During the K-material injection molding process, ejector pin breakage is a common production problem, impacting not only production efficiency but also potentially leading to product scrap and increased production costs. K-material, a styrene-butadiene copolymer, exhibits excellent transparency and toughness. However, during injection molding, its melt flow and cooling shrinkage characteristics can exert significant stress on the ejector pins, making it prone to breakage if not handled properly. Addressing this issue requires a comprehensive approach encompassing mold design, process parameter adjustment, and ejector pin material selection.
First, optimizing mold ejector pin design is fundamental to reducing ejector pin breakage. The ejector pin diameter must be selected to match the product structure. If the pin diameter is too small, stress concentration can lead to breakage when subjected to demolding forces. For K-material parts, especially those with thin walls or complex structures, the ejector pin diameter should be appropriately increased. The pin should also be positioned appropriately to avoid areas of concentrated stress on the product. Furthermore, the clearance between the ejector pin and the mold plate must be strictly controlled. Excessive clearance can cause the ejector pin to wobble during movement, increasing wear and the risk of breakage. Excessive clearance can lead to frictional overheating and pin jamming. A guide sleeve can be used to assist in positioning, improving ejector pin stability and reducing radial force damage.
Secondly, adjusting the injection molding process parameters can effectively reduce ejector pin forces. K-materials have a narrow molding temperature range. If the melt temperature is too high, the part will cool and shrink unevenly, increasing demolding resistance. If the temperature is too low, the melt will have poor fluidity, increasing adhesion between the molded part and the mold cavity. This will require greater ejector pin force to release the part, increasing the probability of breakage. Therefore, precise control of barrel and mold temperatures is crucial. Generally, the barrel temperature is set between 170-200°C and the mold temperature between 40-60°C to ensure uniform cooling of the part and reduce demolding resistance. Furthermore, appropriately extending the holding and cooling times allows the part to fully solidify and set within the mold, minimizing demolding difficulties caused by excessive internal stress.
Furthermore, strengthening the ejector pin’s material selection and heat treatment process is key to improving its fracture resistance. Ordinary carbon steel ejectors are prone to plastic deformation or fracture when faced with the high demolding stress of K material. Therefore, ejectors should be made of high-strength alloy materials, such as SKD61 hot-work die steel. These materials offer high hardness, wear resistance, and toughness, effectively withstanding the impact and friction during demolding. Simultaneously, the ejector pin undergoes rigorous heat treatment, using a quenching and tempering process to achieve a hardness of HRC50-55. This ensures sufficient strength while avoiding increased brittleness due to excessive hardness. Furthermore, the ejector pin’s surface can be nitrided to form a hardened layer, further enhancing its wear resistance and fatigue resistance.
Additionally, regular maintenance of the mold ejector pin system is essential. Over long-term production, the ejector pins can lose precision due to wear, rust, and other factors, increasing the risk of breakage. Therefore, a comprehensive maintenance plan is necessary, regularly inspecting the ejector pins for wear, curvature, and surface condition, and promptly replacing them if any problems are detected. Also, the ejector pin holes must be kept clean, with plastic residue and oil stains regularly removed to prevent impurity accumulation that could hinder the pin’s movement. Apply high-temperature grease to the mating area between the ejector pin and the mold plate to reduce friction and slow down the wear of the ejector pin. Furthermore, ensure the ejector pin’s verticality when installing it to avoid breakage due to uneven force applied due to skewed installation.
Finally, optimize the product structure design to reduce the stress on the ejector pins from the source. During the product design stage, deep cavities, thin walls, or complex undercut structures should be avoided as much as possible. These structures will increase the difficulty of demolding and cause the ejector pins to be subjected to excessive stress. For complex structures that must exist, core pulling mechanisms or parting demolding can be used to disperse the stress on the ejector pins. At the same time, the ejector pin imprint position should be reasonably set on the product surface, and the ejector pins should be placed on the non-appearance surface of the product or in areas with greater stress to avoid local stress concentration due to improper ejector pin position. Through the coordinated optimization of product structure and mold design, the probability of ejector pin breakage can be fundamentally reduced, and the stability and efficiency of production can be improved.
