Typical structure of injection mold base
The injection mold frame is the mold’s foundational framework, supporting and securing various functional components. Its typical structure consists of a fixed die plate, a fixed die plate, a movable die plate, a movable die plate, guide pins and bushings, and tie rods. These components are connected by screws to form a stable load-bearing system. The fixed die plate is the fixed portion of the mold frame, mounted directly on the fixed die plate of the injection molding machine. It is typically 25-50mm thick and made of 45 or Q235 steel. It must be sufficiently rigid to withstand the clamping force. Positioning and mounting holes are provided on its surface to align with the injection molding machine’s locating rings, with a positioning accuracy of ≤0.02mm. The fixed die plate of large molds also requires reinforcing ribs to prevent deformation under stress. The ribs should be 1-1.5 times the thickness of the plate and spaced 200-300mm apart.
The fixed and movable platens are the core components for mounting the cavity and core. The fixed platen is fixed to the fixed mold base and houses the cavity inserts. The movable platen, connected to the movable mold base, houses the core and ejection mechanism. The thickness of both plates is determined by the cavity depth and stress conditions, typically ranging from 50-150mm. The platen is made of HT300 gray cast iron or 45 steel, and undergoes an aging treatment to eliminate internal stresses and prevent deformation during use. The platen is machined with holes for cavity mounting, guide pins and sleeves, and cooling channels. Hole positioning accuracy is controlled to ±0.01mm, and hole verticality is ≤0.01mm/100mm, ensuring accurate assembly of all components. For large platens, stress analysis is required to reduce weight while maintaining strength, such as by using hollow structures or pre-recorded weight-reducing holes during casting.
The guide pin and sleeve assembly is the guiding and positioning core of the mold base, ensuring precise alignment of the movable and fixed molds during opening and closing. Typically, a set of guide pins and sleeves is located at each corner of the mold base, with large mold bases typically increasing to 6-8 sets. The guide pins are constructed of 20CrMnTi alloy structural steel, carburized and hardened to a surface hardness of 58-62 HRC. They have a diameter of 16-50mm and are 20-30mm longer than the mold base thickness, ensuring effective guidance during initial mold closing. The guide sleeves are constructed of tin bronze or composite bearing material, with an inner bore clearance of 0.01-0.03mm and an outer diameter interference fit (H7/r6) with the mold base to prevent loosening. The mounting holes for the guide pins and sleeves are precision-machined to a coaxiality tolerance of ≤0.01mm to ensure smooth, binding-free movement. Over time, wear of the clearance must not exceed 0.05mm.
The movable mold base and spacers form the support portion of the mold frame. The movable mold base is mounted on the ejector mechanism of the injection molding machine. Its thickness is comparable to the fixed mold base and serves to secure the ejector mechanism’s base plate and guide posts. The spacers, located between the movable mold plate and the movable mold base, create a clearance for the ejector mechanism. Their height is determined by the ejection stroke and is typically 50-150mm. Made of 45-grade steel, the spacers should be parallel to the upper and lower surfaces to ≤0.02mm, ensuring uniform force distribution. For large mold frames, spacers should be used in pairs, symmetrically spaced to prevent uneven force on the movable mold plate and resulting in bending. The bottom of the movable mold base should also have an ejector hole that mates with the injection molding machine’s ejector pin. The hole diameter should be 2-5mm larger than the ejector pin to ensure smooth transmission of ejection force.
Tie rods and limiters are used to limit the mold opening and closing strokes, ensuring safe mold operation. Tie rods extend through both the fixed and movable mold plates, with their ends fixed to the fixed and movable mold base plates, respectively. Four tie rods are provided, and their diameters are determined by the mold base dimensions (16-30mm). They are constructed of quenched and tempered 40Cr steel with a tensile strength of ≥800MPa. Limit blocks or travel switches can be installed on the tie rods to control the maximum mold opening distance and prevent excessive separation of the mold plates, which could cause the guide pins to disengage from the guide sleeves. Furthermore, the mold base requires auxiliary components such as reset rods and waste pins. The reset rod ensures accurate reset of the ejector mechanism during mold closing and is 0.02-0.05mm longer than the ejector pins. The waste pins support the ejector plate, preventing wear and tear from direct contact with the movable mold base plate. The height error is ≤0.01mm. Through the precise coordination of various components, the mold base provides stable structural support for the mold, ensuring a safe and reliable molding process.
