Precautions for ejection of injection molding push rod
Ejection by push rod is the most basic and commonly used method for mold release in injection molding. Its ejection effect is directly related to the integrity and surface quality of the plastic part, so many details must be paid attention to in actual application. The core of push rod ejection is to use the push rod to eject the plastic part from the mold cavity smoothly and evenly, avoiding defects such as deformation, strain, and whitening of the plastic part. The layout, number, size, ejection speed, force and other parameters of the push rod need to be precisely designed in combination with the structural characteristics and material properties of the plastic part. Negligence in any link may affect the final demolding effect. For example, if the number of push rods is insufficient or the distribution is uneven for large plastic parts, it will lead to unbalanced force on the plastic part, causing warping and deformation. For plastic parts made of brittle materials, if the ejection force is too strong, it may cause the plastic part to crack.
The placement of push rods is paramount to ensuring effective ejection, adhering to the principles of uniform force distribution and avoiding weak areas. When placing push rods, priority should be given to rigid areas of the plastic part, such as ribs, bosses, and thicker wall thicknesses. These areas can withstand significant ejection forces without deformation. Push rods should also be distributed as evenly as possible around the center of gravity of the part to ensure balanced force distribution across all parts during ejection and avoid damage due to excessive localized forces. For parts with complex shapes, the number of push rods should be appropriately increased based on the cavity structure to ensure sufficient support for the ejection force in each critical area. Furthermore, push rods should be placed away from the exterior and mating surfaces of the part. If push rods must be placed on exterior surfaces, their tips should be designed to match the surface of the part and polished with high precision to minimize the impact of ejection marks. For example, when producing plastic parts with high aesthetic requirements, such as mobile phone cases, push rods are often placed on the inner ribs of the case to avoid leaving noticeable marks on the exterior.
The selection of push rod size parameters requires comprehensive consideration of the weight of the plastic part, material properties, and ejection stroke. The diameter of the push rod is one of the key parameters. A diameter that is too small will cause the push rod to bend or break due to excessive force during the ejection process; a diameter that is too large will increase the difficulty of mold processing and may leave large ejection marks on the surface of the plastic part. Generally speaking, the push rod diameter should not be less than 3mm. For plastic parts that are heavier or made of higher-strength materials, the diameter needs to be appropriately increased. For example, a diameter of 5-8mm can be used. The length of the push rod should be determined according to the ejection stroke to ensure that at the maximum ejection position, there is still enough length at the bottom of the push rod to remain in the guide sleeve to prevent the push rod from falling off or tilting. In addition, the clearance between the push rod and the template must be strictly controlled. The clearance is usually 0.02-0.05mm. A clearance that is too large can easily cause the melt to overflow from the gap and form flash; a clearance that is too small will affect the flexibility of the push rod’s movement and may even cause it to get stuck.
The ejection process parameters significantly impact the ejection performance of the ejector pin and require dynamic adjustment based on the part’s material and structure. The ejection speed of the injection molding machine should not be too fast, as this will instantly subject the part to significant impact force, causing intense friction between the part and the mold cavity, resulting in surface scratches or ejection whitening. For tough materials like ABS and PP, the ejection speed can be increased appropriately, generally within 50-100 mm/s. For brittle materials like PMMA and PC, the ejection speed should be reduced to 30-50 mm/s to ensure smooth part release. The ejection pressure should be commensurate with the part’s release resistance. Insufficient pressure will prevent the part from being fully ejected, while excessive pressure may cause deformation or damage the ejector pin. In actual production, the ejection pressure can be gradually adjusted through trial molds to achieve a level that just ejects the part without noticeable deformation. The ejection stroke should ensure that the part is completely clear of the mold cavity, typically 5-10 mm longer than the part’s maximum height, to prevent the part from remaining in contact with the cavity surface after ejection and causing sticking.
Routine maintenance and troubleshooting of push rod ejectors are crucial for ensuring continuous production. Before each production run, check the push rod’s motion to ensure there are no signs of sticking or unusual noise. If any push rod is bent, deformed, or has surface wear, it should be replaced promptly. Regularly clean the clearance between the push rod and the guide sleeve to remove residual plastic debris and oil, preventing impurities from accumulating and affecting push rod movement. During production, if whitening occurs on plastic parts, this may be due to insufficient push rod-part contact area or excessive ejection pressure. This can be addressed by increasing the push rod diameter, adding more push rods, or reducing the ejection pressure. If part strain occurs, check the push rod surface for smoothness and the mold cavity for scratches. Polish the push rod or repair the cavity surface. Furthermore, for molds used over a long period of time, regularly inspect the push rod for wear. If wear on the push rod tip exceeds 0.1mm, replace it promptly to prevent ejection failure due to insufficient push rod length. Through effective maintenance and timely troubleshooting, the stability and reliability of push rod ejection can be effectively improved.
