Arrangement of injection molding push rod
The placement of injection molding push rods is a core element in mold release system design, directly impacting the release and appearance quality of the molded part. As the most commonly used demolding element, the push rod ejects the molded part from the core or cavity after mold opening. The rationality of its placement is crucial to ensuring smooth demolding and preventing deformation or surface damage. The basic principles of push rod placement are: ensuring uniform force distribution during demolding to avoid excessive localized forces that can cause warping, cracking, or whitening. Push rods should be located in areas of high strength and rigidity, such as ribs, bosses, and thicker walls, to prevent damage during ejection. Furthermore, push rods should be positioned away from the exterior and mating surfaces of the molded part to avoid ejection marks that could impact product performance. For example, in the production of television housings, push rods should be located on the inner ribs, away from visible exterior surfaces, to ensure sufficient release force without affecting the product’s appearance.
The number of push rods required for mold release should be determined based on the part’s size, weight, material properties, and structural strength. For small parts (weighing less than 100g), 2-4 push rods are generally sufficient for demolding; medium-sized parts (weighing 100-500g) require 4-8 push rods; and large parts (weighing over 500g) require 8 or more push rods, with the number of push rods increasing as part weight increases. From a material perspective, for rigid, non-deformable plastics like PS and ABS, the number of push rods can be reduced. For tough, high-shrinkage plastics like PE and PP, the greater clamping force exerted by the part on the core necessitates a greater number of push rods to distribute the release force. For example, a PP part of the same size requires 20%-30% more push rods than a PS part. Insufficient push rods can result in uneven force distribution and deformation, while excessive numbers increase mold complexity and processing costs. Therefore, a reasonable number of push rods should be maintained while ensuring reliable mold release.
The placement of ejector pins should adhere to the principle of “even distribution and local support.” Ejector pins should be symmetrically distributed along the central axis of the part to balance the ejection force and prevent part deflection during ejection. For example, for circular parts, ejector pins should be evenly spaced along the circumference. For rectangular parts, ejector pins should be located near the four corners and at the midpoints of the long sides to ensure symmetrical force distribution. For parts with complex shapes, ejector pins should be located near areas with greater ejection resistance, such as the bottom of deep cavities and the base of bosses. These areas experience strong clamping forces between the part and the core and require greater ejection force. The distance between ejector pins and the edge of the part should not be too close, generally greater than 5mm, to avoid edge cracking during ejection. Furthermore, the distance between ejector pins should be moderate, typically 50-100mm, to ensure even distribution of ejection force across the part. For parts with inserts, ejector pins should be positioned away from the inserts, maintaining a minimum distance of 3mm from the edges to prevent damage or loosening.
The size of the push rod should be determined based on the ejection force and the part structure. The rod diameter should meet strength requirements, calculated using the formula: d ≥ √(4F/(π[σ])), where d is the rod diameter (mm), F is the ejection force per rod (N), and [σ] is the allowable stress of the rod material (MPa, generally 300-500MPa). For example, if a single push rod must withstand an ejection force of 5000N, the rod diameter should be no less than √(4×5000/(3.14×300))≈4.6mm. This value should be rounded up to the nearest 5mm. The rod length should match the mold thickness and ejection stroke, generally extending 0.05-0.1mm beyond the core to ensure the rod can fully return to its original position during mold closing without affecting part formation. The material of the push rod is usually high-quality carbon tool steel such as T8A, T10A, or alloy tool steel such as Cr12MoV. After heat treatment, the hardness reaches HRC50-55 to ensure sufficient wear resistance and strength.
Managing the specific circumstances of ejector pin placement is crucial for ensuring smooth demolding of complex plastic parts. For thin-walled parts, due to their poor rigidity, a large ejector plate or multiple small ejectors should be used to distribute the ejection force and avoid localized deformation of the part caused by excessively large ejector pin diameters. For example, a 1mm thick PET sheet part requires at least six ejectors with a diameter of 2-3mm, evenly distributed around the part’s edges. For parts with deep cavities or undercuts, a combined ejector pin configuration can be employed, such as one at the bottom of the deep cavity and one at an angle on the side of the cavity, to facilitate demolding. For transparent parts, ejectors should be placed in opaque areas. If an ejector pin must be placed in a translucent area, a specialized design, such as a spherical ejector pin, should be used to minimize the impact of ejection marks on light transmittance. Furthermore, the clearance between the ejector pin and the mold must be strictly controlled within 0.02-0.05mm to prevent melt overflow and flash while ensuring flexible ejector pin movement. Through special treatment of different plastic part structures, we can effectively solve the demoulding problems in complex situations and ensure product quality.
