Gas generation during plastic injection molding is closely and multifacetedly related to the mold, primarily manifested in key areas such as mold design, mold manufacturing and processing, and mold use and maintenance.
Mold Design Aspects
Gate Design: As the entry point for molten plastic into the mold cavity during injection molding, the rationality of its design is crucial. If the gate location is improperly chosen, such as not placing it at a thick-walled section of the plastic part, it may lead to discontinuous flow of the molten material within the mold cavity. During the filling of the cavity by the molten material, the air originally present in the cavity needs to be gradually expelled. However, an unreasonable gate location can obstruct the air expulsion channel, preventing the molten material from smoothly pushing the air out of the cavity, thus trapping the gas inside the plastic part and ultimately forming defects such as bubbles and voids in the finished product. In addition, the gate size is also a critical factor. A gate that is too small will cause greater resistance to the molten material entering the cavity, reducing the flow rate and prolonging the filling time. During this process, the time for gas expulsion is also correspondingly reduced, increasing the risk of gas residue. For multi-gate parts, asymmetrical gate arrangement leads to inconsistent molten material flow rates and velocities into the mold cavity, creating complex flow patterns. This makes it difficult for gases to find a unified and smooth exit path, resulting in various gas-related quality problems in the finished product.
Runner Design: The runner is the channel through which molten plastic flows from the injection molding machine nozzle to the mold cavity, and its design also affects gas discharge. Long and narrow main runners and branch runners significantly increase the resistance to molten plastic flow. This not only consumes more injection pressure and slows the molten material flow rate but also prolongs gas discharge time. Within a limited injection cycle, gases may not be completely discharged, remaining in the product. Furthermore, dead zones within the runner, such as poorly designed bends, corners, or abrupt changes in cross-section, easily accumulate gas. Because the molten material cannot completely carry away the gas from these dead zones during flow, over time, this accumulated gas will adversely affect the quality of the product during injection molding.
Venting System Design: The venting system is a crucial part of mold design, specifically designed to expel air from the cavity and volatile gases generated by the molten plastic during injection molding. If the mold parting surface lacks necessary venting channels, or if the number of venting channels is insufficient or their size is too small, the requirement for rapid gas venting cannot be met. During injection molding, as the molten plastic rapidly fills the cavity, the air within the cavity needs to be expelled within a short time. If the venting channels cannot provide sufficient venting area, the air will be compressed within the cavity, creating high pressure, hindering the normal flow of the molten material, and leading to defects such as short shots, trapped air, and scorching in the product. Furthermore, the location of the venting channels is also critical. If the location is improperly chosen, even with sufficient venting channels, it may not be able to effectively vent gas from specific areas. In addition, in some molds, the lack of full utilization of machining gaps such as inserts and ejector pins to assist venting will also significantly reduce the venting effect. Especially during high-speed injection molding, the molten plastic fills the cavity extremely quickly, placing even higher demands on the venting system. If the venting system cannot effectively and promptly expel the gas, the gas will be encapsulated by the rapidly advancing molten material, forming numerous bubbles and streaks in the product, severely affecting its appearance and performance. Cooling System Design: While the cooling system primarily controls mold temperature to ensure rapid and uniform cooling and solidification of the molten plastic within the cavity, its design is also indirectly related to gas generation. When the cooling system is poorly designed, leading to significant temperature differences in the mold, the plastic will shrink unevenly during cooling. This uneven shrinkage can create internal stress in the product and may even form voids. These voids act as tiny gas storage spaces, easily trapping gas and forming bubbles. For example, if some areas of the mold cool too quickly, the plastic solidifies rapidly, while adjacent areas cool more slowly and continue to shrink. This difference in shrinkage creates tensile stress within the product. When this stress exceeds the plastic's strength, voids form, which may trap gas, affecting product quality.
Mold Manufacturing and Processing:
Mold Surface Quality: The roughness of the mold surface directly affects gas generation during injection molding. If the mold surface is rough and not smooth enough, the molten plastic will experience greater frictional resistance during flow. This friction can lead to localized temperature increases, creating hot spots. At these hot spots, the plastic may decompose due to overheating, releasing volatile gases. Furthermore, if burrs, grooves, or other defects exist on the inner surface of the mold, gases can easily accumulate in these microscopic unevennesses. Because the molten material cannot completely expel the gas from these tiny depressions or protrusions during flow, a potential gas residue problem arises during injection molding. Even minor surface defects can gradually accumulate and have an impact during mass injection molding production, leading to significant gas-related defects in the finished product.
Mold assembly issues: Molds are assembled from multiple components, and the precision and quality of the assembly directly affect the mold's performance. If problems occur during mold assembly, such as poor mold closure or gaps during mold closing, outside air may be carried into the cavity through these gaps during injection molding. Once air enters the cavity, it mixes with the molten plastic, forming defects such as bubbles and air streaks in the finished product. In addition, the fitting precision between the various mold components is also crucial. For example, excessively large or small clearances between the slider and the groove, or between the core and the mold plate, can affect the normal venting of the mold. Excessive clearance can cause gas to enter the cavity at unexpected locations; insufficient clearance may block the designed venting channels, affecting gas discharge.
Mold Use and Maintenance
Mold Cleaning: Maintaining mold cleanliness is crucial during use. If the mold is not cleaned promptly, oil, dust, mold release agent residue, and other impurities will gradually accumulate in the cavity and runner. During injection molding, these impurities, under high temperature and pressure, may undergo combustion or decomposition reactions, producing various gases. For example, oil decomposes at high temperatures to produce volatile organic compounds. If these gases cannot be discharged from the mold in time, they will mix into the molten plastic, forming defects such as bubbles and black spots in the finished product. Furthermore, with the continuous accumulation of impurities, the cleanliness and smoothness of the mold surface will further decrease, increasing the likelihood of gas generation and affecting the flow properties of the molten plastic, further deteriorating the quality of the finished product.
Mold Wear: Wear is inevitable in molds used for a long time. Mold wear negatively impacts the venting system. For example, during long-term injection molding, venting channels may narrow or become blocked due to erosion by the molten plastic and impurities. Narrowing of the venting channel increases resistance to gas escape, reducing venting efficiency; while blockage completely obstructs the gas escape path, causing gas to accumulate in the mold cavity. Furthermore, moving parts of the mold, such as ejector pins and slides, experience wear during frequent reciprocating motion. When these moving parts wear, the clearance between them and the mold body changes. If the clearance increases, gas may be carried into the mold cavity during the movement of the moving parts; if the clearance decreases, it will affect the normal movement of the moving parts and may also block the originally designed venting channels, ultimately leading to poor gas escape during injection molding and affecting product quality.
Gas generation during injection molding is closely related to all aspects of mold design, manufacturing, and maintenance. Only by fully considering and optimizing these factors throughout the entire lifecycle of the mold can gas generation be effectively reduced and the quality of injection molded products improved.
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