How to choose screws for different raw materials

1. Thermoplastic Raw Materials

• Polyethylene (PE)

• Features: Good fluidity, low viscosity, high crystallinity, low temperature - sensitivity, but prone to oxidative degradation at high temps.

• Screw Selection: Use a universal screw with L/D ratio of 20:1 - 30:1 and a compression ratio of 2.5:1 - 3:1. The feed - section groove can be deep, and the metering - section groove has a moderate depth.

• Polypropylene (PP)

• Characteristics: Similar to PE, good fluidity, faster crystallization, larger shrinkage, and specific shear - rate needs.

• Screw Selection: L/D ratio is 20:1 - 25:1, compression ratio 3:1 - 3.5:1. Ensure good screw cooling and add shear elements (e.g., pins) carefully.

• Polystyrene (PS)

• Features: Amorphous, good fluidity, high transparency, poor thermal stability, and easy to decompose and produce gas.

• Screw Selection: L/D ratio 18:1 - 22:1, compression ratio 2.5:1 - 3:1. Moderate shear rate, not - too - deep feed - section groove, and high - finish screw surface.









• Polyvinyl Chloride (PVC)

• Features: Heat - sensitive, highly sensitive to temperature and shear, easy to decompose, high viscosity, needs additives.

• Screw Selection: Use a special PVC screw with L/D ratio 20:1 - 25:1, compression ratio 3.5:1 - 4.5:1. Shallow screw grooves, especially in the metering section, and corrosion - resistant screw material.

• Polycarbonate (PC)

• Features: Non - crystalline, no distinct melting point, high glass - transition temp, large viscosity, temperature - sensitive, good thermal stability, strong water absorption.

• Screw Selection: High L/D ratio (25:1 - 30:1), gradual - type screw, compression ratio 3:1 - 3.5:1. High - torque screw, and dry raw materials well.

• Polyamide (PA, Nylon)

• Features: Crystalline, strong water absorption, high melting point, viscosity varies with temperature, prone to melt rupture.

• Screw Selection: L/D ratio 20:1 - 25:1, compression ratio 3:1 - 3.5:1. Dry before processing, and design the screw for water and gas discharge, with some special structures in the feed section.

2. Thermosetting Plastic Raw Materials

• Phenolic Plastic

• Characteristics: Cross - links during heating, has fluidity before curing but cures fast, high requirements for temperature and pressure control.

• Screw Selection: L/D ratio 15:1 - 20:1, high compression ratio (3.5:1 - 4:1), and an exhaust device on the screw.

• Epoxy Resin

• Features: High viscosity, curing requires specific temp and catalyst, cured product has good properties.

• Screw Selection: L/D ratio 20:1 - 25:1, compression ratio 3:1 - 3.5:1, high - torque screw, and control temp and catalyst amount.

3. Rubber Raw Materials

• Natural Rubber

• Features: Good elasticity and plasticity, high viscosity, needs high - temp processing, and is prone to oxidation and aging.

• Screw Selection: L/D ratio 10:1 - 15:1, low compression ratio (1.5:1 - 2:1), deep screw grooves, and wear - and - corrosion - resistant screw material.

• Synthetic Rubber

• Characteristics: Varying performance based on synthesis methods. High - viscosity types (e.g., styrene - butadiene rubber) need higher L/D and compression ratios; others (e.g., butadiene rubber) need special groove designs.

• Screw Selection: Adjust L/D ratio and compression ratio according to viscosity, and design screw grooves for specific rubber types.

4. Engineering Plastic Raw Materials

• Polybutylene Terephthalate (PBT)

• Features: Crystalline, high melting point, low viscosity, good properties, but prone to hydrolysis.

• Screw Selection: L/D ratio 20:1 - 25:1, compression ratio 3:1 - 3.5:1. Dry raw materials, and high - finish screw surface.









• Polyethylene Terephthalate (PET)

• Features: Crystalline, high melting point, low viscosity, good properties, needs drying to avoid defects.

• Screw Selection: L/D ratio 20:1 - 25:1, compression ratio 3:1 - 3.5:1. Dry before processing, and set up pre - heating devices in the feed section.

5. Fiber - Reinforced Plastic Raw Materials

• Glass Fiber - Reinforced Plastic (GFRP)

• Features: Reinforced with glass fiber, improved strength, but increased viscosity and processing difficulty, high screw wear.

• Screw Selection: L/D ratio 20:1 - 25:1, compression ratio 3:1 - 3.5:1. High - hardness and wear - resistant screw material, and add shear elements carefully.

• Carbon Fiber - Reinforced Plastics (CFRP)

• Features: High strength and heat resistance, difficult to process, severe screw wear.

• Screw Selection: Similar to GFRP, but use higher - hardness screw materials (e.g., with ceramic coating) and optimize shear - element design.