Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
S.J. van Niekerk Het ontwikkelen van een norm voor de omsteltijd van een productielijn voor bliksoep Masters thesis, Report 2007.TEL.7125, Transport Engineering and Logistics.
Introduction
On customer order basis for entire Europe, the Unilever Sourcing Unit at Oss produces all 100 canned soup products from raw materials for the brands Unox® and Knorr®.
Production is executed during five continuous days of the week according to a schedule. The production process consist of three seperate sub-processes: manufacturing, filling and sterilization. For every product eacht of the sub-processes has a specified process-configuration which depends on the (amount of) raw materials used. In order to prodcue the next product of the schedule, it is necessary to change the configuration of the sub-processes.
Complementary, each sub-process has a dedicated changeover-process consisting of its own machines and operators.
Production of a new product can only commense if all three sub-processes have been changed over correctly. The single standard for the changeover of the entire production process, independant of the current and next process-configuration of the sub-processes, is equal to 45 minutes. Time spent on changeover causes loss of availible production time.
Due to an average exceeding of the stanard changeover time with 55%, the average changeover time in the year 2005 equals 70 minutes. A question of utmost importance is the feasibility of the single standard changeover time of 45 minutes for all the 6.305 different changeovers? If that appears not be the case, then the question remains if substantial evidence can be found in order for a development of a new standard for the changeover time?
Working procedure
For every product the sub-process configurations, consisting of participating machines, their fysical build-up and corresponding process parameters, has been registered. Every change of products then resembles an change of configuration. With the use of a selfmade tool the configurationchange matrix -changeover tasks, for every machine of the three sub-processes and for all the possible change of configurations, are identified and registered.
According to programmed tasktimes and built-in cleaning equipment, the manufacturing machines execute their solely cleaning changeover task. For all product changes is known which machines need to be cleaned, how long it takes to do so and what the mutual relation between the machines is. This enables the calculation, with the use of Microsoft Excel®, of the changeover time for all possible product changes in the form of a matrix. The average time equals 35 minutes with a deviation of the same amount.
Operators manually execute the changeover tasks of the filling proces. Due to the lack of standard operating procedures and correspondng standard task times, the author created these procedures using step 1 (of the possible 3) of the SMED-methodology where a seperation of internal and external working elements takes place. Using these procedures in a Direct Time Study with a video camera, standard times are assigned to the changeover task of the filling process, which are corrected for the circumstances under which they are executed. In order to minimize the error in and increase the responsebility for the changeover task, the way of working is that one operator executes all occuring changeover tasks of one single machine. Using this way of working, the total changeover time of one machine equals the serial sum of its changeover tasktimes. The changeover time of the filling process as a whole now depends on the number of machines that need changeover, the changeover time for each of those machines and the number of availible operators to execute the tasks. To assign operators to machines, a 'First-Come-First-Serve'-tactic is adopted for 2 and 3 availible operators. Doing so, the first availible operators gets the first availible machine assigned. From there, the next first availible operator is assigned to the next availible machine for changeover until all required machines have been processed. The tactic has been implemented in Microsoft Excel®.
The changeover task of the sterilization process purely consists of waiting until the 'Continuous Stork 4' machine has processed a number of product carriers. Every product uses a sterilization programm, which consist of two paramaters: speed and temperature. Duet to specified differences a short, a long or no waiting time at all occurs. For all possible product changes, these differences have been determined and the waiting time is calculated as the ratio of the number of product carriers to process and the speed at which they are processed. It appears that in only 7% (73-familiy: 15%, 99-family: 85%) of all possible product changes a waiting time occurs. How ever, if a waiting time accours then the average is equal to 123 minutes with an deviation of 57 minutes.
The availibility of changeover times of the sub-processes enables the calculation of the changeover time for the entire production process. The deviation in changeover time of the sub-processes causes a deviation in the changeover time of the entire production process as well. The average changeover time when 2 filling operators are present is 75 minutes with a deviation of 117 minutes. When 3 filling operators are present, the average changeover time equals 68 minutes with a deviation of 124 minutes. Changeover within the stadard of 45 minutes is merely possible in 1% if 2 filling operators are present. With 3 availible filling operators this equals 2%.
appears to the case in 92%.
A mutual bottleneck analysis amongst the sub-processes points out that the changeover of the filling process, with 3 availible operators, forms the primay delay for the changeover of the entire productionproces in 93% of all product changes within the 73-familiy. When 2 operators are present, this percentage accumulates to 96%. Within the 99-family, and 3 availible operators, the changeover of the filling process is only in 28% a bottleneck. With 68% the changeover of the sterilization process is primary bottleneck.
The changeover time between all products is sequence dependent and distributed asymmetrical. Due to adjusting the production sequence, an average changeover time of 42 minutes is possible in week 17 of the year 2005. Even when adjusting the production sequence on a weekly basis in 2005, the best average changeover time is 58 minutes. Therby prooving that the standard changeover time of 45 minutes in 2005 was not feasible. Reallocating the gained production time for extending solely the executed runs, increases the annual output by 2,25 million cans. Allowing additional average runs increases the annual output by 1,8 million cans.
Recommendations
By executing additional measurements, the confidence of the measured tasktimes of the changeover tasks can be increased. Complementay it is advisory to primarily sustain the created standard operating procedures for every changeover task within the organization, and to educate the operators in these procedures. For further reduction of the changeover tasktimes, the design oriented step 2 and 3 of the SMED-methodolgy can be executed.
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