Life of injection molds
The lifespan of an injection mold is a crucial indicator of mold performance. It refers to the number of parts a mold can produce while ensuring quality. Its length directly impacts production costs and efficiency. Mold life is typically divided into sharpening life and total lifespan. The sharpening life refers to the number of parts a mold can produce after a single sharpening cycle, while the total lifespan represents the total number of parts produced after multiple sharpening cycles until the mold becomes irreparable. The lifespan of different mold types varies significantly. Small precision molds typically have a total lifespan of 500,000 to 1,000,000 cycles, while large automotive molds can reach 1,000,000 to 5,000,000 cycles, and even extra-large appliance molds can exceed 10,000,000 cycles. The specific lifespan depends on factors such as mold material, structural design, manufacturing process, and maintenance.
The choice of mold material is a key factor in determining its lifespan. It should be carefully selected based on the material properties of the plastic part, production volume, and precision requirements. For molds with large production runs and parts containing reinforcing agents such as glass fiber (e.g., PA66 with 30% glass fiber), high-hardness, high-wear-resistance materials are essential. For example, pre-hardened plastic mold steel 718H (hardness HRC 32-35) or quenched and tempered steel S136 (hardness HRC 48-52) are suitable. These materials, after proper heat treatment, exhibit excellent wear and corrosion resistance, significantly extending mold life. For molds with smaller production runs and parts made from common plastics (e.g., PP and PE), less expensive carbon structural steel (e.g., S50C) can be used. Surface hardening (hardness HRC 50-55) can meet basic requirements. Furthermore, key areas such as the mold cavity and core can be treated with surface treatments (e.g., nitriding or PVD coating) to create a harder surface layer (e.g., nitrided layers can reach hardness HV 800-1000), further enhancing wear resistance.
The impact of mold structural design on mold life is primarily reflected in stress distribution, guiding accuracy, and the layout of consumable parts. A sound structural design can reduce stress concentration during mold operation, preventing premature cracking or deformation. For example, sufficiently large radius corners (R ≥ 3mm) at the cavity corners can reduce stress concentration. The mold plate thickness is strength-checked based on the clamping force and injection pressure to ensure deformation ≤ 0.01mm/m. The guiding mechanism utilizes high-precision ball guide pins and bushings with a clearance controlled to 0.001-0.003mm, minimizing wear on the cavity caused by lateral forces during mold closing. Standardized designs for consumable parts (such as ejector pins, sprue bushings, and springs) with ample replacement space allow for quick replacement, preventing the entire mold from being scrapped due to damage to a single component. For complex molds, a modular design can be adopted, with consumable parts designed as replaceable modules, significantly extending the mold’s overall life.
The precision and stability of the manufacturing process directly impact the initial lifespan and performance consistency of the mold. Machining errors within mold components must be strictly controlled, with cavity dimensional tolerances ≤ ±0.005mm and surface roughness Ra ≤ 0.4μm, to avoid stress concentration and increased wear caused by machining defects. Heat treatment is crucial for ensuring mold material performance. Advanced processes such as vacuum quenching and isothermal tempering are employed to ensure uniform material hardness (hardness difference ≤ HRC2) and a microstructure free of cracks, porosity, and other defects. For example, after vacuum quenching at 1050°C and tempering at 450°C, S136 steel achieves an impact toughness exceeding 25J/cm², significantly improving fatigue resistance. Assembly is performed in a constant temperature workshop (20±2°C). Precision measuring instruments (such as a three-dimensional coordinate measuring machine) are used to verify assembly accuracy, ensuring smooth operation of all moving parts without binding or interference. A 100% pass rate for mold trials after assembly is required to prevent premature failure caused by improper trial runs.
Mold use and maintenance are crucial for extending its lifespan, necessitating the development of sound operating procedures and maintenance plans. During operation, mold opening and closing speeds must be controlled within a reasonable range (typically 20-50 mm/s) to avoid impact loads caused by excessive speed. Injection and holding pressures must not exceed the mold’s design capacity (typically 150-200 MPa) to prevent cavity overload and deformation. Regularly clean and lubricate the mold. Lubricate the parting surfaces, guide pins and bushings, and ejector mechanism at least once per shift, using a dedicated high-temperature grease (resistant to temperatures ≥ 150°C) to prevent wear on moving parts. Molds should be treated with rust prevention during storage (e.g., spraying with anti-rust oil or placing desiccant) to prevent rust caused by humid environments. Molds that have been out of service for extended periods should undergo regular opening and closing operations to prevent moving parts from seizing. Maintain a mold maintenance log, recording the number of parts produced, maintenance details, and part replacements. If mold abnormalities occur (e.g., increased flash on parts or scratches on the cavity surface), promptly shut down the machine for inspection and repair to prevent further problems. Through scientific use and careful maintenance, the actual life of the mold can be extended by 20%-50% compared to the designed life.
