Study And Application Of Robust Design In Plastic Injection Molding

Plastic injection molding is a key polymer processing technique to transform granules into various plastic parts. It is also one of the most important disciplines of material process engineering. There are many factors affecting the quality of injection molded products, and unreasonable design parameters would lead to serious product defects. In the factors affecting injection molding, material properties fluctuations, process fluctuations and other noise factors will lead to fluctuations in the product qualities, even lead to the emergence of waster products. Considering quality and cost with fluctuations in a variety of conditions, robust design has become one of the leading subjects in plastic injection molding to improve the stability of product qualities.Because of the important status and great role of three stage design in robust design methods, three stage design was chosen in this paper to improve product quality, which includes three design stages: system design, parameter design, and tolerance design. Based on the function discussion of injection molding, it was proposed that the multi-level quality-cost design model of injection molded product, whose focus was multi-level product qualities for injection molding. The product evaluations were established with specific product quality requirements based on the multi-level quality-cost design model. They would provide objectives and constraints for the following robust design.In the noise factors of injection molding, the material properties fluctuations were approximated by the property parameters'distributions of series materials in the database of numerical simulation software. Before fluctuations study, it was necessary to construct the materials based on the materials property distributing to analyze the technical property, and Plackett-Burman Design was introduced to reduce experiment times. The study of technical property provided significant material noise factors for three stage design, and provided the leading material properties for the choosing of material and process drawing of injection molding. The direct product of inner-outer array was applied to improve the dimension stability with small warpage in the parameter design of significant process variables, considering the material properties fluctuations and process fluctuations respectively. While, an integrated noise factor method was introduced in the parameter design of controllable factors under the comprehensive fluctuations of material properties and process.After the system design and parameter design, tolerance design with material properties fluctuations and process fluctuations were introduced to further improve the dimension stability of injection molded products. The interaction between the noise factors and significance of noise factors were analyzed by orthogonal test to determine the target of tolerance design. The tolerances of significant noise factors were adjusted according to the affection of the noise factors on product dimension.Discussion and application on two products showed that the three stage design would observably improve robustness of injection molded product quality. In the three stage design of injection molding, the result of system design would be applied to evaluation and design of injection molding, parameter design had got good process conditions with better quality and quality robustness, and tolerance design improved the quality robustness further. During the parameter design, the study on material technical properties proposed significant material properties for plastic injection molding, which providing information for design of injection molding process. During the tolerance design, it was detected that the PVT parameters b2s was the most significant noise factors for dimension robustness of injection-molded products, and the dimension robustness was improved by tolerance adjustment of b2s. This noise factor should be the key material property parameter to test and control in the precision injection molding.By the discussion and application of three stage design in injection molding, this paper studied the application and signification of robust design in injection molding. The injection molding process with robust design will introduce better quality of the products and stability of processing. The process and methods of this paper provide reference for optimizations and robust designs of injection molding, and the ideas and conclusions provided theoretical basis for quality management of injection-molded products.