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Effects of part geometry and injection molding conditions on the tensile properties of ultra-high molecular weight polyethylene polymer
Materials and Design 31 (2010) 884–893
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Effects of part geometry and injection molding conditions on the tensile properties of ultra-high molecular weight polyethylene polymer Hsien-Chang Kuo, Ming-Chang Jeng * Department of Mechanical Engineering, National Central University, Chung-Li 32054, Taiwan
a r t i c l e
i n f o
Article history: Received 17 April 2009 Accepted 23 July 2009 Available online 29 July 2009 Keywords: Tensile strength Confirmation test Weld line Taguchi method UHMWPE
a b s t r a c t In this study, the influences of process parameters and different cross-sectional dimensions on the tensile strength of specimens with and without a weld line are investigated. In addition, the weld line characteristics of structures with different cross-sections are explored as well. With the Taguchi method and confirmation test, it can be concluded that Taguchi method is suitable to improve the mechanical properties occurring in the injection molded UHMWPE parts. The single-factor experimental results indicate that the process parametric influence is relatively smaller than the cross-sectional dimensions on the tensile strength of specimens without weld line. The experimental observations present that the frictional heating can be enhanced the molecular bonding and self-diffusion in the frozen layer of two sides on the weld line region. The SEM images showed that the micro-voids, cracks and incomplete molecular bonding are the major surface defects on the weld line region. Ó 2009 Published by Elsevier Ltd.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) possesses many excellent properties, such as impact strength, chemical stability, low friction, resistance to high wear, abrasion resistance and auto-lubricant behavior, thus it has been used to fabricate mechanical components, such as bearing, gear and cam. However, UHMWPE has some disadvantages as well, such as hardness, low Young’s modulus and easy creeping under load [1–4]. While many prototype plastic micro-devices are fabricated by using precision manufacturing methods, such as laser machining, microinjection molding is currently being researched all over the world [5,6]. Although, the injection molding process and microinjection molding process is the present important method in the manufacture of plastics, the characteristics of the product are easily affected by the type of flow of the melt and heat transfer effect. Hence, injection molding conditions, cavity geometries or runner will induce different molecular orientations [7–9]. Weld lines, molecular distribution on the surface of the part and sink marks are examples of surface quality defects [10]. Weld lines are formed when two or more separate melt fronts rejoin during injection molding, and the injection molded parts are easily damaged by this. Hence, some methods are proposed to obtain the good quality of weld lines. The methods include optimization of material composition, mold design and process parameters [11]. * Corresponding author. Tel.: +886 3 4267331. E-mail address: [email protected] (M.-C. Jeng). 0261-3069/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.matdes.2009.07.041
Reduced strength of a weld line results largely from four causes, including: (i) incomplete molecular entanglement or diffusion, (ii) formation of V-notches at the weld surface, (iii) presence of contamination or micro-voids at the weld interface and (iv) unfavorable molecular or fiber orientation at the weld [12,13]. Improving the mechanical properties of microstructures and understanding more molded characteristics of UHMWPE is one of the most important issues in injection molding and microinjection molding. The characteristics of a weld line are also investigated in this study, especially for a microstructure. The characteristic of UHMWPE has been the subject of many studies, and a lot different methods have been used to improve the mechanical and tribological performance. However, in these literatures, the test samples are always made by compression molding or machining [1–4]. To make wider applications of UHMWPE, the injection molding process is used in this study. An important advantage of injection molding is that it can be used to make complex geometries in one production step in an automated process. This study discusses the effect of various process parameters on the properties of UHMWPE of an injection molded part. The influence of cross-sectional dimension on the weld line strength is also studied. Furthermore, the Taguchi method can provide a complete system for improving and optimizing product and process quality, in the shortest experiment time. To understand more about the correlation between the tensile strength and the process parameter, more experiments were carried out by changing one factor at a time and keeping the others constant. The microstructure of weld lines was clearly observed from the SEM photographs.
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2. Experiment details 2.1. Materials The material used in this study is an injection molding graded of ultra-high molecular weight polyethylene (UHMWPE, GUR5113, from Ticona, USA). It is a linear polyolefin resin. The properties of the UHMWPE resin are listed in Table 1. The recommended nozzle temperature is between 250 and 260 °C and the recommended mold temperature is between 30 and 90 °C. The material was preheated at 100 °C for 3 h using a dehumidifying drier before use in the injection molding machine. 2.2. Part geometry and mold design A mold was used to generate five tensile specimens with different cross-sections at a time. It was designed to be a dog-bone form that dimensions as shown in Fig. 1. Because of the compact nature of the mold and the injection molding machine, the dimensions are based on ASTM procedure D638 (Type IV) with slight modifications. The cross-sectional width and thickness at the dog-bone region of each specimen is listed in Table 2. Specimen #1 represents a conventional injection molded part. The cross-sections of specimens #2 and #4 are similar except that their widths and depths are interchanged. Specimen #5, which has an aspect ratio of 33.3, is used to study the weld line of an injection molded part with high aspect ratio. Specimen #3, with a small cross-section, is used to investigate the mechanical characteristics of a microstructure. The mold is made of tool steel. The cavity is fed from the center by a sprue of 5 mm in diameter. Five semi-circular rotary plugs are designed to allow the parts to be molded with weld lines or without (as shown in Fig. 2). When five plugs are turned to an ‘‘open” condition, both sides of the gates are open. The molded specimens
have weld lines in the middle position due to symmetric geometry (as shown in Fig. 2(a)). When five plugs are turned to a ‘‘closed” condition, one side of the gates is blocked. The samples are molded without weld lines (as shown in Fig. 2(b)). From the tensile testing, it can be found that the specimens broken in the middle. Hence, it can be known that the weld lines are formed in the middle (as shown in Fig. 2(c)). To obtain accurate tensile strength data, all the flow paths within multi-cavity should be filled simultaneously and uniformly. Otherwise, moldings of different qualities and properties would be produced during one shot. To fill five cavities uniformly, Wu and Liang [9] designed the mold using Moldflow analytical software. Firstly, the thickness of five gates is initially designed to be 0.3 mm. However, it is not filled at the same time. Following, the thicknesses of five gates are set to be 0.31 mm, 0.3 mm, 0.3 mm, 0.14 mm and 0.2 mm (from #1 to #5), respectively. Their simulation results show that a more uniform filling can be obtained at the same time. 2.3. Injection molding All tensile specimens are prepared on a FANUC electric injection molding machine (ROBOSHOT S-2000i 50A). The machine can offer a maximum clamping force 50 tons, and maximum injection velocity 330 mm s1. The screw diameter is 22 mm and the maximum injection volume is 29 cm3. A mold temperature controller is used to prepare the specimens at various mold temperature. Under each set of injection molding conditions, 10 shots are made to ensure that the process is stable before specimens are collected. If no signification variation is observed during these first 10 runs, the specimens from the next 5 runs are collected as the samples for tensile properties. 2.4. Tensile testing Tensile specimens are tested with a tensile tester (PT-1000, Perfect International Instrument Co. Ltd.) at a crosshead rate of
Table 1 Material properties data sheet. Property
Unit
GUR 5113
Density Viscosity Volume density Tensile modulus Tensile stress at yield Tensile strain at yield Vicat softening temperature Molecular weight
g cm3 mg L1 g cm3 MPa MPa % °C g mol1
0.93 2000 0.5 750 17 20 80 3,900,000
Table 2 Cross-sectional and gate dimension of five specimens. Specimen number
1
2
3
4
5
Width (mm) Depth (mm) Width of gate (mm) Depth of gate (mm)
5 2 2 0.31
0.3 2 2 0.3
0.3 0.3 2 0.3
2 0.3 2 0.14
10 0.3 2 0.2
Fig. 1. Dimensions of tensile specimen.
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Fig. 2. Plastics parts for specimens (a) with weld lines and (b) without weld lines, (c) tensile break in the middle along a weld line.
25 mm min1. The clamping and the marking intervals are 15 mm and 10 mm, respectively. Three consecutive molded specimens are used for tensile testing. Mean values and standard deviation are calculated in determining the tensile strength. A load cell with a maximum loading of 1 kN is used for all specimens. 2.5. Microscopy A microscope is used to observe the microstructures of the molded part. Tensile specimens with weld lines and fractures are also examined in an optical microscope (OM) and a scanning electron microscope (SEM). Specimens are cut into smaller pieces, rinsed, dried and gold is sputtered onto the surface, followed by inspection in the SEM. 3. Taguchi analysis The Taguchi experiment method was carried out to analyze the influence of testing parameters on tensile strength of UHMWPE specimens. This is a powerful tool applied to determine the effects of molding factors on the UHMWPE strength and the optimum set of factors that would maximize the tensile strength. In this study, six molding factors are selected for injection molding processes: (i) melt temperature, (ii) mold temperature, (iii) injection velocity, (iv) packing pressure, (v) packing time and (vi) cooling time. Because the influence of these factors may vary nonlinearly, each factor is assigned at three levels. The levels of the molding factors are selected through our initial tests or recommended by the manufacturer. The values of the levels are listed in Table 3. The data in this article are the mean values of three samples at each set of experimental conditions for each tensile specimen. The measured data of Table 3 Factors and levels for the injection molding experiments. Factors
Melt temperature, A (°C) Mold temperature, B (°C) Injection velocity, C (mm s1) Packing pressure, D (MPa) Packing time, E (s) Cooling time, F (s)
Levels 1
2
3
250 50 150 20 2 3
265 70 180 40 4 6
280 90 210 60 6 9
the mean tensile strength are listed in Table 4. According to the levels of each factor, an L18 (21 37) orthogonal array is chosen, which has eighteen rows, one column at two levels and seven columns at three levels. The plan of the experiments is as follows: the second column is assigned to melt temperature (A), third column to mold temperature (B), fourth column to injection velocity (C), fifth column to packing pressure (D), sixth column to packing time (E), seventh column to cooling time (F) and the remaining column is assigned to error item as shown in Table 5. In the analysis, a signal-to-noise (S/N) ratio is the statistical quantity representing the power of a response signal divided by the power of the variation in the signal due to noise. The maximization of the S/N ratio leads to the minimization of any property that is sensitive to noise. The response to be studied was the tensile strength with the objective of larger-the-better. The quality characteristic equation describing the larger-the-better characteristic can be used for the analysis:
g ¼ 10log10
n 1X 1 n i¼1 y2i
! ð1Þ
Table 4 Experimental data of tensile strength. Experimental run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tensile strength (MPa) #1
#2
#3
#4
#5
29.17 28.30 29.03 27.57 28.50 29.63 28.35 25.93 28.43 29.57 29.00 29.65 28.10 28.00 29.45 26.90 27.07 28.33
32.20 31.00 29.87 30.23 32.27 24.65 34.03 27.37 31.75 28.80 31.40 25.00 31.13 29.80 28.17 30.25 27.20 27.75
10.33 25.90 28.75 19.10 20.35 22.90 15.67 23.07 25.00 26.85 26.00 21.30 28.80 27.57 20.33 21.03 20.20 29.30
30.90 38.35 41.80 33.10 37.37 37.05 37.55 40.33 32.37 36.53 34.75 36.85 35.55 40.33 34.90 39.07 35.80 35.70
30.40 32.73 24.45 31.05 33.03 32.30 31.77 33.40 30.87 32.83 31.20 33.50 32.60 33.40 30.93 33.90 31.50 32.73
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Factors and levels
S/N ratio(dB)
–
A
B
C
D
E
F
–
#1
#2
#3
#4
#5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
1 2 3 3 1 2 3 1 2 1 2 3 2 3 1 2 3 1
29.30 29.04 29.26 28.81 29.10 29.44 29.05 28.28 29.08 29.42 29.25 29.44 28.97 28.94 29.38 28.60 28.65 29.05
30.16 29.83 29.50 29.61 30.18 27.84 30.64 28.74 30.03 29.19 29.94 27.96 29.86 29.48 28.99 29.61 28.69 28.87
20.28 28.27 29.17 25.62 26.17 27.20 23.90 27.26 27.96 28.58 28.30 26.57 29.19 28.81 26.16 26.46 26.11 29.34
29.80 31.68 32.42 30.40 31.45 31.38 31.49 32.11 30.20 31.25 30.82 31.33 31.02 32.11 30.86 31.84 31.08 31.05
29.66 30.30 27.77 29.84 30.38 30.18 30.04 30.47 29.79 30.33 29.88 30.50 30.26 30.47 29.81 30.60 29.97 30.30
where g is the S/N ratio for the larger-the-better case, yi is the measured data obtained from experiment and n corresponds to the number of samples in each test trial. The optimum factor levels with the largest S/N ratios can then be summarized, which will minimize sensitivity over the range of noises. Table 5 shows the S/N ratios obtained using the Taguchi method. Then, the contribution percentage of each factor can be found and the optimum set of parameters driving the effective factors in this molding processes can be determined to obtain a product with maximum tensile strength. 3.1. Significance of processing parameters The experimental data can also be analyzed using Analysis of Variance (ANOVA) method. The purpose of the ANOVA is to investigate which process parameter significantly affects the quality characteristic. From this method, the percentage contribution has been calculated to determine which factor will affect the tensile strength significantly. 3.2. Confirmation experiments The final step of Taguchi method is to predict and confirm the improvement in the tensile properties using the optimal level for processing tensile specimens. One major purpose of the confirmation experiments is to provide evidence that shows the additive equation applies and that interactions are low. If the experimental results are within the range of the confidence interval (CI), the experiments have succeeded. Otherwise, it reveals that interactions are significant. The experimental procedure should be redesigned. In this case, confidence intervals are typically stated at the 95% confidence level. The 95% confidence interval for the predicted mean on a confirmation experiments can be calculated using the following equation [14]:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 1 1 CI ¼ F a;ð1;fe Þ V e þ neff R
Then $
g ¼ g CI
ð3Þ
$
where g = the confidence interval of the predicted optimal tensile strength, g = the predicted mean of tensile strength. 4. Results and discussion 4.1. Taguchi analysis Based on the values listed in Table 3, the Taguchi method was applied to analyze the UHMWPE strength of specimens #1 to #5. Since the strength without weld lines is the ‘‘Larger-the-Better” type, the optimal combination of studied factors for the largest strength can be identified from the response plots. Fig. 3(a) illustrates the effects of processing factors on the without weld line strength from experimental results. The factor levels that will generate the largest without weld line strength are found to be at melt temperature of 250 °C, mold temperature of 90 °C, injection velocity of 180 mm s1, packing pressure of 20 MPa, packing time of 6 s and cooling time of 6 s (A1B3C2D1E3F2). Other effects of processing factors of specimen #2 to #5 are shown in Figs. 3(b) to (e), respectively. The confirmation test is very important in Taguchi analysis to validate maximum tensile strength resulted from optimization process. The 95% CI of specimens #1 to #5 are 29.53–29.70 dB, 29.91–30.97 dB, 25.03–35.43 dB, 32.01–32.78 dB and 30.25– 31.18 dB, respectively. Based on the optimum set of processing variables, the tensile strength of specimens #1 to #5 is measured to confirm the experiment. S/N ratios based on experimental results are 29.65 dB, 30.76 dB, 29.62 dB, 32.37 dB and 30.79 dB which are within the CI range, respectively. This means that the Taguchi method is applied appropriately in this case. The quality product have mean values of tensile strength are 30.37 MPa, 34.51 MPa, 30.27 MPa, 41.55 MPa and 34.63 MPa, respectively. 4.2. Parametric analysis
ð2Þ
where Fa;(1,fe) = F ratio required for a, a = risk, fe = error DOF (degrees of freedom), Ve = error variance, neff = effective number of replications = N/1 + (total DOF associated in the estimate of mean), N = total number of experiments, R = number of replications for confirmation experiments.
In order to investigation the effect of injection molding parameters for UHMWPE, more experiments are carried out by changing one factor at a time and keeping the others constant. Melt temperature, mold temperature and injection velocity are investigated to study their effects on the mechanical strength of specimens with and without weld lines. Table 6 identifies the specimens and gives the single-factor experiment parameters.
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Fig. 3. S/N graph for (a) specimen #1; (b) specimen #2; (c) specimen #3; (d) specimen #4 and (e) specimen #5.
Table 6 Parameters set of single-factor experiments. Specimen
Melt temperature (°C)
Mold temperature (°C)
Injection velocity (mm s1)
Packing pressure (MPa)
Packing time (s)
Cooling time (s)
M250 M265 M280 m50 m70 m90 V150 V180 V210
250 265 280 265 265 265 265 265 265
70 70 70 50 70 90 70 70 70
180 180 180 180 180 180 150 180 210
40 40 40 40 40 40 40 40 40
4 4 4 4 4 4 4 4 4
6 6 6 6 6 6 6 6 6
4.2.1. Tensile strength of without a weld line The effects of melt temperature, mold temperature and injection velocity on the tensile strength are shown in Fig. 4. Packing pressure, Packing time, cooling time and back pressure are constant and the values are 40 MPa, 4 s, 6 s and 8 MPa, respectively. It can be found that the process parameters have little effect on the tensile strength. However, the dimensions of the molded specimens strongly affect the tensile strength. When the melt temperature, mold temperature, injection velocity, packing pressure, packing time and cooling time are set to 265 °C, 70 °C, 180 mm s1,
40 MPa, 4 s and 6 s, the measured tensile strength of the five specimens is 28.5 MPa, 31.25 MPa, 24.6 MPa, 37.37 MPa and 33.03 MPa, respectively. Specimen #1, with a normal cross-section of 5 mm 2 mm, has a small tensile strength. Specimen #3, with the smallest cross-section of 0.3 mm 0.3 mm, has the smallest tensile strength. In the previous study [9], if the viscosity of polymer was lower (e.g. polypropylene, PP) and the cross-section was the smallest (e.g. 0.3 mm 0.3 mm), then it presented the largest tensile strength. However, in this case, the UHMWPE is a polymer of high viscosity, which may lead to frictional heat produced in
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Fig. 4. Effects of (a) melt temperature, (b) mold temperature and (c) injection velocity of UHMWPE specimens.
filling stage. This further indicates that the decrease in tensile strength, due to the viscosity of polymer, is more related to the cross-section dimensions. Specimens #4 and #5 present the largest tensile strengths because of having a thickness of only 0.3 mm. The tensile strength of specimen #4 is higher than that of specimen #5 because the width of the former is smaller than the width of the latter.
Albeit the areas of specimens #2 and #4 are the same, the latter is stronger than the former. This result can be interpreted by molecular orientation. If the shape of the dog-bone region is alike to the shape of the gate (as in specimens #4 and #5), the tensile strength is larger. The reason is that the molecular orientation can be kept through the gate to the dog-bone region. The gate for specimen #2 has a cross-section of 2 mm in width
Fig. 5. Effects of (a) melt temperature, (b) mold temperature and (c) injection velocity on the weld line of UHMWPE specimens.
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and 0.14 mm in thickness. The dog-bone region has a cross-section of 0.3 mm in width and 2 mm in thickness. These two cross-sections are not alike. These results agree with [9], which
indicated that when the polymer flows through the gate and then enters into the dog-bone region, the molecular orientation will be constrained and rearranged by the gate. This behavior
Fig. 6. SEM and OM micrographs of tensile fracture surfaces of UHMWPE specimen: (a) without weld line on #1, (b) with weld line on #1, (c) without weld line on #2, (d) with weld line on #2, (e) without weld line on #4, (f) with weld line on #4, (g) without weld line on #5 and (h) with weld line on #5.
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influences molecular orientation and thereby decreases tensile strength. The earlier results can be interpreted by the characteristic of molecular orientation and crystallinity. A larger tensile strength is obtained in the melt flow direction. This could be attributed to the molecular orientation parallel with the melt flow direction. Constrained by the walls of a cavity, flow-induced crystallization occurs. A thin and narrow cavity restricts the molecular orientation and forces the molecules to stretch parallel to the cavity walls. Fig. 6(a) shows that the highest tensile strength of five specimens occurred at a melt temperature of 265 °C. The tensile strength of five specimens decreases whether the melt temperature increases or decreases. However, the lowest tensile strength of these five specimens was obtained at a melt temperature of 280 °C. The tensile strength of five specimens for different mold temperature is shown in Fig. 6(b). It indicates that highest tensile strength was mostly obtained at a mold temperature of 90 °C. Nevertheless, specimen #3 and #5 has the smallest tensile strength for mold temperature at 90 °C. The tensile strength of five specimens for different injection velocity is shown in Fig. 6(c). The highest tensile strength occurs for an injection velocity of 210 mm s1 except the specimen #3. The smallest cross-section causes a higher
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frictional heating which leads to decrease in tensile strength during filling process. From the above experimental observations, four conclusions can be summarized as follows: (1) The related parameters have little effect on the tensile strength for UHMWPE specimens. (2) Although specimens #2 and #4 have the same area, the gate shape is very different from the cross-section of specimen #2. Hence, the strength of specimen #2 is weaker than specimen #4. The phenomenon can be explained by the truth that specimen #4 has a faster molecular rearrangement rate and better molecular orientation than specimen #2. (3) Specimen #4 has the largest tensile strength. Specimen #5 has a lower tensile strength, followed by #2 and #1. Specimen #3 has the lowest tensile strength. (4) The tensile strength is significantly influenced by the crosssectional dimensions than the process parameters. 4.2.2. Tensile strength of with a weld line The process parameters are the same as those for the withoutweld-line specimen. In Fig. 5, the influences of the process param-
Fig. 7. Scanning electron micrographs of welded region on UHMWPE specimens: (a) specimen #1, (b) specimen #2, (c) specimen #3, (d) specimen #4 and (e) specimen #5.
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eters on the specimen with a weld line brought by conducting a single-factor experiment are shown. Effects of melt temperature, mold temperature and injection velocity on the tensile strength can be obtained. To clarify the parametric effects on the weld line strength, the with-weld-line strength are compared with that without weld lines. From these figures, it can be found that the process parameters create greater influences on the tensile strength of specimens with a weld line than the cross-sectional dimension of the dog-bone region. Given that the width and thickness of the cross-section of the dog-bone region are smaller (e.g. specimen #4 (2 mm 0.3 mm)), the tensile strength of the specimen drops at a more significant range (compared with the specimen without a weld line). Contrarily, given that the width and thickness of the cross-section are bigger (e.g. specimen #1 (5 mm 2 mm)), the tensile strength of the specimen drops within a less significant range (compared with the specimen without a weld line). There are two possible reasons leading to such phenomenon. First, the weld line of a specimen is formed when two or more melt fronts meet and bond. The orientation of molecules at melt front is usually parallel to each other, which creates less molecular bonding at weld line and further weakens the tensile strength of weld line. As a result, a specimen with smaller crosssectional dimensions presents less molecular bonding than the one with bigger cross-sectional dimensions. The other reason involves the thickness of a specimen. With smaller specimen thickness, the melt temperature drops faster in the process when melt fronts from the two sides meet. Thus incomplete bonding of the molecular chains at the meeting point of melt fronts appears. The specimen with bigger thickness carries sufficiently high temperature enabling the molecular chains from both sides to pass through melt front interface and create more bonding. Therefore, its tensile strength drops within a smaller range compared with that presented from a specimen with smaller thickness. In addition, specimen #2 presents greater tensile strength than specimen #4 under the condition of having a weld line, which explains that the molecular orientation rearrangement in the melt filling process, due to bigger width and thickness differences between the gate and the cross-section of the dog-bone region, reduces the influence of a weld line on tensile strength. 4.3. Microscopy Using the optimum set of effective factors obtained from the previous analysis, photographs of the tensile fracture surfaces of UHMWPE are taken. Pictures of specimens #1 to #5 are shown in Fig. 6. The specimen #1(a) without a weld line shows the neck fracture at the center. The specimen #1(b) presents the broken part at the center of a weld line. This phenomenon can be explained by frictional heating in mold walls. The frictional heating creates a better molecular bonding or self-diffusion in the frozen layer of two sides on the weld line region. This is why the tensile strength of specimen #1 has smaller difference than others between specimens with and without a weld line. Photos (c), (e) and (g) show the random tensile fracture surfaces at the center of specimen. Photos (d), (f) and (h) show the tensile fracture surfaces along the weld line. These photographs show that the thickness, width, with or without a weld line cause the effect on determining the specimen fracture shape. SEM was applied to UHMWPE specimens with a weld line. SEM images of UHMWPE specimens are shown in Fig. 7. These figures are the magnified pictures at the center of the welded region. Observing from the pictures, the weld lines and V-shaped notches were either absent or difficult to distinguish from surface. However, severer surface defects like scratches appeared on the welded region of UHMWPE. Besides, the micro-voids, cracks and incomplete molecular bonding also appeared at the welded region. These
observations demonstrate that surface defects result in weak and unstable mechanical strength in welded specimens.
5. Conclusions From the experimental analysis and results, the following conclusions can be drawn: (1) UHMWPE can be successfully filled in small channels and cavities by injection molding processes. (2) Taguchi optimization method is applied to find the optimal process parameters leading to maximum tensile strength during injection molded process. Taguchi method is suitable to improve the mechanical properties occurring the injection molded UHMWPE parts. (3) For the UHMWPE specimens without weld line, the crosssectional dimensions cause a higher effect on the tensile strength than the process parameters do. For the most part, the better injection molding parameters for higher tensile strength are a melt temperature of 265 °C, mold temperature of 90 °C and injection velocity of 210 mm s1. (4) The production of weld line reduces the effects of process parameters and cross-sectional dimensions on tensile strength. (5) In the thicker specimen, such as specimen #1, the frictional heating in the filling process leads to an increased molecular bonding and self-diffusion of the UHMWPE. Therefore, it can be seen as a benefit in term as enhancing strength on the weld lines. (6) Micro-voids, cracks and incomplete molecular bonding are the major surface defects on the weld line region. The most severely surface defect of weld line is always found in the specimen #1 due to the thickness effect.
Acknowledgements We would like to thanks to Prof. Cheng-Hsien Wu of the Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Taiwan, Prof. Hai-Wei Chi of the Department of Mechanical and Automation Engineering, Da-Yeh University, Taiwan and Prof. Chung-Te Lee of the Department of Environment Engineering, National Central University, Taiwan. Their assistance with experimental equipment support is appreciated. This study was funded by the National Science Council, Taiwan. Project No. NSC-97-2221-E-008-013-MY3. References [1] Xiong DS. Friction and wear properties of UHMWPE composites reinforced with carbon fiber. Mater Lett 2005;59:175–9. [2] Boscoletto AB, Franco R, Scapin M, Tavan M. An investigation on rheological and impact behavior of high density and ultrahigh molecular weight polyethylene mixtures. Eur Polym J 1997;33(1):97–105. [3] Wang SB, Ge SR. The mechanical property and tribological behavior of UHMWPE: effect of molding pressure. Wear 2007;263:949–56. [4] Song J, Liu P, Cremens M, Bonutti P. Effects of machining on tribological behavior of ultrahigh molecular weight polyethylene (UHMWPE) under dry reciprocating sliding. Wear 1999;225–229:716–23. [5] Yao D, Kim B. Simulation of the filling process in micro channels for polymeric materials. J Micromech Microeng 2002;12:604–10. [6] Wimberger-Friedl R. Injection molding of sub-mm grating optical elements. J Inject Mold Technol 2000;4:78–83. [7] Fujiama M, Awaya H. Mechanical anisotropy in injection-molded polypropylene. J Appl Polym Sci 1977;7:3291–309. [8] Huilier D, Lenfant C, Terrisse J, Deterre R. Modeling the packing stage in injection molding of thermoplastics. Polym Eng Sci 1988;28:1637–43. [9] Wu CH, Liang WJ. Effects of geometry and injection-molding parameters on weld-line strength. Polym Eng Sci 2005;45:1021–30.
H.-C. Kuo, M.-C. Jeng / Materials and Design 31 (2010) 884–893 [10] Lacrampe MF, Pabiot J. Defects in surface appearance of injection molded thermoplastic parts. A review of some problems in surface gloss distribution. J Inject Mold Technol 2000;4:167–76. [11] Fellahi S, Meddad A, Fisa B, Favis BD. Weldlines in injection-molded parts: a review. Adv Polym Technol 1995;14(3):169–95.
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[12] Chang TC, Faison E. Optimization of weld line quality in injection molding using an experimental design approach. J Inject Mold Technol 1999;3(2):61–6. [13] Selden R. Effect of processing on weld line strength in five thermoplastics. Polym Eng Sci 1997;37(1):205–17. [14] Ross PJ. Taguchi techniques for quality engineering. 2nd ed. McGraw-Hill; 1996.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effects of part geometry and injection molding conditions on the tensile properties of ultra-high molecular weight polyethylene polymer Hsien-Chang Kuo, Ming-Chang Jeng * Department of Mechanical Engineering, National Central University, Chung-Li 32054, Taiwan
a r t i c l e
i n f o
Article history: Received 17 April 2009 Accepted 23 July 2009 Available online 29 July 2009 Keywords: Tensile strength Confirmation test Weld line Taguchi method UHMWPE
a b s t r a c t In this study, the influences of process parameters and different cross-sectional dimensions on the tensile strength of specimens with and without a weld line are investigated. In addition, the weld line characteristics of structures with different cross-sections are explored as well. With the Taguchi method and confirmation test, it can be concluded that Taguchi method is suitable to improve the mechanical properties occurring in the injection molded UHMWPE parts. The single-factor experimental results indicate that the process parametric influence is relatively smaller than the cross-sectional dimensions on the tensile strength of specimens without weld line. The experimental observations present that the frictional heating can be enhanced the molecular bonding and self-diffusion in the frozen layer of two sides on the weld line region. The SEM images showed that the micro-voids, cracks and incomplete molecular bonding are the major surface defects on the weld line region. Ó 2009 Published by Elsevier Ltd.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) possesses many excellent properties, such as impact strength, chemical stability, low friction, resistance to high wear, abrasion resistance and auto-lubricant behavior, thus it has been used to fabricate mechanical components, such as bearing, gear and cam. However, UHMWPE has some disadvantages as well, such as hardness, low Young’s modulus and easy creeping under load [1–4]. While many prototype plastic micro-devices are fabricated by using precision manufacturing methods, such as laser machining, microinjection molding is currently being researched all over the world [5,6]. Although, the injection molding process and microinjection molding process is the present important method in the manufacture of plastics, the characteristics of the product are easily affected by the type of flow of the melt and heat transfer effect. Hence, injection molding conditions, cavity geometries or runner will induce different molecular orientations [7–9]. Weld lines, molecular distribution on the surface of the part and sink marks are examples of surface quality defects [10]. Weld lines are formed when two or more separate melt fronts rejoin during injection molding, and the injection molded parts are easily damaged by this. Hence, some methods are proposed to obtain the good quality of weld lines. The methods include optimization of material composition, mold design and process parameters [11]. * Corresponding author. Tel.: +886 3 4267331. E-mail address: [email protected] (M.-C. Jeng). 0261-3069/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.matdes.2009.07.041
Reduced strength of a weld line results largely from four causes, including: (i) incomplete molecular entanglement or diffusion, (ii) formation of V-notches at the weld surface, (iii) presence of contamination or micro-voids at the weld interface and (iv) unfavorable molecular or fiber orientation at the weld [12,13]. Improving the mechanical properties of microstructures and understanding more molded characteristics of UHMWPE is one of the most important issues in injection molding and microinjection molding. The characteristics of a weld line are also investigated in this study, especially for a microstructure. The characteristic of UHMWPE has been the subject of many studies, and a lot different methods have been used to improve the mechanical and tribological performance. However, in these literatures, the test samples are always made by compression molding or machining [1–4]. To make wider applications of UHMWPE, the injection molding process is used in this study. An important advantage of injection molding is that it can be used to make complex geometries in one production step in an automated process. This study discusses the effect of various process parameters on the properties of UHMWPE of an injection molded part. The influence of cross-sectional dimension on the weld line strength is also studied. Furthermore, the Taguchi method can provide a complete system for improving and optimizing product and process quality, in the shortest experiment time. To understand more about the correlation between the tensile strength and the process parameter, more experiments were carried out by changing one factor at a time and keeping the others constant. The microstructure of weld lines was clearly observed from the SEM photographs.
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2. Experiment details 2.1. Materials The material used in this study is an injection molding graded of ultra-high molecular weight polyethylene (UHMWPE, GUR5113, from Ticona, USA). It is a linear polyolefin resin. The properties of the UHMWPE resin are listed in Table 1. The recommended nozzle temperature is between 250 and 260 °C and the recommended mold temperature is between 30 and 90 °C. The material was preheated at 100 °C for 3 h using a dehumidifying drier before use in the injection molding machine. 2.2. Part geometry and mold design A mold was used to generate five tensile specimens with different cross-sections at a time. It was designed to be a dog-bone form that dimensions as shown in Fig. 1. Because of the compact nature of the mold and the injection molding machine, the dimensions are based on ASTM procedure D638 (Type IV) with slight modifications. The cross-sectional width and thickness at the dog-bone region of each specimen is listed in Table 2. Specimen #1 represents a conventional injection molded part. The cross-sections of specimens #2 and #4 are similar except that their widths and depths are interchanged. Specimen #5, which has an aspect ratio of 33.3, is used to study the weld line of an injection molded part with high aspect ratio. Specimen #3, with a small cross-section, is used to investigate the mechanical characteristics of a microstructure. The mold is made of tool steel. The cavity is fed from the center by a sprue of 5 mm in diameter. Five semi-circular rotary plugs are designed to allow the parts to be molded with weld lines or without (as shown in Fig. 2). When five plugs are turned to an ‘‘open” condition, both sides of the gates are open. The molded specimens
have weld lines in the middle position due to symmetric geometry (as shown in Fig. 2(a)). When five plugs are turned to a ‘‘closed” condition, one side of the gates is blocked. The samples are molded without weld lines (as shown in Fig. 2(b)). From the tensile testing, it can be found that the specimens broken in the middle. Hence, it can be known that the weld lines are formed in the middle (as shown in Fig. 2(c)). To obtain accurate tensile strength data, all the flow paths within multi-cavity should be filled simultaneously and uniformly. Otherwise, moldings of different qualities and properties would be produced during one shot. To fill five cavities uniformly, Wu and Liang [9] designed the mold using Moldflow analytical software. Firstly, the thickness of five gates is initially designed to be 0.3 mm. However, it is not filled at the same time. Following, the thicknesses of five gates are set to be 0.31 mm, 0.3 mm, 0.3 mm, 0.14 mm and 0.2 mm (from #1 to #5), respectively. Their simulation results show that a more uniform filling can be obtained at the same time. 2.3. Injection molding All tensile specimens are prepared on a FANUC electric injection molding machine (ROBOSHOT S-2000i 50A). The machine can offer a maximum clamping force 50 tons, and maximum injection velocity 330 mm s1. The screw diameter is 22 mm and the maximum injection volume is 29 cm3. A mold temperature controller is used to prepare the specimens at various mold temperature. Under each set of injection molding conditions, 10 shots are made to ensure that the process is stable before specimens are collected. If no signification variation is observed during these first 10 runs, the specimens from the next 5 runs are collected as the samples for tensile properties. 2.4. Tensile testing Tensile specimens are tested with a tensile tester (PT-1000, Perfect International Instrument Co. Ltd.) at a crosshead rate of
Table 1 Material properties data sheet. Property
Unit
GUR 5113
Density Viscosity Volume density Tensile modulus Tensile stress at yield Tensile strain at yield Vicat softening temperature Molecular weight
g cm3 mg L1 g cm3 MPa MPa % °C g mol1
0.93 2000 0.5 750 17 20 80 3,900,000
Table 2 Cross-sectional and gate dimension of five specimens. Specimen number
1
2
3
4
5
Width (mm) Depth (mm) Width of gate (mm) Depth of gate (mm)
5 2 2 0.31
0.3 2 2 0.3
0.3 0.3 2 0.3
2 0.3 2 0.14
10 0.3 2 0.2
Fig. 1. Dimensions of tensile specimen.
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Fig. 2. Plastics parts for specimens (a) with weld lines and (b) without weld lines, (c) tensile break in the middle along a weld line.
25 mm min1. The clamping and the marking intervals are 15 mm and 10 mm, respectively. Three consecutive molded specimens are used for tensile testing. Mean values and standard deviation are calculated in determining the tensile strength. A load cell with a maximum loading of 1 kN is used for all specimens. 2.5. Microscopy A microscope is used to observe the microstructures of the molded part. Tensile specimens with weld lines and fractures are also examined in an optical microscope (OM) and a scanning electron microscope (SEM). Specimens are cut into smaller pieces, rinsed, dried and gold is sputtered onto the surface, followed by inspection in the SEM. 3. Taguchi analysis The Taguchi experiment method was carried out to analyze the influence of testing parameters on tensile strength of UHMWPE specimens. This is a powerful tool applied to determine the effects of molding factors on the UHMWPE strength and the optimum set of factors that would maximize the tensile strength. In this study, six molding factors are selected for injection molding processes: (i) melt temperature, (ii) mold temperature, (iii) injection velocity, (iv) packing pressure, (v) packing time and (vi) cooling time. Because the influence of these factors may vary nonlinearly, each factor is assigned at three levels. The levels of the molding factors are selected through our initial tests or recommended by the manufacturer. The values of the levels are listed in Table 3. The data in this article are the mean values of three samples at each set of experimental conditions for each tensile specimen. The measured data of Table 3 Factors and levels for the injection molding experiments. Factors
Melt temperature, A (°C) Mold temperature, B (°C) Injection velocity, C (mm s1) Packing pressure, D (MPa) Packing time, E (s) Cooling time, F (s)
Levels 1
2
3
250 50 150 20 2 3
265 70 180 40 4 6
280 90 210 60 6 9
the mean tensile strength are listed in Table 4. According to the levels of each factor, an L18 (21 37) orthogonal array is chosen, which has eighteen rows, one column at two levels and seven columns at three levels. The plan of the experiments is as follows: the second column is assigned to melt temperature (A), third column to mold temperature (B), fourth column to injection velocity (C), fifth column to packing pressure (D), sixth column to packing time (E), seventh column to cooling time (F) and the remaining column is assigned to error item as shown in Table 5. In the analysis, a signal-to-noise (S/N) ratio is the statistical quantity representing the power of a response signal divided by the power of the variation in the signal due to noise. The maximization of the S/N ratio leads to the minimization of any property that is sensitive to noise. The response to be studied was the tensile strength with the objective of larger-the-better. The quality characteristic equation describing the larger-the-better characteristic can be used for the analysis:
g ¼ 10log10
n 1X 1 n i¼1 y2i
! ð1Þ
Table 4 Experimental data of tensile strength. Experimental run
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Tensile strength (MPa) #1
#2
#3
#4
#5
29.17 28.30 29.03 27.57 28.50 29.63 28.35 25.93 28.43 29.57 29.00 29.65 28.10 28.00 29.45 26.90 27.07 28.33
32.20 31.00 29.87 30.23 32.27 24.65 34.03 27.37 31.75 28.80 31.40 25.00 31.13 29.80 28.17 30.25 27.20 27.75
10.33 25.90 28.75 19.10 20.35 22.90 15.67 23.07 25.00 26.85 26.00 21.30 28.80 27.57 20.33 21.03 20.20 29.30
30.90 38.35 41.80 33.10 37.37 37.05 37.55 40.33 32.37 36.53 34.75 36.85 35.55 40.33 34.90 39.07 35.80 35.70
30.40 32.73 24.45 31.05 33.03 32.30 31.77 33.40 30.87 32.83 31.20 33.50 32.60 33.40 30.93 33.90 31.50 32.73
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H.-C. Kuo, M.-C. Jeng / Materials and Design 31 (2010) 884–893 Table 5 Taguchi L18 orthogonal array design and S/N ratio. Experimental run
Factors and levels
S/N ratio(dB)
–
A
B
C
D
E
F
–
#1
#2
#3
#4
#5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2
1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
1 2 3 1 2 3 2 3 1 3 1 2 2 3 1 3 1 2
1 2 3 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1
1 2 3 2 3 1 3 1 2 2 3 1 1 2 3 3 1 2
1 2 3 3 1 2 2 3 1 2 3 1 3 1 2 1 2 3
1 2 3 3 1 2 3 1 2 1 2 3 2 3 1 2 3 1
29.30 29.04 29.26 28.81 29.10 29.44 29.05 28.28 29.08 29.42 29.25 29.44 28.97 28.94 29.38 28.60 28.65 29.05
30.16 29.83 29.50 29.61 30.18 27.84 30.64 28.74 30.03 29.19 29.94 27.96 29.86 29.48 28.99 29.61 28.69 28.87
20.28 28.27 29.17 25.62 26.17 27.20 23.90 27.26 27.96 28.58 28.30 26.57 29.19 28.81 26.16 26.46 26.11 29.34
29.80 31.68 32.42 30.40 31.45 31.38 31.49 32.11 30.20 31.25 30.82 31.33 31.02 32.11 30.86 31.84 31.08 31.05
29.66 30.30 27.77 29.84 30.38 30.18 30.04 30.47 29.79 30.33 29.88 30.50 30.26 30.47 29.81 30.60 29.97 30.30
where g is the S/N ratio for the larger-the-better case, yi is the measured data obtained from experiment and n corresponds to the number of samples in each test trial. The optimum factor levels with the largest S/N ratios can then be summarized, which will minimize sensitivity over the range of noises. Table 5 shows the S/N ratios obtained using the Taguchi method. Then, the contribution percentage of each factor can be found and the optimum set of parameters driving the effective factors in this molding processes can be determined to obtain a product with maximum tensile strength. 3.1. Significance of processing parameters The experimental data can also be analyzed using Analysis of Variance (ANOVA) method. The purpose of the ANOVA is to investigate which process parameter significantly affects the quality characteristic. From this method, the percentage contribution has been calculated to determine which factor will affect the tensile strength significantly. 3.2. Confirmation experiments The final step of Taguchi method is to predict and confirm the improvement in the tensile properties using the optimal level for processing tensile specimens. One major purpose of the confirmation experiments is to provide evidence that shows the additive equation applies and that interactions are low. If the experimental results are within the range of the confidence interval (CI), the experiments have succeeded. Otherwise, it reveals that interactions are significant. The experimental procedure should be redesigned. In this case, confidence intervals are typically stated at the 95% confidence level. The 95% confidence interval for the predicted mean on a confirmation experiments can be calculated using the following equation [14]:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 1 1 CI ¼ F a;ð1;fe Þ V e þ neff R
Then $
g ¼ g CI
ð3Þ
$
where g = the confidence interval of the predicted optimal tensile strength, g = the predicted mean of tensile strength. 4. Results and discussion 4.1. Taguchi analysis Based on the values listed in Table 3, the Taguchi method was applied to analyze the UHMWPE strength of specimens #1 to #5. Since the strength without weld lines is the ‘‘Larger-the-Better” type, the optimal combination of studied factors for the largest strength can be identified from the response plots. Fig. 3(a) illustrates the effects of processing factors on the without weld line strength from experimental results. The factor levels that will generate the largest without weld line strength are found to be at melt temperature of 250 °C, mold temperature of 90 °C, injection velocity of 180 mm s1, packing pressure of 20 MPa, packing time of 6 s and cooling time of 6 s (A1B3C2D1E3F2). Other effects of processing factors of specimen #2 to #5 are shown in Figs. 3(b) to (e), respectively. The confirmation test is very important in Taguchi analysis to validate maximum tensile strength resulted from optimization process. The 95% CI of specimens #1 to #5 are 29.53–29.70 dB, 29.91–30.97 dB, 25.03–35.43 dB, 32.01–32.78 dB and 30.25– 31.18 dB, respectively. Based on the optimum set of processing variables, the tensile strength of specimens #1 to #5 is measured to confirm the experiment. S/N ratios based on experimental results are 29.65 dB, 30.76 dB, 29.62 dB, 32.37 dB and 30.79 dB which are within the CI range, respectively. This means that the Taguchi method is applied appropriately in this case. The quality product have mean values of tensile strength are 30.37 MPa, 34.51 MPa, 30.27 MPa, 41.55 MPa and 34.63 MPa, respectively. 4.2. Parametric analysis
ð2Þ
where Fa;(1,fe) = F ratio required for a, a = risk, fe = error DOF (degrees of freedom), Ve = error variance, neff = effective number of replications = N/1 + (total DOF associated in the estimate of mean), N = total number of experiments, R = number of replications for confirmation experiments.
In order to investigation the effect of injection molding parameters for UHMWPE, more experiments are carried out by changing one factor at a time and keeping the others constant. Melt temperature, mold temperature and injection velocity are investigated to study their effects on the mechanical strength of specimens with and without weld lines. Table 6 identifies the specimens and gives the single-factor experiment parameters.
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Fig. 3. S/N graph for (a) specimen #1; (b) specimen #2; (c) specimen #3; (d) specimen #4 and (e) specimen #5.
Table 6 Parameters set of single-factor experiments. Specimen
Melt temperature (°C)
Mold temperature (°C)
Injection velocity (mm s1)
Packing pressure (MPa)
Packing time (s)
Cooling time (s)
M250 M265 M280 m50 m70 m90 V150 V180 V210
250 265 280 265 265 265 265 265 265
70 70 70 50 70 90 70 70 70
180 180 180 180 180 180 150 180 210
40 40 40 40 40 40 40 40 40
4 4 4 4 4 4 4 4 4
6 6 6 6 6 6 6 6 6
4.2.1. Tensile strength of without a weld line The effects of melt temperature, mold temperature and injection velocity on the tensile strength are shown in Fig. 4. Packing pressure, Packing time, cooling time and back pressure are constant and the values are 40 MPa, 4 s, 6 s and 8 MPa, respectively. It can be found that the process parameters have little effect on the tensile strength. However, the dimensions of the molded specimens strongly affect the tensile strength. When the melt temperature, mold temperature, injection velocity, packing pressure, packing time and cooling time are set to 265 °C, 70 °C, 180 mm s1,
40 MPa, 4 s and 6 s, the measured tensile strength of the five specimens is 28.5 MPa, 31.25 MPa, 24.6 MPa, 37.37 MPa and 33.03 MPa, respectively. Specimen #1, with a normal cross-section of 5 mm 2 mm, has a small tensile strength. Specimen #3, with the smallest cross-section of 0.3 mm 0.3 mm, has the smallest tensile strength. In the previous study [9], if the viscosity of polymer was lower (e.g. polypropylene, PP) and the cross-section was the smallest (e.g. 0.3 mm 0.3 mm), then it presented the largest tensile strength. However, in this case, the UHMWPE is a polymer of high viscosity, which may lead to frictional heat produced in
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Fig. 4. Effects of (a) melt temperature, (b) mold temperature and (c) injection velocity of UHMWPE specimens.
filling stage. This further indicates that the decrease in tensile strength, due to the viscosity of polymer, is more related to the cross-section dimensions. Specimens #4 and #5 present the largest tensile strengths because of having a thickness of only 0.3 mm. The tensile strength of specimen #4 is higher than that of specimen #5 because the width of the former is smaller than the width of the latter.
Albeit the areas of specimens #2 and #4 are the same, the latter is stronger than the former. This result can be interpreted by molecular orientation. If the shape of the dog-bone region is alike to the shape of the gate (as in specimens #4 and #5), the tensile strength is larger. The reason is that the molecular orientation can be kept through the gate to the dog-bone region. The gate for specimen #2 has a cross-section of 2 mm in width
Fig. 5. Effects of (a) melt temperature, (b) mold temperature and (c) injection velocity on the weld line of UHMWPE specimens.
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and 0.14 mm in thickness. The dog-bone region has a cross-section of 0.3 mm in width and 2 mm in thickness. These two cross-sections are not alike. These results agree with [9], which
indicated that when the polymer flows through the gate and then enters into the dog-bone region, the molecular orientation will be constrained and rearranged by the gate. This behavior
Fig. 6. SEM and OM micrographs of tensile fracture surfaces of UHMWPE specimen: (a) without weld line on #1, (b) with weld line on #1, (c) without weld line on #2, (d) with weld line on #2, (e) without weld line on #4, (f) with weld line on #4, (g) without weld line on #5 and (h) with weld line on #5.
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influences molecular orientation and thereby decreases tensile strength. The earlier results can be interpreted by the characteristic of molecular orientation and crystallinity. A larger tensile strength is obtained in the melt flow direction. This could be attributed to the molecular orientation parallel with the melt flow direction. Constrained by the walls of a cavity, flow-induced crystallization occurs. A thin and narrow cavity restricts the molecular orientation and forces the molecules to stretch parallel to the cavity walls. Fig. 6(a) shows that the highest tensile strength of five specimens occurred at a melt temperature of 265 °C. The tensile strength of five specimens decreases whether the melt temperature increases or decreases. However, the lowest tensile strength of these five specimens was obtained at a melt temperature of 280 °C. The tensile strength of five specimens for different mold temperature is shown in Fig. 6(b). It indicates that highest tensile strength was mostly obtained at a mold temperature of 90 °C. Nevertheless, specimen #3 and #5 has the smallest tensile strength for mold temperature at 90 °C. The tensile strength of five specimens for different injection velocity is shown in Fig. 6(c). The highest tensile strength occurs for an injection velocity of 210 mm s1 except the specimen #3. The smallest cross-section causes a higher
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frictional heating which leads to decrease in tensile strength during filling process. From the above experimental observations, four conclusions can be summarized as follows: (1) The related parameters have little effect on the tensile strength for UHMWPE specimens. (2) Although specimens #2 and #4 have the same area, the gate shape is very different from the cross-section of specimen #2. Hence, the strength of specimen #2 is weaker than specimen #4. The phenomenon can be explained by the truth that specimen #4 has a faster molecular rearrangement rate and better molecular orientation than specimen #2. (3) Specimen #4 has the largest tensile strength. Specimen #5 has a lower tensile strength, followed by #2 and #1. Specimen #3 has the lowest tensile strength. (4) The tensile strength is significantly influenced by the crosssectional dimensions than the process parameters. 4.2.2. Tensile strength of with a weld line The process parameters are the same as those for the withoutweld-line specimen. In Fig. 5, the influences of the process param-
Fig. 7. Scanning electron micrographs of welded region on UHMWPE specimens: (a) specimen #1, (b) specimen #2, (c) specimen #3, (d) specimen #4 and (e) specimen #5.
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eters on the specimen with a weld line brought by conducting a single-factor experiment are shown. Effects of melt temperature, mold temperature and injection velocity on the tensile strength can be obtained. To clarify the parametric effects on the weld line strength, the with-weld-line strength are compared with that without weld lines. From these figures, it can be found that the process parameters create greater influences on the tensile strength of specimens with a weld line than the cross-sectional dimension of the dog-bone region. Given that the width and thickness of the cross-section of the dog-bone region are smaller (e.g. specimen #4 (2 mm 0.3 mm)), the tensile strength of the specimen drops at a more significant range (compared with the specimen without a weld line). Contrarily, given that the width and thickness of the cross-section are bigger (e.g. specimen #1 (5 mm 2 mm)), the tensile strength of the specimen drops within a less significant range (compared with the specimen without a weld line). There are two possible reasons leading to such phenomenon. First, the weld line of a specimen is formed when two or more melt fronts meet and bond. The orientation of molecules at melt front is usually parallel to each other, which creates less molecular bonding at weld line and further weakens the tensile strength of weld line. As a result, a specimen with smaller crosssectional dimensions presents less molecular bonding than the one with bigger cross-sectional dimensions. The other reason involves the thickness of a specimen. With smaller specimen thickness, the melt temperature drops faster in the process when melt fronts from the two sides meet. Thus incomplete bonding of the molecular chains at the meeting point of melt fronts appears. The specimen with bigger thickness carries sufficiently high temperature enabling the molecular chains from both sides to pass through melt front interface and create more bonding. Therefore, its tensile strength drops within a smaller range compared with that presented from a specimen with smaller thickness. In addition, specimen #2 presents greater tensile strength than specimen #4 under the condition of having a weld line, which explains that the molecular orientation rearrangement in the melt filling process, due to bigger width and thickness differences between the gate and the cross-section of the dog-bone region, reduces the influence of a weld line on tensile strength. 4.3. Microscopy Using the optimum set of effective factors obtained from the previous analysis, photographs of the tensile fracture surfaces of UHMWPE are taken. Pictures of specimens #1 to #5 are shown in Fig. 6. The specimen #1(a) without a weld line shows the neck fracture at the center. The specimen #1(b) presents the broken part at the center of a weld line. This phenomenon can be explained by frictional heating in mold walls. The frictional heating creates a better molecular bonding or self-diffusion in the frozen layer of two sides on the weld line region. This is why the tensile strength of specimen #1 has smaller difference than others between specimens with and without a weld line. Photos (c), (e) and (g) show the random tensile fracture surfaces at the center of specimen. Photos (d), (f) and (h) show the tensile fracture surfaces along the weld line. These photographs show that the thickness, width, with or without a weld line cause the effect on determining the specimen fracture shape. SEM was applied to UHMWPE specimens with a weld line. SEM images of UHMWPE specimens are shown in Fig. 7. These figures are the magnified pictures at the center of the welded region. Observing from the pictures, the weld lines and V-shaped notches were either absent or difficult to distinguish from surface. However, severer surface defects like scratches appeared on the welded region of UHMWPE. Besides, the micro-voids, cracks and incomplete molecular bonding also appeared at the welded region. These
observations demonstrate that surface defects result in weak and unstable mechanical strength in welded specimens.
5. Conclusions From the experimental analysis and results, the following conclusions can be drawn: (1) UHMWPE can be successfully filled in small channels and cavities by injection molding processes. (2) Taguchi optimization method is applied to find the optimal process parameters leading to maximum tensile strength during injection molded process. Taguchi method is suitable to improve the mechanical properties occurring the injection molded UHMWPE parts. (3) For the UHMWPE specimens without weld line, the crosssectional dimensions cause a higher effect on the tensile strength than the process parameters do. For the most part, the better injection molding parameters for higher tensile strength are a melt temperature of 265 °C, mold temperature of 90 °C and injection velocity of 210 mm s1. (4) The production of weld line reduces the effects of process parameters and cross-sectional dimensions on tensile strength. (5) In the thicker specimen, such as specimen #1, the frictional heating in the filling process leads to an increased molecular bonding and self-diffusion of the UHMWPE. Therefore, it can be seen as a benefit in term as enhancing strength on the weld lines. (6) Micro-voids, cracks and incomplete molecular bonding are the major surface defects on the weld line region. The most severely surface defect of weld line is always found in the specimen #1 due to the thickness effect.
Acknowledgements We would like to thanks to Prof. Cheng-Hsien Wu of the Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences, Taiwan, Prof. Hai-Wei Chi of the Department of Mechanical and Automation Engineering, Da-Yeh University, Taiwan and Prof. Chung-Te Lee of the Department of Environment Engineering, National Central University, Taiwan. Their assistance with experimental equipment support is appreciated. This study was funded by the National Science Council, Taiwan. Project No. NSC-97-2221-E-008-013-MY3. References [1] Xiong DS. Friction and wear properties of UHMWPE composites reinforced with carbon fiber. Mater Lett 2005;59:175–9. [2] Boscoletto AB, Franco R, Scapin M, Tavan M. An investigation on rheological and impact behavior of high density and ultrahigh molecular weight polyethylene mixtures. Eur Polym J 1997;33(1):97–105. [3] Wang SB, Ge SR. The mechanical property and tribological behavior of UHMWPE: effect of molding pressure. Wear 2007;263:949–56. [4] Song J, Liu P, Cremens M, Bonutti P. Effects of machining on tribological behavior of ultrahigh molecular weight polyethylene (UHMWPE) under dry reciprocating sliding. Wear 1999;225–229:716–23. [5] Yao D, Kim B. Simulation of the filling process in micro channels for polymeric materials. J Micromech Microeng 2002;12:604–10. [6] Wimberger-Friedl R. Injection molding of sub-mm grating optical elements. J Inject Mold Technol 2000;4:78–83. [7] Fujiama M, Awaya H. Mechanical anisotropy in injection-molded polypropylene. J Appl Polym Sci 1977;7:3291–309. [8] Huilier D, Lenfant C, Terrisse J, Deterre R. Modeling the packing stage in injection molding of thermoplastics. Polym Eng Sci 1988;28:1637–43. [9] Wu CH, Liang WJ. Effects of geometry and injection-molding parameters on weld-line strength. Polym Eng Sci 2005;45:1021–30.
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