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Optimization of injection parameters for mechanical properties of specimens with weld line of polypropylene using Taguchi method
International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Optimization of injection parameters for mechanical properties of specimens with weld line of polypropylene using Taguchi method☆ Babur Ozcelik Department of Mechanical Engineering, Gebze Institute of Technology, 41400 Gebze-Kocaeli, Turkey
a r t i c l e
i n f o
Available online 10 May 2011 Keywords: Plastic injection molding Weld line Mechanical properties Taguchi method ANOVA Regression analysis
a b s t r a c t This study optimized effect of injection parameters and weld line on the mechanical properties of polypropylene (PP) moldings. The mold with an insert was designed to create weld line in the experimental specimen. Melt temperature, packing pressure and injection pressure were investigated to study their effects on the mechanical strength of specimens with/without weld lines. Taguchi's L9 (3 3) orthogonal array design was employed for the experimental plan. Mechanical properties such as maximum tensile load, extension at break and charpy impact strength (notched) of the specimens were measured. Signal to noise ratio for mechanical properties of PP using Taguchi method was calculated and effect of the injection parameters and weld line on mechanical properties was determined using the analysis of variance (ANOVA). Linear models were also created by using regression analysis. The most important parameter affecting the maximum tensile load and the extension at break (for specimen without/with weld line) was injection pressure and melt temperature, and for charpy impact strength (notched) (without/with weld line) was melt temperature and injection pressure, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The plastic injection molding is one of the most important methods in the plastic sector. In this method, the plastic products have multi-gated molds and inserts can be produced for industry. Weld lines can be occurred in the plastic product when two flow fronts meet due to either multi-gated molds or inserts. The weld line influences the visual and structural of the products. The weld lines decrease the mechanical properties of injection molded products. The compensation of this problem can be reduced through optimization of the injection molding parameters and design conditions. It can be minimized the visibility of weld lines by adjusting processing conditions or the locations of gates. Researches on the mechanical properties of injection molded of weld line and injection molding parameters are realized. Selden investigated the effects of the injection molding process parameters including holding pressure, injection velocity, melt temperature, and mold temperature which could affect weld line, impact strength and flexural strength of injection molded parts (PA 6, PPS, PP with 40% talc, PPO and ABS) [1]. Wu and Liang studied the individual contribution of process parameters on the weld line strength of polypropylene (PP) and high density polyethylene (HDPE), using Taguchi's orthogonal arrays under different conditions of injection molding such as mold temperature, packing pressure, melt temperature, injection speed, injection acceleration, and packing time. In the ☆ Communicated by W.J. Minkowycz. E-mail address: [email protected]. 0735-1933/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2011.04.025
study, they studied the influence of cross-sectional dimensions on the weld line strength [2]. Li et al. analyzed effect of process parameters namely, melt temperatures, injection speed and injection pressure to determine the influence of weld lines on appearance of PP products using Taguchi experimental design method [3]. Yamada et al. investigated influence of flow behavior at the weld line on the mechanical properties (tensile strength) by applying the surface milling technique for general purpose polystyrene [4]. Chen et al. examined effects of cavity surface coating, melt temperature and mold temperature on weld line strength and the quality of ABS material [5]. Chen et al. investigated the influence of processing conditions on the weld line strength of thin-wall ABS parts for singlegate molded specimens (without weld line) and double-gate molded specimens (with weld line)[6]. In this study, the effect of injection parameters and specimens both with weld line and without weld line on the mechanical properties of PP material was optimized. Taguchi's L9 (33) orthogonal array design was used for experimental plan. Signal to noise ratio (S/N) for mechanical properties was obtained and optimum levels of the injection parameters was determined through S/N values to achieve maximum mechanical results. Influence of injection parameters on mechanical properties of the specimen was carried out using ANOVA. Linear mechanical models were also obtained from regression analysis. 2. Definition of Taguchi method Taguchi method developed by Taguchi consists of three stages which are system, parameters, and tolerance designs, respectively [7]. The
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 1 The process parameters and levels.
Table 3 Physical and mechanical properties of PP.
Process parameters Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa)
A B C
Level 1
Level 2
Level 3
200 14 22
220 17 25
240 20 28
system design involves the application of scientific and engineering knowledge required in manufacturing of a product. The parameter design is employed to find optimal process values for improving of the quality characteristics. The tolerance design is used for determining and analyzing of the tolerances in optimal settings recommended by the parameter design. By applying Taguchi method based on orthogonal arrays, time and cost required for conducting of the experiments can be reduced. Taguchi recommends the use of the S/N ratio for determination of the quality characteristics implemented in engineering design problems. The S/N ratio characteristics with signed-target type can be divided into three stages: the smaller is the better, the nominal is the best, and the larger is the better [7]. In this study, the larger is better approach was used for maximizing mechanical properties of the product. In addition to the S/N ratio, ANOVA is employed to obtain the effect of the process parameters on mechanical properties. In this way, optimal levels of the process parameters can be estimated.
3. Experimental 3.1. Taguchi's orthogonal arrays The effects of several process parameters based on the Taguchi's orthogonal design could be determined effectively from matrix experiments [7]. The process parameters and levels were shown in Table 1. Tests were organized in using Taguchi's L9 (33) orthogonal array (Table 1). An experimental plan for three parameters with three levels was organized by Taguchi method (Table 2). Melt temperature taken from the PP data sheet was set to 200 °C for level 1 and 240 °C for level 3, and level 2 was set to 220 °C which was an average value of these two levels. Values of packing pressure and injection pressure for level 1 were obtained from data sheet and parameters for levels 2 and 3 were selected as 1.15 and 1.3 times of level 1, respectively. Other injection parameters such as packing time, cooling time and mold temperature are constant and their values are 7 s, 20 s, and 30 °C, respectively. PP (Dow 765-15NA) material was used for this study. The properties of PP compound had a crystal structure were shown in Table 3.
Physical properties Melt flow rate, g/10 min Density, g/cm3
ISO 1133 ISO 1183
230 °C, 2,16 kg
Mechanical properties Tensile strength at yield, MPa Elongation at yield, % Flexural modules, MPa
ISO 527 ISO 527 ISO 178
– – –
Impact properties CHARPY (notched) impact strength, kJ/m2 ISO 179/1ea 23 °C
4. Results and discussion 4.1. Analysis of the S/N ratio The test results were evaluated in terms of signal/noise (S/N) ratio. The S/N was calculated by larger is better for determining effect of
Table 2 An orthogonal array L9 (33) of Taguchi. A
B
C
200 200 200 220 220 220 240 240 240
14 17 20 14 17 20 14 17 20
22 25 28 25 28 22 28 22 25
12
Experimental specimens were prepared by cutting with air jet method according to ISO 527-2 and ISO 179 standards. The specimen for experimental study was injected as 160 × 80 × 4 mm (Fig. 1.). A suitable dimension was cut from Fig. 1a and b to carry out the tensile and impact tests, respectively. The specimen dimensions for the tensile and impact tests were shown in Figs. 2 and 3. The tensile test speeds were applied as 5 mm/min (ISO 527-2). The impact tests were performed with ISO 179 standards. 7.5 J of the impact hammer was used in the impact test and specimen used for the test was shown in Fig. 3. The tensile strength and impact tests were determined from INSTRON 5560 and Zwick 5113.100 (Universal Testing Machines), respectively. The tests were repeated five times and the mean values were presented.
The cavities of the mold were manufactured in the CNC machine. A test part shown in Fig. 1a and b was injected by a plastic injection machine (Mitsubishi 50 MetIII) which had a clamping force of 490 kN and an injection pressure of 275 MPa, respectively.
1 2 3 4 5 6 7 8 9
26 10 1200
3.3. Experimental testing methods and equipment
3.2. Injection molding machine
Experiment no
15 0.9
Fig. 1. Test specimens for a) tensile test, and b) impact test.
B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
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Fig. 2. Specimen dimensions for tensile strength (ISO 527-2).
values of specimen without weld line decreased 7% for maximum tensile load and 30% for extension at break by process parameters as shown in Table 4. When the specimen without and with weld line compared, the mechanical properties of specimen with weld line decreased 10.14% for maximum tensile load, 28.90% for charpy impact strength and 88.04% for extension at break (Table 4). 4.2. ANOVA results
Fig. 3. Specimen dimensions for charpy impact test (ISO 179).
injection parameters on mechanical properties of the product. The formula of S/N ratio was shown in Eq.1 " S = N = −10 log10
1 n 1 ∑ n i = 1 y2i
# ð1Þ
where S is the standard deviation, yi is the measured experimental results and n explain to the number of specimens in each test trial [7,8]. Results from measurements of maximum (M) tensile load, charpy impact strength (notched) and extension at break were presented for mechanical properties of the products both with weld line (W) and without weld line (Table 4) which determined the optimal levels of three process parameters for PP samples. The lowest results of maximum tensile load and charpy impact strength (notched) were realized in the lowest values of melt temperature, packing pressure, and injection pressure. The lowest results of extension at break were obtained during the high injection conditions. The highest mechanical results were also obtained in the highest injection parameters conditions for specimen with/without weld line in general. The rate of maximum and minimum mechanical
The influence of process parameters and weld line on the mechanical properties of PP material was analyzed by ANOVA in Tables 5–10. The percentage contribution of variance was calculated by ANOVA results. Results from the parameters and weld line for maximum tensile load, charpy impact strength (notched) and extension at break were illustrated in Tables 5–10. F ratio was accurately computed corresponding 95% confidence level in calculation of process parameters. P value refers to the significance level. It was seen from Tables 5 and 6 that the most important parameter for maximum tensile load (without/with weld line) was injection pressure and its percentage for the tensile load was obtained by 64.140% for specimen without W. and 52.437% for specimen with W., respectively. The most important parameters for charpy impact strength (notched) (without W. and with W.) were melt temperature and injection pressure and their percentage were obtained for the parameters by 51.568% and 48.985%, respectively as shown in Tables 7 and 8. The most important parameter affecting the extension at break (without W. and with W.) in Tables 9 and 10 was melt temperature by 50.500% and 89.070%. In the literature, the most influential injection parameter on the weld line strength was found the holding pressure [1], and melt temperature (for the tensile strength) [2,9] when PP (Talc) [1], PP [2] and PS [9] materials used, respectively. The average S/N values in Tables 5–10 were showed and the most suitable injection parameters for each mechanical property taking
Table 4 Experimental and S/N ratio results of mechanical properties and weld line. Exp. no
1 2 3 4 5 6 7 8 9 Average Decrease
M. tensile load (without weld line) (N)
M. tensile load (with weld line) (N)
Charpy impact strength (notched) (without W.) (kJ/m2)
Charpy impact strength (notched) (with W.) (kJ/m2)
Extension at break (without W.) (mm)
Extension at break (with W.)(mm)
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
858 893 906 882 910 880 900 874 922 891.67 N 10.14%
586697 59.0170 59.1426 58.7904 59.1808 58.8897 59.0849 58.8302 59.2946
774 802 811 790 810 800 815 794 815 801.22 N
57.7748 58.0835 58.1804 57.9525 58.1697 58.0618 58.2232 57.9964 58.2232
14.14 15.28 15.92 13.02 14.30 13.96 12.34 12.98 14.50 14.05 kJ/m2 28.90%
23.0090 23.6825 24.0389 22.2922 23.1067 22.8977 21.8263 22.2655 23.2274
9.71 10.02 10.30 9.72 10.15 9.90 10.05 9.85 10.24 9.99 kJ/m2
19.7444 20,0174 20.2567 19.7533 20.1293 19.9127 20.0433 19.8687 20.2060
34.49 29.78 22.57 24.54 18.48 24.22 22.12 21.82 17.15 23.91 mm 88.04%
30.7539 29.4785 27.0706 27.7975 25.3340 27.6835 26.8957 26.7771 24.6853
3.19 3.31 3.20 2.97 2.83 2.83 2.30 2.67 2.42 2.86 mm
10.0758 10.3966 10.1030 9.4551 9.0357 9.0357 7.2346 8.5302 7.6763
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 5 Summary of ANOVA results for maximum tensile load (without W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom (DF) 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
58.94 58.89 58.80 – –
58.99 59.01 59.07 – –
59.07 59.11 59.14 – –
Sum of squares
Mean square
F
P (%)
0.024462 0.073470 0.195982 0.011640 0.305553
0.012231 0.036735 0.097991 0.005820 –
2.10 6.31 16.84 – –
8.006 24.045 64.140 3.809 100
Sum of squares
Mean square
F
P (%)
0.027915 0.044565 0.092024 0.010992 0.175496
0.013958 0.022283 0.046012 0.005496 –
2.54 4.05 8.37 – –
15.906 25.394 52.437 6.263 100
Table 6 Summary of ANOVA results for maximum tensile load (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
58.01 57.98 57.94 – –
58.06 58.08 58.09 – –
58.15 58.16 58.19 – –
Table 7 Summary of ANOVA results for charpy impact strength (notched) (without W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
2 2 2 2 8
23.58 22.38 22.72 – –
22.77 23.02 23.07 – –
22.44 23.39 22.99 – –
Sum of squares
Mean square
F
P (%)
2.05714 1.57381 0.19479 0.16340 3.98913
1.02857 0.78690 0.09739 0.08170 –
12.59 9.63 1.19 – –
51.568 39.452 4.883 4.097 100
Sum of squares
Mean square
F
P (%)
0.018204 0.116768 0.136074 0.006735 0.277781
0.009102 0.058384 0.068037 0.003368 –
2.70 17.34 20.20 – –
6.551 42.035 48.985 2.422 100
Table 8 Summary of ANOVA results for charpy impact strength (notched) (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
2 2 2 2 8
20.01 19.85 19.84 – –
19.93 20.01 19.99 – –
20.04 20.13 20.14 – –
Table 9 Summary of ANOVA results for extension at break (without W.). Factor
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
2 2 2 2 8
29.10 28.48 28.40 – –
26.94 27.20 27.32 – –
26.12 26.48 26.43 – –
Sum of squares
Mean square
F
P (%)
14.2380 6.1772 5.8489 1.9298 28.1939
7.1190 3.0886 2.9244 0.9649 –
7.38 3.20 3.03 – –
50.500 21.910 20.745 6.845 100
Table 10 Summary of ANOVA results for extension at break (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
10.192 8.922 9.214 – –
9.176 9.321 9.176 – –
7.814 8.938 8.791 – –
Sum of squares
Mean square
F
P (%)
8.5427 0.3058 0.3284 0.4141 9.5909
4.2714 0.1529 0.1642 0.2070 –
20.63 0.74 0.79 – –
89.070 3.188 3.424 4.318 100
B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072 Table 11 The highest S/N values for mechanical properties. Tables no.
Highest S/N values
Table Table Table Table Table Table
A3 A3 A1 A3 A1 A1
5 6 7 8 9 10
B3 B3 B3 B3 B1 B2
C3 C3 C2 C3 C1 C1
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(level 1), 14 MPa packing pressure (level 1), and 22 MPa injection pressure (level 1) (Table 9). ● The optimal injection molding conditions for the extension at break (specimens with W.) were 200 °C melt temperature (level 1), 17 MPa packing pressure (level 2), and 22 MPa injection pressure (level 1) (Table 10).
4.3. Regression analysis results
into account the highest values of S/N were listed in Table 11. Based on this study, the following conclusions can be drawn for the injection molding conditions: ● The optimal injection molding conditions for the maximum tensile load (specimens with/without W.) were 240 °C melt temperature (level 3), 20 MPa packing pressure (level 3), and 28 MPa injection pressure (level 3) (in Tables 5 and 6). ● The optimal injection molding conditions for the charpy impact strength (specimens without W.) were 200 °C melt temperature (level 1), 20 MPa packing pressure (level 3), and 25 MPa injection pressure (level 2) (Table 7). ● The optimal injection molding conditions for the charpy impact strength (specimens with W.) were 240 °C melt temperature (level 3), 20 MPa packing pressure (level 3), and 28 MPa injection pressure (level 3) (Table 8). ● The optimal injection molding conditions for the extension at break (specimens without W.) were 200 °C melt temperature
Regression analysis was a statistical tool for the investigation of relationships between variables. R – Sq is correlation coefficient and should be between 0.8 and 1 in multiple linear regression analyses [7]. The purpose of R – Sq value is the prediction of future outcomes on the basis of other related data. It provides a measure of how well results are appropriately to be predicted by the model. A linear model between injection molding parameters and mechanical properties were created. The model result was best explained by values of regression coefficient, r 2, close to 1. The model equations were given in Table 12. Models from the parameters and weld line for maximum tensile load, charpy impact strength (notched) and extension at break had linear relationships between the injection parameters and mechanical properties. 4.4. Confirmation and comparison tests Confirmation experiments were conducted for regression equations. The results of confirmation and comparison experiments in
Table 12 Linear models between parameters and mechanical properties. Mechanical properties
The equations obtained from calculated
R-sq (%)
R-sq (Adj) (%)
Maximum tensile load (without weld line) Maximum tensile load (with weld line) Charpy impact strength (without weld line) Charpy impact strength (with weld line) Extension at break (with weld line) Extension at break (with weld line)
612 + 0.325 A + 3.78 B + 5.78 C 595 + 0.308 C1 + 2.61 B + 3.78 C 17.5 – 0.0460 A + 0.271 B + 0.0822 C 7.44 + 0.00092 A + 0.0533 B + 0.0578 C 111 – 0.215 A – 0.956 B – 0.964 C 7.60 – 0.0193 A – 0.0006 B – 0.0200 C
88.1 92.8 90.3 91.4 90.8 92.6
81.0 88.4 84.5 86.2 85.3 88.1
Table 13 Results of confirmation experiments and predicted values for regression equations. Test
Point
For regression equations 1a Max. tensile load (without weld line) (N)
1a 1b 1c
Optimum A3 B3 C3 A3 B3 C3 Random A1 B2 C2 Point
2a 2b 2c
Optimum A1 B3 C2 A3 B3 C3 Random A1 B2 C2 Point
3a 3b 3c
Optimum A1 B1 C1 A1 B2 C1 Random A1 B2 C2
1b Max. tensile load (with weld line) (N)
Exp.
Predict.
Error (%)
Exp.
Calculated
Error (%)
922
927.44
0.57
815
826.96
1.45
893
885.76
0.81
802
795.47
0.81
2a Charpy impact strength (without weld line) (N)
2b Charpy impact strength (with weld line) (N)
Exp. (Aver.)
Predict.
Error (%)
Exp.
Predict.
Error (%)
14.05
15.21
7.63
10.24
10.15
0.88
15.28
14.96
2.09
10.02
9.98
0.40
3a Extension at break (without weld line) (N)
3b Extension at break (with weld line) (N)
Exp.
Predict.
Error (%)
Exp. (aver.)
Predict.
Error (%)
34.49
33.41
3.13
2.86
3.29
13.07
29.78
27.65
7.15
3.31
3.23
2.42
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 13 showed the comparison between the foreseen (Equations in Table 12) and experimental values. The predicted and calculated results had very close values with the experimental results. For reliable statistical analyses, error values must be smaller than 20%. The calculated error was greater especially for charpy impact strength (without W.) and extension at break (with/without W.) but these error values were in an acceptable range. Since comparisons were done according to average experimental values (Aver.), the calculated error values were obtained higher.
The most important parameters for charpy impact strength (notched) (without/with weld line) were melt temperature and injection pressure, respectively. The other important parameters for charpy impact strength of the both specimens were packing pressure. The results of the mechanical tests were high under optimum conditions, in general. All mechanical properties gave linear relationships (based on values of r 2) with injection parameters.
References 5. Conclusions This study focused on Taguchi experimental method for investigating of influence of the injection parameters and weld line on the mechanical properties of PP during plastic injection molding. In the injection experiments, melt temperature, packing pressure and injection pressure values as injection parameters and specimens with/without weld lines were utilized. Multiple regression analysis was performed to indicate the fitness of experimental measurements. Regression models obtained for all mechanical properties measurements matched very well with the experimental data. The level of importance of the injection parameters on the mechanical properties and weld line was determined by ANOVA. The most important parameter affecting the maximum tensile load and the extension at break (without/with weld line) was injection pressure and melt temperature, respectively. The other important parameters affecting the maximum tensile load for specimens with both without weld line and with weld line were packing pressure and melt temperature, respectively. The other parameters affecting the extension at break (without/with weld line) were closed to each other.
[1] R. Selden, Effect of processing on weld Line Strength in five thermoplastics, Polymer Engineering and Science 37 (1) (1997) 205–218. [2] C.-H. Wu, W.-J. Liang, Effects of geometry and Injection Molding Parameters on Weld line Strength, Polymer Engineering and Science (2005) 1021–1030. [3] H. Li, Z. Guo, D. Li, Reducing the effects of weld lines on appearance of plastic products by Taguchi experimental method, International Journal of Advanced Manufacturing Technology 32 (2007) 927–931. [4] K. Yamada, K. Tomari, U.S. Ishiaku, H. Hamada, Evaluation of Mechanical Properties of Adjacent Flow Weld line, Polymer Engineering and Science (2005) 1180–1186. [5] S.C. Chen, Y. Chang, Y.P. Chang, Y.C. Chen, C.Y. Tseng, Effect of cavity surface coating on mold temperature variation and the quality of injection molded parts, International Communications in Heat and Mass Transfer 36 (10) (2009) 1030–1035. [6] C.S. Chen, T.J. Chen, R.D. Chien, S.C. Chen, Investigation on the weld line strength of thin-wall injection molded ABS parts, International Communications in Heat and Mass Transfer 34 (4) (2007) 448–455. [7] G. Taguchi, Introduction to quality engineering, McGraw-Hill, New York, 1990. [8] P.J. Ross, Taguchi Techniques for Quality Engineering, second ed. McGraw-Hill, New York, 1996. [9] S.j. Liu, J.Y. Wu, J.H. Chang, An experimental matrix design to optimize the weld line strength in injection molded parts, Polymer Engineering and Science 40 (5) (2000) 1256–1262.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Optimization of injection parameters for mechanical properties of specimens with weld line of polypropylene using Taguchi method☆ Babur Ozcelik Department of Mechanical Engineering, Gebze Institute of Technology, 41400 Gebze-Kocaeli, Turkey
a r t i c l e
i n f o
Available online 10 May 2011 Keywords: Plastic injection molding Weld line Mechanical properties Taguchi method ANOVA Regression analysis
a b s t r a c t This study optimized effect of injection parameters and weld line on the mechanical properties of polypropylene (PP) moldings. The mold with an insert was designed to create weld line in the experimental specimen. Melt temperature, packing pressure and injection pressure were investigated to study their effects on the mechanical strength of specimens with/without weld lines. Taguchi's L9 (3 3) orthogonal array design was employed for the experimental plan. Mechanical properties such as maximum tensile load, extension at break and charpy impact strength (notched) of the specimens were measured. Signal to noise ratio for mechanical properties of PP using Taguchi method was calculated and effect of the injection parameters and weld line on mechanical properties was determined using the analysis of variance (ANOVA). Linear models were also created by using regression analysis. The most important parameter affecting the maximum tensile load and the extension at break (for specimen without/with weld line) was injection pressure and melt temperature, and for charpy impact strength (notched) (without/with weld line) was melt temperature and injection pressure, respectively. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The plastic injection molding is one of the most important methods in the plastic sector. In this method, the plastic products have multi-gated molds and inserts can be produced for industry. Weld lines can be occurred in the plastic product when two flow fronts meet due to either multi-gated molds or inserts. The weld line influences the visual and structural of the products. The weld lines decrease the mechanical properties of injection molded products. The compensation of this problem can be reduced through optimization of the injection molding parameters and design conditions. It can be minimized the visibility of weld lines by adjusting processing conditions or the locations of gates. Researches on the mechanical properties of injection molded of weld line and injection molding parameters are realized. Selden investigated the effects of the injection molding process parameters including holding pressure, injection velocity, melt temperature, and mold temperature which could affect weld line, impact strength and flexural strength of injection molded parts (PA 6, PPS, PP with 40% talc, PPO and ABS) [1]. Wu and Liang studied the individual contribution of process parameters on the weld line strength of polypropylene (PP) and high density polyethylene (HDPE), using Taguchi's orthogonal arrays under different conditions of injection molding such as mold temperature, packing pressure, melt temperature, injection speed, injection acceleration, and packing time. In the ☆ Communicated by W.J. Minkowycz. E-mail address: [email protected]. 0735-1933/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2011.04.025
study, they studied the influence of cross-sectional dimensions on the weld line strength [2]. Li et al. analyzed effect of process parameters namely, melt temperatures, injection speed and injection pressure to determine the influence of weld lines on appearance of PP products using Taguchi experimental design method [3]. Yamada et al. investigated influence of flow behavior at the weld line on the mechanical properties (tensile strength) by applying the surface milling technique for general purpose polystyrene [4]. Chen et al. examined effects of cavity surface coating, melt temperature and mold temperature on weld line strength and the quality of ABS material [5]. Chen et al. investigated the influence of processing conditions on the weld line strength of thin-wall ABS parts for singlegate molded specimens (without weld line) and double-gate molded specimens (with weld line)[6]. In this study, the effect of injection parameters and specimens both with weld line and without weld line on the mechanical properties of PP material was optimized. Taguchi's L9 (33) orthogonal array design was used for experimental plan. Signal to noise ratio (S/N) for mechanical properties was obtained and optimum levels of the injection parameters was determined through S/N values to achieve maximum mechanical results. Influence of injection parameters on mechanical properties of the specimen was carried out using ANOVA. Linear mechanical models were also obtained from regression analysis. 2. Definition of Taguchi method Taguchi method developed by Taguchi consists of three stages which are system, parameters, and tolerance designs, respectively [7]. The
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 1 The process parameters and levels.
Table 3 Physical and mechanical properties of PP.
Process parameters Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa)
A B C
Level 1
Level 2
Level 3
200 14 22
220 17 25
240 20 28
system design involves the application of scientific and engineering knowledge required in manufacturing of a product. The parameter design is employed to find optimal process values for improving of the quality characteristics. The tolerance design is used for determining and analyzing of the tolerances in optimal settings recommended by the parameter design. By applying Taguchi method based on orthogonal arrays, time and cost required for conducting of the experiments can be reduced. Taguchi recommends the use of the S/N ratio for determination of the quality characteristics implemented in engineering design problems. The S/N ratio characteristics with signed-target type can be divided into three stages: the smaller is the better, the nominal is the best, and the larger is the better [7]. In this study, the larger is better approach was used for maximizing mechanical properties of the product. In addition to the S/N ratio, ANOVA is employed to obtain the effect of the process parameters on mechanical properties. In this way, optimal levels of the process parameters can be estimated.
3. Experimental 3.1. Taguchi's orthogonal arrays The effects of several process parameters based on the Taguchi's orthogonal design could be determined effectively from matrix experiments [7]. The process parameters and levels were shown in Table 1. Tests were organized in using Taguchi's L9 (33) orthogonal array (Table 1). An experimental plan for three parameters with three levels was organized by Taguchi method (Table 2). Melt temperature taken from the PP data sheet was set to 200 °C for level 1 and 240 °C for level 3, and level 2 was set to 220 °C which was an average value of these two levels. Values of packing pressure and injection pressure for level 1 were obtained from data sheet and parameters for levels 2 and 3 were selected as 1.15 and 1.3 times of level 1, respectively. Other injection parameters such as packing time, cooling time and mold temperature are constant and their values are 7 s, 20 s, and 30 °C, respectively. PP (Dow 765-15NA) material was used for this study. The properties of PP compound had a crystal structure were shown in Table 3.
Physical properties Melt flow rate, g/10 min Density, g/cm3
ISO 1133 ISO 1183
230 °C, 2,16 kg
Mechanical properties Tensile strength at yield, MPa Elongation at yield, % Flexural modules, MPa
ISO 527 ISO 527 ISO 178
– – –
Impact properties CHARPY (notched) impact strength, kJ/m2 ISO 179/1ea 23 °C
4. Results and discussion 4.1. Analysis of the S/N ratio The test results were evaluated in terms of signal/noise (S/N) ratio. The S/N was calculated by larger is better for determining effect of
Table 2 An orthogonal array L9 (33) of Taguchi. A
B
C
200 200 200 220 220 220 240 240 240
14 17 20 14 17 20 14 17 20
22 25 28 25 28 22 28 22 25
12
Experimental specimens were prepared by cutting with air jet method according to ISO 527-2 and ISO 179 standards. The specimen for experimental study was injected as 160 × 80 × 4 mm (Fig. 1.). A suitable dimension was cut from Fig. 1a and b to carry out the tensile and impact tests, respectively. The specimen dimensions for the tensile and impact tests were shown in Figs. 2 and 3. The tensile test speeds were applied as 5 mm/min (ISO 527-2). The impact tests were performed with ISO 179 standards. 7.5 J of the impact hammer was used in the impact test and specimen used for the test was shown in Fig. 3. The tensile strength and impact tests were determined from INSTRON 5560 and Zwick 5113.100 (Universal Testing Machines), respectively. The tests were repeated five times and the mean values were presented.
The cavities of the mold were manufactured in the CNC machine. A test part shown in Fig. 1a and b was injected by a plastic injection machine (Mitsubishi 50 MetIII) which had a clamping force of 490 kN and an injection pressure of 275 MPa, respectively.
1 2 3 4 5 6 7 8 9
26 10 1200
3.3. Experimental testing methods and equipment
3.2. Injection molding machine
Experiment no
15 0.9
Fig. 1. Test specimens for a) tensile test, and b) impact test.
B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
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Fig. 2. Specimen dimensions for tensile strength (ISO 527-2).
values of specimen without weld line decreased 7% for maximum tensile load and 30% for extension at break by process parameters as shown in Table 4. When the specimen without and with weld line compared, the mechanical properties of specimen with weld line decreased 10.14% for maximum tensile load, 28.90% for charpy impact strength and 88.04% for extension at break (Table 4). 4.2. ANOVA results
Fig. 3. Specimen dimensions for charpy impact test (ISO 179).
injection parameters on mechanical properties of the product. The formula of S/N ratio was shown in Eq.1 " S = N = −10 log10
1 n 1 ∑ n i = 1 y2i
# ð1Þ
where S is the standard deviation, yi is the measured experimental results and n explain to the number of specimens in each test trial [7,8]. Results from measurements of maximum (M) tensile load, charpy impact strength (notched) and extension at break were presented for mechanical properties of the products both with weld line (W) and without weld line (Table 4) which determined the optimal levels of three process parameters for PP samples. The lowest results of maximum tensile load and charpy impact strength (notched) were realized in the lowest values of melt temperature, packing pressure, and injection pressure. The lowest results of extension at break were obtained during the high injection conditions. The highest mechanical results were also obtained in the highest injection parameters conditions for specimen with/without weld line in general. The rate of maximum and minimum mechanical
The influence of process parameters and weld line on the mechanical properties of PP material was analyzed by ANOVA in Tables 5–10. The percentage contribution of variance was calculated by ANOVA results. Results from the parameters and weld line for maximum tensile load, charpy impact strength (notched) and extension at break were illustrated in Tables 5–10. F ratio was accurately computed corresponding 95% confidence level in calculation of process parameters. P value refers to the significance level. It was seen from Tables 5 and 6 that the most important parameter for maximum tensile load (without/with weld line) was injection pressure and its percentage for the tensile load was obtained by 64.140% for specimen without W. and 52.437% for specimen with W., respectively. The most important parameters for charpy impact strength (notched) (without W. and with W.) were melt temperature and injection pressure and their percentage were obtained for the parameters by 51.568% and 48.985%, respectively as shown in Tables 7 and 8. The most important parameter affecting the extension at break (without W. and with W.) in Tables 9 and 10 was melt temperature by 50.500% and 89.070%. In the literature, the most influential injection parameter on the weld line strength was found the holding pressure [1], and melt temperature (for the tensile strength) [2,9] when PP (Talc) [1], PP [2] and PS [9] materials used, respectively. The average S/N values in Tables 5–10 were showed and the most suitable injection parameters for each mechanical property taking
Table 4 Experimental and S/N ratio results of mechanical properties and weld line. Exp. no
1 2 3 4 5 6 7 8 9 Average Decrease
M. tensile load (without weld line) (N)
M. tensile load (with weld line) (N)
Charpy impact strength (notched) (without W.) (kJ/m2)
Charpy impact strength (notched) (with W.) (kJ/m2)
Extension at break (without W.) (mm)
Extension at break (with W.)(mm)
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
Exp. result
S/N
858 893 906 882 910 880 900 874 922 891.67 N 10.14%
586697 59.0170 59.1426 58.7904 59.1808 58.8897 59.0849 58.8302 59.2946
774 802 811 790 810 800 815 794 815 801.22 N
57.7748 58.0835 58.1804 57.9525 58.1697 58.0618 58.2232 57.9964 58.2232
14.14 15.28 15.92 13.02 14.30 13.96 12.34 12.98 14.50 14.05 kJ/m2 28.90%
23.0090 23.6825 24.0389 22.2922 23.1067 22.8977 21.8263 22.2655 23.2274
9.71 10.02 10.30 9.72 10.15 9.90 10.05 9.85 10.24 9.99 kJ/m2
19.7444 20,0174 20.2567 19.7533 20.1293 19.9127 20.0433 19.8687 20.2060
34.49 29.78 22.57 24.54 18.48 24.22 22.12 21.82 17.15 23.91 mm 88.04%
30.7539 29.4785 27.0706 27.7975 25.3340 27.6835 26.8957 26.7771 24.6853
3.19 3.31 3.20 2.97 2.83 2.83 2.30 2.67 2.42 2.86 mm
10.0758 10.3966 10.1030 9.4551 9.0357 9.0357 7.2346 8.5302 7.6763
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 5 Summary of ANOVA results for maximum tensile load (without W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom (DF) 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
58.94 58.89 58.80 – –
58.99 59.01 59.07 – –
59.07 59.11 59.14 – –
Sum of squares
Mean square
F
P (%)
0.024462 0.073470 0.195982 0.011640 0.305553
0.012231 0.036735 0.097991 0.005820 –
2.10 6.31 16.84 – –
8.006 24.045 64.140 3.809 100
Sum of squares
Mean square
F
P (%)
0.027915 0.044565 0.092024 0.010992 0.175496
0.013958 0.022283 0.046012 0.005496 –
2.54 4.05 8.37 – –
15.906 25.394 52.437 6.263 100
Table 6 Summary of ANOVA results for maximum tensile load (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
58.01 57.98 57.94 – –
58.06 58.08 58.09 – –
58.15 58.16 58.19 – –
Table 7 Summary of ANOVA results for charpy impact strength (notched) (without W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
2 2 2 2 8
23.58 22.38 22.72 – –
22.77 23.02 23.07 – –
22.44 23.39 22.99 – –
Sum of squares
Mean square
F
P (%)
2.05714 1.57381 0.19479 0.16340 3.98913
1.02857 0.78690 0.09739 0.08170 –
12.59 9.63 1.19 – –
51.568 39.452 4.883 4.097 100
Sum of squares
Mean square
F
P (%)
0.018204 0.116768 0.136074 0.006735 0.277781
0.009102 0.058384 0.068037 0.003368 –
2.70 17.34 20.20 – –
6.551 42.035 48.985 2.422 100
Table 8 Summary of ANOVA results for charpy impact strength (notched) (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
2 2 2 2 8
20.01 19.85 19.84 – –
19.93 20.01 19.99 – –
20.04 20.13 20.14 – –
Table 9 Summary of ANOVA results for extension at break (without W.). Factor
Degree of freedom
Average S/N values Level 1
Level 2
Level 3
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
2 2 2 2 8
29.10 28.48 28.40 – –
26.94 27.20 27.32 – –
26.12 26.48 26.43 – –
Sum of squares
Mean square
F
P (%)
14.2380 6.1772 5.8489 1.9298 28.1939
7.1190 3.0886 2.9244 0.9649 –
7.38 3.20 3.03 – –
50.500 21.910 20.745 6.845 100
Table 10 Summary of ANOVA results for extension at break (with W.). Factor
Melt temperature (°C) Packing pressure (MPa) Injection pressure (MPa) Error Total
Degree of freedom 2 2 2 2 8
Average S/N values Level 1
Level 2
Level 3
10.192 8.922 9.214 – –
9.176 9.321 9.176 – –
7.814 8.938 8.791 – –
Sum of squares
Mean square
F
P (%)
8.5427 0.3058 0.3284 0.4141 9.5909
4.2714 0.1529 0.1642 0.2070 –
20.63 0.74 0.79 – –
89.070 3.188 3.424 4.318 100
B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072 Table 11 The highest S/N values for mechanical properties. Tables no.
Highest S/N values
Table Table Table Table Table Table
A3 A3 A1 A3 A1 A1
5 6 7 8 9 10
B3 B3 B3 B3 B1 B2
C3 C3 C2 C3 C1 C1
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(level 1), 14 MPa packing pressure (level 1), and 22 MPa injection pressure (level 1) (Table 9). ● The optimal injection molding conditions for the extension at break (specimens with W.) were 200 °C melt temperature (level 1), 17 MPa packing pressure (level 2), and 22 MPa injection pressure (level 1) (Table 10).
4.3. Regression analysis results
into account the highest values of S/N were listed in Table 11. Based on this study, the following conclusions can be drawn for the injection molding conditions: ● The optimal injection molding conditions for the maximum tensile load (specimens with/without W.) were 240 °C melt temperature (level 3), 20 MPa packing pressure (level 3), and 28 MPa injection pressure (level 3) (in Tables 5 and 6). ● The optimal injection molding conditions for the charpy impact strength (specimens without W.) were 200 °C melt temperature (level 1), 20 MPa packing pressure (level 3), and 25 MPa injection pressure (level 2) (Table 7). ● The optimal injection molding conditions for the charpy impact strength (specimens with W.) were 240 °C melt temperature (level 3), 20 MPa packing pressure (level 3), and 28 MPa injection pressure (level 3) (Table 8). ● The optimal injection molding conditions for the extension at break (specimens without W.) were 200 °C melt temperature
Regression analysis was a statistical tool for the investigation of relationships between variables. R – Sq is correlation coefficient and should be between 0.8 and 1 in multiple linear regression analyses [7]. The purpose of R – Sq value is the prediction of future outcomes on the basis of other related data. It provides a measure of how well results are appropriately to be predicted by the model. A linear model between injection molding parameters and mechanical properties were created. The model result was best explained by values of regression coefficient, r 2, close to 1. The model equations were given in Table 12. Models from the parameters and weld line for maximum tensile load, charpy impact strength (notched) and extension at break had linear relationships between the injection parameters and mechanical properties. 4.4. Confirmation and comparison tests Confirmation experiments were conducted for regression equations. The results of confirmation and comparison experiments in
Table 12 Linear models between parameters and mechanical properties. Mechanical properties
The equations obtained from calculated
R-sq (%)
R-sq (Adj) (%)
Maximum tensile load (without weld line) Maximum tensile load (with weld line) Charpy impact strength (without weld line) Charpy impact strength (with weld line) Extension at break (with weld line) Extension at break (with weld line)
612 + 0.325 A + 3.78 B + 5.78 C 595 + 0.308 C1 + 2.61 B + 3.78 C 17.5 – 0.0460 A + 0.271 B + 0.0822 C 7.44 + 0.00092 A + 0.0533 B + 0.0578 C 111 – 0.215 A – 0.956 B – 0.964 C 7.60 – 0.0193 A – 0.0006 B – 0.0200 C
88.1 92.8 90.3 91.4 90.8 92.6
81.0 88.4 84.5 86.2 85.3 88.1
Table 13 Results of confirmation experiments and predicted values for regression equations. Test
Point
For regression equations 1a Max. tensile load (without weld line) (N)
1a 1b 1c
Optimum A3 B3 C3 A3 B3 C3 Random A1 B2 C2 Point
2a 2b 2c
Optimum A1 B3 C2 A3 B3 C3 Random A1 B2 C2 Point
3a 3b 3c
Optimum A1 B1 C1 A1 B2 C1 Random A1 B2 C2
1b Max. tensile load (with weld line) (N)
Exp.
Predict.
Error (%)
Exp.
Calculated
Error (%)
922
927.44
0.57
815
826.96
1.45
893
885.76
0.81
802
795.47
0.81
2a Charpy impact strength (without weld line) (N)
2b Charpy impact strength (with weld line) (N)
Exp. (Aver.)
Predict.
Error (%)
Exp.
Predict.
Error (%)
14.05
15.21
7.63
10.24
10.15
0.88
15.28
14.96
2.09
10.02
9.98
0.40
3a Extension at break (without weld line) (N)
3b Extension at break (with weld line) (N)
Exp.
Predict.
Error (%)
Exp. (aver.)
Predict.
Error (%)
34.49
33.41
3.13
2.86
3.29
13.07
29.78
27.65
7.15
3.31
3.23
2.42
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B. Ozcelik / International Communications in Heat and Mass Transfer 38 (2011) 1067–1072
Table 13 showed the comparison between the foreseen (Equations in Table 12) and experimental values. The predicted and calculated results had very close values with the experimental results. For reliable statistical analyses, error values must be smaller than 20%. The calculated error was greater especially for charpy impact strength (without W.) and extension at break (with/without W.) but these error values were in an acceptable range. Since comparisons were done according to average experimental values (Aver.), the calculated error values were obtained higher.
The most important parameters for charpy impact strength (notched) (without/with weld line) were melt temperature and injection pressure, respectively. The other important parameters for charpy impact strength of the both specimens were packing pressure. The results of the mechanical tests were high under optimum conditions, in general. All mechanical properties gave linear relationships (based on values of r 2) with injection parameters.
References 5. Conclusions This study focused on Taguchi experimental method for investigating of influence of the injection parameters and weld line on the mechanical properties of PP during plastic injection molding. In the injection experiments, melt temperature, packing pressure and injection pressure values as injection parameters and specimens with/without weld lines were utilized. Multiple regression analysis was performed to indicate the fitness of experimental measurements. Regression models obtained for all mechanical properties measurements matched very well with the experimental data. The level of importance of the injection parameters on the mechanical properties and weld line was determined by ANOVA. The most important parameter affecting the maximum tensile load and the extension at break (without/with weld line) was injection pressure and melt temperature, respectively. The other important parameters affecting the maximum tensile load for specimens with both without weld line and with weld line were packing pressure and melt temperature, respectively. The other parameters affecting the extension at break (without/with weld line) were closed to each other.
[1] R. Selden, Effect of processing on weld Line Strength in five thermoplastics, Polymer Engineering and Science 37 (1) (1997) 205–218. [2] C.-H. Wu, W.-J. Liang, Effects of geometry and Injection Molding Parameters on Weld line Strength, Polymer Engineering and Science (2005) 1021–1030. [3] H. Li, Z. Guo, D. Li, Reducing the effects of weld lines on appearance of plastic products by Taguchi experimental method, International Journal of Advanced Manufacturing Technology 32 (2007) 927–931. [4] K. Yamada, K. Tomari, U.S. Ishiaku, H. Hamada, Evaluation of Mechanical Properties of Adjacent Flow Weld line, Polymer Engineering and Science (2005) 1180–1186. [5] S.C. Chen, Y. Chang, Y.P. Chang, Y.C. Chen, C.Y. Tseng, Effect of cavity surface coating on mold temperature variation and the quality of injection molded parts, International Communications in Heat and Mass Transfer 36 (10) (2009) 1030–1035. [6] C.S. Chen, T.J. Chen, R.D. Chien, S.C. Chen, Investigation on the weld line strength of thin-wall injection molded ABS parts, International Communications in Heat and Mass Transfer 34 (4) (2007) 448–455. [7] G. Taguchi, Introduction to quality engineering, McGraw-Hill, New York, 1990. [8] P.J. Ross, Taguchi Techniques for Quality Engineering, second ed. McGraw-Hill, New York, 1996. [9] S.j. Liu, J.Y. Wu, J.H. Chang, An experimental matrix design to optimize the weld line strength in injection molded parts, Polymer Engineering and Science 40 (5) (2000) 1256–1262.