Lot size in batch production Slovak University of Technology Faculty of Material Science and Technology in Trnava
Dec 13, 2015
Lot size in batch production
Slovak University of TechnologyFaculty of Material Science and Technology in Trnava
What is the lot size?
Lot size is number of parts that is processed on one workstation at the same time, when the set up costs are spent only once.
Lot size is one of norms of production that markedly influences production costs.
Problem of correct lot size
Advantages of big lot size Decline of set up time Reductiom of the number of arrangement The existence of inventories allows usage of capacity
proportionally
Disadvantages High inventories Long flow time High production costs
Classic methods for determination of lot size
These methods use different mathematical models Minimal lot size (capacity) Optimal lot size Empirically determined lot size Periodic lot size
None of classic methods is not able to involve many real factors, that influences lot size.
How to set up lot size?
Conflict between systems, which go out from MRP and JIT
Use reductionistic alebo holistic approach? It is possible to expect more complex
aproach such as simulation optimization
Simulation?
Simulation allows predicate reactions of production system on selected lot size
It is possible to include crucial factors that influence on lot size into simulation model.
Values of goal parameters of production system are gained directly by simulation: system performance flow time work in process costs capacity utilisation
Usage of simulation optimization The solving for determination of accurate
(optimal) lot size offers simulation optimization
Simulation model of production system in Witness
SkupinypracovískSkupiny
pracovískSkupiny
pracovískWorkstation
groups
IF No_out_parts () < default value of finished parts AND Machine utilisation () < default value of machine utilisation AND Flow time () > default value of flow time
Unit_Costs = SumCosts / No_out_partsRETURN Unit_Costs ELSEUnit_Costs = SumCosts / No_out_parts +
constantRETURN Unit_Costs ENDIF
quantitative values of goals
Definition of objective function
Preparatory experiments
model verification; set up simulation and warm up period; find out lower and upper limit of system
loading; determination of quantitative values of
production goals.
Objective function
155
100120140160180200220240
2 2 2 2 1 2 1 2 2 2 2 2 1 1 2 2 2 1 2 2
3 2 2 1 3 3 3 2 2 2 1 2 2 2 2 1 3 2 2 2
11 12 10 10 6 11 7 10 11 11 11 10 6 6 11 10 12 7 10 12
11 8 9 5 12 12 11 10 9 10 5 11 8 9 8 6 12 7 12 9
Lot size VD2,Lot size VD1,
Input interval P2,Input interval VIP1
Co
sts
per
1 p
art
Otpimization results
Optimal lot size
Values of the others goals:· flow time (42,26 min );· machine utilisation (76%);· number of finished parts (1344).
2
1
Final evaluation
It is possible to set up optimal lot size by simulation optimization, while the factors that influence lot size in real are respected.
Simulation optimization is possible to use also for opposite production goals