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How to choose the screw and barrel configuration for your injection moulding machine?

2025/07/16 By Topstar

screw and barrel of injection moulding machine

Selecting a screw and barrel configuration for an injection moulding machine requires consideration of multiple factors, including polymer type, target shot size, cycle time requirements, and final part specifications. First, we need to evaluate the melt flow index, moisture sensitivity, and thermal stability of the product raw material, which determine the necessary plasticising capacity and affect the shear requirements of the screw. Then, consider the production volume and the complexity of the parts produced. Additionally, environmental operating conditions and temperature control accuracy in the barrel will also impact the heat transfer efficiency and melt uniformity. At the same time, we need to match the screw geometry, length-to-diameter ratio (L/D), and compression curve of the plastic injection moulding machine with these key parameters to ensure stable melt quality.

Material Properties and Screw Selection for Injection Moulding Machine

When configuring the screw and barrel of an injection moulding machine, the first step is to match the screw design to the material properties. For example, amorphous resins such as ABS require moderate shear and high back pressure, so a compression ratio of 2.5:1 to 3:1 is recommended to ensure proper melting without degradation. Additionally, semi-crystalline polymers, such as polypropylene, benefit from lower shear rates to prevent over-orientation, so compression ratios closer to 2:1 are preferred. We select screw flight and root diameters based on the melt viscosity of the resin to ensure uniform melt and minimal residence time. Additionally, hygroscopic polymers such as PET or nylon often require the use of a barrier or mixing screw to break up the water and achieve a uniform melt, reducing the risk of hydrolysis and part defects.

Shot Size, Plasticization Rate, and Length-to-Diameter Ratio

Next, shot size and desired throughput determine the length-to-diameter (L/D) ratio and diameter selection of the screw in the injection moulding machine. In addition to the baseline shot size calculation, cycle time goals also affect the desired plasticization rate. Therefore, an L/D ratio of 20:1 may be ideal for standard applications, while high-throughput systems often use ratios of up to 30:1 to enhance melt capacity and feed efficiency. We calculate the required screw diameter by dividing the maximum shot weight by the specific volume of the material and the target throughput, ensuring the screw can handle the necessary mass during the cooling and injection stages. This approach balances injection speed, melt uniformity, and energy consumption on a plastic injection moulding machine, while also considering factors such as regrind percentage and barrel fill level to prevent under- or over-plasticization.

Shot Size, Plasticization Rate, and Length-to-Diameter Ratio

Screw Geometry for Injection Moulding Machine

Optimising screw geometry enables consistent melt quality on an injection moulding machine. For example, a three-zone screw (feed, compression, metering) with a compression ratio of 3:1 provides a sharp melt profile for most thermoplastics. Thus, materials that require thorough mixing, such as filled or coloured compounds, benefit from a Maddock or barrier mixing section integrated into the metering zone. At the same time, we configure mixing elements based on filler content and desired dispersion, ensuring that the screw produces a uniform melt without excessive shear heating. Additionally, a mixing screw with separate channels can handle highly fibre-reinforced materials, preventing fibre breakage and maintaining the mechanical properties of the final part. Customizing the screw geometry to the part requirements can further improve the injection process.

screw design of all-electric injection molding machine 1

Barrel Heating Zones and Venting Considerations

In addition to the screw, the barrel heating and venting configuration can significantly affect the performance of an injection moulding machine. Dividing the barrel into multiple independently controlled heating zones improves melt temperature uniformity across the entire screw length. Additionally, installing a vacuum vent in the feed section removes volatiles from moisture-sensitive resins, thereby enhancing the clarity of the final part. We set zone temperatures based on the resin’s melt curve and use thermocouple feedback to maintain ±2°C accuracy. Proper venting locations ensure that the melt does not leak out when venting, maintaining screw integrity and preventing flow instabilities. Optimising barrel insulation and heater band power further improves energy efficiency and reduces process variability.

Special configurations for advanced applications

Certain high-performance parts require specialised screw and barrel configurations on the plastic injection moulding machine. In addition to barrier screws for moisture control, co-rotating twin screws can be used to process reactive or high-viscosity materials. As a result, plastic injection moulding machines can perform on-site compounding or masterbatch mixing without the need for separate extrusion equipment. I would recommend a custom-designed variable-pitch screw to optimise the shear profile for processing demanding engineering plastics, such as PEEK or high-impact polycarbonate. Spring-assisted check valves and extended nozzle assemblies further improve backflow prevention and shot consistency for micro moulding applications. Wear-resistant bimetallic barrels and nitrided screws extend service life when processing abrasive fillers such as glass fibre or mineral reinforcements.

Screw and barrel selection

The screw and barrel configuration for your injection moulding machine is tailored to account for material properties, shot size, machine throughput, mixing needs, and product requirements. We select screw aspect ratios, compression and mixing elements, and barrel zone controls based on empirical data, rigorous testing, and product-specific performance criteria. By applying a structured decision matrix and utilising specialised configurations, you can ensure consistent melt quality, optimised cycle times, and superior part performance.

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