Reasons why the injection molding lifter cannot be demoulded or cannot be withdrawn
The inability of the injection molding ejector pin to release or exit the mold is a common failure in mold production. This not only interrupts production but can also damage the plastic part or deform mold components, resulting in significant financial losses for the company. This failure is often related to multiple factors, including mold design, process parameters, and material properties, requiring a comprehensive analysis from multiple perspectives. In actual production, if the ejector pin fails to release smoothly, it can cause scratches, deformation, or even cracks on the part surface. Failure to exit the mold can prevent the mold from closing properly, impacting subsequent production. Therefore, accurately identifying the cause of the failure and implementing targeted measures are key to ensuring smooth injection molding production.
Improper mold structure design is one of the main reasons for the ejector pin to fail to release or exit the mold. First, an improperly designed ejector pin angle can cause motion interference. If the angle is too large, the lateral force on the ejector pin increases during ejection, making it prone to collision with the mold cavity or other components. If the angle is too small, the core pulling distance may be insufficient, preventing the part from fully releasing from the ejector pin. Second, flaws in the guide mechanism design can also cause jamming. For example, insufficient clearance between the guide sleeve and the ejector pin, or insufficient guide sleeve length, can hinder the ejector pin’s movement. Furthermore, insufficient ejector pin rigidity is a significant factor. When the ejector pin length-to-diameter ratio exceeds 15, it is prone to bending and deformation during ejection, preventing proper ejection. Furthermore, if the contact area between the ejector pin and the part is too large, it increases the clamping force, increasing demolding resistance and further hindering demolding.
Improper process parameter settings can also cause ejector movement failures. Excessively high injection temperatures increase the fluidity of the plastic melt, strengthening the adhesion between the part and the ejector after cooling, leading to increased demolding resistance. Excessively low temperatures, on the other hand, can generate significant internal stress within the part, leading to uneven shrinkage and increasing the difficulty of ejecting the ejector. Excessive injection and holding pressures are also significant contributing factors. Excessive pressures can cause the part to adhere tightly to the ejector, creating a significant clamping force. This can also lead to flash on the part, which can become lodged between the ejector and the cavity, hindering ejector movement. Excessively long holding times can overfill the part, further increasing the adhesion to the ejector, while insufficient cooling time can prevent the part from fully solidifying, causing deformation during ejection and interference with the ejector. Furthermore, excessive ejection speeds can subject the ejector to significant impact forces, potentially causing motion jams. Excessively slow ejection speeds can increase ejection resistance due to excessive shrinkage of the part as it cools.
The characteristics and pretreatment of the plastic raw materials also significantly affect the movement of the lifter. Different types of plastics have different shrinkage rates and friction coefficients. For example, polyvinyl chloride (PVC) has a low shrinkage rate and a certain degree of adhesion to metal, which can make demolding the lifter difficult. Polyethylene (PE), on the other hand, has a higher shrinkage rate and better separation from the lifter after cooling, making demolding relatively easy. If the raw materials contain a large number of impurities or additives, they may precipitate during the molding process and adhere to the lifter surface, increasing frictional resistance and preventing the lifter from being ejected smoothly. Furthermore, if the raw materials are not dried sufficiently and contain excessive moisture, gases will be generated at high temperatures, forming bubbles or depressions within the plastic part, increasing the contact area between the part and the lifter and further increasing demolding resistance. Furthermore, a high proportion of recycled material can affect the fluidity and stability of the plastic, resulting in reduced molding quality and indirectly causing lifter movement failure.
Inadequate mold maintenance is also a common cause of ejector failure. The ejector guide mechanism wears out over time, increasing clearance, causing the ejector to wobble during movement and even interfering with other components. Failure to promptly remove plastic debris, oil, and other impurities from the guide grooves can result in poor guidance and increased resistance to the ejector’s movement. Inadequate lubrication on the ejector surface increases friction and wear, and in severe cases, can lead to seizure, preventing the ejector from moving properly. Furthermore, a poorly functioning mold cooling system, such as clogged or improperly laid out cooling channels, can lead to uneven mold temperature distribution, excessively high local temperatures in the ejector, and thermal expansion and deformation, compromising its precision. Furthermore, if the ejector’s return spring ages and fails, lacking sufficient force, it can prevent the ejector from returning to its original position, causing it to collide with the cavity during mold closing, leading to failure. Furthermore, if operators fail to promptly detect problems such as sticking or flashing and continue production, this can exacerbate wear on the ejector and even cause permanent damage.
