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Injection Molding Screws and Barrels – Design & Tips

Injection molding screws and barrels are integral components. Injection molding screws and barrels work together to plasticize and inject molten plastic into molds. The screw rotates inside the barrel, mixing and heating the plastic material until it reaches a molten state. Once the plastic is sufficiently plasticized, the screw advances, forcing the molten plastic through the nozzle into the mold cavity.

The barrel, which houses the screw, provides a controlled environment for the plasticization process, ensuring consistent material temperature and pressure. This article provides a detailed overview of injection molding screws and barrels, including their design tips.

The Role of Screws and Barrels in Injection Molding

The primary function of the screw is to melt and transport plastic material through the barrel by applying heat and mechanical pressure. the feed zone, the compression zone, and the metering zone. Each zone has specific roles in handling the plastic from its solid state to a homogenous melt before injection.

The barrel encases the screw and maintains temperature control through heated zones. This ensures consistent melting of the material as it is conveyed by the screw. Any wear or damage in the barrel can lead to material degradation, uneven heating, and poor part quality.

Injection molding screws and barrels for plastic molding machines

Design Considerations for Injection Molding Screws

Screw design should be customized based on the material being processed and the specific application. The right screw design can significantly improve melt uniformity, throughput, and overall process efficiency. Here are some key design considerations:

Screw Size and Dimensioning

The diameter of the screw (D) is closely related to the injection volume. The relationship is defined as:

Injection Volume (V) = 1/4π × D² × (Injection Stroke) × 0.85

Generally, the square of the screw diameter (D²) is inversely proportional to the maximum injection pressure. A larger screw diameter increases the extrusion rate, which can be expressed as:

Extrusion Rate (Q) = 1.29 × D² × Hm × Nr × 60 / 1000 (kg/hr)

Where:

  • Nr is the screw speed.
  • D is the screw diameter,
  • Hm is the flight depth,

L/D Ratio (Length-to-Diameter Ratio)

The L/D ratio is the ratio of the working length of the screw to its diameter. A higher L/D ratio generally improves plasticization, as it allows more time for mixing and melting. For heat-stable materials like PC and POM, an L/D ratio of 22:1 to 24:1 is recommended. For heat-sensitive materials, a shorter L/D ratio (14:1 to 18:1) helps avoid excessive heating and material degradation.

Compression Ratio

The compression ratio refers to the ratio of the depth of the screw channel in the feed zone to that in the metering zone. A higher compression ratio increases material density and improves melt quality, while also helping to expel trapped air.

For non-crystalline plastics, a longer compression zone is necessary to prevent clogging, as the material may not shrink in volume quickly enough. In the case of crystalline plastics, which typically occupy about 25% of the screw length, specific materials like nylon require a shorter compression zone of around 15%. For highly viscous, flame-resistant materials, this zone can extend to 40–50% of the screw length.

Optimizing Operational Parameters

In addition to screw design, the operational parameters of the injection molding process significantly influence the quality of plasticization and the final product. Below are key parameters to consider:

Screw Speed

Screw rotation speed affects the shear forces exerted on the plastic in the screw channels, which in turn influences plasticization. For smaller screws, higher speeds can increase plasticization efficiency, while for larger screws, excessive speeds can lead to uneven melting and excessive frictional heat. Typically, screw speeds are maintained between 100 to 150 rpm. For heat-sensitive materials like PVC, surface speeds should be kept below 0.5 m/s to prevent degradation.

Back Pressure

Back pressure during metering increases the density and uniformity of the molten plastic, while helping to eliminate un-melted particles. However, excessive back pressure may cause degradation in heat-sensitive plastics, while insufficient back pressure can result in air bubbles in the final product. Back pressure should be adjusted according to the material to optimize melt quality and ensure smooth injection.

Heating Temperature

The heating temperature of the screw and barrel directly affects the plastic’s melting state. The temperature settings should generally be slightly lower than the plastic’s melting point to prevent overheating. For crystalline materials like PE and PP, segmented temperature control ensures that different zones of the screw are heated appropriately, while heat-sensitive materials like PVC require more precise control to prevent decomposition.

Crystalline plastics temperature control:

Plastic TypeBarrel Temperature (°C)Nozzle Temperature (°C)Injection Pressure (Kg/cm²)
PP200 – 270210 – 280400 – 1,000
HDPE210 (cooling to 180)200 – 220500 – 1,500
Acetal220 – 270230 – 280400 – 1,000
PA6/66260 – 280270 – 310600 – 1,500
Delrin180 – 200190 – 220800 – 1,100
PA6225 – 280240 – 280700 – 1,000

Non-crystalline plastics temperature control:

Plastic TypeBarrel Temperature (°C)Nozzle Temperature (°C)Injection Pressure (Kg/cm²)
ABS200 – 230200 – 240800 – 1,500
PC260 – 310280 – 320800 – 1,500
Modified PPO240 – 280250 – 300850 – 1,400
PMMA180 – 220200 – 230700 – 1,500
PS180 – 240190 – 260400 – 1,300
Rigid PVC165 – 185175 – 1951,000 – 1,500

Effects of L/D Ratio

A higher L/D ratio facilitates more uniform feeding of the material but may also lead to overheating of the plastic. For plastics with good thermal stability, a longer screw can be used to enhance mixing without the risk of burning. Conversely, for heat-sensitive plastics, a shorter screw or a screw design without threads at the end is advisable.

Material TypeL/D RatioCharacteristics/Effect
Thermosetting Plastics14–16Suitable for processing heat-sensitive materials with minimal risk of degradation.
Rigid PVC, High-Viscosity PU17–18Recommended for heat-sensitive materials, ensuring proper plasticizing without overheating.
General Plastics18–22Ideal for a wide range of plastics, balancing mixing and melting efficiency.
High-Temperature Stable Plastics (PC, POM)22–24Suitable for plastics with good thermal stability, enhancing mixing without risk of material degradation.
Pre-colored Pellets (Color Mixing)12–16Ensures minimal color variation during molding of pre-colored pellets.
Color Masterbatch Mixing16–18Suitable for mixing color masterbatches within the feed zone, providing consistent quality and reduced color variance.
High Dispersion and Mixing (Color)20–24Ensures uniform dispersion of colorants in the feed zone, maintaining the physical properties of the final product.
injection molding screws and barrels

Barrel Design and Material Considerations

The barrel must be made from durable materials capable of withstanding high temperatures and pressure while resisting wear and corrosion.

Material Composition

Injection molding screws and barrels are typically made from high-performance materials that can withstand the high temperatures and pressures involved in the molding process. Common materials include 38CrMoAlA, 42CrMo, SKD61, and bimetallic compositions such as Fe-based, Ni-based, and Tungsten Carbide. These materials offer wear resistance, corrosion resistance, and high hardness, ensuring long service life and consistent performance.

The barrel’s internal diameter must match the screw’s external diameter precisely to ensure a tight fit and prevent material leakage. The barrel is also divided into different zones, each with its own heating element to control the temperature of the material as it moves through the barrel.

Nitriding and Chromium-plating

To further enhance the durability of screws and barrels, surface treatments such as nitriding and chromium-plating are often applied. Nitriding increases the hardness of the surface layer, typically achieving a case depth of 0.45-0.7mm, while chromium-plating provides a protective layer with a depth of 0.025-0.10mm. These treatments significantly improve wear resistance and corrosion resistance, extending the lifespan of the components.

Injection Molding Screws and Barrels Maintenance and Inspection

Regular maintenance and inspection of injection molding screws and barrels are essential for preventing premature wear and ensuring consistent performance. Key maintenance tasks include:

  • Cleaning: Regularly clean the screw and barrel to remove any residual plastic or contaminants that may affect melt quality.
  • Inspection: Inspect the screw and barrel for signs of wear, cracking, or corrosion. Replace worn components promptly to avoid downtime and product quality issues.
  • Lubrication: Ensure that the screw and barrel are properly lubricated to reduce friction and wear.

Conclusion

Injection molding screws and barrels are essential components of the injection molding process, and their design and maintenance are critical to achieving consistent product quality and reducing downtime. By understanding the material composition, surface treatments, screw design, and barrel design, as well as following practical tips for maintenance, temperature control, and material handling, manufacturers can optimize the performance of their injection molding machines and produce high-quality plastic products.

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