The paper aims at evaluating the impact of technological parameters of a drilling process on the parameter
of surface roughness Rz (maximum height of roughness profile) of wood-based material (WPC – Wood
Plastic Composite). During drilling with tools 3.0, 5.0 and 7.0 mm in diameter, the spindle speed nc
 and feed
rate f were changed. The evaluation of statistical significance was performed using the ANOVA mathematical
tool (two-way with repetition). Using a simple conversion, it is possible to assess which of the set parameters
affects the final surface quality, which serves as supporting information for the work of an engineer.Web of Science52332932

Harničárová, Marta

Hutyrová, Zuzana

Kušnerová, Milena

Valíček, Jan

Zajac, Jozef

Materiale Plastice

DSpace at VSB Technical University of Ostrava

MATERIALE PLASTICE ♦ 52♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 325Statistical Evaluation of Impact of Technological Factors onSurface Texture of WPC CompositesZUZANA HUTYROVÁ1,2*, MARTA HARNICÁROVÁ1,4, JAN VALÍÈEK3,4, MILENA KUŠNEROVÁ3,4, JOZEF ZAJAC21 Nanotechnology  Centre, VŠB-TUO Czech Republic2 Faculty of Manufacturing Technologies of Technical University of Košice with a seat in  Prešov, Slovakia3,4 Institute of Physics, Faculty of Mining and Geology, VŠB-TOU, Czech Republic; RMTVC, Faculty of Metallurgy and MaterialsEngineering, VŠB-TUO, Czech RepublicThe paper aims at evaluating the impact of technological parameters of a drilling process on the parameterof surface roughness Rz (maximum height of roughness profile) of wood-based material (WPC – WoodPlastic Composite). During drilling with tools 3.0, 5.0 and 7.0 mm in diameter, the spindle speed nc and feedrate f were changed. The evaluation of statistical significance was performed using the ANOVA mathematicaltool (two-way with repetition). Using a simple conversion, it is possible to assess which  of  the set parametersaffects the final surface quality, which serves as supporting information for the work of an engineer.Keywords: statistical evaluation, composite, surface textureWPC materials are formed by combining two substances– discontinuous reinforcement (wood particles, or cellu-lose microfibers) and a continuous binder (plastic matrix),in a certain proportion. The final properties of the productdepend on the ratio of individual phases and the method ofproduction. The components are mixed under the influ-ence of heat, and through the use of an extrusion head of apress obtain their required shape, size. Composites withnatural reinforcements replace traditional wood in flooringindustry, in the field of garden furniture, pallets, construc-tion, but their representation they also found in dynamicautomotive production (door panels, dashboard compo-nents, etc.) [1]. Presence of wood in the plastic matrixincreases the strength (and stiffness), and reduces the costof materials. Unlike the plastic products, they are referredto as “friendly environment”, because it is possible to usein their production besides the original plastic material alsorecycled plastic (from PVC bottles), or the waste fromwood processing industry (wood flour, sawdust). Assump-tions for the use in the future: with the increasing applica-tion of those materials, also secondary processing shouldbe considered. Despite the used manufacturing technolo-gies, there are many reasons for the application of classi-cal machining (number of technological operations canbe reduced to: grinding, drilling and finishing operationswhich currently can be performed on just one CNC ma-chining center) and thus focus attention to emerging ma-chined surfaces, the tools used, their wear, dimensionalaccuracy and set technological parameters of the process.Buehlmann (2011) compares the tool wear after grindingand routing five samples of commercially purchased woodcomposites with a comparative sample (white pine).  Theauthor registers a lower degree of wear of conventionalwood and assumes that the increased wear of tools formachining WPCs was due to their composition (the pig-ments to achieve the desired shade) [2]. The authors [3]describe WPC machined surfaces after cutting, and thetemperature arising in connection with the change of cut-ting depth, speed, width of cut and the feed rate. Theystate that due to the low softening point of wood compos-* email: zuzana.hutyrova@tuke.skites, it is necessary to keep the temperature as low aspossible during the cutting operation. The publication byGuo XL. et al.  (2010) describes the relationship betweenthe speed of cutting when sawing and the quality of ma-chined surfaces of three samples of WPCs with a differentmatrix (PE/PP/PVC + rice hull flour). The results show thatthe spindle speed has significant effects on the surfacequality of the machined samples. The advantage of usinga high cutting speed in sawing WPCs is evident [4]. Thepublication of authors Šomšáková et al. describes the qual-ity of machined surfaces after turning with a monolithictool from HSS when changing the feed rate of machining(for a constant cutting speed and depth of cut). In conclu-sions, there is a formulated consideration related to fol-lowing research in the given field and the recommenda-tion to set a lower feed rate and select tools with a largeradius of curvature (for turning) [5, 6]. From the perspec-tive of an engineer in machining in general, it is necessaryto specify which of the parameters set has effect on themachining, and in contrary, which can be corrected(changed) without its influence to be crucial. In case ofthe experimental assessment mentioned below, a math-ematical model was specified to assess the significanceof the impact of technological parameters during the op-eration of machining (with respect to the quality param-eter of surface – the greatest height of profile inequalities).Experimental partMaterials and methodsA composite material with wooden reinforcement anda HDPE matrix in a ratio 30:70 % (matrix: reinforcement).A machined profile manufactured by extrusion, size 60 x40 x 3600 mm. Mechanical properties of the compositeswere determined in the laboratories of VÚHZ Dobrá. Tensiletesting (table 1) was made in accordance with ISO 6892-1, with a constant load speed of 0.015 mm·s-1 (uncertainty± 0.8 %), three-point bending test (table 2) carried out inaccordance with ISO 178-1, with a constant load velocityof 0.08 mm·s-1 (uncertainty ± 1 %). The samples weretaken from the middle part of the profile, in the direction ofthe axis of extrusion. The images captured in figure 1 wereMATERIALE PLASTICE ♦ 52 ♦ No. 3 ♦ 2015http://www.revmaterialeplastice.ro326taken from the front of the extruded profile prior tomachining, in the dark matrix of the material, narrow woodchips, from tens of micrometers to about 2 mm in length,are visible (particles are completely or partiallysupersaturated with polymer). The material comprisesmicro-cracks localized in the area of a wood-plasticcontact. Isolated cracks (size 20 μm to 4 mm) pass eventrough wood chips in the direction of their longitudinal axis.Particles are unevenly distributed throughout the volumeof the material. Orientation of wood particles follows theflow of the polymer.In the drilling operation, TiN coated HSS drill bits wereused, diameters: from 3.0, 5.0 and 7.0 mm (lengths ofcutting part l3=33 mm, l5=52 mm, l7=69 mm, point anglesεr=118° and inclination angles λ=30°); the cutting of thesemi-finished product to the required length dimensions(40 x 60 x 230 mm) was made using the ERGONOMIC275.230 DG frame saw; holes made by means of the CNCvertical 3-axis machining center Pinnacle VMC 650S. Themachining process was carried out without cutting media(due to the natural composite reinforcement), made holesopen, a drilling cycle with irrigation. During machining, twoparameters were changed – the feed rate f and the speedof spindle nc. For each set cutting conditions, 3 holes weremade (fig. 2 and table 3). To measure the parameters ofsurface roughness, the MITUTOYO SJ-400 device withcontact operation was used with the automaticcompensation of radius and inclination, an evaluationlength lr = 4.0 mm (filter profile λc = 0.8 mm). Prior tomeasurement, the samples were divided along the holes;surface roughness was measured at a distance of 20 mmfrom the beginning of the hole on the right and on the leftsides with repeating 3 times. The values entered in table 5represent arithmetic averages of 3 measurements of theright and the left sides of a marginal and an inner hole.After drilling the holes along the extruded profile, it ispossible to assess the significance of the impact of thechanged technological parameters on the surface textureby applying the ANOVA tool – two factors with repetition (abuilt mathematical model of a spreadsheet that performscalculations based on the deviation between the values ofa particular variable).  Considering the fact that this is anapplication of sole technology for one type of workpiece(when changing the diameters of a drill bit), it can beassumed that in all three cases, the result is the same, andthus the impact of the input factors being changed on thefinal characteristics will be the same.Assesing the level of impact of each variable parameterson the outcome assessed parameter of surface roughnessRz was performed using the ANOVA analytical tool – twofactors with repetition (a built mathematical model of aspreadsheet that performs calculations based on thedeviation between the values of the variable). Theparameter Rz is a dependent variable Y, the feed rate andTable 1VALUES OF MECHANICAL PROPERTIESAFTER THE TENSILE TESTTable 2VALUES OF MECHANICAL PROPERTIESAFTER THE TENSILE TESTFig. 1. An image from the opticalmicroscope Nikon Eclise 80i; a microscopicstructure of the WPC material used in theexperiment (samples taken from the frontof the extruded profile – prior to machining)Table 3CONDITIONS OF CUTTINGPROCESSFig. 2. Marking the holes under the conditions of thecutting process; the marked length of the evaluatedsection l=4.0 mm (cut along the holes)MATERIALE PLASTICE ♦ 52♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 327the spindle speed are factors A, B. The variable Y has itsvariability measured by the sum of squares of deviationsfrom the total average. At the beginning of the analysis, abasic assumption is pronounced and that is – the variabilityof values of the variable Y is affected by the factor A, or bythe factor B (or by their interaction). The factor A has levels:a1, a2, a3, ... aa. The factor B has b levels: b1, b2, b3, ... bb. Ateach desired level, multiple replications can be performed.The basic consideration results from the total of sums ofsquares caused by individual levels, their interactions, orby the influence of residues and errors:(1)where:SSTOT – total sum of squares (Sum of Squares)SSA – sum of squares caused by level A (impact oflevel A)SSB – sum of squares caused by level B (impact oflevel B)SSAB – sum of squares caused by interactions of levelsA and BSSE – sum of squares caused by effect of residues,errors, etc.In the resulting tables (table 6 to table 8) for individualtools, the values are obtained, based on which it is assessedwhether the specified factors (or their interactions)influence the final variable Y or not. The values acquired bymeans of the ANOVA tool can be calculated manually usingthe matrix in table 4 [7].Through the matrix, standard deviations of individuallevels are calculated (SSA – in the table of the ANOVAanalytical tool referred to as Selection, SSB → referred toas Columns, SSAB →  referred to as Interaction). Calculating  and y = 680.4,  thus the total sum ofsquares is as follows:      (2)Calculating   subsequently SST:    (3)Table 4MATRIX OF CALCULATIONTable 5VALUES OF THE PARAMETER RzSUBSTITUED TO THE MATRIX (A TOLLDIAMETER OF 7.0 mm)a=3 number of changed spindle speeds (2000, 4000, 6000 rpm)b=3 number of changed feed rates (100, 200, 300 mm.min-1)m=4 number of measurements under individual specified conditionsN=36       total number of values of the variable Y mentioned  in the matrix of the experiment              (as the product of the previous three: a’ . b . m)MATERIALE PLASTICE ♦ 52 ♦ No. 3 ♦ 2015http://www.revmaterialeplastice.ro328Calculating  and the calculation of the sum of squares caused by the levelA: SSA,  the sum of squares caused by the level B: SSB,  thesum of squares caused by their interaction SSAB, and thesum of squares caused by the influence of  residues SSE:Based on SST, SSTOT, the values SSAB and SSE aredetermined:Calculating the degrees of freedom (in the table ofANOVA analytical tool referred to as Differnece) is asfollows:Calculating the value MS (Mean of Square) as a quotientof the value SS and the corresponding degree of freedomdf. In conclusion, the value F is a calculated as the quotientof the revelant MS and MSTOT:Similarly, the same procedure is performed in calculatingthe values FB and FAB. The calculated values are comparedwith Fkrit (a tabular value for the respective degrees offreedom) and the statistical significance of each variableparameters is assessed. (13) (12) (11) (10) (9) (8) (7) (6) (5) (4)Table 8EVALUATION OF THE EFFECT OF INDIVIDUAL FACTORS– A, B ON THE PARAMETER OF SURFACE ROUGHNESS,3.0 MM DIAMETER TOOL (DIF. = DIFFERENCE)If the following applies:F > Fkrit (14)then the effect of the respective level is statisticallysignificant.Results and discussionsComparing the values F and Fkrit, the significance ofeffects of a relevant technological parameter on the surfaceroughness parameter Rz is assessed. If the relationship(14) does not apply, the corresponding value in the columnF is denoted with the sign x (if so – the sign √  is used).From the acquired results mentioned above, it is clear thatthe signification differs for diameters of individual tools.For the instrument with a diameter of 7.0 mm, only the linedenoted as Selection is statistically significant – and thusonly the spindle speed of the tool affects the nature of thesurface. For the instrument with a diameter of 5.0 mm,only the line denoted as  Columns is considered statisticallysignificant (table 7) – feed rate. The interaction of the factorsA, B and the actual impact of speed have not been proven.In case of the latter mentioned instrument, 3.0 mm indiameter, the two factors, and the interaction betweenthem are not statistically significant.  The presumptionreferred to in the description of the experiment – thesignificance of the effect of technological parametersbeing changed on the surface texture will be identical –was not confirmed. The differences between the resultswere caused by the material itself – its inhomogeneity(diversity along the volume of the extruded profile).ConclusionsThe quality of the resulting surface is given by thematerial properties that are different along the extrudedprofile – as demonstrated by the results of the tests: tensionand triax bending tests. The values of the ultimate tensilestrength differ by less than 10 MPa (differences betweenthe samples listed in table 1). Given that the examinedsamples were taken from the center of the profile alongthe axis of extrusion, it can be assumed that in thepreparation of the samples from the profile edge, thedifferences between the values of mechanical propertiesto be more pronounced. Images taken by opticalmicroscopy (fig.  1) point out to microcracks in the area ofTable 6 EVALUATION OF THE EFFECT OF INDIVIDUAL FACTORS– A, B ON THE PARAMETER OF SURFACEROUGHNESS, 7.0 MM DIAMETER TOOL (DIF. =DIFFERENCE)Table 7EVALUATION OF THE EFFECT OF INDIVIDUAL FACTORS –A, B ON THE PARAMETER OF SURFACE ROUGHNESS, 5.0MM DIAMETER TOOL (DIF. = DIFFERENCE)MATERIALE PLASTICE ♦ 52♦ No. 3 ♦ 2015 http://www.revmaterialeplastice.ro 329contact between the plastic matrix and reinforcement, andthe cracks in wood chips. Given that it is only onetechnology being assessed – drilling, it can be assumedthat in case of three instruments differing from each otheronly by their diameters (while maintaining constant spindlespeeds and feed rates), the impact of input factors on thefinal characteristics should be the same. However, the giventheory is not valid due to the fact that the material is notuniform throughout its cross-section, which caused thedifferences between the individual instruments withrespect to the output parameter Rz being assessed. Theapplied mathematical model pointed out that in the processof assessing the statistical significance, also thehomogeneity of the material plays a role (which issupported by both the tests of mechanical properties andthe images of ground surfaces of the extruded profile).The model helps to fast orientation in assessing thesignificance of the factors for the emerging surface textureof WPC composite materials. The solution in the presentpaper is original and useful in both the research plane andin practice.Acknowledgement: This reserach was done in the framework ofIT4Innovations Centre of Excellence project, reg. No. CZ.1.05./1.1.00/02.0070 supported by the Operational Programme „Research andDevelopment for Innovations” funded by the Structural Funds of theEuropena Union from the state budget of the Czech Republic andframework of the project New creative teams in priorites of scientificreserach, reg. No. CZ.1.07/2.3.00/30.0055, supported by OperationalProgramme Education for Competitiveness and co-financed by theEuropean Social Fund and state budget of the Czech Republic, and bythe project RMTVC no. CZ.1.05./2.1.00/01.0040.References1. MIGNEAULT, S., KOUBAA, A., PERRE, P., J. Wood Chem. Technol.,34, nr. 4, 2014, p. 2412. SALONI, D., BUEHLMANN, U., LEMASTER, R. L. Forest Prod. J., 61,nr. 2, 2011, p. 1493. ZHU, N. F, HE, W. J., LIU, P. Forest Prod. J., 61, nr. 2, 2011, p. 1494. GUO, X. L., LIU, N. H., GAO, W., CAO, P. X., GUO, Y. Advances inFunctional Manufacturing Technologies, 33, 2010, p. 4875. ŠOMŠÁKOVÁ, Z., ZAJAC, J., MICHALIK, P., KASINA, M. Mater. Plast.49, 2012, p. 556. HUTYROVÁ, Z., HARNIÈÁROVÁ, M., ZAJAC, J., VALÍÈEK, J., MIHOK,J. Material Science and Engineering Technology II.: 2nd InternationalConference on Material Science And Engineering Technology, TransTech Publications, Zurich, 2013, p. 1087. GIRDEN, E. R.: ANOVA: Repeated Measures, SEGE Publications,Inc. California, 1992, 77 pManuscript received: 15.01.2015

Statistical evaluation of impact of technological factors on surface texture of WPC composites

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Statistical evaluation of impact of technological factors on surface texture of WPC composites

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