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A Study of Taguchi and Design of Experiments Method in Injection Molding Process for Polypropylene Components

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    http://jrp.sagepub.com/  CompositesJournal of Reinforced Plastics and  http://jrp.sagepub.com/content/27/8/819The online version of this article can be found at: DOI: 10.1177/07316844070849882008 2008 27: 819 srcinally published online 31 January Journal of Reinforced Plastics and Composites  Yung-Kuang Yang, Jie-Ren Shie, Hsin-Te Liao, Jeong-Lian Wen and Rong-Tai Yang for Polypropylene ComponentsA Study of Taguchi and Design of Experiments Method in Injection Molding Process  Published by:  http://www.sagepublications.com  can be found at: Journal of Reinforced Plastics and Composites  Additional services and information for http://jrp.sagepub.com/cgi/alerts Email Alerts:  http://jrp.sagepub.com/subscriptions Subscriptions:  http://www.sagepub.com/journalsReprints.nav Reprints:  http://www.sagepub.com/journalsPermissions.nav Permissions: http://jrp.sagepub.com/content/27/8/819.refs.html Citations: at Victoria Uni of Technology on August 26, 2011 jrp.sagepub.comDownloaded from   A Study of Taguchi and Design of ExperimentsMethod in Injection Molding Process forPolypropylene Components Y UNG -K UANG  Y ANG ,* J IE -R EN  S HIE , H SIN -T E  L IAO ,J EONG -L IAN  W EN AND  R ONG -T AI  Y ANG Department of Mechanical EngineeringMing Hsin University of Science and TechnologyHsin Feng, 304, Hsinchu, Taiwan ABSTRACT:  This study analyzes contour distortions, wear mass losses and tensile properties of polypropylene (PP) composite components applied to the interior coffer of automobiles. Thespecimens are prepared under different injection molding conditions by changing meltingtemperatures, injection speeds, and injection pressures via three computer-controlled progressivestrokes. The contour distortions, wear and tensile properties are selected as quality targets.The arrangement of sixteen experiments is based on an orthogonal array table. Both the Taguchimethod and the design of experiments (DOE) method are applied to determine an optimal parametersetting. In addition, a side-by-side comparison of two different approaches is provided. In this study,regression models that link the controlled parameters and the targeted outputs are developed, andthe mathematic models can be utilized to predict the contour distortions, wear and tensile propertiesat various injection molding conditions. KEY WORDS:  contour distortions, wear mass losses, tensile strength, injection molding, Taguchi,DOE, optimization. INTRODUCTION R ECENTLY, BY USING  the polypropylene (PP) with a new catalyst, metallocene,the production efficiency and the properties of fibers have been greatly improved.Besides, with the benefit of low production costs, PP becomes popular in variousapplications such as consumer electronic products, automotive components, the chemicalindustry, and raw materials for packing and sealing, etc.The mechanical properties of PP composites have been studied intensively, mostlythrough experiments. Chien et al. [1] investigated the effects of molding factors on themechanical properties of injection molded foaming PP parts and coinjection moldingPP parts of foaming core material embedded in non-foaming skin material.The controlling parameters were the injection velocity, the melting temperature, themolding temperature and the back-pressure; the quality targets of the parts were theweights and mechanical properties such as the tensile strength, the flexural strength and *Author to whom correspondence should be addressed. E-mail: yky@must.edu.twFigures 2 and 4 appear in color online: http://jrp.sagepub.com Journal of   R EINFORCED  P LASTICS AND  C OMPOSITES ,  Vol. 27, No. 8/2008  819 0731-6844/08/08 0819–16 $10.00/0 DOI: 10.1177/0731684407084988   SAGE Publications 2008Los Angeles, London, New Delhi and Singapore  at Victoria Uni of Technology on August 26, 2011 jrp.sagepub.comDownloaded from   the stiffness. Ismail and Suryadiansyah [2] examined the degree of degradations on thetensile and morphological properties of the PP/natural rubber (NR) and the PP/recycledrubber (RR) blended at different rubber contents. With similar rubber contents anddegradation conditions, PP/RR blends exhibited a higher percentage of retention of thetensile strength and the Young’s modulus but a lower elongation at break than the PP/NRblends. Sain et al. [3] studied the mechanical properties of the tensile, the flexural andthe un-notched impact strength of the PP composites with a variety of natural fiberssuch as old newsprint, kraft pulp, hemp, and glass fibers. Yao et al. [4] consideredlarge deformation behavior of two commercial biaxially oriented polypropylene (BOPP)resins with different processing properties under high temperatures and high strainrate conditions by means of a Meissner rheometer. Modesti et al. [5] inspected theinfluence of processing conditions on the nano-composites structure, i.e., intercalated orexfoliated, and on the enhancement of mechanical properties of the PP nano-composites.In order to optimize the process parameters, both the screw speed and the barreltemperature profile were varied. In addition, Yang et al. [6] applied the design of experiments (DOE) method on the optimization of the injection molding process for thecontour distortions of PP components. The developed regression models can be utilizedto predict the contour distortions at each location under different injection moldingconditions.This study applied Taguchi’s L 16  (2 4 ) orthogonal table to plan the experiments.Nine controlling factors with two levels for each factor were selected. The contourdistortions, wear mass losses and the tensile strengths are the three selected qualityobjectives. Typically, lower contour distortions, lower wear mass losses, and higher tensilestrengths in the PP components are desirable for products by the injection moldingprocess. For each experiment, the Taguchi method is used to calculate a normalized  S/ Nratio, which represents the quality characteristic of the considered processes. Choosing thelargest possible  S/N   ratio then identifies an optimal process parameter level/factorcombination. For side-by-side comparison purposes, regression models that relate thedesired outputs and the significant factors are established through the DOE approach;furthermore, an optimized parameter setting is then determined from those regressionequations. TAGUCHI EXPERIMENTS AND DESIGN OF EXPERIMENTS METHODTaguchi Method The Taguchi method is a novel approach for improving quality, which applies severalstatistical concepts to quality engineering. The factorial experimental design developedby Taguchi concentrates on reducing variation in the presence of noises. Theseexperiments are established using orthogonal arrays. The Taguchi method combines theeffects of each noise factor and calculates a signal-to-noise ratio ( S/N   ratio) for eachexperiment. The Taguchi method has been used successfully in several industrialapplications, such as manufacturing processes, mechanical component designs, andprocess optimizations [7–11].Depending on the objective, there are three different varieties for the Taguchi method asdefine the signal–noise ( S/N  ) ratios, which are the nominal-the-better, larger-the-better,and smaller-the-better. The utilization of a mean square deviation can be considered,calculating the average performance characteristic values for each experiment.820  Y.-K. Y ANG ET AL .  at Victoria Uni of Technology on August 26, 2011 jrp.sagepub.comDownloaded from   Three different  S/N   ratios, corresponding to n experiments, are presented as follows.Nominal-the-better: S N  ratio ¼ 10log 1 n X ni  ¼ 1 ð  y i   m Þ 2 # ¼ 10log  ð   y  m Þ 2 þ S  2  :  ð 1 Þ Larger-the-better: S N  ratio ¼ 10log 1 n X ni  ¼ 1 1  y 2 i  ! :  ð 2 Þ Smaller-the-better: S N  ratio ¼ 10log 1 n X ni  ¼ 1  y 2 i  ! ¼ 10log    y 2   ð 3 Þ ‘‘where S at the right-hand-side of the equation (1), denotes the standard deviation’’;  y i   theexperimental data;  n  the number of performed experiments;  m  is a targeted value. Design of Experiments (DOE) Method The DOE method has been widely applied to various fields: Chao and Hwang [12]proposed an improved Taguchi method for milling CFRP composite; Puertas and Luis[13] optimized the machining parameters for electrical discharge machining of boroncarbide via the DOE method; Lin and Chananda [14] improved the injection moldingquality by four-factor full-factorial design; Yang [15] implemented the DOE to determinethe optimal parameters of photo resist coating process for photolithography in wafermanufacturing; and so on.By the DOE approach, the objective is to identify an optimal setting that minimizes themeasured average distortion and the mass loss under wear conditions as well as maximizesthe tensile strength for the PP produced components. To resolve this type of multi-outputparameter design problem, an objective function,  F  ( x ), is defined as follows [16]: DF  ¼ Y ni  ¼ 1 d  w i  i  ! 1 P n j  ¼ 1  w i  F  ð x Þ¼ DF  ð 4 Þ where the  d  i   is the desirability defined for the  i  th targeted output; the  w i   is the weighting of the  d  i  . For various goals of each targeted output, the desirability,  d  i  , is defined in differentforms. If a goal is to reach a specific value of   T  i  , the desirability  d  i   is: d  i   ¼ 0 if   Y  i   < ¼ Low i  d  i   ¼  Y  i   Low i  T  i   Low i    i  f Low i   <  Y  i   <  T  i  d  i   ¼  Y  i   High i  T  i   High i    if   T  i   <  Y  i   <  High i  d  i   ¼ 0 if   Y  i   > ¼ High i  :  ð 5 Þ Taguchi and Design of Experiments in Injection Molding  821  at Victoria Uni of Technology on August 26, 2011 jrp.sagepub.comDownloaded from 
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