During the K-material injection molding process, ejector pin breakage is a common problem that impacts production efficiency and product quality. This problem is often linked to multiple factors, including mold design, process parameter settings, and material properties. Frequent ejector pin breakage, a critical component for demolding plastic parts, can not only lead to downtime and repairs, increasing production costs, but can also cause scrapped parts due to residual ejector pin fragments and even damage the mold cavity. Therefore, in-depth analysis of the root causes of ejector pin breakage and the development of targeted measures are crucial to ensuring production stability.
From a mold design perspective, improper ejector pin selection and layout are the primary causes of breakage. In pursuit of a more effective demolding effect, some molds blindly increase the number of ejector pins or select pins with too small a diameter, causing the pins to break due to insufficient strength when subjected to the demolding force. Furthermore, improperly controlling the clearance between the ejector pin and the template hole can lead to jamming due to excessive frictional resistance, while excessive clearance can cause radial oscillation during movement, exacerbating wear and stress concentration. To address this issue, the ejector pin layout should be optimized based on the structural characteristics of the plastic part to ensure even distribution of demolding force. At the same time, high-strength alloy materials, such as SKD61, should be used for ejector pins, which have a hardness of up to HRC50-55 and can effectively improve fatigue resistance. For slender ejector pins, reinforcing ribs can be added to the tail or a stepped design can be adopted to enhance overall rigidity.
Improper process parameter settings are also a significant factor in ejector pin breakage. K material is a styrene-butadiene copolymer, which exhibits a certain degree of elasticity and toughness. If the holding time during injection molding is too long or the pressure is too high, the part will adhere too tightly to the cavity wall, resulting in a sudden increase in ejection resistance. This sudden overload of force on the ejector pin can easily cause breakage. Furthermore, insufficient cooling time prevents the part’s internal stress from being fully released. This leads to additional tension during ejection due to uneven shrinkage, further increasing the ejector pin load. To address this issue, process parameters must be gradually optimized through trial molds: appropriately shorten the holding time, controlling the holding pressure to 60%-80% of the injection pressure; extend the cooling time based on the part’s thickness to ensure the part is fully solidified before ejection. Furthermore, lowering the mold temperature (typically between 30-50°C) can reduce adhesion between the part and the cavity, thereby reducing ejection resistance.
Improper installation and maintenance of ejectors can also lead to frequent breakage. If the ejector is skewed or loose during installation, it will be subjected to lateral force during movement, and it is prone to bending or even breaking after long-term use. In addition, if there are foreign objects (such as plastic debris or metal impurities) between the ejector and the template hole, it will increase the wear of both, resulting in poor movement of the ejector and an increased risk of breakage. To avoid such problems, it is necessary to strengthen the quality control of the ejector installation process, use a dial indicator to calibrate the verticality of the ejector, and ensure that the error does not exceed 0.02mm/m; regularly check the tightness of the ejector to prevent loosening due to vibration. In daily maintenance, foreign objects in the ejector and template hole should be cleaned regularly, the ejector should be lubricated weekly (high-temperature resistant lubricating oil can be used), the wear of the ejector should be checked monthly, and any scratched or deformed ejector should be replaced in a timely manner to avoid breakage due to local damage.
Finally, unreasonable structural design of plastic parts may also indirectly lead to ejector pin breakage. If the plastic part has deep ribs, protrusions or undercut structures, it will increase the difficulty of demolding, and the ejector pin will need to withstand greater force to eject the plastic part. For such situations, the plastic part structure can be optimized during the design stage, such as setting a demolding slope (usually 1°-3°) at the bottom of the rib, increasing the ejector pin contact area at the protruding part, or using a core pulling mechanism to replace part of the ejector pin function to reduce the ejector pin’s force load. In addition, for plastic parts with complex shapes, large-area demolding elements such as ejector tubes and ejector plates can be used to disperse the demolding force and reduce the burden on individual ejector pins. By reducing demolding resistance from the source of plastic part design, the probability of ejector pin breakage can be fundamentally reduced, and the stability and economy of production can be improved.
