For injection molding torsion bar bushings, ensuring smooth and efficient demolding is crucial for improving product quality and production efficiency during mold design. Achieving this requires a comprehensive approach, considering mold structure, material selection, process parameters, and auxiliary systems to develop a systematic solution.
The demolding system design is the core element. For the geometric characteristics of the torsion bar bushing, a suitable demolding method must be selected based on its shape, size, and wall thickness distribution. For planar or shallow cavity structures, ejector pin demolding is the preferred choice due to its simplicity and ease of manufacturing; however, it is essential to ensure that the ejector pin diameter matches the stress requirements of the plastic part to avoid the risk of bending or breakage. If the plastic part has thin-walled ribs or bosses, flat ejector pins can increase the contact area and reduce localized stress concentration. For thin-walled cylindrical torsion bar bushings or surfaces where markings are not permitted, top plate demolding effectively prevents deformation through large-area, uniform ejection. Angled ejector demolding is suitable for complex structures with internal undercuts, achieving simultaneous lateral core pulling and demolding through tilting motion. Furthermore, the layout of ejector components must follow a balanced principle, prioritizing their placement in areas of high rigidity on the plastic part, furthest from the core, avoiding weak areas and surface finishes. Guide pillars, bushings, or ejector plate guide pillars should ensure movement accuracy and prevent jamming due to misalignment.
The suitability of mold materials directly affects demolding performance and mold life. The molding material characteristics of torsion bar bushing dictate that the mold steel must possess high hardness, wear resistance, and corrosion resistance. For example, when molding high-flow-rate plastics, the mold steel needs to be hardened to increase surface hardness and resist molten material erosion; if molding transparent parts or highly polished parts, pre-hardened steel with strong corrosion resistance should be selected, combined with chrome plating to enhance surface finish and reduce the risk of sticking. Material selection must also consider machinability, ensuring that the polishing direction of the mold cavity is consistent with the molten material filling direction to avoid demolding difficulties caused by excessive surface roughness.
Precise control of process parameters is a crucial means of optimizing the demolding process. Mold temperature adjustment needs to be dynamically adjusted according to the type of plastic: for materials with high crystallization temperature and slow crystallization rate, the mold temperature needs to be increased to promote molecular chain alignment; while for products requiring controlled dimensional stability, the mold temperature needs to be decreased to reduce shrinkage stress. Matching injection pressure and speed is equally crucial. Excessive injection pressure may lead to overfilling of the plastic part, increasing demolding resistance; while excessive injection speed can easily cause scorching or air bubbles in the melt, affecting surface quality. The setting of holding pressure and time needs to balance the density and internal stress of the plastic part, avoiding shrinkage marks due to insufficient holding pressure or demolding difficulties due to excessive holding pressure. The design of the cooling system must ensure uniform cooling of the plastic part. By optimizing the water channel layout and water flow rate, local deformation caused by uneven cooling can be reduced, thereby reducing the risk of jamming during demolding.
The design of the venting system has an indirect but crucial impact on demolding smoothness. Poor mold venting can lead to insufficient melt filling or trapped air, causing increased internal pressure in the plastic part and increasing demolding resistance. Therefore, venting grooves should be installed at the ends of cavities, the roots of ribs, and corners. Their depth must be strictly controlled according to the material's flowability to prevent overflow. For complex cavities, porous sintered metal inserts can be used to enhance venting, ensuring smooth gas discharge during molten material filling and preventing the plastic part from adhering to the cavity due to air pressure.
Integrating auxiliary systems can further improve demolding efficiency. For example, pneumatic ejection systems use compressed air for assisted ejection, suitable for large or easily deformable plastic parts, reducing ejector pin marks and lowering demolding force requirements. The use of robotic parts-retrieving devices enables automated production, reducing manual intervention and shortening the molding cycle. Furthermore, the rigidity design of the mold needs to be optimized through finite element analysis to ensure no deformation under injection pressure, avoiding demolding difficulties caused by mold misalignment.
Mold maintenance and upkeep are fundamental to ensuring long-term demolding performance. Wear parts such as ejector pins, sliders, and sprue bushings should have detachable structures for easy periodic replacement; cooling and venting systems need to be cleaned regularly to prevent scale or impurities from clogging and affecting functionality. By using standardized design and selecting standard mold bases and standard parts, the design cycle can be shortened and the processing cost reduced, while improving the interchangeability and ease of maintenance of the mold.
