Cagliari, May, 31, 2003 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

Cagliari, May, 31, 2003 2

Agenda

Introduction and background

Objects of the experiment

Factors, Responses and Design

Statistical analysis and modeling

Final remarks and discussion

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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.

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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.

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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.

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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.

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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

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… 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.

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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

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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

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Factors, Responses and Design (3)

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

Surface treatment of metal component: for solving problems with unprotected metal surfaces (possible trouble)

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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

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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.

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Factors, Responses and Design (6)

Initial design:

251 fractional design

Resolution IV (I = ABCDE)

two replications

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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.

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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.