A MULTIFACTOR EXPERIMENT IN AN INDUSTRIAL ENVIROMENT: SOME PECULIAR PROBLEMS

1. A MULTIFACTOR EXPERIMENT IN AN INDUSTRIAL ENVIROMENT: SOME PECULIAR PROBLEMS Ing. Marco Testa – Pianelli&Traversa S.p.A. Prof. Grazia Vicario - Dipartimento di Matematica Prof. Raffaello Levi - Dip. di Sistemi di Produzione ed Economia dell’Azienda Politecnico di Torino

2. Agenda • Introduction and background • Objects of the experiment • Factors, Responses and Design • Statistical analysis and modeling • Final remarks and discussion

3. Introduction and background (1) Production process is under control, product quality meets requirements; yet management feel there’s definitely room for improvement, in terms of production rate, costs, and product performance. O.V.a.T. experiments (One Variable at a Time) were unable to deliver meaningful information, so the proposal of trying a new tack was accepted.

4. Introduction and background (2) Experiments were performed on motor car subassemblies, manufactured as production items and tested to destruction. Some constraints led to an odd plan, that is namely a replicated fractional factorial. Sizable interactions, peculiar failure modes and scatter pattern suggested critical analysis of results.

5. Objects of the experiment (1) • Metal and glass components are bonded by a plastic interface, cured under controlled temperature and pressure in an automated robotic cell. • Proprietary end effectors handle components. • A heating unit and a pneumatic actuator act under robot control in a tight sequence.

6. Objects of the experiment (2) • Storage affects to some extent bond resistance, as evaluated by peel and torsion tests. • Tensile load increases up to failure at constant rate at room temperature. • As a rule failure occurs in glass component, as both metal part and bond exhibit superior resistance.

7. Typical Initial Problems …. • Fear of “new methods”, especially if rather different from traditional ones • Lack of awareness of importance of interactions among concerned people • Managerial resistance to shift from traditional O.V.a.T. strategy to D.o.E. • Blind faith in former beliefs not supported by experimental evidence • Lack of specific know-how – expert procedure had to be explained in detail, step by step

8. … and Steps to Overcome Them • Gather a solid knowledge of process, best with the help of an insider • Track down and collect systematically all relevant information • Stimulate key player’s curiosity and interest about experimental approach Group meetings proved helpful, a blend of technical knowledge and intuition.

9. Factors, Responses and Design (1) Quantitative Factors • Plastic film temperature at assembly • Conditions of fitting plastic film to metal component • Preheating temperature of glass component prior to assembly • Pressure applied to components during assembly • Duration of application of pressure during assembly • Plastic film thickness • Heat treatment parameters • Glass component shape • Drying cycle parameters

10. Factors, Responses and Design (2) Qualitative factors • Surface conditioning of metal component • Shape of metal component • Surface treatment of metal component • Type of plastic film • Presence/absence of enamel on glass • Percentage humidity on stored plastic film

11. Factors considered in the experiment • Plastic film temperature at assembly: • Substantial effects expected, no affect cycle time, • lower limit getting film “sticky”, • upper limit melting temperature • Force applied to components during assembly: upper limit resistance of glass, loading rate constant over all tests • Duration of application of force during assembly: covers bonding phase, for tightly scheduling production • Surface conditioning of metal component: comparison with standard procedure in terms of cost and performance • Surfacetreatment of metal component: for solving problems with unprotected metal surfaces (possible trouble) Factors, Responses and Design (3)

12. Factors, Responses and Design (4) List of responses • Time to failure under constant load at elevated temperature: evaluation in creep tests, either short term (LSL one hour) or long term (LSL three days) • Static tension tests: repeatable thanks to self aligning features, may fail to evaluate ultimate bond resistance, retained for checking purposes only N.B.Resistance in torsion was not considered among responses analysed owing to poor repeatibility, traceble to excessive sensitivity to minor misalignment of test fixture

13. Factors, Responses and Design (5) • Possible initial model • Educated guesses could be obtained about some main effects; • a priori estimates of existence, let alone magnitude, of • interactions were not available, some suspects being however • entertained by experienced foremen. • A simple model was therefore initially considered, catering for • linear combinations of single effects and two factor interactions only.

14. Factors, Responses and Design (6) Initial design: • 25-1 fractional design • Resolution IV (I = ABCDE) • two replications

15. Problems with Communication To exploit results people must grasp their meaning – and if they are to understand facts must be explained appropriately, that is in a crystal clear way. Easier said than done, especially on production floor. All sorts of graphs, charts, diagrams are helpful – if only because most people try hard to understand what they feel you do your best to explain. On the other hand, to expose any audience to statistical treatment beyond their grasp is self defeating, as it seldom fails to originate antagonistic attitudes.

16. Some final Remarks Results were accepted with positive remarks by managers concerned. That such an amount of meaningful information could be inferred from such a small set of data came as a pleasant surprise to plant management, and the reward came as a request to carry on with further investigations.