A study of tolerance allocation and stack-up analysis to improve the assembly precision of an injection mold

ABSTRACT Tolerance designing is a critical job for the assemblies to ensure their precision and functions. Tolerance design must consider manufacturing capability for ensuring the feasibility of the designs in the following processing. This paper presents the available methods of determining tolerance sizes and tolerance analysis to improve assembly precision. An injection mold consisting of hundreds of parts is used as an example to depict the methods of tolerance designing. The precision grades and the corresponding tolerance ranges applied in engineering fits are introduced for giving reasonable tolerances in designing. The defect rates of the designed tolerances to the machining precisions are investigated for observing the effects of manufacturing capabilities. An approach based on block assembly is proposed for eliminating tolerance stack-up, as well as improving assembly precision. The assembly tolerances can be reduced from 0.071 to 0.032 mm by using the proposed method, and the defect rates in manufacturing can be improved from 9.05% to 0.02%. It means that a larger range of tolerances can be designed to meet the specifications, as well as reduce the manufacturing costs.