When processing waste plastics, single-screw plastic recycling machines require effective venting design to remove volatiles from the material, ensuring the quality of the recycled plastic and preventing equipment malfunctions. Volatile substances mainly originate from residual moisture, low-molecular-weight additives, and gases generated by thermal degradation in the plastic. If not vented promptly, these substances can lead to defects such as porosity, bubbles, or a dull surface in the finished product, and may even affect its physical and mechanical properties. The venting design of a single-screw plastic recycling machine needs to consider the screw structure, the location of the venting section, and the configuration of the vacuum system, achieving efficient volatilization through multi-stage synergy.
The screw structure is the core of the venting design; its length-to-diameter ratio and screw channel depth directly affect the venting effect. Vented single-screw machines typically use a larger length-to-diameter ratio to extend the residence time of the material in the barrel, ensuring full release of volatiles. The screw channel depth in the venting section is greater than in other areas. When the material enters this section, the pressure drops sharply to near atmospheric pressure or negative pressure, causing the pressurized gas and vaporized volatiles to escape rapidly. Simultaneously, the screw's stirring and shearing action promotes bubble breakage, preventing gas retention and further enhancing volatilization efficiency.
The location of the venting section must be matched with the screw's compression ratio and melting section. Typically, the venting section is located in the middle of the screw, where the material has partially melted but not fully compacted, facilitating gas discharge. If the venting section is set too early, the material may not be fully melted, and volatiles may not be released due to solid particles obstructing their release; if it is set too late, the material is already highly compacted, making gas escape difficult. Furthermore, the venting section must smoothly transition from the feeding and metering sections to maintain stable material delivery and pressure distribution, preventing material overflow or poor venting due to pressure fluctuations.
The configuration of the vacuum system is a crucial aspect of the venting design. The vent port is usually connected to a vacuum pump, accelerating the extraction of volatiles through a negative pressure environment. To prevent melt accumulation at the vent port, the vent port structure needs to be optimized, for example, using a radial opening design so that the vent port is perpendicular to the barrel radius, reducing melt residence time. Simultaneously, the vent port width must be greater than the rolling material flow to ensure unobstructed gas passage, but the opening should not be too large to prevent excessive melt expansion and blockage. Some designs also incorporate anti-screw ridges or suction angles at the venting port to mechanically pull the melt back into the screw channel, preventing overflow.
Multi-stage screw designs can further enhance venting performance. Two-stage or three-stage screws control material delivery and pressure distribution in segments, maintaining the venting section at atmospheric pressure. For example, in a two-stage screw, the first stage handles feeding and initial melting, while the second stage focuses on venting and final plasticizing. Material transfer between the two stages is achieved through bypasses or anti-screw ridges. This design ensures the screw channel in the venting section is not completely filled, maintaining an atmospheric pressure environment, while increasing screw speed reduces material residence time, lowering the risk of secondary dissolution of volatiles.
The coordinated control of venting temperature and screw speed is crucial for effective devolatilization. Appropriately increasing the venting temperature can reduce material viscosity and promote volatile release, but excessively high temperatures must be avoided to prevent thermal degradation of the plastic. Screw speed needs to be adjusted according to material characteristics and venting requirements. Too low a speed may lead to excessively long material residence time, causing volatiles to be reabsorbed; too high a speed may exacerbate degradation due to shear heat generation. In practice, the optimal parameter combination needs to be determined through experimentation to balance volatilization efficiency and plastic quality.
Optimization of the exhaust design also needs to consider equipment maintenance and ease of operation. Regularly cleaning the exhaust ports and the screw's reverse screw edge can prevent carbon deposits from clogging the channels and ensure unobstructed exhaust. Furthermore, equipping the machine with an online monitoring system to track exhaust pressure and temperature in real time helps to promptly detect anomalies and adjust process parameters. By comprehensively applying the above design strategies, the single screw plastic recycling machine can achieve efficient and stable volatile matter discharge, ensuring high-quality plastic recycling.
