I. From Pipes to Foam Boards: Process Requirements for Different PVC Products Pipe extrusion demands high output and high density. The melt must be fully plasticized without pores, placing strict requirements on the extruder's thermal stability and pressure uniformity. Profiles have complex cross-sections, requiring greater geometric accuracy and surface gloss, which depend on excellent melt homogeneity and pressure stability. For rigid foam boards (Celuka foam), the material must not foam prematurely inside the barrel. Temperature must be precisely controlled below the decomposition point, and the blowing agent must be gently mixed to avoid shear overheating that could cause cell collapse. Wood-plastic composites (WPC), with filler content as high as 50% or more, have extremely poor flowability. They pose severe challenges regarding screw wear resistance, torque, and prevention of fiber carbonization. Sheet products such as waterproof membranes and flooring require uniform width and consistent thickness, demanding excellent control of melt transverse temperature distribution.
II. Requirements for Conical Twin Screws in Extrusion Facing the vastly different process needs above, what hard specifications must suzhou Jwellmech 's conical twin screws meet?
High torque at low rotational speed – Whether for high pipe output or highly filled WPC, the screw must generate sufficient thrust at low speeds. Excessive speed increases shear heating, which is fatal for heat-sensitive PVC.
Precisely controllable shear intensity – Pipes need enough shear to ensure plasticization degree, but foam boards need only "just enough" shear – enough to disperse the blowing agent without causing localized overheating. Shear action must be stable, uniform, and adjustable.
Very short material residence time – For PVC, each extra second at high temperature exponentially increases decomposition risk. The effective screw length must be optimized to complete plasticization and mixing while allowing material to "flow in and out quickly."
Excellent wear and corrosion resistance – Wood powder, calcium carbonate in WPC, and chloride ions from PVC decomposition pose dual attacks on the screw and barrel. The screw material must withstand long-term abrasion and chemical erosion.
Flexibility for wide‑range processing – From pipes a few dozen millimeters in diameter to foam boards over one meter wide, the extrusion system must be able to quickly adapt to different dies and maintain stable melt delivery under varying backpressure conditions.
III. How Conical Twin Screws Meet the Demands and Mutually Reinforce the Process suzhou Jwellmech 's conical twin screws have two parallel axes with diameters gradually increasing from the feeding section to the metering section. They are inherently designed to meet the above requirements. Their three core advantages create a positive cycle with actual production.
Advantage 1: Low speed, large volume, mild shear – perfectly matching heat‑sensitive PVC. The large diameter and high volume in the metering section allow the screw to run at low speeds of 20–40 rpm, far below the usual speeds of parallel twin screws. Low speed dramatically reduces shear heat; combined with a short screw and residence time of only 30–60 seconds, this achieves high PVC output without decomposition and precise foam control for foam boards. In actual production of PVC rigid foam board, melt temperature fluctuation is controlled within ±2°C, with cell uniformity significantly better than with other screw types.
Advantage 2: Gradient shear – from pipe compactness to foam board uniformity. The intermeshing clearance gradually changes along the screw axis. In the feeding section, strong kneading quickly absorbs additives; in the compression section, gentle plasticization avoids overheating; in the large‑diameter metering section, low shear homogenizes and builds pressure. This "first strong, then mild" gradient shear allows the same screw set to meet both the high plasticization requirement for pipes and the gentle mixing requirement for foam boards. In practice, switching products only requires adjusting temperature settings and formulation – no screw change – greatly improving production line flexibility.
Advantage 3: High strength, wear resistance – facing WPC and highly filled challenges. The conical design gives a larger root diameter, providing torque capacity far exceeding that of parallel screws with the same center distance. Combined with bimetallic hardfacing or nitriding treatment, surface hardness reaches HRC 58–62, effectively resisting abrasive wear from wood powder and calcium carbonate. A WPC plant that switched to high‑quality conical screws maintained stable output for 10 consecutive months, whereas previously with parallel screws they needed repair every 3 months. Long life ensures process consistency, avoiding product color differences and density fluctuations caused by screw wear.
Mutual reinforcement between technology and process: In practice, screw design continues to evolve – introducing barrier sections to optimize distribution, improving dispersion mixing without increasing residence time; adding helical grooves at the discharge section and multi‑zone independent cooling to keep temperature difference within ±1°C. These improvements in turn expand application boundaries. Today, conical twin screws have been successfully used for PE and PP wood‑plastic composites and even for some engineering plastics filled with modifiers.
Summary: PVC pipes, foam boards, WPC, etc., have widely differing process requirements, demanding screws with high torque at low speed, precise shear, short residence time, and wear resistance. suzhou Jwellmech 's conical twin screws precisely match these diverse requirements with their three advantages: low‑speed mild shear, gradient shear, and high wear resistance. Technology and process mutually reinforce each other, and applications have expanded to PE, PP wood‑plastic composites, and engineering plastics.
