Increasing the screw speed during plasticization can also improve the gas mark defect of PVC
During the PVC (polyvinyl chloride) injection molding process, gas streaks are a common quality issue. These defects manifest as silvery or cloud-like streaks on the surface of the plasticized part, affecting not only the part’s appearance but also potentially degrading its mechanical properties. Gas streaks are often caused by the failure to promptly expel gases generated during the plasticization process. These gases primarily consist of moisture, volatiles, and air contained in the plastic raw material. When PVC plastic is plasticized in the barrel, insufficient or uneven plasticization can trap gases in the molten plastic. These gases enter the mold cavity during injection molding, ultimately forming gas streaks on the surface of the part. Increasing the screw speed during plasticization can improve PVC gas streaks to a certain extent, which is closely related to the influence of screw speed on the plasticization process.
Increasing screw speed can enhance the shearing effect on PVC plastic, promoting melting and uniform mixing of the plastic, and reducing gas generation and retention. Increasing screw speed accelerates the relative motion between the screw and barrel, increasing the shear friction on the PVC plastic particles and generating more shear heat. This accelerates the melting of the plastic, allowing it to transition from a solid state to a molten state in a shorter time. Furthermore, the intense shearing action breaks up agglomerated particles in the PVC plastic, allowing the plastic raw material and additives to be thoroughly mixed, avoiding localized overheating or poor plasticization caused by uneven mixing. This allows the moisture and volatiles in the plastic to be fully heated and evaporated during the plasticization process and discharged through the barrel’s vents, reducing the amount of gas remaining in the molten plastic and thus reducing the likelihood of gas mark defects.
Appropriately increasing the screw speed can shorten the residence time of PVC plastic in the barrel and reduce the amount of gas generated by thermal degradation. PVC plastic has poor thermal stability and is prone to thermal degradation reactions when exposed to high temperatures for extended periods, producing gases such as hydrogen chloride. If these gases are not promptly expelled, they can form gas marks on the surface of the plastic part, causing discoloration and performance degradation. Increasing the screw speed accelerates the plastic’s movement through the barrel, shortening its residence time, reducing the likelihood of thermal degradation and the amount of gas generated. However, it is important to note that the screw speed should not be too high, as this will cause excessive shearing of the plastic in the barrel, generating significant shear heat and rapidly increasing the temperature inside the barrel, which in turn exacerbates thermal degradation of the PVC and increases gas generation. Therefore, the appropriate screw speed range should be determined based on the specific production situation.
Increasing the screw speed can improve the fluidity of the PVC molten plastic, which is beneficial for the discharge of gas during the injection process. The increase in screw speed allows for more thorough stirring of the molten plastic, enhancing the uniformity and fluidity of the melt. During the injection molding process, the molten plastic can more smoothly fill the mold cavity, reducing the gas entrapment caused by poor flow. At the same time, the improved fluidity makes the pressure distribution of the molten plastic in the mold cavity more uniform, helping to push the air in the cavity toward the venting groove for discharge. In addition, good fluidity can also reduce the shear stress of the plastic at the mold gate, avoiding the generation of gas due to shear overheating, and further reducing the risk of gas mark defects. For example, when producing some PVC plastic parts with complex structures and thin walls, increasing the screw speed to improve the fluidity of the plastic can effectively avoid the occurrence of gas mark defects.
In actual production, improving PVC gas mark defects by increasing screw speed requires comprehensive adjustments in conjunction with other process parameters to achieve optimal results. First, the barrel temperature must be appropriately adjusted. Increasing screw speed increases shear heat, so the barrel temperature can be appropriately lowered to prevent thermal degradation of the plastic due to excessive temperature. Second, the back pressure must be properly adjusted. Properly increasing back pressure enhances the compaction of the molten plastic and helps expel gases. However, excessive back pressure increases screw load and energy consumption, and may also cause the plastic to overheat . Furthermore, the dryness of the PVC plastic raw material must be ensured. Pre-production should be thorough drying of the raw material to reduce its moisture content and mitigate gas generation at the source. By comprehensively adjusting process parameters such as screw speed, barrel temperature, and back pressure, PVC gas mark defects can be minimized and the quality of the plastic parts improved.
