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The objective of this research is to investigate optimum parameters for STR 20 block rubber production process while all possible constrains are carefully taken into consideration. According to the current problem study, it was found that the proportion of the achieved rubber with white-spot problems, caused by inappropriate drying process, went higher than the factory acceptable range. The analysis was done by applying the Cause-and-Effect diagram and Failure Mode and Effect Analyze method (FMEA). It revealed that the white-spot problems were mainly caused by improper working parameters operated in the production process. In order to systematically solve the problems, the Lean Six Sigma approach was integrated. The 2K Factorial experiment design was used to screen for system variables and Box-Behnken experiment design was applied to get the optimal production conditions. The experimental designs provided the optimum drying process parameters. For the STR 20 block rubber drying process, the appropriate moisture discharging and drying temperatures were 110 and 120 degree Celsius, respectively. The 3-millimeter-thick mixed rubber should be dried in 210 minutes. The achieved parameters were then used in the onsite production line. The results showed that the new parameter process decreased the fraction the problematic white-spot rubber by 17.16 percent. The capability analysis (Cpk) was 1.45 with 0.05 significance level. It indicated the highly efficient process as the confidence interval of the process was 1.04 < Cpk < 1.87. The estimated reduced cost from white-spot block rubber reprocessing was 924,630 Baht per year.
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