Key Factors in Extruder Screw Performance: Length, Thickness, Material, And Shear

Screw Length (L/D Ratio)

The length of an extruder screw is typically expressed not in centimeters (cm) or millimeters (mm), but in terms of its L/D ratio. Here, L stands for the length of the screw, and D represents the screw’s diameter.

For example, if a screw has a diameter of 100 mm and an L/D ratio of 34, the actual length of the screw would be 100 mm × 34 = 3,400 mm.

In general, increasing the screw’s rotational speed tends to increase the output. However, if the speed is too high, the residence time for melting the resin becomes insufficient, resulting in incomplete melting. To address this, it is often necessary to increase the L/D ratio to 36 or 40, allowing for complete melting even at higher rotation speeds.

Long Screws: Easier to increase production volume, and they provide better mixing compared to short screws. However, they are more prone to resin degradation and burning.

Short Screws: Reduce thermal history and help prevent material degradation but are less effective in increasing output.

For instance, a longer screw generally allows better mixing of the resin. Conversely, a shorter screw is suitable when thorough mixing is not critical. Materials like LDPE, which melt easily, often use shorter screws to minimize thermal degradation during recycling processes by allowing the resin to exit quickly after melting.

Longer screws make it easier to increase production output because they allow for a higher screw rotation speed. As the resin travels a longer distance inside the cylinder, there is more time for it to melt properly. Thus, even with increased screw speeds, the resin can fully melt, enhancing production efficiency.

Screw Diameter

Thicker Screws: Enable higher production volumes but result in greater material loss during material changes or when troubleshooting.

Thinner Screws: Easier to clean during material changes and ideal for small-batch production, but they limit output.

The diameter of the extruder screw significantly affects the production rate. For example, a screw with a diameter of 100 mm may produce 300 kg per hour. By increasing the diameter to 120 mm, production can rise to 400–500 kg per hour. Choosing a screw with a larger diameter can thus significantly boost output.

Screw Material

The screw material must be selected to match the type of resin being processed. It is essential to choose a steel material with a mirror-like finish that ensures the resin flows smoothly without sticking.

Below are common materials and surface treatments:

Nitriding Treatment
Extruder screws are constantly exposed to resin, sometimes containing fillers like calcium carbonate or glass fibers that promote wear. Therefore, high wear resistance is crucial. Resistance to corrosion is equally important, especially when recycling plastics containing corrosive components like PVC.

For general recycling applications, nitrided screws are commonly used. Nitriding enhances both corrosion and wear resistance, significantly extending screw life. While untreated screws are cheaper, they suffer from rapid corrosion and wear, resulting in frequent replacements and higher long-term costs.

The typical material used is SACM645 (chromium-molybdenum steel). After nitriding, SACM645 offers excellent wear resistance and is considered an ideal screw material. SACM645 is almost synonymous with nitrided screws.

Quenching (Hardening) Treatment
If even higher wear resistance is required, quenching can significantly improve durability. For materials containing around 30% glass fiber (GF30), quenching provides sufficient protection.

Stainless Steel
Using stainless steel as the base material, combined with various treatments, can further enhance performance:

Stainless Steel + Nitriding: Improves both corrosion and wear resistance.

Stainless Steel + Quenching: Further enhances wear resistance beyond what nitriding alone can achieve, while also maintaining corrosion resistance.

Powder Metallurgy Quenching
For maximum wear resistance, powder metallurgy quenching can be employed, offering superior durability compared to standard quenching. The hardened surface also improves resin release properties. Screws treated this way are typically used when processing materials with over 50% glass fiber content.

Shear by Screws

Resin melting occurs through external heaters and shear heat generated by the screw’s rotation. In general, high-viscosity (thicker) resins produce more shear heat.

Simply put, when the resin is mixed by the screw, "shear heat" is generated. This happens as the motor’s mechanical energy is transferred to the resin through screw rotation, resulting in heat.

Thus, the resin's temperature can rise not only from the heaters but also from the internal shear heat generated by the resin itself. As a result of shearing, the resin’s viscosity decreases, making it more fluid.

The degree of shear applied to the resin depends on the screw’s compression ratio (how strongly the resin is compressed). A lower compression ratio results in less shear, while a higher compression ratio (causing more friction) increases shear.

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