Abstract:
This research proposes a method for automotive parts production scheduling in plastic injection molding machines that aims to reduce labor costs and electical energy costs. The case study is the production scheduling of 75 models on 30 parallel machines. The daily production schedule was created based on the planner's experience, causing the workload of each machine to be inappropriate, which results have overtime costs. This research therefore proposes a mathematical scheduling model for solving this production scheduling problem. The model decides on each machine from five variations: 8, 10, 16, 18, and 20 hours, and sequences the jobs on each machine according to this maximum run time condition. Ten scheduling problems from the past production data are used to test the effectiveness of the proposed method in two types: Model 1 is to set the number of machines to be fixed at 30 machines, and Model 2 is to find the appropriate number of machines as well. The research results reveal that the proposed Model 1 was able to find an optimal solution for 6 problems with an average processing time of 212.5 seconds. For problems with unfounded optimal solution, the percentage of gap between the best feasible solution and the lower bound is 0.006% on average. For the proposed Model 2, an optimal solution was found for 3 problems with an average processing time of 631 seconds. The percentage of gap between the best feasible solution and the lower bound is 0.104% for problems with unfounded optimal solution. The cost comparison results found that production schedules with the proposed Model 1 and 2 have production costs reduced from the previous method by 14,926 baht and 18,949 baht, or a reduction of 4.6% and 5.8%