Question 1

You are managing a factory that manufactures washing machines. The factory is open 8 hours per day. The washing machines are built upon order. Orders enter the system with a distribution of EXP(X) (defined above) minutes. The machine is then constructed, which requires two staff members working simultaneously a duration of TRIA(30,45,60) minutes. All machines are then tested, which requires one staff member a duration of TRIA(10,12,30) minutes. 85% of tested washing machines pass the test and are shipped out (leaving the model). 80% of the failed components are repaired (which requires two staff members a duration of TRIA(10,30,60) minutes and the remaining 20% are scrapped and have to be rebuilt from raw components. Run the simulation for 10 working days using various inspection staff combinations. Record the number of scrapped components. Determine the preferred number of staff members for this operation.

Assume that all staff members work equally well in every role. Your requirements are to ensure that employee utilisation is kept high and more than 95% of machines are exported within one working day from receiving the order. Use five replications for your study.

Question 2

Parts arrive into a system in batches of 2 with an exponential interarrival distribution of mean 20 minutes; the first batch arrives after time t = 0. Upon arrival, the parts are processed at one of two parallel machines, with equal probability to visit either machine. Each machine has a processing-time distribution of TRIA(10,15,19) minutes. Each of these machines is taken down for maintenance with uptimes EXPO(25) minutes and downtimes EXPO(5) minutes.

The parts are inspected at a station with a processing time of UNIF(1,2) minutes and about 15% are sent back to the same machines to be reprocessed. The rest pass inspection and are shipped externally. Run for 50,000 minutes with 5 replications. Find the average and maximum number of times a part is processed by either machine, the average number of parts in the machine queues and the average part cycle time. Analyse and highlight the important results obtained from the simulation.

What, if any, changes would you propose to the system to improve the performance without significantly increasing the cost?