Injection mold dimensional tolerances are a key parameter for ensuring product precision. They directly impact the product’s assembly performance and performance. They must be determined by comprehensively considering factors such as plastic shrinkage, product tolerance requirements, and mold manufacturing precision. Mold dimensional tolerances generally adhere to the “one-way deviation” principle: the cavity dimension takes a lower tolerance (to ensure the maximum part dimension does not exceed the upper limit), while the core dimension takes an upper tolerance (to ensure the minimum part dimension does not fall below the lower limit). For example, for a part with an outer contour dimension of 100 ± 0.2 mm, the mold cavity dimensions should be set to 102 (accounting for a 2% shrinkage) – 0.05/0 mm to ensure that the part remains within the acceptable tolerance range after shrinkage. The mold tolerance grade is generally two to three grades higher than the part. For example, if the part tolerance is IT12, the mold tolerance should be IT9-IT10 to offset the effects of mold wear and plastic shrinkage fluctuations.
Fluctuations in plastic shrinkage are a key factor affecting molding dimensional tolerances. The shrinkage ranges of different plastics vary significantly. Even for the same plastic, shrinkage can fluctuate by 0.5%-2% depending on the grade and molding process. When designing a mold, the cavity dimensions must be calculated based on the maximum shrinkage, and the core dimensions based on the minimum shrinkage, allowing for shrinkage compensation. For example, the shrinkage of PP plastic is 1.5%-2.5%. When molding a 100mm product, the cavity dimensions must be designed as 100×(1+2.5%)=102.5mm, and the core dimensions as 100× (1+1.5%)=101.5mm . Shrinkage fluctuations are offset by a reasonable distribution of tolerance bands. For precision products, multiple mold trials are required to correct the shrinkage, and the mold dimensions must be adjusted using the actual measured shrinkage values to ensure that the tolerances are within the required range.
Mold manufacturing precision is crucial to achieving mold dimensional tolerances, and the accuracy of machining equipment directly determines the dimensional errors of mold components. Cavities and cores must be machined using high-precision CNC machine tools (such as CNC machining centers and EDM machines) to ensure dimensional errors of ≤0.01mm and surface roughness Ra ≤0.8μm. During assembly, the clearances between components must be controlled: the clearance between guide pins and guide sleeves must be ≤0.02mm, and the clearance between ejector pins and ejector holes must be ≤0.03mm to avoid dimensional deviations due to assembly errors. For multi-cavity molds, the dimensions of each cavity must be consistent, with dimensional variations of ≤0.02mm to ensure interchangeability between products with different cavities.
Wear during mold use can cause mold dimensional tolerances to gradually increase, so allowance for wear must be made during design. The initial dimensions of areas susceptible to wear (such as cavity edges and core working surfaces) should be 0.03-0.05mm larger than the theoretical value. Repairs can be made through grinding before wear reaches the limit. For example, the threaded core of a bottle cap mold can have its initial diameter increased by 0.05mm. This will ensure thread fit accuracy even after 0.03mm of wear. Regular mold dimension testing is crucial for tolerance control. It is recommended to inspect key dimensions every 100,000 molds. If wear exceeds one-third of the tolerance, repair or replace the part promptly to avoid batches of defective products.
Specialized control methods are required for the dimensional tolerances of molded parts with special structures. For dimensions with slopes or radians, both linear and angular tolerances must be controlled separately. Calculating the tolerance for slope dimensions requires considering the dual effects of shrinkage on both the length and height of the bevel. For example, the bevel length tolerance for a 30° bevel requires controlling both length and angular errors (≤0.5°) to avoid misalignment of the bevel endpoints. For precision structures like threads and gears, mold dimensional tolerances must be controlled within a range of 0.01-0.02mm. Specialized measuring tools (such as thread gauges and gear testers) are used for inspection, and if necessary, precision finishing is performed on a jig grinder to ensure smooth thread fit and accurate gear meshing after molding.
