The venting design of a single-screw plastic recycling machine requires a multi-dimensional technological synergy to resolve the contradiction between gas escape from the melt and degradation control. Its core requirements encompass four major areas: structural parameters, process control, material properties, and equipment synergy.
The screw structure in the venting section must meet specific geometric parameter requirements. The screw groove depth in the venting section must be significantly greater than that in the metering section, typically 2.5 to 6 times the depth of the metering section's screw groove. This design reduces the pressure of the melt in the venting section, promoting gas release from the melt. Simultaneously, the length of the venting section needs to be optimized based on the screw diameter, generally 2 to 6 times the screw diameter. Too short a length will result in insufficient gas release, while too long a length may cause melt retention and overheating. Furthermore, the vent position should be offset from the barrel axis by several millimeters to utilize centrifugal force to reduce melt splashing, and a 20° chamfer should be set on the inner wall of the barrel to prevent the elastically expanding melt from clogging the vent.
Precise control of process parameters is crucial to preventing degradation. The temperature in the venting section must be lower than the material's thermal decomposition temperature, typically 10 to 30°C lower than the metering section, to inhibit thermal degradation reactions. Simultaneously, a reasonable back pressure system needs to be established to ensure that the conveying capacity of the second-stage metering section is greater than that of the first stage, preventing melt backflow into the venting section during injection. The screw speed and back pressure must be matched to the material characteristics. For example, high-viscosity materials require a lower speed to reduce shear heat generation, while low-viscosity materials can have a higher speed to enhance venting efficiency.
Material characteristics impose differentiated requirements on venting design. For heat-sensitive plastics, such as PVC, a bypass or hollow venting structure is required to reduce the residence time of the melt in the high-temperature zone. For recycled materials containing volatiles, the stirring function of the venting section needs to be enhanced, using reverse screw ridges or special thread designs to break up air bubbles and improve gas release efficiency. Furthermore, the shape of the vent needs optimization; rectangular vents are commonly chosen due to their larger area and more uniform venting, with their long side typically being 1 to 3 times the screw diameter.
The design of collaborative and auxiliary systems is indispensable for single-screw plastic recycling machines. The vacuum pump needs to be directly connected to the vent to ensure that the pressure in the venting section is close to negative pressure, improving gas extraction efficiency. A cooler needs to locally cool the venting section barrel to prevent melt degradation due to excessive temperature. Meanwhile, the gap between the screw and the barrel must be strictly controlled. Too large a gap will cause melt backflow, while too small a gap will increase mechanical wear. A gap of 0.1 to 0.3 mm is typically used, and nitriding treatment is employed to improve surface hardness and extend equipment life.
The design of the connection between the venting section and subsequent sections affects overall stability. The length of the second-stage pressure-reducing section needs to be reasonable; too long a section will increase the length-to-diameter ratio, causing melt stagnation; too short a section may block the vent, terminating the venting process. Furthermore, the screw groove depth in the pressure-reducing section needs to be adjusted according to the material viscosity. High-viscosity materials require deeper grooves to avoid foaming, while low-viscosity materials require shallower grooves to enhance shearing. The design of the shear transition section is also crucial; its high-shear zone can thin the melt layer, increase the gas contact area, and promote escape.
The anti-overflow design of the vent must balance functionality and safety. A one-way vent valve can prevent external air from entering the barrel and avoid melt oxidation, but a low-stiffness spring must be selected to ensure the working pressure difference is less than 3.3 kPa. In addition, a protective cover should be installed around the exhaust port to prevent injury from molten metal splashing and to facilitate the cleaning of accumulated impurities.
Verification and optimization of the exhaust design must be completed through iterative experiments. By simulating the exhaust effect under different process conditions and analyzing changes in melt temperature, pressure, and gas content, the screw structure or process parameters can be adjusted accordingly. For example, for easily degradable materials, the length of the exhaust section can be shortened and the temperature lowered; for high-gas-content recovery materials, the number of exhaust ports can be increased or the screw stirring function can be optimized.
