearboxes. Proper flange alignment and bolt torquing are essential for secure mounting.
• Alignment: The screws should intermesh without metal-to-metal contact. A 0.015 mm linearity tolerance helps ensure perfect alignment, but manual rotation at low speed is required during commissioning to verify the gap.
• Cooling and Heating: Ensure all barrel cooling jackets and external heating resistors are securely connected. Zoned temperature controllers should be calibrated before any polymer feeding.
• Machine Compatibility: The main extruder must provide sufficient torque for the 138 mm wide feed section. Verify that the L/D ratio and mounting flanges match the existing machine architecture.

Maintenance and Lifespan

• Wear Monitoring: Regularly monitor production volume and melt quality. Check the radial gap between screw pitch and barrel wall. If the bimetallic or nitrided layer wears beyond standard tolerances, replacement or refurbishment is necessary.
• Cleaning: Clean the barrel immediately after processing PVC to prevent the release of corrosive HCl gas (purge). Using standard commercial cleaning compounds helps maintain Ra 0.4 µm surface quality.
• Preventive Maintenance: Regularly check end bearings, gearbox oil, and screw couplings. Ensure heaters and cooling channels are descaled and fully operational.
• Life Expectancy: A well-maintained 65/138 barrel with a 2–5 mm bimetallic layer can last tens of thousands of hours even when continuously operated with highly filled PVC and WPC compounds.

Safety and Quality Standards

• Quality Assurance: Components are manufactured to tight tolerances (Nitriding brittleness: Class II). Suppliers typically comply with ISO standards and provide full material and heat treatment certification.
• Safety Practices: Implement standard extruder safety protocols. Ensure that hot barrel areas are insulated and enclosed. Provide adequate ventilation for potential polymer gas emissions, especially in PVC formulations.
• Operational Safety: Operators must strictly adhere to the manufacturer's startup and shutdown sequences to prevent cold starts or excessive back pressure buildup.