Computers & Software


Getting a grip with data analysis


A tyre and rubber company designed an experiment to solve challenges overcoming a production bottleneck and certifying new tires meet government standards. Eston Martz reports.


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ridgestone Corporation, the world’s largest tyre and rubber company, manufactures tyres in locations around the world for use in a wide variety of applications. Among the company’s facilities is its Costa Rica plant, which produces


12,000 tyres every day for 23 markets in Central America and the Caribbean. But the plant faced two challenges: certifying a new tyre by meeting US Department of Transportation regulations, and overcoming a bottleneck that limited the overall production of light truck tires at the plant. Six Sigma black belt Kenneth Quirós Acuña embarked


on a project that, if successful, would address both challenges. He and his project team used Minitab Statistical Software to identify critical factors affecting both production levels and the certification tests, design an experiment to find optimal settings for those factors, and develop a model the company could use to produce and certify more tyres with more speed.


The challenge


When engineers at the plant looked at how capacity might be increased, they found that the chief bottleneck in production was a lack of tread. The plant had two types of tread-making machines: an extruder that makes tread in a single piece, and a ribbon-tread machine that makes tread by extruding many small ribbons. The ribbon-tread machine was being underused. But efforts


to get more use out of the machine faltered when tyres created with it failed to meet the standards needed for certification. Plant managers believed it was too difficult to make


qualified light truck tyres using the ribbon-tread machine, which involved multiple steps and complicated variables such as feed rate and increments width. “It is not easy to reproduce a tread shape using ribbons


because you have to find the correct combination of factors,” Quirós says. “The feed rate, drum velocity, extrusion velocity, and other factors all need to be in balance to produce a good tread.”


Quirós and his team realised that with so many steps and


potential factors in the tyre-making process, he first needed to narrow his focus to create a manageable project. “When we looked at different combinations of factors to better understand why we had issues with the ribbon-tread machine, we realised that about 80 per cent of the problems involved a specific tire certification test.” Tyres are built and cured according to exacting technical


specifications. If a tyre passes preliminary building and curing tests, it proceeds to other tests required by the US Department of Transportation. Tyres that meet these specifications go on


Fig. 1. A project team at Bridgestone’s Costa Rica plant used Minitab Statistical Software to increase capacity and certify new tyres more efficiently.


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to the plunger test, in which a rounded plunger is forced into the centre of the tread of an inflated tyre. Testers measure the energy required to either penetrate the tire, or contact the surface of the tire rim. The Costa Rica plant’s ribbon-tread tyres were not meeting the guidelines. “Some tests have a bit of flexibility,” Quirós explains,


“but a tyre that can’t pass the plunger test can’t be certified, so that’s where we focused our efforts. We set out to create a model that yielded specifications for tires that passed the plunger test without hampering their performance on other tests.”


Solving the problem


First, the team members ranked variables in the tyre-making process according to their importance for certification. After an initial analysis, the team prioritised four factors for further investigation.


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