68/25 Parallel Twin Screw Barrel

Overview

The 68/25 parallel twin screw barrel refers to a set of extruder screws and barrels with a screw diameter of 68 mm and a length-to-diameter ratio of 25:1. This assembly is designed for heavy-duty plastic extrusion applications (e.g., PVC pipe/profile, WPC flooring, compounds). The screws, which can rotate together or in opposite directions, provide stable cutting and mixing for consistent melt quality by rotating sequentially within the barrel.

Such barrels are typically made from high-strength alloy steel (e.g., 38CrMoAl) with precision machining and surface hardening (nitriding) to withstand abrasive fillers and high back pressure. In practice, a 68 mm screw with an L/D = 25:1 provides high efficiency while maintaining precise dimensional control for profiles and sheets. Manufacturers often supply these barrels in a bimetallic form (hardened alloy-lined carbon steel base) to enhance wear resistance.

Key Point: Parallel twin barrels have a fixed screw diameter (no taper), which allows for homogeneous pressure and easy integration of feed or vent ports. Compared to single screw extruders, twin screw systems offer superior mixing and thermal balance, making them ideal for filled or recycled compounds. Opposite rotating twin barrels are commonly used for PVC and WPC as they produce precise, glossy products by generating high compression and homogeneous pressure.

🛠️ Technical Specifications Table

Notes: The screw pitch (distance between pitches) is typically about twice the diameter for coarse feeding but can be customized. The gap between the screw pitch and the barrel hole (radial clearance) is critical: the optimum gap for a 68 mm screw is roughly 0.10–0.18 mm. A very wide gap can cause backflow of the melt and reduce efficiency, while a very narrow gap increases friction and wear.

Design Features and Benefits

• Bimetallic Barrel Liner: Many 68/25 barrels use a bimetallic structure (liner made from alloy steel) that provides excellent wear and corrosion resistance. This liner can be coated for ultra-hard surfaces (58–62 HRC) in abrasive compound applications (e.g., HVOF tungsten carbide). The hardened liner extends the service life by 3-5 times compared to unlined barrels.

• Nitrided Screw: Screw pitches and roots are precisely ground and gas nitrided up to ~850-1000 HV. This provides a hard and wear-resistant surface while keeping the core intact (HRC ~28–32).

• Thermal Control: The barrel is often divided into independent heating/cooling zones (feed, compression, metering zones). Advanced designs include optimized cooling channels in the barrels to minimize temperature fluctuations. Homogeneous thermal control prevents material degradation and ensures consistent output.

• Standard Flanges and Modular Segments: The design uses industry-standard flanges and modular barrel segments for easy installation. Interchangeable screw sections allow for adapted compression ratios for different materials (e.g., higher feed zone for rigid PVC, deeper compression for compounds).

• Precision Machining: The matching surfaces of the screw/barrel are ground to very tight tolerances (surface roughness Ra ≤0.4 μm, linearity ≤0.015 mm). These tight tolerances reduce leakage (increasing efficiency) and provide balanced torque between screws, enhancing stability. The result is smooth, stable extrusion and consistent profile quality.

• Opposite Rotation Option: While co-rotating screws self-clean for mixing, opposite rotating barrels generate higher pressure and shear force, making them ideal for rigid PVC and filled compounds. The opposite rotating 68/25 twin barrel can achieve outputs of 250–450 kg/h for PVC with lower energy consumption due to improved melt efficiency.

Manufacturing and Quality Control

• CNC Machining: Screw roots and pitches are milled on CNC lathes, and shape tools grind the profile according to technical specifications. Multi-axis machining ensures homogeneous pitch depth and profile along the length. CNC grinding brings pitch surfaces to the desired smoothness.

• Alloy Sourcing (Bimetal Casting): For bimetallic barrels, a thick (5-8 mm) liner made from high alloy steel is welded or centrifugally cast into the carbon steel barrel. The liner material (e.g., 41CrAlMo7) is selected based on its wear and corrosion resistance; welding follows stress relief patterns to prevent distortion.

• Gas Nitriding: Both the screw and barrel are typically gas nitrided; this diffusion process creates a hardened surface layer with hardness up to 900-1000 HV (~0.5 mm deep). Nitriding increases wear resistance without significant distortion; components are tempered post-nitriding to adjust core hardness (typically ~HB 260–280).

• Surface Inspection: The linearity of mounted screws and the roundness of barrel holes are measured (typically ≤0.015 mm tolerance). Surface roughness is checked with profilometers (target Ra ≤0.4 μm); these inspections ensure that the screw-barrel clearance remains within design tolerances.

• Quality Management: Reputable suppliers use ISO 9001 quality systems with material traceability and hardness test reports. Before shipment, each barrel set is typically trialed and checked for leakage and alignment. Non-destructive testing (e.g., liquid penetrant or ultrasonic) may be used to verify weld integrity.

• Core Benefit: These stringent manufacturing and quality control practices ensure that the 68/25 twin screw barrel operates reliably under high pressure. A precision-hardened barrel is resistant to wear and maintains tight tolerances during long runs, reducing downtime.

Installation and Maintenance

• Alignment: During installation, ensure that both barrels are precisely aligned within the extruder frame; misalignment can lead to uneven wear or jamming. Use the manufacturer's alignment tools and adhere to torque specifications for bolts.

• Lubrication and Cooling: Before first use, coat the screws with a separating agent to assist with break-in. Check that all cooling channels are open and provide fluid as designed. Consistent cooling prevents localized overheating of the barrel.

• Temperature Setting: Follow the recommended barrel heating profiles for the material; gradually increase temperatures initially to avoid thermal shock. Verify that all thermocouples in each barrel zone are functioning to maintain a stable profile.

• Regular Inspection: Schedule screw withdrawal operations at regular intervals; inspect screws for signs of wear (worn pitches, scratches) and barrels for scoring or deposits. Check the screw-barrel clearance with feeler gauges; excessive clearance (greater than >0.3 mm) indicates the need for refurbishment or replacement.

• Preventive Maintenance: When changing materials, mechanically clean or use a cleaning compound to remove residues and color remnants. Preventive cleaning and filtering can minimize abrasive contamination; for critical operations, consider reverse cleaning to clear vents and ports.

• Component Wear: If continuous operation is required, keep spare liners or screw sections in stock. Bimetallic liners can sometimes (depending on the supplier's capabilities) be recoated or reground.

• Best Practice: Keep a log of operating hours and any issues encountered. Gradual performance decline (e.g., slowly increasing melt pressure or lower output) often indicates wear; timely maintenance will prevent unplanned downtime.

Material Compatibility and Processing

• General Plastics (PVC, PE, PP): Standard nitrided barrels (no special liner) are cost-effective for unfilled plastics. Nitrided steel is resistant to wear in normal service; for rigid PVC (especially CaCO₃ filled), a design with opposite rotation and a bimetallic barrel is recommended to manage high shear forces and reduce wear.

• Filled/Composite Materials: High filler materials (glass, talc, etc.) or wood-plastic composites are abrasive. Here, a thick bimetallic or even hard filler (Tungsten or Hard Chrome plating) is recommended. Extra hard liners minimize erosion and optimized cooling keeps filled melts stable.

• High Temperature Polymers: Higher temperature resistant steels (e.g., Ni-based alloys) and robust cooling are used for engineering resins or flame-retardant blends. While less common in PVC lines, solid carbide screws/barrels can be used for ultra-hard service.

• Rubber and Elastomers: If thermoplastic elastomers or rubber are processed, the design may shift to non-intermeshing screws (separate flow channels) to prevent excessive shearing. Flexible PVC and TPEs often require light compression; a low pitch co-rotating screw may be specified.

• Processing Tips: Always dry hygroscopic materials before feeding; use gradual feeding rates when starting a new run. In filled systems, feed injection (side feeding) ports or venting areas can help remove volatiles and improve mixing.

Applications

• PVC Pipe and Profile Extrusion: Common in rigid PVC pipe lines and profile extrusion where precise dimensional control and smooth surfaces are required. Opposite rotating twin screws provide the high pressure needed for wall thickness consistency.

• WPC Flooring: Wood-plastic composites for flooring/decking typically use 68 mm and larger twin screw extruders. The twin screw design ensures thorough mixing of wood filler and PVC, producing a uniform composite profile.

• Plastic Compounding: In masterbatch and pelletizing facilities, 68 mm parallel twins can be used for small to medium scale compounding of additives or recycled polymers, leveraging their good mixing capabilities.

• Rubber Extrusion: For specific rubber or TPE applications, these barrels (with suitable screw designs) can process high viscosity elastomers using intermeshing or non-intermeshing configurations to control shearing.

• Fiber Reinforced Extrusion: Glass fiber reinforced PVC or other polymers (e.g., glass-filled PA) can be extruded with twin screws; the intermeshing variant is particularly suitable for distributing the fiber mix.

• Usage Note: Exact output and melt quality depend on screw configuration and material. A 68 mm opposite rotating extruder can produce 200-400 kg/h of PVC (depending on L/D and operating conditions), while co-rotating units may operate at slightly higher capacities with flexible polymers.

Troubleshooting and Tips

• Inconsistent Feeding (Material Bridging): If pellets or powders are bridging in the hopper or feed throat, try using a cleaner feeder or mechanical wiper. Ensuring a homogeneous particle size (eliminating fines) can also prevent clumping.

• Overheating/Melt Degradation: Closely monitor barrel temperatures; if material degrades (burnt smell, black specks), lower the barrel temperature or increase cooling. A clogged cooling channel or faulty thermocouple can cause overheating – check the equipment's operation.

• Decreasing Output / Screw Wear: A gradual drop in output typically indicates wear; verify screw-barrel clearance with feeler gauges. If the clearance is greater than >0.2 mm above specification, consider regrinding or replacing. Use wear-resistant screw sections or replace worn pitches to regain capacity.

• Irregular Product (Flow Fluctuations): Quality fluctuations (e.g., weight variation, dimensional inconsistency) can stem from mold issues or contamination. Check for blockages in the mold and ensure smooth feeding of the extruder screw. Thoroughly clean the system when changing resins to prevent mixing incompatible polymers.

• Gel/Black Specks: These can occur if the material formulation is closed or if separation occurs. Review the raw material batch for contaminants; adjust processing conditions (e.g., slower speeds, lower temperature increases) to prevent overheating of pellets. Periodic cleaning with a cleaning compound can remove residual gels.

• Barrel Leaks: Any melt leaking from barrel connections or flanges should be addressed immediately by re-tightening or replacing seals. Persistent leaks indicate a cracked barrel segment or worn flange face; investigate and repair as necessary.

• Preventive Advice: Establish a routine maintenance program; regularly replace barrel bolts (they stretch over time) and grease screw seals. When material changes occur, use a compatible cleaning compound to clean residues. Training operators on these practices can significantly reduce downtime and defects.