Report of visit to Hydro Aluminium Rolled Products in Holmestrand, Norway

Dates of visit: 15th and 16th December 2004 Visit to: Hydro Aluminium Rolled Products, N-3081 Holmestrand, Norway Visitors involved: Tony Greenfield, Greenfield Research, Conway House, 9 Casterbridge Lane, Weyhill, Hampshire, SP11 OSY, England. Tel + 44 (0)845 330 2292 E-mail: [email protected] Shirley Coleman and Oystein Evandt, ISRU (Industrial Statistics Research Unit), Stephenson Group, University of Newcastle upon Tyne, Ne1 7RU, England. E-mail: [email protected]. Tel: +44 191 222 7096 E-mail : [email protected] / [email protected]. Tel + 47 930 89 118.

Reason for choosing the company Hydro Aluminium Rolled Products has invested heavily in installing state of the art equipment. The company has many opportunities for SPC and the assessment of measurement uncertainty. The pro-ENBIS visitors felt that it would be very useful to visit the company to explore applications of SPC, DoE and measurement systems analysis in this environment. The pro-ENBIS community would benefit from the visit report. The company were also very enthusiastic about receiving us as visitors.

Prior to the visit the company sent the visitors a one page description of matters the company wanted to discuss during the two days of the visit, an agenda for the two days and a PowerPoint presentation, with a flow chart, showing the connections between the most important stages of the production processes and management processes. During the visit the emphasis was mainly on the production processes. The three documents mentioned above are found in appendix 1.

Basic purpose of the visit: At the time of the visit the company had for a while been discussing how they could benefit from applying statistical methods more than they did. The basic purpose of the visit was to discuss improvement possibilities, based on statistical methods, with involved representatives of the company, and to facilitate this discussion by presenting and discussing the processes in question while they were running.

The company had already started to apply statistical methods, with emphasis on applying retrospective SPC. Real time SPC was not considered equally relevant, since automatic control procedures were introduced in most of the main processes in question, and these procedures seemed to work satisfactorily. Still, retrospective SPC had been found useful in order to obtain better understanding of the processes. In addition, requirements of demonstrating process capabilities during audits according to the technical specification ISO/TS 16949, necessitates use of SPC since capability indices can only be computed if SPC is used. (ISO/TS 16949 replaces the quality standard QS 9000 for vendors to the car industry. ISO/TS 16949 has not yet become a quality standard, but is likely to become so in the future. Apart from the QS 9000 standard document itself, ISO/TS 16949 contains the same documents as QS 9000.

Pro-ENBIS visit report The requirements for process capabilities are found in the PPAP document, PPAP = Production Part Approval Process. ) All in all, DoE (Design of Experiments) seems to have more scope for process improvement in the company than SPC, even if SPC is very important.

Activities conducted: The company was presented to the visitors, and the processes in question for discussion were explained in more detail than the PowerPoint presentation sent to the visitors in advance. Tony Greenfield presented the PRO-ENBIS program, and the teaching material containing a simulation program for a production process for aluminium wheels, that he has developed. The material is well suited for training in DoE as well as in SPC.

The visitors were taken on a tour to the hot mill and the cold mill. At the hot mill, the main challenge is to roll, in an efficient way, an aluminium slab of length about 7 meters, width 120 cm and thickness 33 cm to an approximately 4 mm thick strip, of width about 33 cm, to be wound into a coil. The plate is rolled backwards and forwards many times during the rolling process. Particularly for clad material the uniformity specifications concern thickness across the plate as well as in the longitudinal direction. It is important to avoid the formation of “swallow tails” at the end of the plate during the rolling process, as much as possible, since such tails must be cut off and melted again. In the cold mill area the thickness is reduced further, to a minimum of 0.2 mm on the break down mill, and on the thin strip mill sometimes further to 0.05 mm. At 0.05mm the strip length of a coil is about 45 km and the coil weight approximately 7 tons.

Another part of the process involves welding on a cladding of liner at one end of an aluminium plate and heating it for rolling. The rolling combined with 500 0C temperature makes it bond. There is mutual diffusion across the boundary. One of the main problems is occasional formation of surface blisters. Tony took the representatives of the company through the planning, and analysis of a ‘thought experiment’ related to the quality of clad material.

First the employees were asked to list important factors they thought could influence the formation of blisters. They came up with 10 factors, of which 5 could be controlled. Low and high experimental values for the 5 controllable factors were decided on for the thought experiment. It was deemed that 3 two-factor interactions could be important, and that two of the controllable factors should have quadratic terms in the response function. The experiment with the smallest number of runs of factor combinations, which gives this possibility, consists of 22 runs. (This “smallest experiment” was found automatically by the DoE program WinDex, developed by Tony. To the knowledge of the visitors, no other program for DoE can find such an experimental design automatically. This ability of the program can sometimes be very time saving.) Of the 22 runs in question, 16 of them constituted a half fraction of a 25 = 32 run full factorial design. In addition 6 runs, represented by axial points and centre points, were needed in order to estimate the quadratic terms. (The 5 non controllable factors were considered as concomitant variables.) The 5 employees present at the time were asked to guess the result for each of the 22 runs in terms of a Likert scale from 1 to 5, where 1 represents no problem and 5 represents scrap. The mean values of the guesses were analysed with respect to the factors. The specified

2 response model was fitted and graphs drawn. No factors or interactions were statistically significant, but some factors were close to being significant. In a real experiment, a replication would be recommended. The staff were very taken with this thought experiment and wanted to make use of this methodology in practice.

Another issue: A central ISO/TS 16949 audit issue has to do with the PPAP requirements of sample sizes for computing capability indices. Section 1.2.1 in the PPAP document insists on a minimum of 300 parts, and yet a roll is counted as one part even if it contains 45 km of strip. In section 1.2.2.9.1 of the PPAP document, note 4, the specification for Xbar-R charts requires a minimum of 25 sub groups from a minimum of 100 sampled parts. The auditors and customers sometimes want Cpk values for single rolls. But Cpk is defined by independent measurements on independent parts. Measurement along a single coil will tend to be statistically dependent. The definition of Cpk does not apply to a single coil. In addition, only one sample is usually taken for measuring the thickness of a coil. It would be very expensive to take many thickness measurements along a coil. It could however be done occasionally, but not on a regular basis. Further, usually the measurements are assumed to stem from a normal distribution. This eases the interpretation of capability indices. However, measurements which are constrained to be within specification and which are re-processed if outside specification because of high values, and scrapped if outside the specification because of low values, tend to have a uniform rather than a normal distribution, because the parts in the tails of the distribution are preferentially removed (if too small) and re- or further processed (if too wide), so that they are within specification. Such a tendency will be present for strips of specified width produced by cutting a coil in the longitudinal direction. The theoretical Cp of a uniform distribution with specification limits at the distribution limits is 0.58 (= 12 / 6), so that a requirement of Cpk = 1.33 or Cpk = 1.67 cannot be fulfilled even if all units are within specification. The problem of computing meaningful Cpk values for deliveries of coils must be considered thoroughly.

The problem of getting good, representative measurements from a roll without too much waste was discussed. The visitors recommended the following: To assess the rest of a roll from a sample, test samples taken from multiple points on an abandoned roll can be used. To assess the uniformity of strip width throughout the whole of the roll, dynamic testing of the entire roll during processing can be done, and these measurements can be compared with, and matched with, the measurements made after processing by a digital micrometer.

In general the visitors suggested that the company focuses on problems considered important by their customers, preferably in cooperation with the customer’s technical service personnel. In this connection historic data should be studied first to identify prioritised areas, and then SPC and DoE should be considered. It should be noted that DoE should preferably be applied only to processes that are shown to have, at least, a reasonable degree of statistical stability, which can only be confirmed by application of SPC. This is so since if one experiments with an unstable process, and changes are observed, one can not know whether the changes are due to the experimentation or that the process is unstable.

On the second day there were fewer discussions, and more visits to production areas, than the first day. The paint process was studied in depth. There were very tight

3 specifications on the paint depth. The colour matching and gloss were the features of greatest importance to the customers. Sequential batches are to be produced within a short time frame, because customers want the batches to look the same. The resolution on the colour measurer was rather low, with only 5 possible values in the area of interest. One effect of this is that the standard deviation of the measurements is underestimated. (See chapter one, “Inadequate measurement Units”, in Wheeler and Lyday (1989)). There were also considerable trends in the data, which indicates that more advanced methods than “regular” SPC may be useful, e.g. EWMA charts (EWMA = Exponentially Weighted Moving Averages).

The cast house was also visited. 50% of the raw material is new aluminium and 50% comes from scrap. Their processor was so good that there was no discernible quality difference between aluminium from the different sources. They sample from the blocks of scrap to check its composition and that it has the recorded proportion of aluminium in it. It is difficult to take a random sample from anywhere except the surface.

Attendance list: Jon Dag Evensen, Manager Technical Customer Service Jan Einar Selseth, Quality System Manager Bjørn S. Berg, Technical Customer Service Hans Erik Vatne, Production Manager Rolled Products Tore Kallestad, Technical Manager Robert Grønli , Process Engineer Cold Rolling Knut Nilsen, Process Manager Lacquered Products

Outcomes from visit: The company acknowledges that at all the stages of hot and cold rolling and painting, they have scrap. The visit has confirmed their perception that this problem can be significantly reduced by means of relevant statistical methods. Also, in-process stock could be reduced. They have already reduced from 42 days worth of stock to 30 days, and they think that a sensible amount would be 24 days.

The company is interested in having some DoE training based on the computer program WinDex, which was demonstrated. When it comes to DoE, WinDex has capabilities for additional types of analysis compared to Minitab, Statgraphics and other packages, e.g. finding the design with fewest runs required for a specified model. WinDex also produces analysis to suit a production analysis engineer’s report layout, and does good charts and predictions. The company is also interested in some statistical consultancy and data analysis. They will send some real data for the visitors to analyse. They currently grade the problem in only 3 levels, but will try to grade it in 5 levels in the data to be sent. This data will be studied by means of multiple regression analysis, since the (better) analyses methods which are part of DoE cannot be applied unless the data are collected by means of a properly designed experiment.

An additional outcome is that Oystein is going to visit the company for another day of statistical discussions in near future.

Referenc: Donald J. Wheeler and Richard W. Lyday: Evaluating the Measurement process. SPC Press, Inc., Knoxville, Tennessee

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