In precision injection molding, ejector pin selection is crucial to avoiding ejection whitening. Ejection whitening typically occurs when the ejector pin thrust exceeds the plastic's tolerance limit during demolding, leading to localized stress concentration and causing white streaks or hazy marks at the ejector pin location, severely impacting the product's appearance and structural strength. Therefore, a comprehensive consideration of ejector pin diameter, arrangement principles, material selection, fixing method, integration with the cooling system, and special structural applications is necessary to achieve ejection force balance, reduce stress concentration, and thus prevent ejection whitening.
The selection of ejector pin diameter should follow the principle of "choose larger rather than smaller." While meeting mold function and product requirements, priority should be given to ejector pins with larger diameters, generally not less than 3mm and not more than 16mm. Larger diameter ejector pins increase the contact area with the plastic part, dispersing ejection pressure, reducing localized stress, and minimizing the risk of ejection whitening. For example, for products with thicker walls or higher demolding resistance, using larger diameter ejector pins can effectively prevent ejection whitening caused by concentrated ejection force.
Ejector pin arrangement should adhere to the principles of balance and effectiveness. Ejector pins should be placed in areas with complex structures and high demolding resistance, such as ribs, pillars, steps, and metal inserts, and their arrangement should balance the ejection force as much as possible. Complex structures require greater demolding force; increasing the number of ejector pins can distribute the pressure. At the same time, ejector pins should be avoided at rounded corners, sharp corners, bevels, and too close to the glue edge to facilitate machining and prevent sharp corners from affecting the mold's strength. For thin-walled parts, the ejector pin arrangement must consider shrinkage to avoid the product getting stuck on the ejector pins, making removal difficult.
Material selection directly affects the wear resistance and service life of the ejector pins, thus affecting the occurrence of ejector whitening. In precision injection molds, ejector pins are generally made of high-speed steel, such as SKD61 and SKH51. These materials have high hardness and good wear resistance, and can withstand high ejection pressure. SKD61 ejector pins can withstand temperatures up to 1600℃, and their wear resistance is further improved after surface nitriding; SKH51 ejector pins have better toughness than SKD61 and are suitable for applications requiring higher toughness. Avoid using inferior materials, such as 65Mn, as they are brittle and prone to causing whitening issues.
The ejector pin fixing method must ensure stability and reliability to prevent uneven ejection force due to loosening or rotation during production. Common fixing methods include direct fixing with headless screws and fixing with screws in conjunction with a pressure plate. Headless screw fixing is suitable for situations with a small number of ejector pins; pressure plate fixing is suitable for situations with a large number of ejector pins or situations requiring anti-rotation. For ejector pins with a diameter less than 2.5mm, a support ejector pin design should be used to enhance their strength and prevent bending deformation. Simultaneously, the ejector pin surface should be 0.03-0.05mm higher than the core plane. For situations where the glue surface has requirements, a countersunk platform can be added around the ejector pin to reduce ejection resistance.
Ejector pin selection must also consider compatibility with the cooling system. Cooling water channels should avoid ejector pins and other components as much as possible, and the distance between the cooling pipes and the ejector pins should be at least 4mm to prevent penetration and leakage. If the water cooling system interferes with the ejector pins, and the water cooling system cannot be moved, the ejector pin position can be finely adjusted without affecting product ejection. A well-designed cooling system ensures uniform mold temperature, reduces internal stress caused by temperature differences, and thus lowers the risk of ejection whitening.
For products with special structures, such as deep ribs and tube pillars, special ejector pin types must be selected. For long ribs (over 10mm high), flat ejector pins are recommended, as shorter flat sections offer better strength and are easier to process. For tube pillars (over 10mm high), push-tube ejection is recommended, providing stable and reliable ejection and preventing part deformation. These special ejector pin types better adapt to product structures, distribute ejection pressure, and prevent ejection whitening.
