Selection of tolerance grades for commonly used plastic parts
The selection of common tolerance grades for plastic parts must first consider the part’s operating environment and functional requirements, which are the core factors in determining the tolerance grade. Different application scenarios have significantly different requirements for plastic part dimensional accuracy. For example, transmission plastic parts used in precision instruments require strict clearance control to prevent impacts on the instrument’s measurement accuracy or transmission stability. In these cases, higher tolerance grades, such as MT3 or MT4 in GB/T 14486, are typically selected. For decorative plastic parts, such as toy casings and furniture accessories, the primary focus is on appearance quality, with lower dimensional accuracy requirements. Grades MT6 to MT8 can meet these requirements. In the electrical field, the fit between plastic and metal parts is particularly important. For connector insulation housings, the aperture and pin tolerances must be precisely matched to each other, otherwise poor contact or difficulty in insertion and removal can result. Grades MT4 to MT5 are generally selected. For cable sheaths, however, only a certain dimensional range is required; MT7 to MT9 are sufficient. In addition, the plastic tolerance grade of moving parts must be higher than that of stationary parts. For example, the tooth thickness and pitch tolerance of plastic gears in rack and pinion transmission must be controlled within MT3 level to reduce transmission noise and wear.
The material properties of plastic parts are key factors influencing the selection of tolerance grades. Different plastics have varying shrinkage fluctuations, which directly limits the achievable precision grade. Crystalline plastics such as PE, PP, and POM have large shrinkage rates with wide fluctuations, resulting in poor dimensional stability after molding. Therefore, achieving high-precision tolerance grades is generally difficult. For example, PP has a molding shrinkage of 1.0%-2.5%. Even with strict control of process parameters, its tolerance grade can generally only reach MT5 to MT6. POM, on the other hand, has a shrinkage of 1.5%-3.5%, resulting in a tolerance grade of MT6 to MT7. Amorphous plastics such as PS, PMMA, and PC have lower shrinkage rates with narrow fluctuations, resulting in excellent dimensional stability, allowing for higher tolerance grades. For example, PC has a shrinkage of 0.5%-0.8%. After optimizing the mold and process, tolerance grades of MT3 to MT4 can be achieved, making it suitable for the production of high-precision plastic parts such as optical lenses. In addition, the shrinkage rate of fiber-reinforced plastics is smaller than that of pure resins, and they are obviously anisotropic. When selecting the tolerance grade, the shrinkage difference in different directions must be considered. Usually, different tolerance grades are selected in the flow direction and the vertical direction. For example, for glass fiber reinforced PA6, if the shrinkage rate in the flow direction is 0.3%-0.8%, MT4 grade can be selected, and if it is 0.8%-1.5% in the vertical direction, MT5 grade can be selected.
The mold’s manufacturing accuracy and structure directly impact the tolerance grade selected for plastic parts. The mold’s machining accuracy must match the required tolerance grade for the part. If the machining accuracy of the mold cavity and core falls below the required tolerance grade, even the best process cannot produce a qualified part. For example, if a part requires an MT3 tolerance, the mold cavity’s manufacturing tolerance must reach IT7 to IT8, and high-precision guiding and positioning devices must be installed to ensure mold closing accuracy. Mold structures such as the parting surface and core pulling mechanism also affect part dimensional accuracy. For parts with lateral core pulling, the clearance between the core pulling components can cause deviations in the part’s lateral dimensions. Therefore, the tolerance grade for such parts typically needs to be reduced by one or two grades. For example, a part that could originally achieve MT4 may only achieve MT5 if a hydraulic lateral core pulling mechanism is used due to the clearance between the core pulling slider and the guide groove. In addition, mold wear will cause the size of the plastic part to gradually change. Therefore, for plastic parts produced over a long period of time, a certain amount of wear allowance must be reserved when selecting the initial tolerance grade. For example, for mass-produced home appliance plastic parts, the MT5 grade is initially selected. Taking mold wear into account, the upper tolerance limit can be appropriately relaxed to ensure that the plastic part size is qualified within the life cycle of the mold.
Production volume and cost are also important considerations when selecting tolerance grades for plastic parts. A higher tolerance grade implies higher production costs and stricter process control. In small-batch production, the tolerance grade can be appropriately lowered to reduce mold costs. For example, for parts in the pilot production phase, selecting MT7 to MT8 grades can meet assembly and testing requirements, avoiding the increased difficulty and expense of mold manufacturing due to the pursuit of high precision. In large-scale production, while the initial investment in high-precision molds is higher, the cost per part is lower and consistent product quality is ensured, making it reasonable to select a higher tolerance grade. For example, automotive parts are typically produced in large batches, and MT4 to MT5 tolerance grades are selected to achieve efficient and stable production through automated production and strict process control. Furthermore, higher tolerance grades place greater demands on raw material purity, molding equipment precision, and the stability of the production environment, all of which increase production costs. Therefore, when selecting a tolerance grade, it is important to minimize costs while meeting application requirements and avoid unnecessary waste of precision. For example, the dimensional tolerance of plastic water cups used in daily life can be selected from MT6 to MT7. Too high a precision level will only increase production costs without significantly improving performance.
The assembly relationship and subsequent processing steps of plastic parts significantly influence the selection of tolerance grades. An appropriate tolerance range must be determined based on the assembly method and subsequent processing requirements. For plastic parts with an interference fit, such as the bearing outer ring and plastic housing, a higher tolerance grade is required to ensure a secure and reliable fit, typically MT4 to MT5. For parts with a clearance fit, such as the guide rail and slider, the tolerance grade can be lowered to MT5 to MT6 to reduce assembly difficulty. If a plastic part requires subsequent processing, such as drilling, tapping, or grinding, allowance should be made when selecting the tolerance grade. In this case, the initial molding tolerance grade can be relaxed. For example, if a plastic part requires drilling on the side, the tolerance grade for the side dimension during molding can be MT6, with a 0.5mm machining allowance. After drilling, machining can ensure the final dimensional accuracy reaches MT4. Furthermore, when assembling multiple plastic parts, the impact of cumulative tolerances must be considered, and the tolerance grades of each part should be allocated based on the assembly accuracy requirements. For example, if an assembly of three plastic parts requires an overall assembly tolerance of ±0.2mm, the tolerance grade of each plastic part must be controlled within ±0.07mm (approximately MT5) to avoid cumulative tolerance violations. Therefore, when selecting a tolerance grade, it is necessary to fully consider every link in the assembly chain to ensure overall assembly accuracy.
