Lean 5S + Safety and Work-Related Injuries in an Aluminum Casting Plant: A Five-Year Department-Stratified Analysis

Lean 5S programs are widely promoted to improve occupational safety and health, but quantitative evaluations in metal casting are scarce. This study examined whether a Lean 5S + Safety program was associated with changes in injury outcomes in an aluminum casting plant. Safety records from 2021 to 2025 for production and shopfloor support workers included injuries, lost workdays, and person–hours. Injury frequency (IFR) and severity (SR) rates per 1,000,000 person–hours were computed, and multivariable Poisson regression with log(person–hours) offsets estimated incidence rate ratios (IRRs) comparing the program period (2022–2025) with the pre-implementation year (2021); negative binomial models were also fitted. Over 3.53 million person–hours, 170 lost-time injuries, and 1848 lost workdays were recorded, and the annual IFR declined from 68.2 to 29.3 per 1,000,000 person–hours. Across model specifications, the program period was associated with 30–40% lower injury rates and roughly halved lost workday rates versus baseline. Injuries and lost workdays were concentrated in foundry and machining, and young short-tenure workers and those with lower secondary education had the highest rates; exploratory analyses suggested a late-summer shift in injuries from day to evening and night shifts. Overall, this multi-component Lean 5S + Safety-centered occupational safety and health (OSH) improvement program was associated with improved safety performance, while residual risk remained concentrated in specific departments and worker groups.