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The influence of injection molding on tribological characteristics of ultra-high molecular weight polyethylene under dry sliding
Wear 268 (2010) 803–810
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The influence of injection molding on tribological characteristics of ultra-high molecular weight polyethylene under dry sliding Hsien-Chang Kuo, Ming-Chang Jeng ∗ Department of Mechanical Engineering, National Central University, Chung-Li 32054, Taiwan
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Article history: Received 8 July 2009 Received in revised form 26 November 2009 Accepted 7 December 2009 Available online 4 January 2010 Keywords: Tribology Ultra-high molecular weight polyethylene Friction Wear Injection molding
a b s t r a c t The objective of this study is to investigate the effects of various injection molding process parameters on the tribological properties of ultra-high molecular weight polyethylene (UHMWPE). The tribological properties, such as the friction coefficient and wear volume loss, were obtained using the Schwingum Reibung Verschleiss (SRV, oscillation friction wear) ball-on-plane wear tester. In addition, the mechanical properties of UHMWPE were investigated as well. The variable parameters of the injection molding process were melt temperature, mold temperature and injection velocity. Experimental results show that different wear contact loads and varied injection molding conditions influence the friction coefficient and wear volume loss of the UHMWPE specimens. As the sliding contact loads increased, the friction coefficient also increased. Moreover, the lowest wear volume loss mostly occurred in highest injection molding conditions. The morphologies of the worn surfaces and the specimen cross sections were examined with an optical microscope and a scanning electron microscope, respectively. Plastic deformations, grooves and wavelike formations are the main wear mechanism on the surface in the UHMWPE wear tests. Experimental results also showed that the tensile strength and surface hardness are affected by injection molding conditions and sliding contact loads. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a polymer with extremely high molecular weight. It possesses excellent wear resistance, high impact strength, good sliding quality, and low friction loss, and its self-lubrication performance can be widely used in engineering applications [1–3]. Unal and Mimaroglu [4] studied the wear behavior of contact loads and sliding speeds. The authors found that the friction coefficient of UHMWPE slightly increases as the load increases, but the wear rate was not affected. Moreover, the worn surface of UHMWPE was wrinkled. Atkinson et al. [5] indicated that adhesive and fatigue on the surface of UHMWPE were the two main wear mechanisms. The adhesive process occurred quickly after sliding, and fatigue only appeared after long periods of sliding. Song et al. [6] studied the effects of machining on the tribological behavior of UHMWPE under dry wear; they found that severe plastic deformation and ploughing are the main wear mechanisms. Bartenev and Lavrentev [7] observed wear mechanisms; when the UHMWPE is against rigid bodies, the main wear mechanisms are fatigue and abrasion. Da Silva and Sinatora [8] developed severity conditions for UHMWPE wear; they observed the worn surface analysis and found that the three wear
∗ Corresponding author. Tel.: +886 3 4267331. E-mail address: [email protected] (M.-C. Jeng). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.12.012
mechanisms include abrasion, fatigue and adhesion. Some reports have studied the relationship between wear behavior in contact loads and/or UHMWPE sliding speeds and its composites. However, these samples of UHMWPE are mostly made by compression molding [2,4,6,9,10,15,16,19]. In order to more understand the difference in mechanical properties for common UHMWPE and some relevant polymers with this study, such as the wear test conditions, friction coefficient, hardness and molecular weight are listed in Table 1. Furthermore, some papers reported that the thermal effect on wear mechanisms of UHMWPE, frictional heating, might be related to the wear of UHMWPE through softening or some other effects [11–15].The review of the above mentioned publications shows that most UHMWPE specimens are processed by compression molding or extrusion molding because the viscosity of UHMWPE is very high, which means it does not flow well. Therefore, it is difficult to mold by injection molding [16]. Recently, an UHMWPE material has been improved such that it is suitable for injection molding technology. Furthermore, injection molding is one of the most important processes. An important advantage of injection molding is that the material can be easily shaped into complex geometries in a short production cycle or in a single production step with an automated process. Therefore, the main purposes of this study are the following: (1) to investigate the relationships between molecular orientation and different injection molding conditions; (2) to understand the effects of tribological characteristics of UHMWPE wear parameters; (3) to observe the
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Table 1 Referenced polymers and their mechanical and physical properties. Materials Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE POM POM PA 6 PA 6 PA 66 a
Against material
Molecular weight
Wear load
Wear speed
Friction coefficient
Hardness
Reference
316 L stainless steel X5CrNi18-10 Stainless steel 45 # steel AISI D2 steel 316 L stainless steel ZrO2 ball Stainless steel AISI D2 steel 316 L stainless steel Stainless steel AISI D2 steel
1.5 × 10 g/mol 5.0 × 106 g/mol n/a 3.5 × 106 g/mol n/a 3.0 × 106 g/mol 3.0 × 106 g/mol n/a n/a 3.5 × 104 g/mol n/a n/a
1, 2.5 MPa 21.2 N 20, 30, 40 N 15, 30, 50 kg 0.35, 0.70, 1.05 MPa 196 N 5N 20, 30, 40 N 0.35, 0.70, 1.05 MPa 1, 2.5 MPa 20, 30, 40 N 0.35, 0.70, 1.05 MPa
0.5 m/s 28.2 mm/s 0.88 m/s 200 rev/min 1 m/s 0.42 m/s 0.19 m/s 0.88 m/s 1 m/s 0.5 m/s 0.88 m/s 1 m/s
0.10, 0.16 0.12 0.15, 0.18, 0.20 0.09-0.10 0.41, 0.34, 0.27 0.21 0.09 0.12, 0.17, 0.15 0.37, 0.35, 0.29 0.20, 0.26 0.09, 0.11, 0.12 0.52, 0.42, 0.37
Hk25 g 21.2 n/a n/a Shore D 65 n/a HB 49 HB 30 n/a n/a Hk25 g 29.8 n/a n/a
[2]a [3] [4]a [11] [12] [15]a [23] [4] [12] [2] [4] [12]
6
The UHMWPE samples are made by compression molding.
wear mechanism on worn surfaces by optical microscope (OM) and scanning electron microscope (SEM). 2. Experimental 2.1. Materials The material used in this study is an injection molding grade of ultra-high molecular weight polyethylene (UHMWPE, GUR5113, from Ticona, USA). It is a linear polyolefin resin. The bulk density is 0.93 g/cm3 , and the molecular weight is 3.9 × 106 g/mol. Other physical and mechanical properties of UHMWPE are listed in Table 2. 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 80 ◦ C for 3 h using a dehumidifying drier before use in the injection molding machine. 2.2. Specimen preparations The mold design is used to generate a tensile specimen, and the dimensions are based on the ASTM procedure D638 (Type IV). The mold is made of tool steel. One semicircular rotary plug is designed to allow the part to be molded with or without a weld line. A wear specimen is cut from the central position of the injection molding specimens. The specimen design and cut are shown in Fig. 1. All wear specimens are prepared on a FANUC electric injection molding machine (ROBOSHOT S-2000i 50A). The machine can offer a maximum clamping force of 50 tons and a maximum injection velocity of 330 mm/s. 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 temperatures. The molded parts are cooled for 6 s in the injection molding period. Under each set of injection molding conditions, 10 shots are made to ensure that the process is stable before specimens are collected. If no significant variation is observed during these first 10 runs, the specimens from the next five runs are collected as the samples for tribological characterization.
2.3. Injection molding conditions In this study, three basic conditions of the injection molding, the melt temperature (250, 265 and 280 ◦ C), mold temperature (50, 70 and 90 ◦ C) and injection velocity (150, 180 and 210 mm/s), are varied. The levels of the molding factors are selected through our initial tests and recommended by the manufacturer. The specimens and the injection molding conditions are listed in Table 3. 2.4. Sliding wear tests The wear tests were done on a Schwingum Reibung Verschleiss (SRV oscillation friction wear) tester, as shown in Fig. 2. The wear test specimens with dimensions 5.0 mm × 5.0 mm × 2.0 mm were cut from the central part of the tensile specimen. Before wear test, the UHMWPE surface was not ground and polished. Surface roughness of the UHMWPE specimens was measured by using a HANDYSURF E-35A profilometer. Three specimens were measured for each injection molding condition to obtain an average roughness value Ra . The measured surface roughness data are listed in Table 4. Furthermore, surface roughness of a weld line was always
Table 2 The physical and mechanical properties of UHMWPE. 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/l g/cm3 MPa MPa % ◦ C g/mol
0.93 2000 0.5 750 17 20 80 3,900,000
Fig. 1. Dimensions of the molded specimens (a) without a weld line and (b) with a weld line.
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Table 3 Injection molding conditions for wear specimens. Specimen
Melt temperature (◦ C)
Mold temperature (◦ C)
Injection velocity (mm/s)
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
wear mass loss and the density of the specimen. The wear mass loss of the specimens was measured by an electronic analytical balance (METTLER AT201) with a precision of 0.01 mg. Each sliding wear test was repeated three times and average values were reported. The friction coefficient data were recorded using a NI PCI-6024E data acquisition board (DAQ card) that was controlled by a LabVIEW virtual instrument program. Both parallel (P-direction) and perpendicular (AP-direction) directions relative to the melt flow were taken to be the slide direction of the SRV wear tests. All tests were carried out under dry conditions in air at an ambient temperature. 2.5. Tensile tests Fig. 2. Schematic diagram of the SRV wear tester.
higher than without a weld line, due to the defects and incomplete molecular bondings occurred on the weld line region, and the surface roughness, Ra , of above 1.5 m is obtained. The SRV wear tests were performed in ball-on-plane contact with a load of 25 and 100 N. The stroke length was fixed at 2 mm, while the frequency of oscillation and test duration time of each specimen were 25 Hz and 4 h, respectively. The above stroke, frequency and test duration conditions resulted in a total sliding distance of 1.44 km. Wear test conditions are listed in detail in Table 5. The ball is a chromium steel ball (AISI E52100), 10 mm in diameter, with an average hardness of 62 ± 2 HRC. The ball and polymer plate specimens were cleaned with alcohols and dried prior to testing. The wear mass loss was measured after each wear test, and the volume loss of the specimen was calculated from the measured
The tensile specimens were tested with a tensile tester (PT1000, Perfect International Instrument Co. Ltd.) at a crosshead rate of 25 mm/min. The clamping and the marking intervals were 15 and 10 mm, respectively. A load cell with a maximum loading of 1 kN was used for all specimens. 2.6. Hardness tests The hardness measurement was taken using a Vickers hardness tester (FUTURE TECH. Corp., Tokyo, Japan), which has a squarebased pyramid diamond indenter with an angle of 136◦ between the opposite faces at the vertex. The test condition was at a load of 9.8 N for a duration of 20 s at room temperature. 3. Results and discussion 3.1. Friction behavior
Table 4 Surface roughness of without weld line specimens by different injection molding conditions, slid parallel (P) and perpendicular (AP) to the melt flow direction. Specimen
Surface roughness, Ra (m) P-direction
M250 M265 M280 m50 m70 m90 V150 V180 V210
0.50 0.57 0.60 0.80 0.70 0.83 0.70 0.80 0.73
± ± ± ± ± ± ± ± ±
0.10 0.06 0.10 0.26 0.10 0.40 0.10 0.35 0.30
AP-direction 0.50 0.50 0.60 0.50 0.57 0.77 0.56 0.48 0.47
± ± ± ± ± ± ± ± ±
0.03 0.10 0.20 0.09 0.06 0.15 0.06 0.07 0.06
Table 5 Wear test conditions. Factor
P-direction
AP-direction
Test duration (h) Contact load (N) Frequency (Hz) Stroke (mm) Sliding distance (m)
4 25/100 25 2 1440
4 25/100 25 2 1440
The friction coefficient values for UHMWPE tested in dry sliding conditions, at room temperature, are 25 and 100 N loads with a fixed sliding velocity (100 mm/s). The mean values of the friction coefficient from the ball-on-plane SRV wear tests for different injection molding conditions are shown in Fig. 3. Fig. 3(a) shows the variation of the specimen friction coefficients with melt temperatures of 250, 265 and 280 ◦ C, the sliding direction was either parallel or perpendicular to the melt flow direction. For a contact load of 100 N, the friction coefficient increases in proportion to the increasing melt temperature. For a contact load of 25 N, the highest friction coefficient occurs at a melt temperature 265 ◦ C. The friction coefficient decreases whether the melt temperature increases or decreases. Fig. 3(b) shows the variation of the specimen friction coefficients with the mold temperature at 50, 70 and 90 ◦ C, the sliding direction was either parallel or perpendicular to the melt flow direction. This indicates that for contact loads of 25 and 100 N, the friction coefficient of parallel and perpendicular to the melt flow direction is close and the friction coefficient decreases with increase in mold temperature. Fig. 3(c) shows the variation of the specimen friction coefficients with injection velocities of 150, 180 and 210 mm/s. It indicates that the friction coefficient for flow directions parallel and perpendicular to the melt flow direction increases
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Fig. 4. Wear volume loss for sliding directions parallel and perpendicular to the melt flow direction with injection molding conditions in (a) melt temperature, (b) mold temperature and (c) injection velocity. Fig. 3. Mean value of the friction coefficient with different sliding contact loads and injection molding conditions: (a) melt temperature, (b) mold temperature and (c) injection velocity.
as the injection velocity increases when the sliding contact loads are 25 and 100 N. These experimental results demonstrate that the friction coefficients for a sliding contact load of 100 N are higher than those for a sliding contact load of 25 N. This phenomenon is in agreement with the results obtained in previous literature [2,4]. It can be attributed to the fact that UHMWPE is a viscoelastic material and its deformation under contact load is viscoelastic. As a result, the variation of the friction coefficient with contact load will increase because of the critical surface energy of the UHMWPE. Moreover, the rising temperature of the friction surface is caused by friction heating because higher frictional temperature could deform UHMWPE molecule chains. For a contact load of 100 N, the friction coefficients in the sliding direction perpendicular to the melt flow direction are higher than those that slide parallel to the melt flow direction. Furthermore, the injection molding conditions also affect the UHMWPE friction coefficient. 3.2. Wear behavior The mean value of specimen wear volume loss sliding either parallel or perpendicular to the melt flow direction from the SRV ball-on-plane wear test (under a contact load of 100 N) is shown
in Fig. 4. Because the wear mass loss is difficult to measure under a contact load of 25 N, it is not discussed in this section. The wear volume loss for different melt temperatures is shown in Fig. 4(a). It indicates that the lowest volume loss was obtained at a melt temperature of 280 ◦ C with a sliding direction perpendicular to the melt flow direction. The same result is obtained with a sliding direction parallel with the melt flow direction. However, the wear volume loss in the sliding direction parallel to the melt flow direction is greater than that of the sliding direction perpendicular to the melt flow direction. The wear volume loss for different mold temperatures is shown in Fig. 4(b). The lowest volume loss occurs at a mold temperature of 90 ◦ C with a sliding direction parallel to the melt flow direction. Similarly, the variation in wear volume loss with mold temperature shows the same trend as for the melt temperature. The wear volume loss in the sliding direction perpendicular to the melt flow direction is generally higher than that in the sliding direction parallel with the melt flow direction. The volume loss for different injection velocities is shown in Fig. 4(c). It also shows the same trend as that of the melt temperature. However, the lowest wear volume loss was obtained for an injection velocity at 210 mm/s with a sliding direction parallel to the melt flow direction. From these experimental results, it can be seen that the highest process conditions in this study cause the lowest wear volume loss. The wear volume loss was between 0.039 and 0.222 mm3 in two different sliding directions. Therefore, it can be concluded that the wear volume loss in the two different
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Fig. 5. Effect of different injection molding conditions on friction and wear of UHMWPE for different sliding directions relative to the melt flow direction: (a) parallel and (b) perpendicular direction.
sliding directions was slightly influenced by the injection molding conditions. Fig. 5 shows the effect of different injection molding conditions on the wear and tensile strength of the UHMWPE for sliding directions parallel and perpendicular to the melt flow direction. The increase in friction coefficient that is observed under different injection molding conditions is attributed to the frozen layer. It can be seen that higher friction coefficients are found at a melt
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temperature of 280 ◦ C, mold temperature of 70 ◦ C and injection velocity of 210 mm/s when the sliding direction is parallel to the melt flow direction. Furthermore, the lowest wear volume loss is found at a melt temperature of 280 ◦ C, mold temperature of 90 ◦ C and injection velocity of 210 mm/s, when the sliding direction is parallel to the melt flow direction. Similarly, the variation in friction coefficient and wear volume loss with different injection molding conditions show the same trend as for the sliding direction perpendicular to the melt flow direction. For the range of different injection molding conditions of this investigation, the wear volume loss values for UHMWPE specimens decrease or increase as the injection molding conditions decrease or increase. The reason for this behavior is because melt temperature, mold temperature and injection velocity change the surface properties, such as the viscosity or the density of the melt, and may result in a higher temperature gradient near the mold wall because of the heat transfer effect [17]. Therefore, the specimen will be formed at different thicknesses of the frozen layers for different injection molding conditions (see Fig. 6). These frozen layers are associated with the friction coefficient and wear volume loss values [17,18]. These papers have been reported that the minimum frozen layer thickness causes a lower friction coefficient and volume loss occurred. Although, the different thicknesses of frozen layers of UHMWPE for different injection molding conditions are also observed in this case, however, the friction coefficient and the wear volume loss are not significantly influenced by the frozen layer thickness. This result could be explained by the effectiveness of the formed transfer film on the worn surface. On the other hand, in the repeated sliding processes, the heat accumulated in the wear process causes thermal softening of the UHMWPE. The softened surface transferred and spread over the chromium steel ball counterface, formed a thin film and acted as a lubricant. Thin film between the sliding surfaces provides good lubrication, which reduces the adhesive interaction between the specimen and contacting surface and reduces the shear stresses due to the lubricating action of the UHMWPE transfer film on the specimen surface; thus the wear volume loss decreased [19]. From
Fig. 6. Microscopy of the cross section of a UHMWPE specimen: (a) melt temperature of 280 ◦ C, (b) mold temperature of 50 ◦ C and (c) injection velocity of 180 mm/s.
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Fig. 7. The OM micrographs on the worn surface of UHMWPE after SRV oscillation friction wear testing under a load of 25 N for a melt temperature of 280 ◦ C, mold temperature of 70 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
Fig. 8. The OM micrographs on the worn surface of UHMWPE after SRV oscillation friction wear testing under a load of 100 N for a melt temperature of 265 ◦ C, mold temperature of 90 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
these observations, it can be concluded that the friction coefficient and the wear volume loss are significantly affected by the injection molding conditions and frictional heating. 3.3. Wear mechanisms The optical microscope micrographs on the worn surface of UHMWPE with and without the weld line after SRV oscillation friction wear testing under different sliding contact loads and injection molding conditions are shown in Figs. 7–9. Fig. 7(a) shows the worn surface of UHMWPE without a weld line under a sliding contact load of 25 N and the following injection molding conditions: melt temperature 280 ◦ C, mold temperature 70 ◦ C, injection velocity 180 mm/s and sliding direction parallel to the melt flow direction. The worn surface shows some grooves and definite wavelike formation. Fig. 7(b) shows the worn surface for sliding direction perpendicular to the melt flow direction. The worn
surface microscopic observation shows that the wear morphologies are scratches that have a small wavelike formation. Fig. 8(a) shows the worn surface of UHMWPE without a weld line under a sliding contact load of 100 N and the following injection molding conditions: melt temperature 265 ◦ C, mold temperature 90 ◦ C, injection velocity 180 mm/s and sliding direction parallel to the melt flow direction. The worn surface shows a few grooves and a few severe wavelike formations. Fig. 8(b) shows the worn surface for sliding direction perpendicular to the melt flow direction. The OM image of the worn surface shows an equal number of grooves and small wavelike formations. Fig. 9(a) and (b) shows the wear mechanisms of a worn surface with a UHMWPE weld line under a sliding contact load of 25 N and the following injection molding conditions: melt temperature 265 ◦ C, mold temperature 70 ◦ C, injection velocity 180 mm/s and sliding direction parallel and perpendicular to the melt flow direction. These wear morphologies appear the same in the wear mechanisms without weld lines. In addition, the weld
Fig. 9. The OM micrographs on the worn surface of weld line of UHMWPE after SRV oscillation friction wear testing under a load of 25 N for a melt temperature of 265 ◦ C, mold temperature of 70 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
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Fig. 10. Effect of different injection molding conditions on wear and tensile strength of UHMWPE for sliding directions (a) parallel and (b) perpendicular to the melt flow direction.
line mark seems to be removed by wear tests. Because of the heating effect of friction, which promoted a temperature rise, and thus softened the UHMWPE surface, there is a wrinkled and wavelike morphology occurrence on the UHMWPE surface. 3.4. Effect of tensile strength and wear volume loss Fig. 10 presents the relationship between the tensile strength and the wear volume loss of the UHMWPE in the sliding direction parallel and perpendicular to the melt flow direction, respectively. It is shown that the tensile strength of UHMWPE specimens slightly increases as the injection molding conditions increase. The wear volume loss mostly decreases with increases in tensile strength. The decrease of the wear volume loss is also due to the frictional heating generation on the specimen surface.
Fig. 11. The hardness of UHMWPE at different injection molding conditions: (a) melt temperature, (b) mold temperature and (c) injection velocity.
the polymer [20–22]. This indicates the contact load is an important factor for the hardness. 4. Conclusions
3.5. Effect of surface hardness Hardness is one of the most important material properties. It can be used as an indicator of the polymer tribological property. Fig. 11(a)–(c) shows the variation of Vickers hardness on the UHMWPE specimen surface under changing melt temperature, mold temperature and injection velocity with a sliding contact load of 25 or 100 N and a sliding velocity of 100 mm/s. It can be seen that the injection molding conditions have little effect on the hardness value of the UHMWPE specimens. Moreover, it can also be seen that the UHMWPE hardness decreases as the contact load increases. The UHMWPE specimen without wear has the greatest hardness. The UHMWPE specimen at a contact load of 25 N causes lesser hardness. The wear test at a contact load of 100 N causes the least hardness. These phenomena are attributed to surface softening due to frictional heating by dry sliding. Hence, the surface hardness decreases as the contact load increases. In other words, frictional heating probably changes the degree of crystallinity or the crosslinking of the polymer chains. In general, hardness is directly related to both the degree of crystallinity and any crosslinking of
The objective of this work is to investigate the tribological properties of UHMWPE by using an SRV wear tester. Parametric analysis was applied to study the influence of molding conditions on the wear specimen. The results were obtained from experimental observation and measurement. Thus, the following can be concluded from the results: 1. The friction coefficient of UHMWPE is influenced by sliding contact load. For the designated contact loads of 25 and 100 N in this work, the UHMWPE specimens sliding at a contact load of 100 N obtain higher friction coefficients. The friction coefficient for a specimen sliding perpendicular to the melt flow direction is slightly higher than that for a specimen sliding parallel to the melt flow direction. In addition, in the range of injection molding studied in the present work, the UHMWPE friction coefficient is affected by different injection molding conditions. 2. The lowest wear volume loss generally occurs under the highest injection molding conditions. Furthermore, the wear volume loss in the sliding direction parallel to the melt flow direction is
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generally lower than that in the sliding direction perpendicular to the melt flow direction. 3. From scanning electron microscopy, plastic deformation, grooves and wavelike formations on the surface of UHMWPE are the main wear mechanisms. A more severe wavelike worn surface is usually found in the sliding direction parallel to the melt flow direction. 4. The UHMWPE tensile strength mostly increases as the injection molding conditions increase, and the lowest wear volume loss mostly occurred in the specimens with the largest tensile strength. 5. The UHMWPE hardness decreases as the sliding contact loads increase, and the injection molding conditions have little impact on the hardness. Thus, the sliding contact load is an important factor for UHMWPE surface hardness.
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The influence of injection molding on tribological characteristics of ultra-high molecular weight polyethylene under dry sliding Hsien-Chang Kuo, Ming-Chang Jeng ∗ Department of Mechanical Engineering, National Central University, Chung-Li 32054, Taiwan
a r t i c l e
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Article history: Received 8 July 2009 Received in revised form 26 November 2009 Accepted 7 December 2009 Available online 4 January 2010 Keywords: Tribology Ultra-high molecular weight polyethylene Friction Wear Injection molding
a b s t r a c t The objective of this study is to investigate the effects of various injection molding process parameters on the tribological properties of ultra-high molecular weight polyethylene (UHMWPE). The tribological properties, such as the friction coefficient and wear volume loss, were obtained using the Schwingum Reibung Verschleiss (SRV, oscillation friction wear) ball-on-plane wear tester. In addition, the mechanical properties of UHMWPE were investigated as well. The variable parameters of the injection molding process were melt temperature, mold temperature and injection velocity. Experimental results show that different wear contact loads and varied injection molding conditions influence the friction coefficient and wear volume loss of the UHMWPE specimens. As the sliding contact loads increased, the friction coefficient also increased. Moreover, the lowest wear volume loss mostly occurred in highest injection molding conditions. The morphologies of the worn surfaces and the specimen cross sections were examined with an optical microscope and a scanning electron microscope, respectively. Plastic deformations, grooves and wavelike formations are the main wear mechanism on the surface in the UHMWPE wear tests. Experimental results also showed that the tensile strength and surface hardness are affected by injection molding conditions and sliding contact loads. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a polymer with extremely high molecular weight. It possesses excellent wear resistance, high impact strength, good sliding quality, and low friction loss, and its self-lubrication performance can be widely used in engineering applications [1–3]. Unal and Mimaroglu [4] studied the wear behavior of contact loads and sliding speeds. The authors found that the friction coefficient of UHMWPE slightly increases as the load increases, but the wear rate was not affected. Moreover, the worn surface of UHMWPE was wrinkled. Atkinson et al. [5] indicated that adhesive and fatigue on the surface of UHMWPE were the two main wear mechanisms. The adhesive process occurred quickly after sliding, and fatigue only appeared after long periods of sliding. Song et al. [6] studied the effects of machining on the tribological behavior of UHMWPE under dry wear; they found that severe plastic deformation and ploughing are the main wear mechanisms. Bartenev and Lavrentev [7] observed wear mechanisms; when the UHMWPE is against rigid bodies, the main wear mechanisms are fatigue and abrasion. Da Silva and Sinatora [8] developed severity conditions for UHMWPE wear; they observed the worn surface analysis and found that the three wear
∗ Corresponding author. Tel.: +886 3 4267331. E-mail address: [email protected] (M.-C. Jeng). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.12.012
mechanisms include abrasion, fatigue and adhesion. Some reports have studied the relationship between wear behavior in contact loads and/or UHMWPE sliding speeds and its composites. However, these samples of UHMWPE are mostly made by compression molding [2,4,6,9,10,15,16,19]. In order to more understand the difference in mechanical properties for common UHMWPE and some relevant polymers with this study, such as the wear test conditions, friction coefficient, hardness and molecular weight are listed in Table 1. Furthermore, some papers reported that the thermal effect on wear mechanisms of UHMWPE, frictional heating, might be related to the wear of UHMWPE through softening or some other effects [11–15].The review of the above mentioned publications shows that most UHMWPE specimens are processed by compression molding or extrusion molding because the viscosity of UHMWPE is very high, which means it does not flow well. Therefore, it is difficult to mold by injection molding [16]. Recently, an UHMWPE material has been improved such that it is suitable for injection molding technology. Furthermore, injection molding is one of the most important processes. An important advantage of injection molding is that the material can be easily shaped into complex geometries in a short production cycle or in a single production step with an automated process. Therefore, the main purposes of this study are the following: (1) to investigate the relationships between molecular orientation and different injection molding conditions; (2) to understand the effects of tribological characteristics of UHMWPE wear parameters; (3) to observe the
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Table 1 Referenced polymers and their mechanical and physical properties. Materials Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE Common UHMWPE POM POM PA 6 PA 6 PA 66 a
Against material
Molecular weight
Wear load
Wear speed
Friction coefficient
Hardness
Reference
316 L stainless steel X5CrNi18-10 Stainless steel 45 # steel AISI D2 steel 316 L stainless steel ZrO2 ball Stainless steel AISI D2 steel 316 L stainless steel Stainless steel AISI D2 steel
1.5 × 10 g/mol 5.0 × 106 g/mol n/a 3.5 × 106 g/mol n/a 3.0 × 106 g/mol 3.0 × 106 g/mol n/a n/a 3.5 × 104 g/mol n/a n/a
1, 2.5 MPa 21.2 N 20, 30, 40 N 15, 30, 50 kg 0.35, 0.70, 1.05 MPa 196 N 5N 20, 30, 40 N 0.35, 0.70, 1.05 MPa 1, 2.5 MPa 20, 30, 40 N 0.35, 0.70, 1.05 MPa
0.5 m/s 28.2 mm/s 0.88 m/s 200 rev/min 1 m/s 0.42 m/s 0.19 m/s 0.88 m/s 1 m/s 0.5 m/s 0.88 m/s 1 m/s
0.10, 0.16 0.12 0.15, 0.18, 0.20 0.09-0.10 0.41, 0.34, 0.27 0.21 0.09 0.12, 0.17, 0.15 0.37, 0.35, 0.29 0.20, 0.26 0.09, 0.11, 0.12 0.52, 0.42, 0.37
Hk25 g 21.2 n/a n/a Shore D 65 n/a HB 49 HB 30 n/a n/a Hk25 g 29.8 n/a n/a
[2]a [3] [4]a [11] [12] [15]a [23] [4] [12] [2] [4] [12]
6
The UHMWPE samples are made by compression molding.
wear mechanism on worn surfaces by optical microscope (OM) and scanning electron microscope (SEM). 2. Experimental 2.1. Materials The material used in this study is an injection molding grade of ultra-high molecular weight polyethylene (UHMWPE, GUR5113, from Ticona, USA). It is a linear polyolefin resin. The bulk density is 0.93 g/cm3 , and the molecular weight is 3.9 × 106 g/mol. Other physical and mechanical properties of UHMWPE are listed in Table 2. 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 80 ◦ C for 3 h using a dehumidifying drier before use in the injection molding machine. 2.2. Specimen preparations The mold design is used to generate a tensile specimen, and the dimensions are based on the ASTM procedure D638 (Type IV). The mold is made of tool steel. One semicircular rotary plug is designed to allow the part to be molded with or without a weld line. A wear specimen is cut from the central position of the injection molding specimens. The specimen design and cut are shown in Fig. 1. All wear specimens are prepared on a FANUC electric injection molding machine (ROBOSHOT S-2000i 50A). The machine can offer a maximum clamping force of 50 tons and a maximum injection velocity of 330 mm/s. 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 temperatures. The molded parts are cooled for 6 s in the injection molding period. Under each set of injection molding conditions, 10 shots are made to ensure that the process is stable before specimens are collected. If no significant variation is observed during these first 10 runs, the specimens from the next five runs are collected as the samples for tribological characterization.
2.3. Injection molding conditions In this study, three basic conditions of the injection molding, the melt temperature (250, 265 and 280 ◦ C), mold temperature (50, 70 and 90 ◦ C) and injection velocity (150, 180 and 210 mm/s), are varied. The levels of the molding factors are selected through our initial tests and recommended by the manufacturer. The specimens and the injection molding conditions are listed in Table 3. 2.4. Sliding wear tests The wear tests were done on a Schwingum Reibung Verschleiss (SRV oscillation friction wear) tester, as shown in Fig. 2. The wear test specimens with dimensions 5.0 mm × 5.0 mm × 2.0 mm were cut from the central part of the tensile specimen. Before wear test, the UHMWPE surface was not ground and polished. Surface roughness of the UHMWPE specimens was measured by using a HANDYSURF E-35A profilometer. Three specimens were measured for each injection molding condition to obtain an average roughness value Ra . The measured surface roughness data are listed in Table 4. Furthermore, surface roughness of a weld line was always
Table 2 The physical and mechanical properties of UHMWPE. 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/l g/cm3 MPa MPa % ◦ C g/mol
0.93 2000 0.5 750 17 20 80 3,900,000
Fig. 1. Dimensions of the molded specimens (a) without a weld line and (b) with a weld line.
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Table 3 Injection molding conditions for wear specimens. Specimen
Melt temperature (◦ C)
Mold temperature (◦ C)
Injection velocity (mm/s)
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
wear mass loss and the density of the specimen. The wear mass loss of the specimens was measured by an electronic analytical balance (METTLER AT201) with a precision of 0.01 mg. Each sliding wear test was repeated three times and average values were reported. The friction coefficient data were recorded using a NI PCI-6024E data acquisition board (DAQ card) that was controlled by a LabVIEW virtual instrument program. Both parallel (P-direction) and perpendicular (AP-direction) directions relative to the melt flow were taken to be the slide direction of the SRV wear tests. All tests were carried out under dry conditions in air at an ambient temperature. 2.5. Tensile tests Fig. 2. Schematic diagram of the SRV wear tester.
higher than without a weld line, due to the defects and incomplete molecular bondings occurred on the weld line region, and the surface roughness, Ra , of above 1.5 m is obtained. The SRV wear tests were performed in ball-on-plane contact with a load of 25 and 100 N. The stroke length was fixed at 2 mm, while the frequency of oscillation and test duration time of each specimen were 25 Hz and 4 h, respectively. The above stroke, frequency and test duration conditions resulted in a total sliding distance of 1.44 km. Wear test conditions are listed in detail in Table 5. The ball is a chromium steel ball (AISI E52100), 10 mm in diameter, with an average hardness of 62 ± 2 HRC. The ball and polymer plate specimens were cleaned with alcohols and dried prior to testing. The wear mass loss was measured after each wear test, and the volume loss of the specimen was calculated from the measured
The tensile specimens were tested with a tensile tester (PT1000, Perfect International Instrument Co. Ltd.) at a crosshead rate of 25 mm/min. The clamping and the marking intervals were 15 and 10 mm, respectively. A load cell with a maximum loading of 1 kN was used for all specimens. 2.6. Hardness tests The hardness measurement was taken using a Vickers hardness tester (FUTURE TECH. Corp., Tokyo, Japan), which has a squarebased pyramid diamond indenter with an angle of 136◦ between the opposite faces at the vertex. The test condition was at a load of 9.8 N for a duration of 20 s at room temperature. 3. Results and discussion 3.1. Friction behavior
Table 4 Surface roughness of without weld line specimens by different injection molding conditions, slid parallel (P) and perpendicular (AP) to the melt flow direction. Specimen
Surface roughness, Ra (m) P-direction
M250 M265 M280 m50 m70 m90 V150 V180 V210
0.50 0.57 0.60 0.80 0.70 0.83 0.70 0.80 0.73
± ± ± ± ± ± ± ± ±
0.10 0.06 0.10 0.26 0.10 0.40 0.10 0.35 0.30
AP-direction 0.50 0.50 0.60 0.50 0.57 0.77 0.56 0.48 0.47
± ± ± ± ± ± ± ± ±
0.03 0.10 0.20 0.09 0.06 0.15 0.06 0.07 0.06
Table 5 Wear test conditions. Factor
P-direction
AP-direction
Test duration (h) Contact load (N) Frequency (Hz) Stroke (mm) Sliding distance (m)
4 25/100 25 2 1440
4 25/100 25 2 1440
The friction coefficient values for UHMWPE tested in dry sliding conditions, at room temperature, are 25 and 100 N loads with a fixed sliding velocity (100 mm/s). The mean values of the friction coefficient from the ball-on-plane SRV wear tests for different injection molding conditions are shown in Fig. 3. Fig. 3(a) shows the variation of the specimen friction coefficients with melt temperatures of 250, 265 and 280 ◦ C, the sliding direction was either parallel or perpendicular to the melt flow direction. For a contact load of 100 N, the friction coefficient increases in proportion to the increasing melt temperature. For a contact load of 25 N, the highest friction coefficient occurs at a melt temperature 265 ◦ C. The friction coefficient decreases whether the melt temperature increases or decreases. Fig. 3(b) shows the variation of the specimen friction coefficients with the mold temperature at 50, 70 and 90 ◦ C, the sliding direction was either parallel or perpendicular to the melt flow direction. This indicates that for contact loads of 25 and 100 N, the friction coefficient of parallel and perpendicular to the melt flow direction is close and the friction coefficient decreases with increase in mold temperature. Fig. 3(c) shows the variation of the specimen friction coefficients with injection velocities of 150, 180 and 210 mm/s. It indicates that the friction coefficient for flow directions parallel and perpendicular to the melt flow direction increases
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Fig. 4. Wear volume loss for sliding directions parallel and perpendicular to the melt flow direction with injection molding conditions in (a) melt temperature, (b) mold temperature and (c) injection velocity. Fig. 3. Mean value of the friction coefficient with different sliding contact loads and injection molding conditions: (a) melt temperature, (b) mold temperature and (c) injection velocity.
as the injection velocity increases when the sliding contact loads are 25 and 100 N. These experimental results demonstrate that the friction coefficients for a sliding contact load of 100 N are higher than those for a sliding contact load of 25 N. This phenomenon is in agreement with the results obtained in previous literature [2,4]. It can be attributed to the fact that UHMWPE is a viscoelastic material and its deformation under contact load is viscoelastic. As a result, the variation of the friction coefficient with contact load will increase because of the critical surface energy of the UHMWPE. Moreover, the rising temperature of the friction surface is caused by friction heating because higher frictional temperature could deform UHMWPE molecule chains. For a contact load of 100 N, the friction coefficients in the sliding direction perpendicular to the melt flow direction are higher than those that slide parallel to the melt flow direction. Furthermore, the injection molding conditions also affect the UHMWPE friction coefficient. 3.2. Wear behavior The mean value of specimen wear volume loss sliding either parallel or perpendicular to the melt flow direction from the SRV ball-on-plane wear test (under a contact load of 100 N) is shown
in Fig. 4. Because the wear mass loss is difficult to measure under a contact load of 25 N, it is not discussed in this section. The wear volume loss for different melt temperatures is shown in Fig. 4(a). It indicates that the lowest volume loss was obtained at a melt temperature of 280 ◦ C with a sliding direction perpendicular to the melt flow direction. The same result is obtained with a sliding direction parallel with the melt flow direction. However, the wear volume loss in the sliding direction parallel to the melt flow direction is greater than that of the sliding direction perpendicular to the melt flow direction. The wear volume loss for different mold temperatures is shown in Fig. 4(b). The lowest volume loss occurs at a mold temperature of 90 ◦ C with a sliding direction parallel to the melt flow direction. Similarly, the variation in wear volume loss with mold temperature shows the same trend as for the melt temperature. The wear volume loss in the sliding direction perpendicular to the melt flow direction is generally higher than that in the sliding direction parallel with the melt flow direction. The volume loss for different injection velocities is shown in Fig. 4(c). It also shows the same trend as that of the melt temperature. However, the lowest wear volume loss was obtained for an injection velocity at 210 mm/s with a sliding direction parallel to the melt flow direction. From these experimental results, it can be seen that the highest process conditions in this study cause the lowest wear volume loss. The wear volume loss was between 0.039 and 0.222 mm3 in two different sliding directions. Therefore, it can be concluded that the wear volume loss in the two different
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Fig. 5. Effect of different injection molding conditions on friction and wear of UHMWPE for different sliding directions relative to the melt flow direction: (a) parallel and (b) perpendicular direction.
sliding directions was slightly influenced by the injection molding conditions. Fig. 5 shows the effect of different injection molding conditions on the wear and tensile strength of the UHMWPE for sliding directions parallel and perpendicular to the melt flow direction. The increase in friction coefficient that is observed under different injection molding conditions is attributed to the frozen layer. It can be seen that higher friction coefficients are found at a melt
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temperature of 280 ◦ C, mold temperature of 70 ◦ C and injection velocity of 210 mm/s when the sliding direction is parallel to the melt flow direction. Furthermore, the lowest wear volume loss is found at a melt temperature of 280 ◦ C, mold temperature of 90 ◦ C and injection velocity of 210 mm/s, when the sliding direction is parallel to the melt flow direction. Similarly, the variation in friction coefficient and wear volume loss with different injection molding conditions show the same trend as for the sliding direction perpendicular to the melt flow direction. For the range of different injection molding conditions of this investigation, the wear volume loss values for UHMWPE specimens decrease or increase as the injection molding conditions decrease or increase. The reason for this behavior is because melt temperature, mold temperature and injection velocity change the surface properties, such as the viscosity or the density of the melt, and may result in a higher temperature gradient near the mold wall because of the heat transfer effect [17]. Therefore, the specimen will be formed at different thicknesses of the frozen layers for different injection molding conditions (see Fig. 6). These frozen layers are associated with the friction coefficient and wear volume loss values [17,18]. These papers have been reported that the minimum frozen layer thickness causes a lower friction coefficient and volume loss occurred. Although, the different thicknesses of frozen layers of UHMWPE for different injection molding conditions are also observed in this case, however, the friction coefficient and the wear volume loss are not significantly influenced by the frozen layer thickness. This result could be explained by the effectiveness of the formed transfer film on the worn surface. On the other hand, in the repeated sliding processes, the heat accumulated in the wear process causes thermal softening of the UHMWPE. The softened surface transferred and spread over the chromium steel ball counterface, formed a thin film and acted as a lubricant. Thin film between the sliding surfaces provides good lubrication, which reduces the adhesive interaction between the specimen and contacting surface and reduces the shear stresses due to the lubricating action of the UHMWPE transfer film on the specimen surface; thus the wear volume loss decreased [19]. From
Fig. 6. Microscopy of the cross section of a UHMWPE specimen: (a) melt temperature of 280 ◦ C, (b) mold temperature of 50 ◦ C and (c) injection velocity of 180 mm/s.
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Fig. 7. The OM micrographs on the worn surface of UHMWPE after SRV oscillation friction wear testing under a load of 25 N for a melt temperature of 280 ◦ C, mold temperature of 70 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
Fig. 8. The OM micrographs on the worn surface of UHMWPE after SRV oscillation friction wear testing under a load of 100 N for a melt temperature of 265 ◦ C, mold temperature of 90 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
these observations, it can be concluded that the friction coefficient and the wear volume loss are significantly affected by the injection molding conditions and frictional heating. 3.3. Wear mechanisms The optical microscope micrographs on the worn surface of UHMWPE with and without the weld line after SRV oscillation friction wear testing under different sliding contact loads and injection molding conditions are shown in Figs. 7–9. Fig. 7(a) shows the worn surface of UHMWPE without a weld line under a sliding contact load of 25 N and the following injection molding conditions: melt temperature 280 ◦ C, mold temperature 70 ◦ C, injection velocity 180 mm/s and sliding direction parallel to the melt flow direction. The worn surface shows some grooves and definite wavelike formation. Fig. 7(b) shows the worn surface for sliding direction perpendicular to the melt flow direction. The worn
surface microscopic observation shows that the wear morphologies are scratches that have a small wavelike formation. Fig. 8(a) shows the worn surface of UHMWPE without a weld line under a sliding contact load of 100 N and the following injection molding conditions: melt temperature 265 ◦ C, mold temperature 90 ◦ C, injection velocity 180 mm/s and sliding direction parallel to the melt flow direction. The worn surface shows a few grooves and a few severe wavelike formations. Fig. 8(b) shows the worn surface for sliding direction perpendicular to the melt flow direction. The OM image of the worn surface shows an equal number of grooves and small wavelike formations. Fig. 9(a) and (b) shows the wear mechanisms of a worn surface with a UHMWPE weld line under a sliding contact load of 25 N and the following injection molding conditions: melt temperature 265 ◦ C, mold temperature 70 ◦ C, injection velocity 180 mm/s and sliding direction parallel and perpendicular to the melt flow direction. These wear morphologies appear the same in the wear mechanisms without weld lines. In addition, the weld
Fig. 9. The OM micrographs on the worn surface of weld line of UHMWPE after SRV oscillation friction wear testing under a load of 25 N for a melt temperature of 265 ◦ C, mold temperature of 70 ◦ C, injection velocity of 180 mm/s, and sliding direction (a) parallel and (b) perpendicular to the melt flow direction.
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Fig. 10. Effect of different injection molding conditions on wear and tensile strength of UHMWPE for sliding directions (a) parallel and (b) perpendicular to the melt flow direction.
line mark seems to be removed by wear tests. Because of the heating effect of friction, which promoted a temperature rise, and thus softened the UHMWPE surface, there is a wrinkled and wavelike morphology occurrence on the UHMWPE surface. 3.4. Effect of tensile strength and wear volume loss Fig. 10 presents the relationship between the tensile strength and the wear volume loss of the UHMWPE in the sliding direction parallel and perpendicular to the melt flow direction, respectively. It is shown that the tensile strength of UHMWPE specimens slightly increases as the injection molding conditions increase. The wear volume loss mostly decreases with increases in tensile strength. The decrease of the wear volume loss is also due to the frictional heating generation on the specimen surface.
Fig. 11. The hardness of UHMWPE at different injection molding conditions: (a) melt temperature, (b) mold temperature and (c) injection velocity.
the polymer [20–22]. This indicates the contact load is an important factor for the hardness. 4. Conclusions
3.5. Effect of surface hardness Hardness is one of the most important material properties. It can be used as an indicator of the polymer tribological property. Fig. 11(a)–(c) shows the variation of Vickers hardness on the UHMWPE specimen surface under changing melt temperature, mold temperature and injection velocity with a sliding contact load of 25 or 100 N and a sliding velocity of 100 mm/s. It can be seen that the injection molding conditions have little effect on the hardness value of the UHMWPE specimens. Moreover, it can also be seen that the UHMWPE hardness decreases as the contact load increases. The UHMWPE specimen without wear has the greatest hardness. The UHMWPE specimen at a contact load of 25 N causes lesser hardness. The wear test at a contact load of 100 N causes the least hardness. These phenomena are attributed to surface softening due to frictional heating by dry sliding. Hence, the surface hardness decreases as the contact load increases. In other words, frictional heating probably changes the degree of crystallinity or the crosslinking of the polymer chains. In general, hardness is directly related to both the degree of crystallinity and any crosslinking of
The objective of this work is to investigate the tribological properties of UHMWPE by using an SRV wear tester. Parametric analysis was applied to study the influence of molding conditions on the wear specimen. The results were obtained from experimental observation and measurement. Thus, the following can be concluded from the results: 1. The friction coefficient of UHMWPE is influenced by sliding contact load. For the designated contact loads of 25 and 100 N in this work, the UHMWPE specimens sliding at a contact load of 100 N obtain higher friction coefficients. The friction coefficient for a specimen sliding perpendicular to the melt flow direction is slightly higher than that for a specimen sliding parallel to the melt flow direction. In addition, in the range of injection molding studied in the present work, the UHMWPE friction coefficient is affected by different injection molding conditions. 2. The lowest wear volume loss generally occurs under the highest injection molding conditions. Furthermore, the wear volume loss in the sliding direction parallel to the melt flow direction is
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generally lower than that in the sliding direction perpendicular to the melt flow direction. 3. From scanning electron microscopy, plastic deformation, grooves and wavelike formations on the surface of UHMWPE are the main wear mechanisms. A more severe wavelike worn surface is usually found in the sliding direction parallel to the melt flow direction. 4. The UHMWPE tensile strength mostly increases as the injection molding conditions increase, and the lowest wear volume loss mostly occurred in the specimens with the largest tensile strength. 5. The UHMWPE hardness decreases as the sliding contact loads increase, and the injection molding conditions have little impact on the hardness. Thus, the sliding contact load is an important factor for UHMWPE surface hardness.
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