Injection mold matching size tolerance
The dimensional tolerances of injection molds are core technical parameters that ensure the precise assembly and stable operation of mold components. They are directly related to the mold’s service life and the quality of the molded part. The setting of dimensional tolerances must be based on the functional characteristics of the part, and the tolerance requirements for parts with different functions vary significantly. For example, the cavity and core are key components in directly molded plastic parts. Excessive clearance between them can lead to flash, while too little friction can cause wear and even jamming. Therefore, a clearance fit of H7/g6 is typically used, keeping the clearance within a range of 0.01-0.03mm. Guide pins and guide sleeves, as the core of the guide mechanism, must ensure smooth movement and guiding accuracy. A clearance fit of H7/f7 is often used to avoid both motion blockage caused by an overly tight fit and positioning deviation caused by excessive clearance.
The determination of mating dimensional tolerances requires a comprehensive consideration of material properties and processing technology. For parts made from high-strength alloy steel (such as Cr12MoV), due to their high hardness and excellent wear resistance after heat treatment, higher tolerance grades (such as IT5-IT6) can be used to meet the precision requirements of long-term use. For auxiliary parts made from ordinary carbon steel (such as 45 steel), the tolerance grade can be appropriately lowered (such as IT8-IT9) to reduce processing costs while ensuring functionality. The accuracy of the processing technology also constrains the choice of tolerance. For example, grinding can achieve IT5 accuracy, while milling can generally only achieve IT8. Therefore, the design should be based on actual processing capabilities to avoid situations where the tolerance requirements are too high and cannot be met.
The impact of temperature changes on fit tolerances cannot be ignored. During the injection molding process, the surface temperature of the mold cavity can reach 100~300℃, while the temperature of parts such as the template is relatively low. This temperature difference will cause the parts to expand and contract, thereby changing the fit clearance. Therefore, when designing fit tolerances, temperature compensation calculations are required. For example, for sliding fits in high-temperature working environments, the initial gap should be appropriately enlarged to offset the gap reduction caused by thermal expansion. Taking nylon material molding molds as an example, the working temperature of the cavity is about 120℃, and the temperature difference with the template is about 80℃. At this time, the fit clearance between the cavity and the template should be increased by 0.02~0.04mm compared to normal temperature to prevent over-tight fit due to thermal expansion.
There are significant differences in the tolerance requirements for dynamic and static fits. Dynamic fits (such as the fit between a push rod and a template hole) must ensure that the parts do not bind or leak during movement. Therefore, the tolerance band should be distributed towards the clearance side, and an H8/f7 fit is typically used. Static fits (such as the fit between an insert and a template) primarily require precise positioning and a secure connection. Transition fits (such as H7/m6) are often used to ensure ease of assembly while preventing loosening during use. For static fits that require the transmission of high torque, an interference fit (such as H7/s6) can be used. This creates friction through the interference between parts to achieve a secure connection, but the interference must be strictly controlled to prevent deformation of the parts due to excessive stress.
Verifying and optimizing the fit tolerances is a crucial step in mold commissioning. During the mold trial phase, actual assembly and molding tests are required to verify the rationality of the fit dimensions. If the guide pin and guide sleeve are found to be too tight, resulting in excessive resistance to mold opening and closing, the clearance can be increased appropriately by grinding the outer diameter of the guide pin. If the clearance between the cavity and core is too large, resulting in flash, the part must be reground and resized. Furthermore, precision testing equipment such as a three-dimensional coordinate measuring machine can be used to quantitatively test key fit dimensions. The measured data can then be compared and analyzed with the design tolerances to provide a basis for tolerance optimization. Through repeated verification and adjustment, a perfect match between the fit tolerances and the mold’s operating conditions can be achieved, ensuring the long-term and stable operation of the mold.
