Slovak University of Technology
Faculty of Material Science and Technology in Trnava
Lot size in batch
production
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
Skupiny
Workstation
groups
pracovísk
Workstation
Definition of objective function
IF No_out_parts () default value of flow time
Unit_Costs = SumCosts / No_out_parts
RETURN Unit_Costs
ELSE
Unit_Costs = SumCosts / No_out_parts +
constant
RETURN Unit_Costs quantitative values of
goals
ENDIF
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
Otpimization results
Objective function
Optimal lot 240
Costs per 1 part
220
size 200
180
160
140 155
120
100
2 2 2 2 1 2 1 2 2 2 2 2 1 1 2 2 2 1 2 2
2
3 2 2 1 3 3 3 2 2 2 1 2 2 2 2 1 3 2 2 2
1
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
Values of the others goals:
Lot size VD2,
· flow time (42,26 min ); Lot size VD1,
· machine utilisation (76%); Input interval P2,
· number of finished parts (1344). Input interval VIP1
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.