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DOI 10 1007 s00170 004 2328 8 ORIGINAL ARTICLE Int J Adv Manuf Technol 2006 28 61 66 Fang Jung Shiou Chao Chang A Chen Wen Tu Li Automated surfacefi nishing of plastic injection mold steel with spherical grinding and ball burnishingprocesses Received 30 March 2004 Accepted 5 July 2004 Published online 30 March 2005 Springer Verlag London Limited 2005 Abstract This study investigates the possibilities of automated spherical grinding and ball burnishing surface fi nishing pro cesses in a freeform surface plastic injection mold steel PDS5 on a CNC machining center The design and manufacture of a grinding tool holder has been accomplished in this study The optimal surface grinding parameters were determined using Taguchi s orthogonal array method for plastic injection molding steel PDS5 on a machining center The optimal surface grind ing parameters for the plastic injection mold steel PDS5 were the combination of an abrasive material of PA Al2O3 a grind ing speed of 18000 rpm a grinding depth of 20 m and a feed of 50 mm min The surface roughness Raof the specimen can be improved from about 1 60 m to 0 35 m by using the optimal parameters for surface grinding Surface roughness Racan be further improved from about 0 343 m to 0 06 m by using the ball burnishing process with the optimal burnishing parameters Applying the optimal surface grinding and burnishing parame ters sequentially to a fi ne milled freeform surface mold insert the surface roughness Raof freeform surface region on the tested part can be improved from about 2 15 m to 0 07 m Keywords Automated surface fi nishing Ball burnishing process Grinding process Surface roughness Taguchi s method 1 Introduction Plastics are important engineering materials due to their specifi c characteristics such as corrosion resistance resistance to chemi cals low density and ease of manufacture and have increasingly F J Shiou u C C A Chen W T Li Department of Mechanical Engineering National Taiwan University of Science and Technology No 43 Section 4 Keelung Road 106 Taipei Taiwan R O C E mail shiou mail ntust edu tw Tel 88 62 2737 6543 Fax 88 62 2737 6460 replaced metallic components in industrial applications Injec tion molding is one of the important forming processes for plas tic products The surface fi nish quality of the plastic injection mold is an essential requirement due to its direct effects on the appearance of the plastic product Finishing processes such as grinding polishing and lapping are commonly used to improve the surface fi nish The mounted grinding tools wheels have been widely used in conventional mold and die fi nishing industries The geometric model of mounted grinding tools for automated surface fi nish ing processes was introduced in 1 A fi nishing process model of spherical grinding tools for automated surface fi nishing sys tems was developed in 2 Grinding speed depth of cut feed rate and wheel properties such as abrasive material and abrasive grain size are the dominant parameters for the spherical grind ing process as shown in Fig 1 The optimal spherical grinding parameters for the injection mold steel have not yet been investi gated based in the literature In recent years some research has been carried out in de termining the optimal parameters of the ball burnishing pro cess Fig 2 For instance it has been found that plastic de formation on the workpiece surface can be reduced by using a tungsten carbide ball or a roller thus improving the surface roughness surface hardness and fatigue resistance 3 6 The burnishing process is accomplished by machining centers 3 4 and lathes 5 6 The main burnishing parameters having signifi cant effects on the surface roughness are ball or roller material burnishing force feed rate burnishing speed lubrication and number of burnishing passes among others 3 The optimal sur face burnishing parameters for the plastic injection mold steel PDS5 were a combination of grease lubricant the tungsten car bide ball a burnishing speed of 200 mm min a burnishing force of 300 N and a feedof 40 m 7 The depth of penetration ofthe burnished surface using the optimal ball burnishing parameters was about 2 5 microns The improvement of the surface rough ness through burnishing process generally ranged between 40 and 90 3 7 The aim of this study was to develop spherical grinding and ball burnishing surface fi nish processes of a freeform surface 62 plastic injection mold on a machining center The fl owchart of automated surface fi nish using spherical grinding and ball bur nishing processes is shown in Fig 3 We began by designing and manufacturing the spherical grinding tool and its alignment de vice for use on a machining center The optimal surface spherical grinding parameters were determined by utilizing a Taguchi s orthogonal array method Four factors and three corresponding levels were then chosen for the Taguchi s L18matrix experiment The optimal mounted spherical grinding parameters for surface grinding were then applied to the surface fi nish of a freeform surface carrier To improve the surface roughness the ground surface was further burnished using the optimal ball burnishing parameters Fig 1 Schematic diagram of the spherical grinding process Fig 2 Schematic diagram of the ball burnishing process Fig 3 Flowchart of automated surface fi nish using spherical grinding and ball burnishing processes 2 Design of the spherical grinding tool and its alignment device To carry out the possible spherical grinding process of a freeform surface the center of the ball grinder should coincide with the z axis of the machining center The mounted spherical grinding tool and its adjustment device was designed as shown in Fig 4 The electric grinder was mounted in a tool holder with two ad justable pivot screws The center of the grinder ball was well aligned with the help of the conic groove of the alignment com ponents Having aligned the grinder ball two adjustable pivot screws were tightened after which the alignment components could be removed The deviation between the center coordi nates of the ball grinder and that of the shank was about 5 m which was measured by a CNC coordinate measuring machine The force induced by the vibration of the machine bed is ab sorbed by a helical spring The manufactured spherical grind ing tool and ball burnishing tool were mounted as shown in Fig 5 The spindle was locked for both the spherical grinding process and the ball burnishing process by a spindle locking mechanism 63 Fig 4 Schematic illustration of the spherical grinding tool and its adjust ment device 3 Planning of the matrixexperiment 3 1 Confi guration of Taguchi s orthogonal array The effects of several parameters can be determined effi ciently by conducting matrix experiments using Taguchi s orthogonal array 8 To match the aforementioned spherical grinding pa rameters the abrasive material of the grinder ball with the diam eter of 10 mm the feed rate the depth of grinding and the revolution of the electric grinder were selected as the four experi mental factors parameters and designated as factor A to D see Table 1 in this research Three levels settings for each factor were confi gured to cover the range of interest and were identi Fig 5 a Photo of the spherical grinding tool b Photo of the ball burnishing tool Table1 The experimental factors and their levels FactorLevel 123 A Abrasive materialSiCAl2O3 WAAl2O3 PA B Feed mm min 50100200 C Depth of grinding m 205080 D Revolution rpm 120001800024000 fi ed by the digits 1 2 and 3 Three types of abrasive materials namely silicon carbide SiC white aluminum oxide Al2O3 WA and pink aluminum oxide Al2O3 PA were selected and studied Three numerical values of each factor were determined based on the pre study results The L18orthogonal array was se lected to conduct the matrix experiment for four 3 level factors of the spherical grinding process 3 2 Defi nition of the data analysis Engineering design problems can be divided into smaller the better types nominal the best types larger the better types signed target types among others 8 The signal to noise S N ratio is used as the objective function for optimizing a product or process design The surface roughness value of the ground sur face via an adequate combination of grinding parameters should be smaller than that of the original surface Consequently the spherical grinding process is an example of a smaller the better type problem The S N ratio is defi ned by the following equation 8 10log10 meansquarequalitycharacteristic 10log10 1 n n i 1 y2 i 1 where yi observations of the quality characteristic under different noise conditions n number of experiment After the S N ratio from the experimental data of each L18 orthogonal array is calculated the main effect of each factor was determined by using an analysis of variance ANOVA tech nique and an F ratio test 8 The optimization strategy of the 64 smaller the better problem is to maximize as defi ned by Eq 1 Levels that maximize will be selected for the factors that have a signifi cant effect on The optimal conditions for spherical grinding can then be determined 4 Experimentalwork and results The material used in this study was PDS5 tool steel equiva lent to AISI P20 9 which is commonly used for the molds of large plastic injection products in the fi eld of automobile com ponents and domestic appliances The hardness of this material is about HRC33 HS46 9 One specifi c advantage of this ma terial is that after machining the mold can be directly used for further fi nishing processes without heat treatment due to its special pre treatment The specimens were designed and manu factured so that they could be mounted on a dynamometer to measure the reaction force The PDS5specimen was roughly ma chined and then mounted on the dynamometer to carry out the fi ne milling on a three axis machining center made by Yang Iron Company type MV 3A equipped with a FUNUC Com pany NC controller type 0M 10 The pre machined surface roughness was measured using Hommelwerke T4000 equip ment to be about 1 6 m Figure 6 shows the experimental set up of the spherical grinding process A MP10 touch trigger probe made by the Renishaw Company was also integrated with the machining center tool magazine to measure and determine the coordinated origin of the specimen to be ground The NC codes needed for the ball burnishing path were generated by PowerMILL CAM software These codes can be transmitted to the CNC controller of the machining center via RS232 serial interface Table 2 summarizes the measured ground surface roughness value Raand the calculated S N ratio of each L18orthogonal ar ray using Eq 1 after having executed the 18 matrix experiments The average S N ratio for each level of the four factors can be obtained as listed in Table 3 by taking the numerical values pro vided in Table 2 The average S N ratio for each level of the four factors is shown graphically in Fig 7 Fig 6 Experimental set up to determine the op timal spherical grinding parameters Table2 Ground surface roughness of PDS5 specimen Exp Inner arrayMeasured surfaceResponse no control factors roughness value Ra ABCDy1y2y3S N ratioMean m m m dB y m 111110 350 350 359 1190 350 212220 370 360 388 6340 370 313330 410 440 407 5970 417 421230 630 650 643 8760 640 522310 730 770 782 3800 760 623120 450 420 397 5200 420 731320 340 310 329 8010 323 832130 270 250 2811 4710 267 933210 320 320 329 8970 320 1011220 350 390 408 3900 380 1112330 410 500 436 9680 447 1213110 400 390 427 8830 403 1321130 330 340 319 7120 327 1422210 480 500 476 3120 483 1523320 570 610 534 8680 570 1631310 590 550 545 0300 560 1732120 360 360 358 9540 357 1833230 570 530 535 2930 543 Table3 Average S N ratios by factor levels dB FactorABCD Level 18 0997 6559 1106 770 Level 25 7787 4537 0678 028 Level 38 4087 1766 1077 486 Effect2 6300 4793 0031 258 Rank2413 Mean7 428 The goal in the spherical grinding process is to minimize the surface roughness value of the ground specimen by determin ing the optimal level of each factor Since log is a monotone decreasing function we should maximize the S N ratio Conse quently we can determine the optimal level for each factor as being the level that has the highest value of Therefore based 65 Fig 7 Plots of control factor effects on the matrix experiment the optimal abrasive material was pink aluminum oxide the optimal feed was 50 mm min the optimal depth of grinding was 20 m and the optimal revolution was 18000 rpm as shown in Table 4 The main effect of each factor was further determined by using an analysis of variance ANOVA technique and an F ratio test in order to determine their signifi cance see Table 5 The F0 10 2 13 is 2 76 for a level of signifi cance equal to 0 10 or 90 confi dence level the factor s degree of freedom is 2 and the degree of freedom for the pooled error is 13 according to F distribution table 11 An F ratio value greater than 2 76 can be concluded as having a signifi cant effect on surface roughness and is identifi ed by an asterisk As a result the feed and the depth of grinding have a signifi cant effect on surface roughness Five verifi cation experiments were carried out to observe the repeatability of using the optimal combination of grinding pa rameters as shown in Table 6 The obtainable surface roughness value Raof such specimen was measured to be about 0 35 m Surface roughness was improved by about 78 in using the op Table4 Optimal combination of spherical grinding parameters FactorLevel AbrasiveAl2O3 PA Feed50 mm min Depth of grinding20 m Revolution18000 rpm Table5 ANOVA table for S N ratio of surface roughness FactorDegreesSumMeanF ratio of freedomof squaressquares A224 79112 3963 620 B20 6920 346 C228 21814 1094 121 D24 7762 388 Error939 043 Total1797 520 Pooled to error1344 5113 424 F ratio value 2 76 has signifi cant effect on surface roughness Table6 Surface roughness value of the tested specimen after verifi cation experiment Exp no Measured value Ra m Mean y m S N ratio y1y2y3 10 300 310 330 31310 073 20 360 370 360 3638 802 30 360 370 370 3678 714 40 350 370 340 3539 031 50 330 360 350 3479 163 Mean0 3499 163 timal combination of spherical grinding parameters The ground surface was further burnished using the optimal ball burnishing parameters A surface roughness value of Ra 0 06 m was ob tainable after ball burnishing Improvement of the burnished sur face roughness observed with a 30 optical microscope is shown in Fig 8 The improvement of pre machined surfaces roughness was about 95 after the burnishing process The optimal parameters for surface spherical grinding ob tained from the Taguchi s matrix experiments were applied to the surface fi nish of the freeform surface mold insert to evalu ate the surface roughness improvement A perfume bottle was selected as the testedcarrier The CNCmachining of the mold in sert for the tested object was simulated with PowerMILL CAM software After fi ne milling the mold insert was further ground with the optimal spherical grinding parameters obtained from the Taguchi s matrix experiment Shortly afterwards the ground surface was burnished with the optimal ball burnishing parame ters to further improve the surface roughness of the tested object see Fig 9 The surface roughness of the mold insert was meas ured with Hommelwerke T4000 equipment The average surface roughness value Ra on a fi ne milled surface of the mold insert was 2 15 m on average that on the ground surface was 0 45 m Fig 8 Comparison between the pre machined surface ground surface and the burnished surface of the tested specimen observed with a toolmaker microscope 30 66 Fig 9 Fine milled ground and burnished mold insert of a perfume bottle on average and that on burnished surface was 0 07 m on aver age The surface roughness improvement of the tested object on ground surface was about 2 15 0 45 2 15 79 1 and that on the burnished surface was about 2 15 0 07 2 15 96 7 5 Conclusion In this work the optimal parameters of automated spheri cal grinding and ball burnishing surface fi nishing processes in a freeform surface plastic injection mold were developed suc cessfully on a machining center The mounted spherical grinding tool and its alignment components was designed and manu factured The optimal spherical grinding parameters for surface grinding were determined by conducting a Taguchi L18matrix experiments The optimal spherical grinding parameters for the plastic injection mold steel PDS5 were the combination of the abrasive material of pink aluminum oxide Al2O3 PA a feed of 50 mm min a depth of grinding 20 m and a revolution of 18000 rpm The surface roughness Raof the specimen can be improved from about 1 6 m to 0 35 m by using the optimal spherical grinding conditions for surface grinding By applying the optimal