U. of Thessaly5th Int'l Conf. on Manuf. Syst.1 Analysis of a Bufferless, Paced, Automatic Transfer Line with Massive Scrapping of Material during Long Failures George Liberopoulos, George Kozanidis, Panagiotis Tsarouhas University of Thessaly, Greece

U. of Thessaly5th Int'l Conf. on Manuf. Syst.2 Outline Introduction Model description Model analysis –Case 1: Material has NO memory of damage during previous stoppages –Case 2: Material has memory of damage during previous stoppages Effect of parameters on performance Comparison with model in which workstation downtimes do not have memoryless distributions

U. of Thessaly5th Int'l Conf. on Manuf. Syst.3 Introduction Manufacturing Setting –Continuous or semi-continuous processing manufacturing –High-speed, paced transfer lines with NO buffers in between workstations Examples –metallurgical products –nonmetallic mineral products (e.g., ceramics, glass, and cement) –basic chemicals –food and beverage products –paper products

U. of Thessaly5th Int'l Conf. on Manuf. Syst.4 Introduction Motivation –When a failure occurs, the section of the line upstream of the failure stops. –This causes a gap in production downstream of the failure (loss of productivity). –The quality of the material that is trapped in the stopped section of the line deteriorates with time. –If the stoppage lasts long enough, the trapped material may have to be scrapped because its quality becomes unacceptable. –RESULT: (1) Havoc, (2) wasted material, and (3) an additional significant gap in production upstream of the failure (additional loss in productivity).

U. of Thessaly5th Int'l Conf. on Manuf. Syst.5 Introduction Our own experience: –In a pizza processing line, approximately half of the 10% drop in efficiency of the line was due to the gap in production caused by failures while the other half was due to the gap caused by scrapping of material during long failures (Liberopoulos & Tsarouhas, 2005). Many other real-life situations (material solidification, too much exposure to heat, humidity, acidity, etc.) The problem is important from a manufacturing systems practitioner’s point of view but has not been studied from a manufacturing systems engineering perspective.

U. of Thessaly5th Int'l Conf. on Manuf. Syst.6 Introduction Literature on transfer lines and scrapping –Okamura & Yamashina (1977), Shanthikumar & Tien (1983), Jafari & Shanthikumar (1987), Buzacott & Shanthikumar (1993), Altiok (1996), Dogan & Altiok (1998) Literature on transfer lines and scrap/rework –Pourbabai (1990), Yu & Bricker (1993), Gopalan & Kannan (1994), Helber (2000), Li (2004) In all of the above works, wherever scrapping is involved, it is assumed that when a failure occurs at a workstation, a single part – that which is on the workstation – is either always scrapped or scrapped with a given stationary probability, independently of the failure time.

U. of Thessaly5th Int'l Conf. on Manuf. Syst.7 Model description Assumptions –M workstations in series with NO buffers in between –Space and time are discretized –N i = Number of discrete positions of workstation i –Processing time at every position is 1 time unit (cycle) –Inexhaustible supply of raw parts upstream of the first workstation –Unlimited storage area for finished parts downstream of the last workstation –Uptime of workstation i geometrically distributed with mean 1/p i –Downtime of workstation i geometrically distributed with mean 1/r i

U. of Thessaly5th Int'l Conf. on Manuf. Syst.8 Model analysis Efficiency (availability) of workstation i in isolation Efficiency (availability) of workstation i in the system Operating time of workstation i geometrically distributed with mean Stoppage time of workstation i geometrically distributed with mean

U. of Thessaly5th Int'l Conf. on Manuf. Syst.9 Model analysis Case 1: Material with NO memory of damage during previous stoppages –R i = stoppage time of workstation i ~ Geom ( ) –n i = maximum allowable stoppage time of workstation i –q i = conditional probability that a part will not be scrapped from a particular position of workstation i, given that it has entered this position –l i = conditional expected time that a part spends in any position of workstation i, given that it has moved into this position

U. of Thessaly5th Int'l Conf. on Manuf. Syst.10 Model analysis –q i,j = conditional probability that a part will enter position j of workstation i, given that it has entered workstation i –l i,j = conditional expected time that a part spends in position j of workstation i, given that it has entered workstation i –L i = conditional expected flow time of a part at workstation i, given that it has entered this workstation –Q i = conditional probability that a part will move from workstation i to workstation i + 1, given that it has entered workstation i, i.e. yield of workstation i

U. of Thessaly5th Int'l Conf. on Manuf. Syst.11 Model analysis – = unconditional probability that a part will move from workstation i to workstation i + 1, given that it has entered the system Note: = yield of the entire line = line availability  line yield = line efficiency – = unconditional expected flow time of a part at workstation i – = total unconditional expected flow time of a part in the line

U. of Thessaly5th Int'l Conf. on Manuf. Syst.12 Model analysis –B i = average number of parts in workstation i –B TOT = average number of parts in the entire line

U. of Thessaly5th Int'l Conf. on Manuf. Syst.13 Model analysis Case 2: Material WITH memory of damage during previous stoppages –S i,j = cumulative time that workstation i is stopped from the moment that a part enters position 1 of workstation i until it exits position j of workstation i –Q: P{S i,j = k}=? A: For k =0, For k > 0, consider the event that in its trajectory from position 1 to position j, a part does not stop in m out of the j positions and stops in the remaining j – m positions, and that the cumulative time that the part is stopped is k, where k > 0. The probability of this event is

U. of Thessaly5th Int'l Conf. on Manuf. Syst.14 Model analysis is the probability that the part does not stop in m positions and stops in the remaining j – m positions (binomial). is the probability that the cumulative time until the (j – m)th resumption of operation of workstation i following a stoppage is equal to k (Pascal or negative binomial). To find P{S i,j = k}, add over all possible values of m:

U. of Thessaly5th Int'l Conf. on Manuf. Syst.15 Model analysis –Each time a part enters a new position j of workstation i, where j = 2, …, N i, its remaining maximum allowable standstill time is n i – S i,j-1 instead of n i, as was the situation in the NO memory case. –Conditional expected flow time of a part in position j of workstation i, given that the part has entered workstation i: –Yield of workstation i: –The rest of the expressions are the same as in the NO memory case.

U. of Thessaly5th Int'l Conf. on Manuf. Syst.16 Effect of parameters on performance Problem instance with M = 6 identical workstations with parameters: N i = 10, p i = 0.1, r i = 0.8, and n i = 10, i = 1,…, M. How do p i, r i, n i affect,, B TOT ?

U. of Thessaly5th Int'l Conf. on Manuf. Syst.17 Effect of parameters on performance

U. of Thessaly5th Int'l Conf. on Manuf. Syst.18 Effect of parameters on performance

U. of Thessaly5th Int'l Conf. on Manuf. Syst.19 Effect of parameters on performance

U. of Thessaly5th Int'l Conf. on Manuf. Syst.20 Effect of parameters on performance

U. of Thessaly5th Int'l Conf. on Manuf. Syst.21 Effect of parameters on performance Other observations: –,, and B TOT are higher in the NO memory case than in the memory case (there is less scrapping in the NO memory case). –The effects of system parameters on system performance are more intense for downstream workstations than for upstream workstations. –The effect of p i on system performance seems to be linear, whereas the effect of r i seems to be concave.

U. of Thessaly5th Int'l Conf. on Manuf. Syst.22 Comparison with modified model in which workstation downtimes do not have memoryless distributions Original model: –M = 6 identical workstations with parameters: N i = 30, p i = 1/1600, r i = 1/30, and n i = 10, 20, 40, 50, i = 1,…, M. E[downtime i ] = 30; Var[downtime i ] = 870 Modified model: –Same as original model except that the workstation downtimes are distributed as the sum of two iid geometrically distributed rv’s, each with mean 15 E[downtime i ] = 30; Var[downtime i ] = 420

U. of Thessaly5th Int'l Conf. on Manuf. Syst.23 nini Memory of damage Model & method B TOT 10 No Oa Os ± ± ± ± M ± ± ± ± Yes Oa Os ± ± ± ± M ± ± ± ± No Oa Os ± ± ± ± M ± ± ± ± Yes Oa Os ± ± ± ± M ± ± ± ± Comparison with modified model

U. of Thessaly5th Int'l Conf. on Manuf. Syst.24 nini Memory of damage Model & method B TOT 40 No Oa Os ± ± ± ± M ± ± ± ± Yes Oa Os ± ± ± ± M ± ± ± ± No Oa Os ± ± ± ± M ± ± ± ± Yes Oa Os ± ± ± ± M ± ± ± ± Comparison with modified model

U. of Thessaly5th Int'l Conf. on Manuf. Syst.25 Observations: –,, and B TOT are higher in the NO memory case than in the memory case for both the original and the modified models (there is less scrapping in the NO memory case). –As n i increases,,, and B TOT increase (there is less scrapping when the material can remain still for a longer period). –The workstation downtimes in the modified model have half the variance of the workstation downtimes in the original model. –When n i > E[downtime i ], there is less scrapping in the modified model. –When n i < E[downtime i ], there is more scrapping in the modified model. –The difference in performance between the original and the modified models is less than 6%. Comparison with modified model

U. of Thessaly5th Int'l Conf. on Manuf. Syst.26 Comparison with modified model nini E[downtime] P{scrapping} = P{downtime > n i }