This study focuses on developing a predictive model for quantitatively assessing variations in the mechanical properties of microcellular foamed products, with particular emphasis on their tensile yield stress. The model allows for a more straightforward and precise estimation of tensile yield stress. Using polyamide 66 (PA 66) and polybutylene terephthalate (PBT), the relationship between lightweighting ratio and tensile yield stress in microcellular foamed products was investigated. The results demonstrate that the proposed predictive model can effectively estimate tensile yield stress based on the lightweighting ratio, offering a practical tool for engineers. Additionally, the study successfully predicted the strength of microcellular foamed products after environmental stress testing, confirming a high correlation with measured values. This ability to predict post-stress strength is significant for practical design considerations. These findings offer valuable insights for the design of plastic products, enabling lightweighting and sufficient mechanical strength. This contributes to reducing environmental impact through efficient material use and enhances product competitiveness in the market. Overall, this research highlights the potential of microcellular molding technology in advancing sustainable manufacturing processes.