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In the core structure of a vertical injection molding machine, the screw system plays a key role in transforming plastic raw materials from solid to molten state. This seemingly simple metal component, with its precise design and efficient motion control, performs dozens of high-precision plasticization cycles per minute. As the "heart" of the injection molding process, the design of the screw directly affects the molding quality and production efficiency.

I. Evolution of Screw System Structure

Modern vertical injection molding machine screws typically adopt a classic three-stage structural design, with each stage serving a distinct function. The feed section is responsible for the stable transportation of raw materials, and the deep screw grooves in this section ensure smooth flow of the granules under the influence of gravity. The compression section generates a mechanical compression effect through gradually narrowing screw grooves, enhancing plasticization efficiency while preventing excessive shear. The metering section, with its shallower screw grooves, ensures uniform melting in a high-pressure environment, thus stabilizing product quality.

The metering section is crucial, and its design typically follows a golden ratio of length-to-diameter (L/D) between 5:1 and 7:1. This not only ensures the homogeneity of the melt but also keeps temperature fluctuations within ±2°C. To prevent melt backflow, the check ring component uses a dual-seal structure, with a response time of less than 0.03 seconds.

II. Coupling of Thermodynamics and Rheology

The shear heat effect generated by the screw rotation follows the rheological formula τ = η(du/dy), with the shear rate varying across different sections. For example, in the feed section, the shear rate typically ranges from 50 to 100 s⁻¹, while in the metering section, it can reach 500 to 1000 s⁻¹. For heat-sensitive materials, such as PC (polycarbonate), a specialized screw design is employed, shortening the compression section length to limit the temperature rise to within 30°C.

The temperature field of the melt exhibits an axial gradient. Using infrared thermography, the temperature curve from the feed opening to the nozzle exit is observed. By optimizing screw speed and back pressure control parameters, the temperature fluctuation coefficient can be reduced to below 0.05, preventing material degradation due to excessive temperature.

III. Engineering Materials and Surface Treatment

To enhance wear resistance, the screw body is made of nitrided steel, which undergoes ion nitriding treatment, resulting in a surface hardness that meets high standards. For fiberglass-reinforced materials, a bi-metallic alloy treatment layer is used, improving wear resistance by 3 to 5 times compared to traditional nitriding treatments. The thread top surface is coated with diamond, reducing the coefficient of friction to below 0.08.

The latest surface texturing technology employs laser cladding to create micron-level groove arrays on the screw surface. Experimental data shows that this structure improves mixing efficiency by 18% and enhances melt temperature uniformity by 25%.

In the field of precision injection molding, screw diameter tolerances are now controlled within IT5-grade precision, with concentricity error not exceeding 0.01mm/m. Additionally, the newly designed wavy screw, optimized using CFD (Computational Fluid Dynamics) simulations, can reduce birefringence to below 3nm/cm when molding optical-grade PC components. With the integration of smart sensing technology, the screw system now allows for real-time monitoring of melt viscosity, coupled with an adaptive control system, ensuring that the plasticization process remains highly stable with a CPK (Process Capability Index) value consistently above 1.67.

This new generation of screw systems, combining electromechanical integration and precise design, is redefining the boundaries of plastic processing accuracy.