Leistritz Reports Q1 2022 Twinscrew Extrusion Market Trends

In modern polymer processing, co-rotating intermeshing twin-screw extruders (TSE) have become essential equipment for achieving efficient, rapid, and precise mixing. Their unique inter-screw fluid dynamics enable superior dispersive and distributive mixing capabilities, whether compounding polymers with fillers/additives or imparting specific properties to final products. The core advantage lies in sophisticated screw design and process parameter optimization, allowing deep material mixing and performance enhancement.

• Screw Speed (TSE screw rpm): Moderate increases enhance mixing, but excessive speeds may cause material degradation due to high shear.
• Degree of Screw Fill: Higher fill rates extend residence time and mixing frequency, while lower fills enable gentler processing.
• Temperature: Directly affects material viscosity, influencing mixing efficiency. Optimal temperature prevents degradation while reducing shear resistance.
• Extensional Mixing & Planar Shear: Critical mechanisms in TSEs, where residence time and mixing frequency determine dispersive mixing effectiveness.
• Sequential Feeding: Introducing components at different stages controls shear stress exposure, enabling precise process management.

In flexible packaging, biaxially oriented films (biax) dominate due to their superior physical, mechanical, and barrier properties. With production capacities often reaching several tons per hour, TSEs demonstrate significant advantages over single-screw extruders (SSE) in terms of cost-to-capacity ratios, making them the preferred choice for new biax film lines.

• Energy Efficiency: TSEs convert motor energy into material melting/heating more efficiently, achieving 20-35% energy savings. Built-in devolatilization eliminates pre-drying for hygroscopic materials like PET.
• Process Advantages: Integrated production reduces thermal/shear history, minimizes gel formation, ensures uniform molecular weight distribution, and prevents material agglomeration.
• Maintenance & Footprint: Modular design allows individual component replacement, while TSEs occupy less than half the space of SSEs.

Polylactic acid (PLA) and polyhydroxyalkanoates (PHA) represent prominent bioplastics with distinct processing characteristics. PLA typically requires high-torque TSEs for pellet processing, while PHA's powder form demands optimized feed zone designs. Leistritz's experiments demonstrate how segmented screw configurations enhance feeding and melting performance.

• PLA Melting Tests: Optimized melt zone designs reduced torque and temperature in NatureWorks™ 4032D PLA processing.
• PHA Processing: Modified screw designs with delayed melting and atmospheric venting increased throughput from 12 kg/hr to 62 kg/hr at 800 rpm.

Combining multiple extruders in series enables cost-effective production of specialized materials. These systems integrate single-screw, co-rotating, and counter-rotating twin-screw extruders to achieve effective length-to-diameter ratios exceeding 100 L/D, facilitating sequential unit operations.

For example, recycling polyvinyl butyral (PVB) from automotive safety glass involves a tandem system with two co-rotating TSEs for dehydration, glass fiber dispersion, filler incorporation, and pelletizing.

The overflight gap—the space between screw tip and barrel wall—is where materials experience intense extensional mixing and planar shear. Peak shear rate is calculated as:

Peak Shear Rate = (π × D × n) / (h × 60)

where D = screw diameter, n = screw speed, and h = overflight gap. For a 77.5 mm screw with 0.55 mm gap at 600 rpm, this reaches 4867 s⁻¹.

Shear stress (shear rate × viscosity) determines dispersive mixing effectiveness. Early-stage high viscosity promotes dispersion but risks degradation for shear-sensitive materials, while later-stage viscosity reduction minimizes stress. Cooling can increase melt viscosity for stronger dispersion when needed.

• Multiple TSEs (ZSE 12 MAXX to ZSE 50 MAXX)
• Diverse pelletizing systems (strand, underwater, hot-face)
• 40+ feeders for pellets, powders, fibers, liquids, and recyclates
• Downstream equipment for films/sheets (50 mm–1 m width), tubing/profiles, and 3D printing filaments
• Supercritical foaming and multi-stage devolatilization systems

For disassembly, heating elements with a propane torch followed by a brass-tipped pneumatic impact hammer (similar to automotive exhaust tools) proves effective:

1. Disassemble while warm or reheat in barrel for 20 minutes if cooled.
2. Support long screws evenly to prevent deflection.
3. Remove end nut; slide off last element if possible. Otherwise, apply heat and patience.
4. Use brass drift and hammer for stubborn elements, maintaining heat during tapping.
5. Clean exposed shafts after each removal.