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1、DOI 10.1007/s00170-004-2328-8 ORIGINAL ARTICLE Int J Adv Manuf Technol (2006) 28: 6166 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 / Publi

2、shed 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 manuf

3、acture of a grinding tool holder has been accomplished in this study. The optimal surface grinding parameters were determined using Taguchis orthogonal array method for plastic injection molding steel PDS5 on a machining center. The optimal surface grind- ing parameters for the plastic injection mol

4、d 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

5、 roughness Racan be further improved from about 0.343 m to 0.06m 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 free

6、form surface region on the tested part can be improved from about 2.15 m to 0.07m. Keywords Automated surface fi nishing Ball burnishing process Grinding process Surface roughness Taguchis method 1 Introduction Plastics are important engineering materials due to their specifi c characteristics, such

7、 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

8、. E-mail: .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 requi

9、rement 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 g

10、eometric 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 abras

11、ive 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

12、 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

13、 resistance 36. 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 pass

14、es, 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 m7. The depth of penetration ofthe burnished s

15、urface 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% 37. The aim of this study was to develop spherical grinding and ball burnishing surface fi nish processes of a freefor

16、m 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 ce

17、nter. The optimal surface spherical grinding parameters were determined by utilizing a Taguchis orthogonal array method. Four factors and three corresponding levels were then chosen for the Taguchis L18matrix experiment. The optimal mounted spherical grinding parameters for surface grinding were the

18、n 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

19、 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

20、-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 th

21、e 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 5m, which was measured by a CNC coordinate

22、 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 pr

23、ocess 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 Taguchis orthogonal array The effects of several parameters can be determined effi ciently by conducting matrix e

24、xperiments using Taguchis 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- ment

25、al 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

26、 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

27、, 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 ni

28、tion 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

29、 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 equa

30、tion 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 ef

31、fect 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 o

32、n . 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

33、 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

34、 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

35、- pany NC-controller (type 0M) 10. The pre-machined surface roughness was measured, using Hommelwerke T4000 equip- ment, to be about 1.6m. 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 ma

36、chining 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 interfac

37、e. 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 n

38、umerical 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 surfaceRespons

39、e 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 832130.270.250.28

40、11.4710.267 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

41、 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

42、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 e

43、ffects 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

44、 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 factors degree of freedom is 2 and the degree of freedom for the pooled error is 13, according to

45、 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 c

46、arried 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

47、of spherical grinding parameters FactorLevel AbrasiveAl2O3, PA Feed50 mm/min Depth of grinding20m 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 Error

48、939.043 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

49、.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 b

50、urnishing. 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 Taguchis matr

51、ix 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

52、ne milling, the mold insert was further ground with the optimal spherical grinding parameters obtained from the Taguchis 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

53、 (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

54、, 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 o

55、bject on ground surface was about (2.150.45)/2.15 = 79.1%, and that on the burnished surface was about (2.150.07)/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

56、 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 sph

57、erical 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 20m, 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 surface grinding and