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Effects of melt temperature and hold pressure on the tensile and fatigue properties of an injection molded talc-filled polypropylene.



INTRODUCTION

Mineral-filled polypropylene polypropylene (pŏl'ēprō`pəlēn), plastic noted for its light weight, being less dense than water; it is a polymer of propylene. It resists moisture, oils, and solvents.  has many potential applications in automobiles, appliances, and other commercial products where creep resistance, stiffness, and some toughness are demanded in addition to weight and cost savings. The mechanical behavior of mineral-filled polypropylene and short glass or carbon fiber-reinforced polypropylene has been the subject of many studies over the last few years [1-5]. Limited research has also been done to examine the relationship between their tensile tensile,
adj having a degree of elasticity; having the ability to be extended or stretched.
 properties, microstructure mi·cro·struc·ture  
n.
The structure of an organism or object as revealed through microscopic examination.


microstructure
Noun

a structure on a microscopic scale, such as that of a metal or a cell
, and processing conditions [6-15]. A few of these studies have focused on talc-filled polypropylene. For example, Dave and Chundury [12] used a "design of experiment" approach to determine the effects of several injection molding injection molding
n.
A manufacturing process for forming objects, as of plastic or metal, by heating the molding material to a fluid state and injecting it into a mold.
 conditions on the tensile strength tensile strength

Ratio of the maximum load a material can support without fracture when being stretched to the original area of a cross section of the material. When stresses less than the tensile strength are removed, a material completely or partially returns to its
 of talc-filled polypropylene. They observed that tensile strength increased with both barrel temperature as well as mold temperature. Guerrica-Echevaria et al. [13] found that modulus See modulo.  and yield strength of 10%, 20%, and 40% talc-filled polypropylenes were affected slightly with increasing mold temperature as well as increasing melt temperature, but injection rate and screw rotation speed did not have much effect on these two properties. However, screw rotation speed had a very severe effect on the ductility ductility, ability of a metal to plastically deform without breaking or fracturing, with the cohesion between the molecules remaining sufficient to hold them together (see adhesion and cohesion). Ductility is important in wire drawing and sheet stamping.  of talc-filled polypropylenes. Diez-Gutierrez et al. [14, 15] studied the heterogeneity het·er·o·ge·ne·i·ty
n.
The quality or state of being heterogeneous.



heterogeneity

the state of being heterogeneous.
 and anisotropy anisotropy /an·isot·ro·py/ (an?i-sot´rah-pe) the quality of being anisotropic.
anisotropy (an´āsôt´r
 of injection-molded talc-filled polypropylene discs using dynamic mechanical analysis and thermomechanical analysis Thermomechanical analysis or TMA measures the change in deformation of a sample under a non-oscillating load with time or variation in temperature. Properties measured by TMA include the coefficient of thermal expansion, softening, sintering, and glass transition temperature. . They reported a high degree of anisotropy in these discs due to preferred orientation of talc particles in the flow direction.

In this article we report the effects of two injection molding parameters, namely, melt temperature and hold pressure, on the tensile and stress-controlled fatigue properties of a 40 wt% talc-filled polypropylene. In an earlier article [16], we reported the effects of specimen orientation, stress concentration, weld lines, and frequency on the stress-controlled fatigue properties of the same material. To our knowledge, the effect of injection molding parameters on the fatigue behavior of talc-filled polypropylene has not been studied in the past.

EXPERIMENTAL

The material investigated in this study was a 40 wt% talc-filled polypropylene homopolymer available from Ferro (Cleveland, OH) under the grade name TP40AC52BK. The average flow rate of this material, as reported by Ferro, was 6.8 g / 10 min (measured under ASTM ASTM
abbr.
American Society for Testing and Materials
 D-1238 condition). The melting point melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and  of the polypropylene matrix was 160[degrees]C. Square plates, 150 X 150 mm, were injection-molded from the pellets in a single edge-gated mold with a central 25-mm diameter core (Fig. 1). The plate thickness was 2.5 mm. A 90-ton Toyo injection molding machine Injection molding machine (also known as injection press) - a machine for making plastic parts. Manufacturing products by injection molding process. Consist of two main parts, an injection unit and a clamping unit.  was used to mold these plates. Three different melt temperatures were considered: 209, 232, and 277[degrees]C. The peak injection pressure was 103 MPa for all plates, but the hold pressure was varied at three levels: 27.6, 55.2, and 82.7 MPa. The mold temperature was maintained at 35[degrees]C. The following two groups of plates were injection molded:

[FIGURE 1 OMITTED]

(1) Group I: Hold pressure = 55.2 MPa and melt temperature = 209, 232, and 277[degrees]C

(2) Group II: Melt temperature = 232[degrees]C and hold pressure = 27.6, 55.2, and 82.7 MPa

Dog-bone-shaped specimens were prepared from the injection-molded plates in three different directions: parallel to the flow direction (L-direction specimens), normal to the flow direction (W-direction specimens), and with weld line (WL specimens). The specimen dimensions were 100 mm in overall length, 25 mm in gage length, and 12.7 mm in gage width. For the WL specimens, the weld line was located at the mid-length. The weld line was formed in the plates as the flow front was divided by the central core in the mold and then joined behind the core.

Uniaxial tensile tests and fatigue tests were performed on an MTS (1) See Microsoft Transaction Server.

(2) (Modular TV System) The stereo channel added to the NTSC standard, which includes the SAP audio channel for special use.

1. MTS - Message Transport System.
2.
 servohydraulic testing machine testing machine

Machine used in materials science to determine the properties of a material. Machines have been devised to measure tensile strength, strength in compression, shear, and bending (see strength of materials), ductility, hardness, impact strength (
. The tensile tests were conducted at 1.25 mm/min, which was approximately equivalent to a strain rate of 0.05 [min.sup.-1]. Three tensile parameters were determined from each stress-strain curve: elastic modulus (E) from the initial slope, yield strength ([[sigma].sub.y]) corresponding to the maximum stress observed, and yield strain ([[epsilon].sub.y]) corresponding to the yield strength. Stress-controlled cyclic cyclic /cyc·lic/ (sik´lik) pertaining to or occurring in a cycle or cycles; applied to chemical compounds containing a ring of atoms in the nucleus.

cy·clic or cy·cli·cal
adj.
1.
 fatigue tests were performed in tension-tension mode at an ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade.  of ~23[degrees]C. The ratio of the minimum cyclic stress Cyclic stress in engineering refers is an internal distribution of forces (a stress) that changes over time in a repetitive fashion. As an example, consider one of the large wheels used to drive an aerial lift such as a ski lift.  to the maximum cyclic stress (i.e., R-ratio) was 0.1. The cyclic frequency was 1 Hz. Such a low frequency was selected so that fatigue failure would occur instead of thermal failure [16].

[FIGURE 2 OMITTED]

RESULTS

Tensile Properties

Figure 2 shows the tensile stress-strain curves of the 40 wt% talc-filled polypropylene specimens parallel to the flow direction (L), normal to the flow direction (W), and with weld line (WL) under the different processing conditions considered. It can be observed in these figures that the stress-strain relationships were nonlinear A system in which the output is not a uniform relationship to the input.

nonlinear - (Scientific computation) A property of a system whose output is not proportional to its input.
 even at strains lower than the yield strain. Each curve shows a maximum stress, which was assumed to be the yield strength of the material. After reaching the maximum stress, the specimen with the weld line failed almost immediately; however, for the L-direction and W-direction specimens, stress decreased steadily with strain until fracture occurred. The mean value and the standard deviation of the tensile properties obtained from three specimens for different processing conditions are listed in Table 1. The yield strength in the L-direction was higher than that in the W-direction. The difference in properties in these two mutually perpendicular directions indicates inherent anisotropy of the injection-molded talc-filled polypropylene plates. The presence of weld line in WL specimens significantly decreased the yield strength and failure strain. However, the weld line did not influence the modulus.

Figure 2a shows the effect of melt temperature on the tensile behavior of the talc-filled polypropylene. The hold pressure for these specimens was 55.2 MPa. For specimens parallel to the flow direction, the yield strengths at different melt temperatures were very close and the stress-strain curves after yielding did not show much difference. For specimens normal to the flow direction, the yield strength and stress level after yielding were significantly lower at melt temperature 277[degrees]C. The tensile orientation factor, defined as the ratio of yield strengths of the L-direction and the W-direction specimens, changed from 1.05 at melt temperature 209[degrees]C to 1.27 at melt temperature 277[degrees]C. For specimens with weld line, yield strength increased with increasing melt temperature. The tensile weld line factor, defined as the ratio of the yield strengths of the L-direction specimens and the WL specimens, was 1.55 at melt temperature 209[degrees]C and decreased to 1.43 at melt temperature 277[degrees]C.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Figure 2b shows the effect of hold pressure on the tensile behavior of the talc-filled polypropylene. The melt temperature for these specimens was 232[degrees]C. Increasing the hold pressure increased the yield strengths for all three types of specimens. But the anisotropy of the talc-filled polypropylene was not very sensitive to hold pressure. The tensile orientation factors were 1.06, 1.06, and 1.07 at hold pressure 27.6 MPa, 55.2 MPa, and 82.7 MPa, respectively. However, the weld-line strength increased with increasing hold pressure. The tensile weld line factor dropped from 1.56 at hold pressure 27.6 MPa to 1.31 at hold pressure 82.7 MPa.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Figure 3 shows that both tensile modulus and yield strength of the talc-filled polypropylene varied linearly with melt temperature and hold pressure. The linear relationships observed in these plots are modeled with the following equations:

[[sigma].sub.Y] = [[sigma].sub.Y0][1 + [J.sub.melt](T - [T.sub.0])][1 + [J.sub.pressure](P - [P.sub.0])] (1)

E = [E.sub.0][1 + [L.sub.melt](T - [T.sub.0])][1 + [L.sub.pressure](P - [P.sub.0])] (2)

where [J.sub.melt], [L.sub.melt] and [J.sub.pressure], [L.sub.pressure] are melt temperature sensitivity factors and hold pressure sensitivity factors, [T.sub.0] and [P.sub.0] are the reference melt temperature and reference mold pressure, and [[sigma].sub.Y0] and [E.sub.0] are the reference yield strength and modulus. The average values of [J.sub.melt], [L.sub.melt], [J.sub.pressure], and [L.sub.pressure] are listed in Table 2.

Fatigue Properties

Figure 4 shows the S-N curves of the 40 wt% talc-filled polypropylene parallel to the flow direction (L), normal to the flow direction (W), and with weld line (WL) at different melt temperatures. The hold pressure for these specimens was 55.2 MPa. The fatigue strength in L-direction was much higher than that in the W-direction. The presence of weld line decreased the fatigue strength even further. Similar to the tensile test results, melt temperature did not have any effect on the fatigue strength of the L-direction specimens. However, for the W-direction specimens fatigue strength decreased with increasing melt temperature and for the WL specimens, fatigue strength increased with increasing melt temperature.

[FIGURE 8 OMITTED]

Figure 4 also shows the effect of hold pressure on the fatigue behavior of the talc-filled polypropylene. The melt temperature in this case was 232[degrees]C. As before, the L-direction specimens had the highest fatigue strength and the WL specimens had the lowest fatigue strength. The fatigue strength for all three types of specimens increased with increasing hold pressure. The highest increase occurred when the hold pressure was increased from 55.2 to 82.7 MPa.

Based on the experimental results, the following empirical equation, similar to the Basquin equation used for stress-life data of metals [17], was fitted to the S-N curves corresponding to each processing condition:

[sigma] = [[sigma].sub.f]([N.sub.f])[.sup.b] (3)

where, [sigma] = fatigue stress level, [N.sub.f] = number of cycle to failure at [sigma], [[sigma].sub.f] = fatigue strength coefficient, b = fatigue strength exponent exponent, in mathematics, a number, letter, or algebraic expression written above and to the right of another number, letter, or expression called the base. In the expressions x2 and xn, the number 2 and the letter n . The values of fatigue strength coefficient [[sigma].sub.f] and fatigue strength exponent b at different processing conditions are listed in Table 1 and are plotted in Figs. 5 and 6 as a function of melt temperature and hold pressure, respectively.

Figure 5 shows that [[sigma].sub.f] for the L-direction specimens and WL specimens increased with increasing melt temperature, while [[sigma].sub.f] for the W-direction specimens decreased with increasing melt temperature. Figure 5 also shows that the variation of fatigue strength exponent b with melt temperature was very small, and therefore, in the melt temperature range considered, b was assumed to be a constant. Figure 6 shows the variation of fatigue strength coefficient [[sigma].sub.f] and fatigue strength exponent b with hold pressure. The values of [[sigma].sub.f] for all three types of specimens increased with increasing hold pressure; however, the fatigue strength exponent b was not much affected by hold pressure. Thus, in the range of hold pressures considered, b was assumed to be a constant. The following empirical relationship In science, an empirical relationship is one based solely on observation rather than theory. An empirical relationship requires only confirmatory data irrespective of theoretical basis.  was found to fit the combined effect of melt temperature and hold pressure on [[sigma].sub.f]:

[FIGURE 9 OMITTED]

[[sigma].sub.f] = [[sigma].sub.f0][1 + [K.sub.melt](T - [T.sub.0])][1 + [K.sub.pressure](P - [P.sub.0])] (4)

where [K.sub.melt] and [K.sub.pressure] are melt temperature sensitivity factor and hold pressure sensitivity factor in fatigue, [T.sub.0] and [P.sub.0] are reference melt temperature and reference mold pressure, and [[sigma].sub.f0] is the reference fatigue strength coefficient. The average values of [K.sub.melt], [K.sub.pressure] and b of the talc-filled polypropylene are listed in Table 2. The value of [K.sub.melt] with weld line was much higher than that in the flow direction and the [K.sub.melt] normal to the flow direction was negative. The values of [K.sub.pressure] in the flow direction and normal to the flow direction were close, but [K.sub.pressure] with weld line was higher than that in the flow direction and normal to the flow direction.

Fracture Surface Observations

The fracture surfaces of several fatigue specimens were examined under scanning electron microscope scan·ning electron microscope
n. Abbr. SEM
An electron microscope that forms a three-dimensional image on a cathode-ray tube by moving a beam of focused electrons across an object and reading both the electrons scattered by the object and
 to determine the differences in the L-direction, W-direction, and WL specimens. Each fracture surface exhibited skin-core morphology morphology

In biology, the study of the size, shape, and structure of organisms in relation to some principle or generalization. Whereas anatomy describes the structure of organisms, morphology explains the shapes and arrangement of parts of organisms in terms of such
 with different degrees of talc particle orientations in the core and in the skin. The relative size of the core and the skin varied with processing condition. In general, the greater the talc particle orientation parallel to the loading direction, the higher will be strength, since they are more effective in stress transfer than those oriented o·ri·ent  
n.
1. Orient The countries of Asia, especially of eastern Asia.

2.
a. The luster characteristic of a pearl of high quality.

b. A pearl having exceptional luster.

3.
 normal to the loading direction. The processing condition also affects the molecular orientation and crystalline Like a crystal. It implies a uniform structure of molecules in all dimensions. For example, phase change technology, widely used for rewritable optical discs, uses crystalline spots (bits) to reflect the laser beam. Amorphous, non-crystalline bits do not reflect light.  morphology in the skins and core of injection-molded polypropylene; however, their effects on mechanical properties may be of less importance than the effect of filler fill·er 1  
n.
One that fills, as:
a. Something added to augment weight or size or fill space.

b. A composition, especially a semisolid that hardens on drying, used to fill pores, cracks, or holes in wood, plaster,
 orientation in a highly filled polypropylene [6].

[FIGURE 10 OMITTED]

Figure 7 shows the fracture surfaces of the L-direction specimens at different processing conditions. On each fracture surface, there was evidence of skin-core morphology, with the skins containing many talc particles oriented normal to the loading direction. Talc particles in the core were oriented parallel to the loading direction, which was also the flow direction for the specimens. The parabolic par·a·bol·ic   also par·a·bol·i·cal
adj.
1. Of or similar to a parable.

2. Of or having the form of a parabola or paraboloid.
 flow pattern in the core was also clearly visible in Figure 7. The skins were relatively thin compared to the core and their thickness did not change much with melt temperature. However, the skin thickness decreased with increasing hold pressure.

Skin-core morphology was also found on the fatigue fracture fatigue fracture
n.
A fracture, usually transverse in orientation, that occurs as a result of repeated or unusual endogenous stress.


fatigue fracture 
 surfaces of the specimens normal to the flow direction (Fig. 8). In this case, many talc particles in the core were oriented normal to the loading direction (Fig. 9). With increasing hold pressure, the talc particle orientation normal to the loading direction was reduced, which explains the increase in yield strength and fatigue strength of these specimens at higher hold pressure.

Figure 10 shows the fracture surfaces of WL specimens containing a weld line. In these specimens, the skin-core morphology was very clearly visible. The white band in the core contained talc particles that were oriented normal to the loading direction. The darker zones in the skins contained talc particles oriented parallel to the loading direction. The thickness of the core decreased and the thickness of the skins increased with increasing melt temperature and increasing hold pressure (Table 3). This explains the reason for increasing yield strength and fatigue strength with either increasing melt temperature or increasing hold pressure.

CONCLUSIONS

Static tensile and fatigue tests were performed on 40 wt% talc-filled polypropylene injection molded at three different melt temperatures and three different hold pressures. It was observed that yield strength and fatigue strength of the talc-filled polypropylene specimens were lower normal to the flow direction than in the flow direction, indicating inherent anisotropy in the material. Yield strength and fatigue strength of the talc-filled polypropylene with weld line were significantly lower than those without weld line. Yield strength and fatigue strength of the talc-filled polypropylene in the flow direction was not influenced by melt temperature, but they increased with increasing hold pressure. Yield strength and fatigue strength of the talc-filled polypropylene normal to the flow direction decreased with increasing melt temperature, but they increased with increasing hold pressure. Yield strength and fatigue strength of the talc-filled polypropylene with weld line increased with increasing melt temperature as well as increasing hold pressure. Both melt temperature and hold pressure influenced skin-core morphology exhibiting different orientations of talc particles in the skins than in the core.
TABLE 1. Tensile and fatigue properties of 40w% talc-filled
polypropylene at different processing conditions.

Melt temp.    Hold pressure             Tensile modulus
([degrees]C)  (MPa)          Direction  (GPa)

209           55.2           L          8.48 [+ or -] 0.24
209                          W          7.92 [+ or -] 0.32
209                          WL         6.56 [+ or -] 0.32
232           55.2           L          8.62 [+ or -] 0.13
232                          W          7.51 [+ or -] 0.25
232                          WL         6.48 [+ or -] 0.43
277           55.2           L          8.82 [+ or -] 0.27
277                          W          7.41 [+ or -] 0.36
277                          WL         6.94 [+ or -] 0.48
232           27.6           L          8.82 [+ or -] 0.25
232                          W          7.22 [+ or -] 0.36
232                          WL         6.71 [+ or -] 0.39
232           82.7           L          9.61 [+ or -] 0.32
232                          W          8.13 [+ or -] 0.30
232                          WL         6.92 [+ or -] 0.26

Melt temp.    Yield strength       Failure strain
([degrees]C)  (MPa)                (%)

209           27.03 [+ or -] 0.16  2.96 [+ or -] 0.12
209           25.70 [+ or -] 0.23  2.39 [+ or -] 0.22
209           17.49 [+ or -] 0.13  0.79 [+ or -] 0.10
232           27.21 [+ or -] 0.12  2.80 [+ or -] 0.23
232           25.39 [+ or -] 0.26  2.57 [+ or -] 0.14
232           17.88 [+ or -] 0.22  0.91 [+ or -] 0.02
277           27.38 [+ or -] 0.08  3.25 [+ or -] 0.17
277           22.57 [+ or -] 0.16  1.87 [+ or -] 0.12
277           19.11 [+ or -] 0.21  1.00 [+ or -] 0.03
232           26.61 [+ or -] 0.02  3.08 [+ or -] 0.11
232           25.15 [+ or -] 0.37  2.69 [+ or -] 0.12
232           17.06 [+ or -] 0.24  0.86 [+ or -] 0.03
232           29.52 [+ or -] 0.19  2.95 [+ or -] 0.10
232           27.62 [+ or -] 0.26  2.67 [+ or -] 0.06
232           22.47 [+ or -] 0.32  1.31 [+ or -] 0.03

Melt temp.
([degrees]C)  [[sigma].sub.f] (MPa)  b

209           32.18                  -0.0306
209           28.49                  -0.0283
209           18.23                  -0.0202
232           32.19                  -0.0312
232           26.28                  -0.0236
232           19.82                  -0.0216
277           32.58                  -0.0303
277           26.04                  -0.0277
277           20.73                  -0.0210
232           31.38                  -0.0304
232           25.23                  -0.0264
232           18.91                  -0.0263
232           34.91                  -0.0299
232           29.35                  -0.0252
232           24.95                  -0.0247

TABLE 2. Processing condition factors of 40w% talc-filled polypropylene.

Property          Parameter         In the flow (L) direction

Yield strength    [J.sub.melt]       0.0050
                  [J.sub.pressure]   0.53
                  [L.sub.melt]       0.0049
                  [L.sub.pressure]   0.20
Fatigue strength  [K.sub.melt]       0.011
                  [K.sub.pressure]   0.64
                  b                 -0.031

Property          Parameter          Normal to the       With weld line
                                     flow (W) direction

Yield strength    [J.sub.melt]       -0.048               0.024
                  [J.sub.pressure]    0.45                0.98
                  [L.sub.melt]       -0.0068              0.0062
                  [L.sub.pressure]    0.17                0.038
Fatigue strength  [K.sub.melt]       -0.032               0.034
                  [K.sub.pressure]    0.75                1.10
                  b                  -0.027              -0.023

TABLE 3. Effect of melt temperature and hold pressure on the core and
skin thickness values in the weld-line specimens.

Melt temp.    Hold pressure  Approx. core    Approx. total skin
([degrees]C)    (MPa)        thickness (mm)  thickness (mm)

209           55.2           1.40            1.10
232           55.2           1.35            1.15
277           55.2           1.17            1.33
232           27.6           1.45            1.05
232           55.2           1.35            1.15
232           82.7           0.97            1.53


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17. J.A. Bannantine, J.J. Comer, and J.L. Handrock, Fundamentals of Metal Fatigue Analysis, Prentice Hall Prentice Hall is a leading educational publisher. It is an imprint of Pearson Education, Inc., based in Upper Saddle River, New Jersey, USA. Prentice Hall publishes print and digital content for the 6-12 and higher education market. History
In 1913, law professor Dr.
, New York New York, state, United States
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 (1990).

Yuanxin Zhou, P.K. Mallick

Center for Lightweighting Automotive Materials and Processing, University of Michigan-Dearborn The University of Michigan-Dearborn, located in Dearborn, Michigan, USA, is part of the University of Michigan system. It was established in 1959 after a gift of 196 acres (793,000 m²) from the Ford Motor Company. , Dearborn, Michigan Dearborn is a city in the U.S. state of Michigan. It is located in the Detroit metropolitan area and Wayne County, and is the tenth largest city in the U.S. state of Michigan. As of the 2000 census, it had a population of 97,775.  48128

Correspondence to: P.K. Mallick; e-mail: pkm@umich.edu
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Author:Zhou, Yuanxin; Mallick, P.K.
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Date:Jun 1, 2005
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