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

Workstation

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.

Optimizer Window

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

Final evaluation

However, simulation model have to be created.

Simulation optimization requires special software aid (plug in module)

Optimization process can take long time.