Injection Foam Molding on. Glass-Fiber Reinforced. Polypropylene. Jonathan Jung Hyun Park. A thesis submitted in partial fulfillment

Transcription

1 Injection Foam Molding on Glass-Fiber Reinforced Polypropylene Jonathan Jung Hyun Park A thesis submitted in partial fulfillment of the requirements for the degree of BACHELOR OF APPLIED SCIENCE Supervisor: C. B. Park Department of Mechanical and Industrial Engineering

2 ABSTRACT A combination of glass-fiber polypropylene composite and foaming process can provide a next generation of polymer composite by offering lighter weight and superior properties. In order to contribute to development of injection foam molding technology guideline which will obtain best material properties, this thesis will investigate the effects of processing parameters of injection foam molding process on cellular properties, dimensional properties, and mechanical properties of glass-fiber reinforced polypropylene as well as the relationships between those properties. Processing parameters are content of physical blowing agent, injection speed, and shot size. Each processing parameter is divided into three levels and for each level of parameter, samples for each property evaluation will be prepared and prepared sample will be tested by corresponding test methodology. Cellular density was affected dominantly by the gas content. The cell density was found to be increased by increasing the amount of PBA content. Another processing parameter that affected cell density, the shot size, or void fraction was found to exhibit as a significant processing parameter only when high injection speed was used. Dimensional properties, warpage and shrinkage, were reduced with the foaming process in comparison to the non-foamed counter parts. Since the degree of warpage was strongly related to the cooling rate of the sample, the slow injection speed with slow cooling rate exhibited less warpage. Shrinkage values exhibited clear decreasing trend as the cell density was increased. Non-foamed counter parts were found to have better mechanical properties than foamed parts. Any increment relates to the cell density, void fraction, or gas content reduces tensile strength and impact strength. The main reason is that cells or bubbles within the foamed samples are acting as stress concentration points and propagate the cracks. The expected effect by addition of glass-fiber filler was not enough to overcome the deteriorated properties, the drawback of foaming technology.

3 i ACKNOWLEDGEMENTS I would like to acknowledge the Microcelluar Plastics Manufacturing Laboratory and its affiliated members. Especially, I would like to thank my thesis supervisor, Peter U. Jung, Professor Chul B. Park and all other friendly members.

4 ii TABLE OF CONTENTS 1. LIST OF TABLES. iv 2. LIST OF FIGURES... v 3. INTRODUCTION BACKGROUND OBJECTIVE EXPERIMENT MATERIALS MATERIAL COMPOSITION EXPERIMENTAL SETUP EXPERIMENTAL PROCEDURE PROPERTY EVALUATIONS CELLULAR PROPERTIES VOID FRACTION CELL DENSITY DIMENSIONAL PROPERTIES WARPAGE SHRINKAGE MECHANICAL PROPERTIES TENSILE TEST IMPACT TEST RESULTS AND DISCUSSION CELLULAR PROPERTIES GAS CONTENT SHOT SIZE INJECTION SPEED.19

5 iii RESULT ANALYSIS DIMENSIONAL PROPERTIES WARPAGE SHRINKAGE RESULT ANALYSIS MECHANICAL PROPERTIES TENSILE PROPERTIES IMPACT STRENGTH RESULT ANALYSIS CONCLUSION REFERENCES.. 34

6 iv 1. LIST OF TABLES Table I. Material composition set-up for compounding process Table II. Processing parameter variables and their values

7 v 2. LIST OF FIGURES Figure 1. Weekly U.S. retail gasoline prices (1) Figure 2. TGA result of sample pellets of glass-fiber and PP compound Figure 3. Schematic diagram for the advanced structural foam molding system (14) Figure 4. Schematic diagram for mold cavity used Figure 5. Experimental Procedure Sequence Figure 6. Schematic of experimental set-up for measuring warpage of injection molded sample Figure 7. Normal plot of the standardized effects on cell density Figure 8. Cell density with respect to gas content (or PBA) Figure 9. Cell density plotted against shot size Figure 10. Cell density against injection speed Figure 11. Normal plot of the standardized effects on warpage Figure 12. Average warpage with respect to injection speed Figure 13. Main effects plot for shrinkage Figure 14. Cell density against shrinkage of foamed sample Figure 15. Main effects plot for tensile strength at breakage and elastic modulus Figure 16. Tensile and specific tensile strength with respect to shot size Figure 17. Elastic and specific elastic modulus against shot size Figure 18. Main effects plot for impact strength Figure 19. Impact strength against gas content Figure 20. Impact strength versus cell density

8 1 3. INTRODUCTION In the past decade, the price of oil has been skyrocketing and plastic resin industry which is hardly dependent on oil suffers a direct hit from the oil price rise. As shown in Figure 1 (1), the average price of regular grade gasoline has been increased approximately 400% for the past decade and approximately 170% for last two years. Figure 1. Weekly U.S. retail gasoline prices (1) Meanwhile, in last two years, the average price of high impact polypropylene has been increased by 160% (2), which is less than increase in oil price. As a result, the industry must be doing their uttermost effort to reduce the material cost without compromising the properties of final product. Additionally, in this oil price skyrocketing era, since the weight of a vehicle is directly related to fuel efficiency, the automotive industry is also eagerly looking for ways to reduce the

9 2 weight of a vehicle. The solution the industry found was the usage of reinforced polymers (3) which has been significantly increased for the last few decades and replaced heavy metal materials. With light weight compared to metals and superior mechanical properties, glass-fiber reinforced polymer composite is at the hub of these changes. There is a favorable forecast for its growth annually 10% until 2010 (4). Nevertheless, among various polymer composites, glassfiber composites often appear to have high material density due to relatively heavy glass-fiber. On the other hand, foaming technology is another solution for weight reduction. Foaming technology reduces product weight and density of the composite by creating bubbles in it by dissolving gas into polymer matrix. While it reduces product weight, it can also offer superior properties when compared with their non-foamed counterparts. Consequently, the combination of glass-fiber polymer composite and foaming technology can provide a next generation of polymer composite that has lighter weight with superior properties BACKGROUND The relative new foaming technology has been studied for the last few decades since Suh et al. developed the first microcellular foam structures in the early 1980s (5). Nowadays, the continuous efforts have been made to implement the technology on various polymer manufacturing process such as batch process (5) (6), extrusion (7) (8) (9) (10) (11), and injection molding (12) (13) (14). The greatest advantage of this foaming technology is a significant weight reduction of polymer products because it then directly leads to a significant material cost reduction.

10 3 Additionally, the foaming technology has been attracting both engineers and industries due to its some superior properties comparing to the solid counter parts. Nevertheless, foaming generally deteriorates the mechanical properties of products. In order to overcome this drawback of foaming technology, two major types of solutions were conducted from various types of researches. First type is to improve necessary properties of foamed product by focusing on exclusively cellular properties such as size, cell density, distribution, and etc. This is because foamed phase governs final properties of product when it is significantly compared to polymer phase (15). Another type is to improve the deteriorated properties by implementing additives or fillers to foamed polymer. Different fillers such as wood, glass-fiber, and nano-composites have been utilized with foaming technology. As it was mentioned earlier, the applications of glass-fiber plastic composites have been increased and the related researches, of course, have been actively conducted as well. In recently, M. R. Thompson et al. have investigated the effects of foaming during injection molding process on breakage of glass-fiber (16). The molten polymer with dissolved gas was developed by using a chemical blowing agent during the plasticizing part of injection molding process. The dissolved gas in the molten polymer decreased shear viscosity of flow. As a result, the occurrence of glass-fiber breakage throughout the injection molding process was significantly reduced. Since the length of glass-fiber is one of the most critical characteristics to its mechanical properties, it was observed that foaming technology may affect mechanical properties in positive way.

11 OBJECTIVE According to literature review, there has not been any research to investigate the effects of processing parameters of injection foam molding process on various mechanical properties. Therefore, this research was designed not only to analyze the relationships between processing parameters and mechanical properties, but also to extensively investigate their effects on foaming behavior and dimensional properties as well as the relationships between those properties. Overall, this thesis was designed to contribute to development of manufacturing technologies that can achieve improved mechanical properties than conventional foamed plastics as well as conventional glass fiber reinforced plastics.

12 5 4. EXPERIMENTAL 4.1. MATERIALS Various materials from industries were used to form the desirable composite for this study. As a matrix resin, one of the most typical matrix resins, Polypropylene (BE170) was supplied by Borealis for this study. Typical matrix resins used in composite include polypropylene, polyethylene, polyurethane, and epoxy. Choice of matrix materials depends on intended mechanical property and also compatibility between the fiber and the matrix. It is found that polypropylene is an acceptable candidate for every fiber due to its compatibility (i.e. capability of blending polymer in molecular level). The material was determined to have a density of g/cm 3, a melting temperature of 230 ºC, and a melting flow index of 13g/10 min. Glass fiber, material made from extremely fine fibers of glass, was used as a reinforcing agent. A glass-fiber composite is the most commonly used fiber reinforcement for plastics due to its superior mechanical properties for reinforcement and the moderate material price (US$ ) (17). Other advantages of glass fibers include high tensile strength of 3450 MPa (18), stiffness, and moisture resistant (19). Also, softening temperature of about 850 C makes glass fiber highly resistance to temperature as well (20). In this study, chopped glass fiber (144A-14C- 4mm) from Owens Corning was used as main filler and N 2 from BOC Gas was utilized as a physical blowing agent. In order to increase the filler acceptability of polymers, a coupling agent between polymers and fillers were used. Without it, the bond between the glass fiber and the resin would

13 6 weaken severely (21). As a coupling agent, Fusabond P MD353D from DuPont was used and its melting point and melting flow index were 136 ºC and 22.4g/10 min, respectively MATERIAL COMPOSITION Prepared materials must be compounded to customize the composition of each ingredient. In this study, the glass-fiber reinforced polypropylene resin was compounded with using a twinscrew extruder from Leistriz. The coupling agent was mixed with polypropylene prior to the compounding process. It was precisely measured using a precision digital scale and dry-mixed with the ratio of 3 phr of polypropylene. A mixture of polypropylene and the coupling agent was fed into the extruder using the main feeder (Brabender Technologies). However, for the chopped glass fiber, the side feeder (Brabender Technologies) was used to feed since the glass fiber can be damaged due to high shear from the extruder screw in the main feeder direction. Therefore, two feeders must be used and the set feeding ratio between two feeders is shown in Table I. Material Polypropylene-Coupling Agent Glass-Fiber Weight % Table I. Material composition set-up for compounding process In order to verify the actual amount of glass-fiber within polypropylene resin, TGA experiment was conducted after the compounding process. As it is illustrated in Figure 2, the average weight percentage of glass-fiber was approximately 14.6 %, which has 5.4 % difference from the set-up value.

14 7 100 Sample 1 Sample 2 Relative Sample Weight [%] Temperature [ C] Figure 2. TGA result of sample pellets of glass-fiber and PP compound 4.3. EXPERIMENTAL SETUP The advanced structural foam molding system shown in Figure 3 (14) was used in this experiment. The system consists of three subsystems: an extrusion system, an injection system, and a control system.

15 8 Gas Injection Port Gas 3 Cylinder 5 4 Gas Pump Hydraulic Systems P2 P1 1 Secondary Accumulator Accumulator 1 Nozzle 1 Shut-Off Valve Shut-Off Valve (or Non-Returnable Check Valve) Mold Extrusion Barrel 1 Screw 2 Gear Pump 7 9 Molded Part Figure 3. Schematic diagram for the advanced structural foam molding system (14) In the extrusion system, there is a 3/4" extruder (Brabender ) with a mixing screw of 30:1 L/D ratio (Brebander, ) for plasticizing the prepared polymer pellets from the previous experimental stage and for dispersing the injected physical blowing gas throughout the pellet melting process; a 5-hp motor (Allen-Bradley 1329R, Rockwell Automation) for rotating the screw; a motor speed controller (Allen-Bradley 1336 Impact, Rockwell Automation); a positive-displacement syringe pump for the blowing gas (ISCO Inc.); a gear pump that controls the flow rate of the melt (Zenith, PEP cc/rev); and a gas injection port for injecting the blowing gas. The injection subsystem also has a plunger-type injection molding machine (Mini-Jector 50, manufactured by Mini-Jector Corp.). The simplest plate design mold cavity was used for this study, which is shown in Figure 4.

16 9 Figure 4. Schematic diagram for mold cavity used The control system includes a screw-speed computer control system for a steady barrel pressure; and a timer for controlling the shot size; twelve temperature controllers and corresponding twelve thermocouples for monitoring and maintaining desirable temperature condition in the system. In this study, the effect of the temperature change on the polymer is not intended for evaluation. Therefore, a consistent temperature is preferred EXPERIMENTAL PROCEDURES As it was briefly mentioned in previous section and shown in Figure 5, the compounding was conducted to customize the composition of each ingredient.

17 10 Figure 5. Experimental Procedure Sequence The twin-screw extrusion system (Micro27-Twin Screw Extruder, Leistriz) was utilized for the compounding materials and its processing temperature was 230ºC. The total feeding rate was 10 kg/hour. The extrudate from the extrusion system was cooled using a water channel. At the end of the water channel, a pelletizer (SheerBay BT25) was set-up to cut the extrudate into appropriate pellet sizes. Then, the pellets were dried before the injection foam molding process in order to eliminate moisture within. Foam injection molding process was taken place once the sample was dried. Its processing temperature was 230 ºC, and the mold cavity was kept in room temperature. While the processing parameters were controlled to be consistent, other processing variables were varied to investigate their effects on glass-fiber reinforced polypropylene. Each

18 11 processing parameter was divided into three levels and their actual set-up values are shown in Table 3. The sampling quantity of each experiment condition was 7 shots. Processing Parameters Physical Blowing Agent Content [wt%] Shot size [vol%] Injection speed [mm/s] Levels Low Medium High Table II. Processing parameter variables and their values

19 12 5. PROPERTY EVALUATIONS 5.1. CELLULAR PROPERTIES In order to evaluate the foaming behavior of samples, two evaluation categories were implemented, which were void fraction and cell density. Void fraction, also known as porosity, describes the fraction of void space in the composite; Cell density is the number of cells per unit volume of un-foamed material VOID FRACTION The foam density was determined by the water displacement method, which is specified in ASTM D The relative density Φ is the ratio of the bulk density of PP and glass-fiber compounds, ρ o, to the measured density of the foam sample, ρ f. The equation to calculate the void fraction is following. 1 Void Fraction 1 100% (1) CELL DENSITY The cell density was calculated from the images of the scanning electron microscopy (SEM). In order to investigate cellular morphology, SEM images of the fractured cross section of the sample were observed. Prior to fracture, the samples were deposited into liquid nitrogen for several seconds. Under normal atmospheric pressure, nitrogen exists as a liquid between the temperatures of 63 K and 77.2 K (22). Cooling in liquid nitrogen makes the samples easier to fracture and promises better quality of the fracture cross-section. Then, the samples were goldcoated using a sputter coater to improve their conductivity for SEM image. Finally, the

20 13 morphology was observed with using a SEM (JSM-6060) from JEOL. The cell density was determined based on the following equation. 3 / 2 2 nm Cell Density (2) A Where n: the number of bubbles in the micrograph A: the area of the micrograph M: the magnification factor of the micrograph 5.2. DIMENSIONAL PROPERTIES In most cases, maintaining consistent dimension of injection molded products is almost impossible because warpage and shrinkage are occurred during the cooling process. For dimensional properties, warpage and shrinkage tests were performed to evaluate the effects of foaming. As the dimensional properties measurements, five samples were evaluated for individual experimental condition WARPAGE For warpage, the height of its highest point subtracts its thickness, Figure 6 shows the setup used for this experiment.. The warpage was measured as recommended in ASTM D A molded sample (4 inch by 6 inch by 3.2 mm) was placed on top of flat surface. Then, the height of its highest point was measured by using a digital height gauge.

21 14 Figure 6. Schematic of experimental set-up for measuring warpage of injection molded sample SHRINKAGE In case of shrinkage, it was measured as ASTM D and total shrinkage, the sum of shrinkage in length and shrinkage in width, was also measured in the similar manner MECHANICAL PROPERTIES Often, mechanical properties are considered as the most important properties of the material. Better mechanical properties of the material are directly implicated to the long term objective of this study; for mechanical properties, tensile and impact tests were performed to evaluate the effects of foaming; Five samples were tested for each experiment condition.

22 TENSILE TEST In case of tensile test, Instron 4465 was utilized base on ASTM D The test specimen Type IV was implemented and the strain rate was 5 mm/min. The measured values were tensile strength at breakage, and Young s modulus IMPACT TEST In case of impact properties, the notched IZOD pendulum impact testing according to ASTM D was used. On the testing system, Model 892 from Tinius Olsen, type A test method was implemented and the impact resistance of sample was investigated.

23 16 6. RESULTS AND DISCUSSION The normal plots of standardized effects for each processing parameter were plotted by using the concept of Analysis Of Variance (ANOVA). An ANOVA is an analysis of the variation observed in an experiment. It is a hypothesis test that the variation in an experiment is no greater than that due to normal variation of individuals' error and characteristics in their measurement (23). On the normal plot of the standardized effects, the greater distance from the curve a factor has, the more significant effect it has CELLUALAR PROPERTIES Figure 7. Normal plot of the standardized effects on cell density

24 GAS CONTENT In case of gas content or physical blowing agent, gas content was found to be the most significant parameter among three processing parameters. As shown in Figure 7, which represents the normal plot of standardized effects of processing parameters on cell density, the gas factor A has the longest distance from the curve and confirms its significance. This trend is shown in Figure 8 as well. As the gas content increases from 0.3 to 0.8 wt%, higher cell density is achieved for sure. As clearly plotted in Figure 8, the effect of gas content is independent from the other two processing parameters. For every case, the highest cell density was obtained at the highest gas content. The highest cell density obtained among all cases was approximately 6.8*10 6 cells/ cm 3 when 0.8 wt% of N2, 80% shot size, and 100 mm/s of injection speed were utilized together Cell Density Vs Gas Content Cell Density [cells/cm 3 ] Gas Content [wt%] ### ### ### ### ### ### Figure 8. Cell density with respect to gas content

25 SHOT SIZE In case of shot size, the cell density was decreased as the shot size was increased based on the results of Figure 8. Although the degree of variation varies for each case, all cases exhibited the same decreasing trend. Interestingly, it was found that the cell densities of higher injection speed samples were much more sensitive to the change in shot size. As shown in Figure 9, the cell density values of samples with 100 mm/s injection speed decrease insignificantly compare to fast injection speed samples. In other words, the shot size has significant effect only on higher injection speed samples, which makes it not as critical as the gas content processing parameter Cell Density Vs Shot Size Cell Density [cells/cm 3 ] ### ### ### ### ### ### ### ### ### Shot size [%] Figure 9. Cell density plotted against shot size

26 INJECTION SPEED Unlike the other two variables, the effect of injection speed was not consistent as shown in Figure 10. Among the six combined cases, four samples had their highest cell density with the slowest injection speed, 100 mm/s and other two cases obtained their highest cell density with the fastest injection speed, 400 mm/s. Relationship with other two processing parameters was not clearly represented as well. Therefore, further investigation is required for the effects of injection speed on glass-fiber polymer composite samples Cell Density Vs Injection Speed 10 6 Cell Density Injection Speed [mm/s] 0.3wt% N 2 & 90% shot size 0.3wt% N 2 & 80% shot size 0.5wt% N 2 & 90% shot size 0.5wt% N 2 & 80% shot size 0.8wt% N 2 & 90% shot size 0.8wt% N 2 & 80% shot size Figure 10. Cell density against injection speed

27 RESULT ANALYSIS For the cellular properties of the samples, gas content was found to be the most significant parameter. Since cell density is a measurement of the number of cells per unit volume of un-foamed material, it theoretically has a thread of connection that the gas content plays the most significant role affecting the cell density. Another processing parameter in relationship with the cell density was shot size which only had significant effect with higher injection speed. Further investigation is suggested for the relationship of shot size and the injection speed of the samples DIMENSIONAL PROPERTIES WARPAGE Figure 11. Normal plot of the standardized effects on warpage

28 21 Figure 11 illustrates the normal possibility plot of the standardized effect on warpage. It was observed that all three parameters had significant impacts, however among the three, the injection speed had more dominant effect. Accordingly, Figure 12 shows the effect of injection speed on average warpage values. As clearly shown in the figure, in all cases, the degree of warpage was significantly increased as the injection speed was increased. Because the higher injection speed leads to higher cooling rate of the samples and the higher cooling rate causes more warpage in the sample dimension, it was found that slow injection speed must be used to minimize the warpage of the sample. 10 Warpage Vs Injection Speed Average Warpage [mm] Injection Speed [mm/s] % (Solid 100% Shotsize % (0.3N 2 90% Shotsize % (0.3N 2 80% Shotsize % (0.5N 2 90% Shotsize % (0.5N 2 80% Shotsize % (0.8N 2 90% Shotsize % (0.8N 2 80% Shotsize Figure 12. Average warpage with respect to injection speed

29 SHRINKAGE Figure 13. Main effects plot for shrinkage Figure 13 is a main effects plot for shrinkage which shows how each processing parameter affects the shrinkage value. From the obtained plot, as gas content was increased from 0.3wt% to 0.8wt%, the shrinkage was significantly reduced. It was also increased when the shot size was increased from 80% to 90%. However, the injection speed did not exhibit any significant effect on shrinkage unlike warpage.

30 RESULT ANALYSIS In order to investigate the effects of foaming process, the warpage of non-foamed samples was measured and compared with the warpage of foamed samples. The maximum warpage for non-foamed sample was mm while the minimum warpage for foamed sample was mm. Thus, the warpage of foamed samples was reduced by approximately 68 % comparing to that of solid counter parts. The maximum shrinkage value for non-foamed samples was % while the minimum shrinkage measured for foamed sample was %. This was approximately 13 % reduction obtained by the foaming process. As shown from warpage and shrinkage reductions, it was found that foaming process improved dimensional properties of glass-fiber composites. In order to support that statement, the relationship between foaming behavior and shrinkage of samples was investigated, which is shown in Figure Cell Density Vs Shrinkage 1.6 Total Shrinkage [%] Linear Fit Shrinkage [%] Cell Density [cells/cm 3 ] Figure 14. Cell density against shrinkage of foamed sample

31 24 According to the Figure 14, the shrinkage was decreased as the cell density was increased in a linear fashion. As a result, it was proven that more foaming, or higher cell density leads to less shrinkage or better dimensional property.

32 MECHANICAL PROPERTIES TENSILE PROPERTIES Figure 15. Main effects plot for tensile strength at breakage and elastic modulus

33 26 For tensile properties of samples, three typical characteristics measured were tensile strength at yield (or yield stress), tensile strength at breakage (or ultimate tensile stress) and elastic modulus (or Young s modulus). As Figure 15 illustrates, in case of the tensile strength, the shot size was the most dominant factor showing the utmost amount of change. In case of the elastic modulus, it seemed more difficult to choose the dominant factor. Gas content and injection speed parameter exhibited neither increasing nor decreasing trend. Despite the other two parameters, the effect of shot size exhibited consistent increasing trend for both of tensile properties. Therefore, all tensile properties were plotted against shot size for further analysis. Tensile Strength [MPa] Tensile strength Vs Shot size % (Yield Strength ) % (Ultimate Strength Shot size [%]

34 27 Specific Tensile Strength [MPacm 3 /g] Specific Tensile Strength Vs Shot size Shot size [%] % (Specific Yield Strengt % (Specific Ultimate Stre Figure 16. Tensile and specific tensile strength with respect to shot size Figure 16 shows tensile and specific tensile strength values with respect to shot size. In the figure, 100 % shot size represents non-foamed samples. As it was expected, both yield and ultimate strength values were decreased as the shot size was decreased accordingly. In case of the tensile strength, the average yield strength value was decreased by 35.2 % when the shot size was 80%, but the average ultimate strength was reduced only by 23.9 % with the identical shot size. Hence, the tensile strength values experienced the reduction, which was rather greater than the weight reduction rate of samples. In case of the specific tensile strength values, the graph shows the specific values of foamed samples were still smaller than solid samples and this illustrates that the weight reduction due to foaming process was not large enough to overcome the original difference between foamed and solid parts.

35 28 Likewise, figure 17 illustrates that both elastic modulus and specific elastic modulus values are reduced as the shot size is decreased. However, In case of the specific modulus, the values of both foamed and non-foamed samples were almost equal. It explains that the modulus was decreased in the rate of weight reduction. Elastic Modulus Vs Shot size Elastic Modulus [MPa] Specific Elastic Modulus [MPacm 3 /g] Shot Size [%] Specific Elastic Modulus Vs Shot Size Shot Size [%] Figure 17. Elastic and specific elastic modulus against shot size

36 IMPACT STRENGHT Figure 18. Main effects plot for impact strength As shown in Figure 18 that represents the main effect plot for impact strength values, the processing parameter with greatest variation, the gas content was the most significant factor on the impact property. As the gas content was increased, the impact strength was significantly reduced.

37 30 90 Impact Strength Vs Gas Content Impact Strength [J/m] Gas Content [wt%] Figure 19. Impact strength against gas content Further investigation on the dominant factor, the gas content is shown in Figure 19 which compares the strength values of foamed and non-foamed samples with the zero wt% gas content. The average impact strength of non-foamed samples was 80.6 J/m while 0.3, 0.5, and 0.8 wt% of gas content had the average values of 72.3, 68.0, and 63.0 J/m, respectively. As previously discussed on cellular properties, the gas content had the most significant effect on the cell density of samples. Since the impact strength shows decreasing trend when the amount of gas content used is increasing, investigation on relationship between the impact strength and the cell density was done.

38 31 80 Impact Strength Vs Cell Density Impact Strength [J/m] Cell Density [cells/cm 3 ] Figure 20. Impact strength versus cell density As an expected result, the impact strength values were decreased as the cell density was increased rather linearly as illustrated in Figure 20. Two figures 19 and 20 clearly illustrate that foamed sample cannot have better impact strength than solid, non-foamed sample RESULT ANALYSIS The dominant processing parameter of the tensile strength was shot size, also known as void fraction. The dominant processing parameter of the impact strength was gas content. For both mechanical properties, any foaming process was found to have adverse effects. The main factor of this phenomenon is that cells within the samples act as stress concentration points of structure. The cells or bubbles within the samples also spread the inner cracks along their empty space within the samples. Consequently, the addition of glass-fiber filler to overcome the deteriorated amount of mechanical properties caused the foaming technology was ended up being a failure.

39 32 7. CONCLUSION It was found in this study that there were various relationships among the chosen processing parameters and evaluated properties. Through investigation of the cellular properties, cell density was affected dominantly by the PBA content of the system. The cell density was found to be increased by increasing the amount of PBA content. Another processing parameter that affected cell density, the shot size, or void fraction was found to exhibit as a significant processing parameter only when high injection speed was used. For the dimensional properties, in general, both warpage and shrinkage were reduced with the foaming process in comparison to the non-foamed counter parts. Since the degree of warpage was strongly related to the cooling rate of the sample, the slow injection speed with slow cooling rate exhibited less warpage. In case of shrinkage, it was strongly related the cell density. As the cell density was increased, shrinkage values exhibited clear decreasing trend; as the shot size was increased, the cell density decreases accordingly, therefore, more shrinkage was exhibited. For the mechanical properties, in general, non-foamed counter parts had better properties. Any increment in cell density, void fraction, or gas content reduces tensile strength and impact strength. The main reason is that cells or bubbles within the foamed samples are acting as stress concentration points and propagate the cracks because they are empty spaces within the structure. The expected effect of the addition of glass-fiber filler was not enough to overcome the drawback of foaming technology.

40 33 For the future work of the thesis, since there is no stanch theory or hypothesis statement on the relationship of those processing parameters, further investigation on relationship between cell density and injection speed, relationship between injection speed and shot size can be prepared in details. Furthermore, within the experiments, the amount of coupling agents was found to have considerable effects on mechanical properties since the coupling agents have lower modulus than that of polypropylene. Further investigation on the amount of coupling agent within the samples can be studied as well.

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