Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Microcellular Foamed Parts

Transcription

1 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh*, Mehdi Mahmoodi, Peyman Shahi Dept. of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran, P.O. Box: 14115/143 Received: 26 August 2009, Accepted: 26 November 2009 ABSTRACT This paper presents an experimental study on microstructural and mechanical properties of injection molded microcellular foamed parts. The effects of shot size, injection pressure and mold temperature on the relative density, unfoamed skin thickness, cell population density, surface hardness and fl exural strength of various regions of the injected parts were investigated. A conventional injection molding machine was modified to produce microcellular acrylonitrile butadiene styrene (ABS) foamed plates. Nitrogen gas was used as the blowing agent. The results showed that surface hardness of microcellular foamed parts are higher and their flexural strength is lower than those of the unfoamed parts. Examining the properties throughout the parts confi rmed distinct variations. 1. INTRODUCTION Usage of thermoplastic foams provides reduction in part weight, manufacturing and transportation costs. Thermoplastic foams can be categorized as conventional and microcellular foams. In conventional foams, cell population density is in the order of 10 6 cells/cm 3 and the cell size is in the order of 100 microns. However, cell population density in microcellular foams is about 10 9 cells/cm 3 and higher and cell size is in the order of 10 microns. The main advantages of microcellular foams are claimed to be that their relative mechanical properties (property-to density) ratio are higher than or at least comparable to those of the unfoamed samples. The proposed reason was that the microcells are smaller than critical natural flaw (crack) (1,2). In microcellular foaming, usage of small molecular size * The author to whom the correspondence should be addressed: amirhb@modares.ac.ir, Tel.: Smithers Rapra Technology, 2009 Cellular Polymers, Vol. 28, No. 6,

2 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi blowing agents (usually CO 2 and N 2 ) is favorable. A very consistent and uniform microcellular structure can be produced in a batch process where the polymeric material is in solid state. The main reason is that the gas saturation into the polymeric matrix is self-controlled so that the amount of dissolved gas never exceeds the solubility level. Besides the cell growth is much more controllable via accurately adjusting the temperature of heating bath and also foaming (heating) time. However, when producing microcellular foams in a continuous process such as extrusion and injection molding, excessive gas injection could produce non-uniform cellular structure consisting of undesirable large voids. The cell growth control involves challenges especially in injection molding process since the nature of injection process in interruptive. Hence, production of sound microcellular foams having a average cell size of 10 μm or less by injection molding size and uniform structure is highly challenging (3-8). To produce microcellular structure, a minimum pressure drop rate of 1 GPa/s is required (9). This high-pressure drop rate results in a thermodynamic instability (sudden super-saturation) and leads to nucleation of myriads of tiny cells in a very short time. As the pressure decreases, the supersaturated gas diffuses into the nucleated cells and thus the cells start to grow. To produce microcellular injection molded parts, the mold cavity has to be partially filled to provide space for expansion. Too short a shot size could lead to an incomplete part (although expanded) and a full shot can lead to the formation of an unfoamed single-phase solution (3). In general, a microcellular foamed part consists of three-layer (sandwich) structure. As for the side layers, also called the frozen skin layers, either the nucleation does not occur or the gas are not diffused into the nucleated cells because of highly stiffened matrix at the cooled surface (9). Consequently, a solid skin is formed. Microcellular foam in injection molding has been an attractive subject of research interests since 1990 s. Wang et al. (10) used polypropylene to produce microcellular parts with CO 2, but the produced samples exhibited a few numbers of micro-cells. Shimbo et al. (11) worked on the microcellular foam process using an in-line injection molding machine. Still showing non-uniformity in microcellular structure, the produced parts had cell sizes in a range of 20 to 100 μm. Shimbo et al. (12) continued the work using a screw and plunger injection molding machine. This method was somewhat less complicated than the in-line foam molding because the requirements to maintain a single-phase solution was decoupled from the reciprocation step. Microcellular injection system was commercially introduced by Trexel under the Mucell name (6). The economical benefits of microcellular foam injection molding process are reduced operating costs up to 50%, reduction in cycle time, reduced scrap rates and lower energy consumption (13). 406 Cellular Polymers, Vol. 28, No. 6, 2009

3 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded To attain a uniform structure, injected gas must be completely dissolved into the molten polymer to form a single-phase gas-polymer solution. The solution is then injected into the mold with a high pressure drop rate. A higher pressure drop rate increases the number of nucleation sites and the cell uniformity (1). In injection molding process, pressure drop rate (dp/dt, Pa/s) is a function of viscosity (μ, Pa.s), barrel diameter (D, m), injection velocity (V, m/s) and injection nozzle diameter (d, m) (6) : dp dt = 32μD4 V 2 d 6 (1) Equation (1) suggests that the pressure drop rate significantly increases with increasing barrel diameter (D) and decreasing nozzle diameter (d), provided that the screw ram speed is maintained unchanged. The mechanical properties of the parts are directly influenced by the cell size, cell population density and the homogeneity of the cell structure (6,14). Processing parameters can have significant influence on the foam morphology. Mold cooling rate, shot size and melt temperature are of these influencing parameters. The unfoamed skin thickness is an important characteristic that could strongly affect the physical and mechanical properties of the injected foamed parts. This frozen layer is influenced by the melt and mold temperatures, thermoplastic glass transition temperature and injection velocity. Adding to the complexity is the non-uniform distribution of pressure and temperature throughout the part that may cause a non-uniform final cell structure. Moreover, different parts with different mold geometries could have different cell growth and nucleation conditions (14-16). As for injection velocity, a slow injection speed increases filling time, allowing a thicker skin to form and consequently resulting in a narrow flow channel (17). Skin layer thickness can be estimated by mathematical formulation using polymer melt temperature, polymer melt freezing temperature, mold temperature, cooling rate and thermal diffusivity of the polymer as input data (18) or by calculating the size of the region affected by the mold temperature (16). Cell sizes decrease from core to skin layer and highest cell population density is obtained at the lowest shot size (19). Shot size has a dominant effect on foam microstructure. Too low a shot size causes a non-uniform microstructure and incomplete foamed part, although higher expansion ratio is achieved. Too high a shot size yields no expansion in injected parts while the gas remained dissolved in the solid polymeric matrix (3,19). In general, via increasing the injection speed, shot size or gas content, relative density and cell diameter decreases, cell population density increases and cellular structure uniformity improves (20-22). It is very hard Cellular Polymers, Vol. 28, No. 6,

4 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi to create a foam structure with average cell size of 10 μm or less by injection molding (as mentioned earlier). Using of special processing technologies like PMO (Precision Mold Opening) and GCP (Gas Counter Pressure) can improve foam quality (8). Also, relative density values are different along the flow path. However, at higher injection velocity, the density difference between areas close to the gate and areas far from the gate decreases (22). Increased surface hardness of unpainted plastic parts contributes to increased scratch resistance (23). Surface hardness of microcellular foamed parts directly depends on the relative density of microcellular injection molded parts. Also, adding the short fibers can lead to increase the surface hardness (24). Xu et al. presented a mathematical formulation to predict the flexural strength ratio in microcellular injection molded parts. The input for this formulation is part weight reduction and average skin thickness. They showed that flexural strength of the foamed parts is lower than that of the unfoamed parts and skin thickness is very important factor for flexural strength. Because the maximum stress of bending test during three-point flexure loading is on the skin. Also, filled materials will have less flexural strength loss than the unfilled materials at the same weight reduction percentage and skin thickness (18). Several models were developed to predict flexural modulus of structural foams. Some of these models are Square Power-law Model, Modified Halpin-Tsai, I-beam Model of Hobbs, Modified I-beam Model of Hobbs and I-beam Model of Throne. The input data for these models are relative density and ratio of total skin thickness over sample thickness (25). Also, flexural strength of foamed sample is a function of their /d (density/average cell size) (26). However, foamed and unfoamed flexural strengths decreases with increasing temperature, while at high temperatures ( C), the difference between their flexural strengths decreases (27). Flexural modulus of foamed samples increases with increasing skin thickness (28). For symmetric foam (equal skin thickness on both side), the flexural modulus does not depend on which side the load is applied. But for asymmetric foams, the apparent flexural modulus was found to be higher when the load was applied on the thicker side (25). The goal of this research work is to experimentally investigate the effect of processing parameters on microstructures including the unfoamed skin thickness, surface hardness and flexural strength at various regions of the microcellular foamed parts. 408 Cellular Polymers, Vol. 28, No. 6, 2009

5 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded 2. EXPERIMENTS 2.1 Equipment A 70-ton clamping tonnage conventional injection molding machine was modified to produce microcellular foamed parts (Figure 1). The major modification were on designing, manufacturing and installing a special screw, a gas (blowing agent) injection system and a special melt injection nozzle. The designed screw consists of four section (zone), i) feeding zone, ii) transition zone, iii) metering zone, and iv) mixing and dissolution zone. Special modification was performed on metering zone by increasing screw channel path in this section. This was to decrease the melt pressure, permitting gas stream from the source to easily enter into the barrel through the non-return valve. The main functions of the mixing and dissolution elements (zone) are assumed to break injected gas pockets into smaller ones and to dissolve them into the polymer melt and create single phase solution. As the expected expansion ratio was low (1.1 to 1.5), the required gas amount was low and thus a high pressure was not required for dissolution, providing that the barrel pressure at the gas injection port is maintained low. This could be achieved via appropriate screw design and injection timings. Thus, a gas cylinder (supply) with pressure of 15 MPa was considered sufficient for the purpose. The amount of injecting gas was adjusted using a sound dozing valve. The gas passes through a special non-return valve and enters into the cylinder (barrel). This non-return valve has a brass filter to prevent the polymer melt to enter into the gas pipes when Figure 1. The schematic of the modified injection molding machine for producing microcellular injection molded parts Cellular Polymers, Vol. 28, No. 6,

6 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi the melt pressure in cylinder rises above the gas pressure in pipes. Another efficiency of using the brass filter related to dividing the total gas stream to several streams that lead to faster and more uniformity of gas dissolution in polymer melt. The melt injection valve nozzle (shut-off nozzle) equipped with a pneumatics actuator was timely adjusted to operate. During the material feeding, melting and mixing, the valve nozzle remained closed to allow forming a single phase solution inside the barrel before injecting into the mold cavity. When melt-gas solution injection was initiated, a delay time was adjusted for valve opening, to pressurize the solution. This could create a high pressure before injection, which eventually could cause a high pressure drop rate. A two-plate mold with rectangular cavity of mm in dimensions was used. A fan gate was machined on the mold plate to ensure a uniform melt distribution across the mold cavity. A mold temperature controller was utilized to adjust mold temperature at the desired level. 2.2 Materials and Process Parameters As the blowing agent, nitrogen gas with 99.5% purity was used. Acrylonitrile butadiene styrene (ABS), produced by Cheil Synthesis Co., grade SD0150 was used as the polymeric material. Prior to the experiments, it was dried at 80 C for at least three hours. The variable parameters were shot size, injection pressure and mold temperature. Other parameters were maintained constant during the experiments. Tables 1 and 2 give the constant and the variable parameters, respectively. Table 3 shows the design of experiments based on full factorial scheme. Table 1. Constant parameters during the experiments Melt temperature ( C) Delay time (s) Nozzle diameter (mm) Screw diameter (mm) Table 2. Variable parameters used for microcellular injection molding during the experiments Shot size (%) Injection pressure (MPa) Mold temperature ( C) 85, 90 20, 50, 90 40, Cellular Polymers, Vol. 28, No. 6, 2009

7 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Table 3. The design of experiments Experiment No. Shot size (%) Injection pressure (MPa) Mold temperature ( C) Measurements To investigate the effect of processing parameters on the relative density of the parts, nine areas of the parts were selected as shown in Figure 2. The specimen size was mm. Bulk density was measured using Archimedes method. The relative bulk density (ρ r ) of each specimen was calculated by dividing its bulk density (ρ f ) to the density of the unfoamed part (ρ p ): r = f p (2) The overall relative bulk density of the part was considered as the average of the relative density of the nine specimens. Same specimens were used for scanning electron microscopy (SEM- Model Philips XL-30) to examine their microstructures. The specimens were dipped in liquid nitrogen and fractured to reveal the cross section as shown in Figure 3, and then gold coated. Using SEM pictures, the unfoamed skin thickness was measured. Also, cell population densities (numbers) were calculated as (29) : 3 N = n 2 p L 2 c (3) Cellular Polymers, Vol. 28, No. 6,

8 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi Figure 2. The nine selected areas of the sample Figure 3. Schematic of the method to prepare the specimens for SEM analysis 412 Cellular Polymers, Vol. 28, No. 6, 2009

9 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded where, N (cells/cm 3 ) is specimen cell population density, n is cell number in a selected L L (μm μm) size area of SEM picture, ρ p (gr/cm 3 ) is the unfoamed bulk density (1.04 gr/cm 3 for ABS Starex SD 105) and ρ c (gr/cm 3 ) is the core density of foamed sample. The core density can be calculated by equation (4) (30) : c = p d ( e 1 + e 2 ) d ( e 1 + e 2 ) f p (4) where, ρ p (gr/cm 3 ) is the unfoamed bulk density, ρ f (gr/cm 3 ) is bulk density of foamed sample was measured using Archimedes method, d is sample thickness (3.2 mm in this study) and e 1 and e 2 are unfoamed skin thickness of each side of the samples measured using SEM pictures. Allowing 24 hours after part ejection from the mold, the surface hardness of three regions (near gate, middle and far from the gate) of each part was measured according to ASTM D2240 Durometer Hardness Shore A (Figure 4). To investigate the surface hardness with more subtlety, three points in each region were selected. Also, the flexural strength and flexural modulus of two areas (near the gate and far from the gate, Figure 4) was measured according to ASTM D790 (three points) using INSTRON 5500R (2 tonnes) with a strain Figure 4. Selected areas for measuring surface hardness and flexural strength Cellular Polymers, Vol. 28, No. 6,

10 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi rate of 5 mm/min. The surface hardness and flexural tests were performed at the ambient temperature of C. Whereas the selected areas for surface hardness and relative density (and SEM analysis) tests were the same, surface hardness test were performed before cutting the samples for relative density and SEM analysis according to Figure RESULTS AND DISCUSSION Figures 5 and 6 show the examples of the microstructures of the injected foams at various conditions. Figure 5 specifically reveals the unfoamed skin thickness variations. For more clarity, only the half section of microstructures is presented. It shows that the unfoamed skin thickness reduces with increasing injection pressure and mold temperature and with decreasing shot size. A large thickness (678 μm) was observed at a condition illustrated in Figure 5c. While as revealed in Figure 5b, unfoamed skin was not created. The skin thicknesses in Figure 5a and 5.d were 285 and 220 μm, respectively. It must be mentioned that Parts No. 1 and 2 were not fully filled because their injection pressure and shot size were too low. Due to possible error in comparisons, SEM analysis, hardness and flexural tests were not performed on these parts. (a) (b) (c) (d) Figure 5. Skin thickness variations of foamed parts: a) Reg. 5 of Part No. 4, b) Reg. 8 of Part No. 6, c) Reg. 4 of Part No. 8 and d) Reg. 5 of Part No Cellular Polymers, Vol. 28, No. 6, 2009

11 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded (a) (b) (c) (d) Figure 6. Foam morphology variations of foamed parts: a) Reg. 2 of Part No. 4, b) Reg. 4 of Part No. 5, c) Reg. 6 of Part No. 8, and d) Reg. 2 of Part No Relative Density Table 4 gives the relative density at nine regions and their average for each part (except Parts No. 1 and 2). Part No. 6 exhibited the minimum relative density. This is attributed to the favorable high injection pressure (and hence high pressure drop rate) and low thickness of the solid skin. Also, in most parts, the relative densities at the regions far from the gate are larger than that of the regions near the gate (Figure 7). The maximum difference is 0.1 g/ cm 3. The reason is related to lower cell number at the regions far from the gate (section 3.3), although the unfoamed skin thickness decrease for these regions (section 3.2). As for Parts No. 7 and 8, the unfoamed skin thicknesses were thicker since the injection pressures were lower. This causes a lower filling velocity which is known to signify the mold cooling effect which in turn increases the solidified layer during mold filling (or reducing the flow channel) (17). In addition, it causes an over-packing of the melt at the region near the gate and underpacking the melt far from the gate. The result will be a higher density at the near gate regions than that of the far regions. For parts No. 9 and 10, because of relatively low injection pressure and high shot size, the conditions is similar to those of parts No. 7 and 8. Cellular Polymers, Vol. 28, No. 6,

12 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi Table 4. Relative density of the parts at nine regions (according to Figure 2) Part No. Reg. 1 Reg. 2 Reg. 3 Reg. 4 Reg. 5 Reg. 6 Reg. 7 Reg. 8 Reg. Average Figure 7. Relative density of various regions of parts 416 Cellular Polymers, Vol. 28, No. 6, 2009

13 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Figure 8. Effect of injection pressure on relative density at various injection conditions Figure 8 shows the effect of injection pressure on the average relative density of the injected parts. In general and especially when injecting with 85% shot size, the relative density decreases as the injection pressure increases. This could be due to faster injection speed and thus shorter filling time, which in turn reduces the time for gas escape. In addition, the relative density decreases as the mold temperature increases, which could be mainly due to the decrease in the unfoamed skin thickness. As expected, the increase in shot size led to an increase in the average relative density of parts, as more material was injected into the mold cavity. 3.2 Unfoamed Skin Thickness Table 5 shows the unfoamed skin thickness at nine regions and their average value for each injected part. The skin thickness at the regions far from the gate is smaller than that for the regions near the gate, which could be due to the lower compression and shorter exposure time to the cooled wall during filling (Figure 9). Although the cell population density at the far regions is slightly smaller than that of the regions near the gate (section 3.3), but the cells are distributed more uniformly across the thickness of the part. Minimum skin thickness was observed in Parts No. 5 and 6 that were injected at a high injection pressure and a low shot size. In the most regions of Part No. 6, no unfoamed skin was noticed. In contrast, maximum skin thickness was formed in Parts No. 7 and 8 that were injected at a low injection pressure and relatively a high shot size. Average skin thickness decreased as the injection pressure increased Cellular Polymers, Vol. 28, No. 6,

14 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi Table 5. Unfoamed skin thickness at nine regions of the specimens (in micron - according to Figure 2) Part Reg. Reg. Reg. Reg. Reg. Reg. Reg. Reg. Reg. Average No Figure 9. Skin thickness of various regions of parts 418 Cellular Polymers, Vol. 28, No. 6, 2009

15 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Figure 10. The effect of injection pressure on skin thickness (Figure 10) due to the faster mold filling. In addition, by increasing the mold temperature, skin thickness decreased because of the slower cooling rate at the mold wall. An increase in shot size increased the skin thickness possibly due to a higher compression and less space for expansion. Also, all parts relatively have equal skin thickness on both sides (e 1 and e 2 in equation (4)). 3.3 Cell Population Density Table 6 shows the cell population density at nine regions and their average value for each injected part. The cell numbers at the areas near the gate are higher than those far from the gate (Figure 11). This could be due to the gas escape phenomenon at the flow front as the melt enters the mold. This melt segment forms the region far from the gate. On the other hand, at the early stage of injection, the pressure drop rate is larger since the opposing pressure at the mold is lower. This, inversely, contributes to a higher nucleation. It appears that the gas escape phenomenon dominates the cell population density. The highest cell population density was produced in Part No. 6 that was injected at a low shot size (85%), high injection pressure (90 MPa) and high mold temperature (80 C). In addition, for all parts, maximum cell population density appeared at the region 2 (near the gate). By increasing the injection pressure, average cell population density relatively remains constant (Figure 12). Cellular Polymers, Vol. 28, No. 6,

16 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi Table 6. Cell population density at nine selected regions of the injected parts (according to Figure 2) Part No. Reg. 1 Reg. 2 Reg. 3 Reg. 4 Reg. 5 Reg. 6 Reg. 7 Reg. 8 Reg. 9 Average 3 2.3E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E Cellular Polymers, Vol. 28, No. 6, 2009

17 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Figure 11. Cell population density at various regions of parts Figure 12. Effect of injection pressure on the cell population density Cellular Polymers, Vol. 28, No. 6,

18 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi 3.4 Surface Hardness Table 7 shows the surface hardness of the three selected regions and their average value for each part. The results indicate that the surface hardness of microcellular specimens is higher than that of the unfoamed specimens (except for Part No. 6 Reg. 3). This seems to be unexpected as the foamed article contains voids, although due to existence of solid skin, the effect of voids could be marginal. The higher hardness in the foamed parts could be attributed to the internal gas pressure during foam expansion (due to nucleation and cell growth phenomenon) that applies compressing effect on the surface layers from inside against the mold wall. Figure 13 illustrates this proposed phenomenon schematically. This effect is similar to the effect of holding pressure on surface hardness for unfoamed parts. Also interesting to note that the surface hardness at the regions far from the gate is higher than that of the regions near the gate (Figure 14). This can be related to more contact time between end part regions and mold wall. Because the end part region forms from flow front of polymergas solution when injected in the mold cavity. For parts No. 7 and 8, surface hardness of end regions is lower than those of the regions near the gate. The reason referred to narrow flow channel that illustrated in section 3.1 of this paper and lower filling velocity. The average value of the surface hardness decreased as the injection pressure increased (Figure 15), because of the decrease in the unfoamed skin thickness and the creation of cells near the surface. Also by increasing the mold temperature, Table 7. Part measured hardness at the three measured regions (according to Figure 4). Unfoamed part was injected at 50 MPa injection pressure, 40 C mold temperature and 40 MPa holding pressure for 6 sec Part No. Reg. 1 Reg. 2 Reg. 3 Average Unfoamed Cellular Polymers, Vol. 28, No. 6, 2009

19 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Figure 13. Schematic of the internal pressure caused by foam expansion and imposed on the layer near the wall Figure 14. Surface hardness of various regions of parts Cellular Polymers, Vol. 28, No. 6,

20 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi Figure 15. The effect of injection pressure on surface hardness surface hardness decreased (except for those parts that were injected at 90 MPa injection pressures and 85% shot size-part No. 6). It is likely due to the slower cooling rate of the melt and thus lower melt viscosity, so that a higher internal pressure can be transferred to compress the layers against the mold wall before solidifying the melt. The effect trend of shot size is not clear. But for major conditions, surface hardness decreased as the shot size increased. 3.5 Flexural Strength and Modulus Table 8 shows the flexural strength and modulus at the two selected regions and their average value for each part. It is seen that the flexural strength of microcellular parts is much lower than that of the unfoamed parts. Even the strength-to-density is lower for the foamed parts. Although the cells are on the order of 10 micron, but the reduction in flexural strength is significant. Similar finding was given in references (2,31) to show that the strength of microcellular foams is not phenomenal as usually perceived. This could depend on many parameters such as materials and processing. The logical method to show the advantages of microcellular foams is to compare them with the conventional foams where the cells are larger (and not with the unfoamed parts). This is especially important when dealing with the injected parts where the creation of microcellular structure is more challenging compared to the batch foaming. The flexural strength at the regions far from the gate is higher than that of the regions near the gate (Figure 16). This is accompanied by the less cell population density at the far regions. However, the SEM pictures showed that 424 Cellular Polymers, Vol. 28, No. 6, 2009

21 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded Table 8. Flexural strength and flexural modulus at the two selected regions of the injected parts Flexural strength (MPa) Flexural modulus (GPa) Part No. Region 1 Region 2 Average Region 1 Region 2 Average Unfoamed Figure 16. Flexural strength at various regions of parts Cellular Polymers, Vol. 28, No. 6,

22 Seyed Abdol Mohammad Rezavand, Amir Hossein Behravesh, Mehdi Mahmoodi and Peyman Shahi the unfoamed skin thickness at the far regions is thinner than that of near gate regions. It seems that the effect of decreasing the cell numbers on increasing the flexural strength, dominate to the effect of decreasing the skin thickness on decreasing the flexural strength., Although Xu et al., 2002 (18) stated that the skin thickness is a critical factor for the promotion of the flexural strength of the microcellular part in the unfilled polymers, but further data are needed to present reliable conclusion. The results show that the average flexural strength increases with decreasing the injection pressure and increasing the shot size (Figure 17), which could be due to the lower cell population density. Mold temperature does not show a consistent effect on the strength. The flexural modulus of foamed parts is lower than that for unfoamed parts. For parts number 7 and 8, the flexural modulus, due to thick unfoamed skin (Table 5), did not decrease. Even by combination of the effects of thick skin and foamed core, the flexural modulus of these two parts are almost equal or higher than flexural modulus of unfoamed part. Figure 17. The effect of injection pressure on flexural strength 4. CONCLUSION A microcellular injection foaming system was implemented via modification of a conventional injection molding machine. Effects of processing parameters on the microstructures and mechanical properties of the injected parts were investigated. Nine regions of the injected molded rectangular shaped part were selected to observe the variations in microstructural properties. Shot 426 Cellular Polymers, Vol. 28, No. 6, 2009

23 Experimental Study on Microstructural, Surface Hardness and Flexural Strength of Injection Molded size, injection pressure and mold temperature were the variable processing parameters. Relative density, unfoamed skin thickness, cell population density, surface hardness and flexural strength were the measured properties. The results indicated that: Microstructural and mechanical properties are different at various regions of the foam injected parts. An increasing injection pressure, decreased relative density, skin thickness, surface hardness and flexural strength. While increasing injection pressure, showed various trends on cell population density. With increasing mold temperature, properties such as relative density, skin thickness, surface hardness and flexural strength decreased and cell population density show various trends. With increasing shot size, relative density, skin thickness and flexural strength increased, surface hardness decreased and cell population density show various trends. REFERENCES 1. Park C.B., The Role of Polymer/gas Solution in Continuous Processing of Microcellular Polymers, PhD Thesis, Massachusetts Institute of Technology (MIT), (1993). 2. Beydokhti K.K., Behravesh A.H., and Azdast T., Iranian Polymer Journal, 15(7), (2006), p Behravesh A.H. and Rajabpour M., Cellular Polymers, 25(2), (2006), Behravesh A.H., Park C.B., Pan M., and Venter R.D., 212th National ACS Meeting - Polymer Preprints, Orlando, (1996). 5. Behravesh A.H., Park C.B., and Venter R.D., SPE ANTEC, Atlanta, (1998). 6. Xu J. and Pierick D., Journal of Injection Molding Technology, 5(6), (2001), Waldman F.A., The processing of microcellular foam, Msc Thesis, Massachusetts Institute of Technology (MIT), (1982). 8. Bledzki A.K., Rohleder M., Kirschling H., and Chate A., Cellular Polymers, 27(6), (2008), Okamoto K., Microcellular processing, Hanser Publisher, Munich, (2003). 10. Wang C., Cox K., and Campbell G.A., SPE ANTEC, Boston, (1995). Cellular Polymers, Vol. 28, No. 6,

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