Paper - Scheduling of Die Casting Dies

8/6/2019 Paper - Scheduling of Die Casting Dies

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Presented by:North American Die Casting Association

This paper is subject to revision. Statements and opinions advanced in this p aper or du ring presentation are the authors and are his/her responsibility, not the Associations. The paper has been editedby NADCA for uniform styling and format. For permission to publish this paper in full or in part, contact NADCA, 9701 W. Higgins Rd., Ste. 880, Rosemont, IL 60018-4721 USA, and the author.

Computer Aided ProcessPlanning (CAPP)

for the Schedul ingof Die Cast ing Dies

Rich Miller, VijayaKumar Sivalingam, Dr.Jerald R Brevick, Dr. Shahrukh Irani

Department of Industrial, Welding and SystemsEngineering,The Ohio State University

Abstract

Short-run die casting requires frequent die changes and

new machine set -ups. To impr ove the responsiveness ofthe die caster to the needs of t he customer, the abil it y t o

change dies in a min imum amount of t ime becomes in-creasingly import ant . By categor izing the physicalatt r ibutes of the die, i ts corresponding geometr y, and the

peri pheral equipment and t ooli ng, the amount of t imerequi red to change from one die to the next can be esti -

mated. The abil it y to esti mate the amount of timerequi red for the die change increases the accur acy of the

schedule. A lso, the total machine production down-timedue to die changes can be minimized, increasing the over-all producti on capacity.

I n thi s paper, a computer aided method for gr ouping

dies based on physical at t r ibutes, geometr y, and per iph-erals to assist in est imating die change t ime is presented.In addit ion, techniques for min imi zing die change ti me

for short -run die casting are descr ibed.

Introduction

As the die casti ng industr y demands more flexibil it y inthe scheduli ng of the par ts to be cast , the effects of di e

changes wil l have a larger impact on t he amount of t otalproduction time. With the demands of downstream com-

panies becoming more variable, the number of die changesneeded to produce a given number of par ts is being in-creased. As lot sizes reduce, the time requir ed for a die

change becomes a more import ant factor in t he consider-ati on of the producti on t ime requir ed for a given number

of part s. Wit hout a reliable model for esti mati ng th istime, the scheduling of the die changes will not be as

accur ate as they should be. Al so, as wi th the machineused in t hi s study, many machines are a capaci ty con-str aint in t he plant. Any ti me lost due to inefficient die

changes and schedul ing is not r ecoverable.The die cast ing industr y does not have a model for est i-

mat ing the amount of ti me a die change wi ll take based

upon the part and die att ri butes. Wit h the att ri butes ofthe die and the part , an estimate for the amount of t ime

needed to complete a die change can be generat ed. This

wi ll aid the industry by givi ng an accur ate esti mate forthe change over t ime needed for a die based upon infor-

mation t hat i s available from the die drawings and partcharacteristics.

Typically, the time that is required for a die change ona die cast machine is an est imate based upon t he previ-

ous knowledge and exper ience of the schedul er. Dur ingthe in it ial lots run wit h a new die, the average amount oft ime needed to do the die change is determi ned. Thi s

number i s then the average amount of t ime requi red forthe changeover. The focus of thi s study i s to develop an

est imate of the t ime needed based upon the unique partcharacteristics using Group Technology. Thi s coding

system wil l al low t he scheduler to establi sh a more accu-rate and pr ecise t ime needed to complete the die changes.

Literature Review

Group Technology (GT) can have a great impact in the

field of die casting. A coding system for the mechani cal,geometri c, and manufactur ing in formati on has been de-

vised (Lewi s and Chou, 1987). This system has the usercode the die and par t based upon par t complexit y, all oycast , tolerances, die materi al, number of cores and in -

sert s, gating, part weight, wall t hickness, and wall depth.Thi s classifi cati on system can adequately descr ibe the

att ri butes of t he part and the die, but i t does not takein to account the addit ional att r ibut es that have a role in

the determi nati on of die change ti me.The understanding of t he time required for die change

and the die attr ibutes that affect th is t ime helps to facil -

i tate the use of schedul ing sequence-dependent setups.Thi s scheduli ng approach can lower the amount of t ime

requi red to complete a die change. The concept is to sched-ule dies in a sequential manner, based upon setup

att ri butes, to minimize the amount of t ime requir ed (Kim


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and Bobrowski , 1994). An example is the t ip and sleeve

diameter change required between certain dies. I f thescheduling is done with the forethought to reduce thenumber of changes needed in t he sleeve and ti p diameter,

th is time in tensive task can be minimized. For smallincreases in setup t ime, larger i ncreases are seen i n t he

rat io of die change ti me to producti on t ime (Kim and Bo-

browski , 1994). As customers demand more flexibil i ty inthe ordering of part s, thi s ratio wil l have a larger role inthe profit abili ty of the die caster.

With t he knowledge generated from t he classifi cati on

of t he par ts using t he GT system, schedul ing algori thmsbased on sequence-dependent set -ups can be used to de-

velop the best order for t he jobs. Several al gor i thms canbe used to calculate the near-optimal sequence of tools

usage. One approach is to develop a binary mat r ix todescr ibe the relati onship between t he parts and the ma-chine set-up t asks (Char les-Owaba and Lambert , 1987).

Simulated annealing is another algori thm that i s used tofind the near optimal solu ti on t o the schedul ing problem

(Parthasarathy and Rajendran, 1997). An aspect not con-sidered in thi s paper, but one the is common for other

manufactu ri ng envir onments is the backt racki ng of t hepart s in the product ion sequence that have set -up t imes

not related to the previous par t . Hwang and Sun, 1997developed a model to take int o considerati on t he twin bol-ster system being used in a punch pr ess to mini mize the

makespan. Whi le one punch was being used, the recip-rocal t ool was being changed. The setup t ime for thi s

tool i s based upon the previous part n , and part n+2 part .Another challenge with scheduling sequence-dependent

set -up i s the release of orders in to the shop. The goal ofthe order r elease is to min imi ze the amount of work -in-

pr ocess (WI P) on t he shop floor. By t aki ng int oconsiderati on t he production pl anning and sequence-de-pendent set -ups when creating the schedule, the abil it y

of the schedule to approach an optimal solu ti on is furt herenhanced (Missbauer, 1997). I n addit ion to WIP, just in

ti me (JIT) is also an import ant consideration t o the sched-uling of production for an ever increasing number ofcompani es. In adding a cost factor for earl iness and tar-

diness factors and the cost of slowing down or speeding-upproducti on to minimize them, (Kolahan and Li ang 1998)

use a Tabu search cri ter ia to compute the least cost sched-ule. As the earlier references show, several different

schedul ing cri teria can be used to develop the schedul e.Kim and Bobrowski, 1994 review several performancemeasures currently used for sequence-dependent sched-

uling.The abil it y t o rapidly change the tooli ng between t he

jobs can also have a signif icant impact on t he ti me andcosts associated wi th changing tooli ng. To provide the

flexibil it y and to produce the vari ety required by the cus-tomer in t he same amount of production t ime, the conceptsof Shigeo Shingos Single Minute Exchange of Dies

(SMED) need t o be implemented (Shingo, 1985).

One of the principles of SMED is to external ize all pos-sible act ivi t ies of t he die change process (Szatkowski andReasor, 1991). Acti vi t ies that do not need to be completed

duri ng the die change whi le the machine is not in pro-ducti on should be completed before the die change starts.

A few examples of such act ivi t ies are assembl ing the nec-

essary tools, gauging, preheating the dies, and performingfunct ional checks (Shingo, 1985). As more act ivi t ies arecompleted before the start of the die change, the timethat is requi red to complete the in ternal acti vit ies wi ll

reduce. This will lead to a reduction of the total timeduri ng which t he machine wil l be out of pr oducti on.

Activities of the Die Change

The ent ire die change procedur e on a typi cal 800-ton diecast machine was documented in a step-by-step l isting ofthe act ivi t ies requi red to successful ly complete the die

change (Refer Appendix Table AI, AI I , AII I ). As a vari a-t ion in the list was encount ered, a branch was inser ted

to indi cate differences in t he die change procedur e. Thesebranches contained all the different possibi li ti es that are

available during a die change based upon the att r ibut esof the die and the part .

Using the procedure described, the amount of t ime need-ed for each individual act ivi ty was assigned. The l ist wasused as the database for t he estimat ion of the times based

on the die and part att ri butes. This list was then catego-r ized in to separat e groups of activi t ies based upon the

item being work ed on or t he ti me dependent sequence ofthe act ivi ty. The act iv i t ies and the assigned ti mes were

reviewed by pract it ioners for accur acy and completeness.Each activi ty was also assigned an i ntensity value to

determine the relati ve amount of effort needed to com-plete the task. The intensit ies were assigned based uponthe amount of effort requi red to complete the task fr om

tur ning a toggle swit ch t o lift ing over 25 pounds. Ident i-fying the in tensit ies of each activi ty had the benefit of

helping a company to target the strenuous tasks thathad t o be completed dur ing a die change and reducingthe in tensit y rating or eli mination of t hese tasks to help

prevent or reduce the li kelihood of i nju ry. The intensit yof each acti vi ty was assigned a value based upon t he rel-

at ive scale in Table 1.

By examining the amount of t ime and int ensiti es re-quir ed for each act ivi ty i n the die change l isted in Table AIV, the act ivi t ies wi th the highest values in each category

could be identi fied to receive the in it ial improvements.

Coding System Used To Record Variationsin Die Changes

Thr ough the understanding of the variati ons that werepresent in the die change procedure as seen in t he branches


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inser ted in the die change procedur e, the root cause foreach var iat ion was ident if ied. With the root cause ident i-

fi ed, a sequence of t he code could be assigned to thi s rootcause to descr ibe the att r ibute. An example would be thepart volume that un iquely ident if ies the size of t ip andshot sleeve and the amount of cooling lines needed for theejector die. A case study i nvolving the dies and t he part sthat were cast on a 800-ton die cast machine was under-taken. A list of att r ibut es was created and the coding t ablein Table 2 was used in the classification of the parts basedupon t he unique att ri butes.

Each of the dies used on the 800 ton machine wereclassified using Table 2 based upon each di es indi vidualatt r ibut es. The result ing codes are shown in Table 3.

Tabl e 1. Int ensit y of Di e Change Acti vi ti es

The variations in t he part s and the dies could be divi d-ed into independent attributes that described thedifferences in the die change procedure needed to accom-modate the vari ati ons. Thr ough the pr incipl es of GroupTechnology, a numeri c coding scheme was created t hatdescr ibed the att r ibutes of the die and the cast ing. Thesedigi ts in the coding scheme could be used t o uniquely

identi fy the vari ati ons in t he amount of setup ti me re-quir ed for a die change. Wit h th is informati on from thecode, the die change ti me could be estimat ed.

ModelThe var iati on between di es was the basis of t hi s modeland the rati onale behind the estimat ion of the time need-ed for a die change. For i nstance, if the die to be takenout had a different sleeve and ti p diameter than the die tobe insert ed, th is would be added to the die change time. Ifthe sleeve and t ip diameters are the same between t hedies, no ti me would be added to the die change time. Theinput of t he variat ions for each die was done thr ough theuse of a fl owchart . As the flowchar t in Fi gure 1 is com-pleted, the values of each var iable are obtained and arethen inser ted in the mathemat ical model .

Figure 1. Coding fl owchart

Table 2. Classif icati on and Coding Scheme

Tabl e 3. Codi ng Scheme for Cl assi fi cation of Di es


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Aft er the values for the model were obtained, they were

entered into the spreadsheet containi ng the equation andthe time database for the acti vit ies. The spreadsheet wouldnow retur n t he estimated die change time. The model

used the values obtained fr om the flowchar t i n Figure 1to estimate t he die change t ime. The ser ies of equati ons

used in the model are li sted in Table 4.

Tabl e 4. Die Change Model Equat ions

I t is evident that the coding scheme employed in t hismodel only takes in to account the var iati ons between the

dif ferent dies and parts. Many of the acti vi ti es performeddur ing the die change were stati c wi th respect to the dif-ferent att ri butes. These fixed times were added by the

model t o output t he complete die change t ime requir ed.

Results

This model comput ed the variable times for the die changeprocedur e. Since the fixed t ime was a constant , it was

not included in the result s for the die change ti mes dur -ing the following analysis.

The die change times required were calculated for al lof t he combinat ions of die changes for t he dies listed in

Table 3. In F igur e 2, the P8 was the die wit h the highestvari able amount of die change t ime. Thi s was due to thefour slide cores on t he ejector die and t he lar ge bot tom

slide core. Al so, the shot sleeve and ti p diameter were inthe 100mm category which r equi red these to be changed

when any of t he P1, P2, P3, P4, P5, P6, or P11 dies wereto be inser ted.

Fi gure 2. Average var iable time for d ie changes

The average t ime requi red for each activi ty is shown

in Table 3. These times are not t he average amount ofti me requi red to complete the tasks, but are the averageamount of t ime spent on t he acti vit ies for all the possible

combinat ions of die changes.The time required to handle the die was the largest

contr ibut or to the variable ti me requir ed in the averagedie change. This acti vit y involved the li fti ng of t he die

in to the machine, the ali gnment of t he dies on t he plat-ens, the removal of t he dies fr om t he machine, and t he

placement of the dies on a pall et or the speciali zed carr ierfor a large bot tom sl ide core.

As shown in Fi gure 3, the handli ng acti vit y resul ts in

35% of the total vari able time in a die change. The chang-ing of the tip and sleeve also cont r ibut ed signi fi cant ly t o

the total t ime requir ed wit h 18% of the vari able time.


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Figure 3. Pareto Analysis of variable time for all die change

combinations

DiscussionThe handling time of t he die and the time requir ed to

complete the ti p and sleeve change represented over one-half of t he variable ti me requi red in a die change. The

amount of t ime requir ed to complete these two act ivi ti esindicates the areas for improvement. I mpr ovement s in

these areas have the potent ial to dramaticall y reduce thet ime requir ed for the average die change.

One of t he most impor tant it ems to be produced in t he

generati on of the act ivi ty li st is the amount of t ime re-quir ed for each act ivi ty. Obtaini ng the indivi dual t imes

for each acti vi ty emphasizes the import ance of each t askas a discrete acti vi ty. By creat ing deli neati on, the acti v-

it ies can be weighed only upon i ts mer it s to the procedureas a whole. The amount of t ime requir ed and the repeti -t iveness of t asks, any task done more than once, can be

examined to reduce the total number of steps requi redand the amount of t ime these steps actual ly require to be

completed.A surpr ise in the complet ion of the die change act iv i -

t ies was the presence of 50 var iat ions in the die changeprocedur e. These vari ati ons account for 51% of all the

Table 5. Average Ti me for Var iable Acti vit ies

die change acti vit ies. Many of the att ri butes used in t his

model ar e present on al l of t he dies. Thi s conceals thet ime spent on these vari ati ons and the impacts they haveto the total ti me requir ed. By reveali ng these act ivi t ies,

the engineers now have a bet ter understanding of whi chact iv i t ies to focus their effort s on t o improve the proce-

dure.

The amount of t ime requir ed to complete the t ip andsleeve change helps to reinforce the impor tance of min i-mizing the number of these changes requi red. Withproper schedul ing, the ti p and sleeve changes can be kept

at a min imum. The amount of ti me the model added tothe die change time for thi s change was 21.25 minut es.

Each t ime a die change is done that requires the tip andsleeve to change, thi s ti me is l ost.

General Recommendations

The reducti on of the handling time in th is study could

have been accompl ished by changing the means of t rans-porting the die into the machine. Many die casters

cur rent ly use an overhead bridge crane for t his pur pose.The dies must be li ft ed approximately 10 feet above the

shop floor to clear al l of t he per ipheral devices of t he ma-chine. The speed of the li ft ing is slow due to the weight of

the dies and the care that must be taken to ensure thesafety of t he operators and the machine.

Another l imi ti ng factor in the handli ng acti vit y is the

amount of t ime needed t o properl y l ocate the dies on t heplatens. Typicall y, the dies are located on the platens by

means of visual and tactile alignment using the shotsleeve and the hole in the cover die for the shot sleeve.

Changing the means of locating to a hard stop on theplat ens would drasticall y reduce the amount of ti me re-

quired to achieve proper locati on.One shor t-t erm drawback to the fixed locator i s the

need for standar dizati on of the distance between the cen-

ter of the shot sleeve and the bot tom of the die. Wi thproper planning of t he new die purchases, thi s system

could be incrementall y implemented as the standardi zeddies are put in to production. By using the distance to thecent er of the shot sleeve, the sleeve diameter woul d not

be a concern as to the locat ion posit ion.I n addit ion to the fixed locators on t he platens, the

amount of t ime requir ed t o bri ng the dies between t heplat ens can be reduced. Current ly the cover die is placed

on t he stationary platen, located, and clamped to the plat-en. The ejector die is then l if ted int o positi on using theoverhead br idge crane. The ejector di e is ali gned wi th

the cover di e thr ough t he use of gui de pins on the coverdie.

Thi s process can be simpl i fied by using a method de-scr ibed by Shingeo Shingo (Shingo, 1985) In t he book he

demonst rates an example using r oll ers t o place the diesin the proper l ocati on. The roll ers could be placed on thebot tom ti e bars of the machine and the dies placed upon


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them using t he crane. The operat or would then roll the

dies int o place wit h assistance fr om t he fixed locators.The platens could then be closed and t he dies clamped tothe platens. An addit ional benefit fr om th is method is

the simul taneous inserti on of both dies. This would eli m-inate a step in t he die change procedur e by having the

dies pre-ali gned wit h r espect to each other.

The use of the rollers and the fixed locators would elim-inate the time-int ensive task of individuall y li fti ng thedies off of t he floor and into the machine. Also, the repeat-abil i ty of the process would be increased wi th t he fixed

locators. The safety benefi ts of t hi s change are anotherconsiderati on because the dies would no longer need to be

l i ft ed above the operator, to be placed in t he machi ne.

Conclusions

The die change model is a versat il e tool that can be ap-plied to a vari ety of industr ies wi th success because many

manufactur ing processes are simi lar t o die cast ing dur-ing the change over of t ooli ng. Forging dies requi re that

the dies be removed fr om t he forging pr ess in a sim i larsequence as in die casting. The amount of accessories

att ached to the die may be fewer, but t he basic principlesare the same. The model i s also valid in the estimat ion of

the time needed in the change over of machining fixturesand tool ing. The tool ing and fix tur ing must be removedfrom t he machine and t he new t ool or fi xtu re must be

placed in the machine, located, and fixed int o place.By using the framework of the proposed model, the user

can modify t he information t o conform to the process thatwi l l be est imated. The key to a successful modifi cati on

wi l l be the coding of t he new pr ocess to ensure that theroot causes of t he variati ons are captu red in the coding

scheme. I f t he root causes of the variati ons are not iden-t i f ied, the t ime model generated wi l l havein terdependencies of t he vari ati ons that wi ll not all ow

the model to be as accurat e as possible.The t ime database wi l l al so need to be modif ied to re-

fl ect the sequence and t imes requi red t o complete theindividual t asks. Dur ing the creation of the models andthe times, the in tensit ies of the act iv it ies should also be

establi shed. The int ensity rat ings of t he acti vit ies wi llhighli ght t he acti vit ies that requir e further modifi cation

to eli minate or r educe the intensit y of the acti vi ti es.The die change model generated is onl y vali d for t he

specific class of machine and for the given die changeprocedure. Thi s li st i s only valid for t he machine and thedies for whi ch it was created. The modi fi cati on of the

flowchart and the ti mes for the acti vit ies wi ll need to bereassessed for a new process. Other machines may be

simi lar, but wi ll need to be modifi ed to reflect the chang-es.

Future Work

One item missing from this work is the listing of thenecessary materials needed to complete the die change

wit hout leaving the work area. This will be beneficial t othe operators to obtain the materi als whi le the machine

is sti ll in producti on t o reduce the amount of t ime themachine is not in pr oduction due to the die change. Some

of t he it ems that could be included in the list ar e:

# and length of hydraul ic hoses

# and l ength of cool ing hoses

# and length of limi t swit ch wi res

# and length of fixed spray l ines

size of t ip and sleeve (if appl icable)

Another useful addit ion to this esti mation model would

be an inter face wit h a CAD system t o complete the classi-ficati on code dir ectl y fr om t he part drawing. This would

all ow t he design engineers to experi ment wi th di fferentdie design to help minimize the die change t ime.

References

1.Charles-Owaba, O.E. and Lambert, B.K., 1998, Sequence

Dependent Machine Set-ups Time and Si mi lar it y of Parts: A

Mathematical Model, IIE Transactions. Vol. 20, No. 1, p.

12-20.

2.Kolahan, F. and Liang M., 1998, An adaptive approach to

JIT sequencing wi th var iable processing t imes and sequence-

dependent setups. European Journal of Operational

Research. Vol. 109, p. 142-159.

3.Ki m, S.C. and Bobrowski , P.M., 1994, Sequence-dependent

setup t im e and j ob shop schedul ing. Int ernati onal Jour nal

of Producti on Research, Vol. 32, N o. 7, p.1503-1520.

4.Lewis, Ronal d L . and Chou, Yeou-Li , 1987, Group Technolo-

gy Codi ng System for Di e Casting. Transacti ons of the 14th

Society of Die Casting Engi neers Di e Casting Expositi on and

Congress. Paper # G-T87-049

5.Missbauer, H. 1997, Order release and sequence-dependent

setup ti mes. I nternati onal J ournal of Production Econom-

ics, Vol. 49, p. 131-143.

6.Parthasarathy, Sri nivasaraghavan and Rajendr an, Chan-

drasekhar an, 1997, An exper imental evaluation of heur isti cs

for t he schedul ing of a real-l i fe fl owshop wi th sequence-de-

pendent setup times of jobs. International Journal ofProducti on Economi cs, Vol. 49, p. 255-263.

7.Shingo, Shi geo, 1985, A Revolut ion i n M anufactur ing: The

SME D System, Producti vi ty Press, Norwalk Connecti cut,

1985.

8.Szatkowski, Paul M. and Reasor, Roderick J., 1991, The

SMED System for Setup Reduction - A Case Study. 1991

I nternat ional I ndustri al Engineer ing Conference Proceed-

ings, p 123-129.


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Appendix

Table AI

Table AI I


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Table AI II


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Table IV

Documents

Paper - Scheduling of Die Casting Dies