Development of Advanced Structural Foam Injection Molding. Kye Kim. A thesis submitted in partial fulfillment of the requirements for the degree of

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1 Development of Advanced Structural Foam Injection Molding Kye Kim A thesis submitted in partial fulfillment of the requirements for the degree of BACHELOR OF APPLIED SCIENCE Supervisor: Park, C.B. Department of Mechanical and Industrial Engineering University of Toronto March, 2007

2 Abstract The structural foam injection molding has a number of advantages over the conventional injection molding including absence of sink marks on part surface, weight reduction, low back pressure, faster cycle time, and high stiffness-to-weight ratio. However, uniform cell structure and high void fraction is unachievable due to nonuniform cell density distribution. This non-uniformity is due to changes in pressure drop and pressure drop rate, which is inevitable in injection molding process during mold filling operation. In this study, processing parameters such as injection speed, melt temperature, blowing agent content, and gate size are studied for their influence towards void fraction and uniform cell structure using advanced structural foam injection molding machine, which allows gas/polymer one-phase solution in preparation stage.

3 i Acknowledgements I wish to thank all those who helped me. Without them, I could not have completed this project. Professor Park, C.B. Ph. D. John W. S. Lee

4 ii Table of Content 1. Introduction Background Fundamentals Structure of the Advanced Structural Foam Molding Machine Use of a Very Small Amount of Blowing Agent Use of an Effective Nucleating Agent Experimental Materials Preparation of HDPE/Talc Compounds Experimental Setup Mold Geometry Experiment Processing Parameters Affecting High Void Fraction Injection Speed and Uniform Void Fraction Distribution Results and Discussions Processing Parameters Affecting High Void Fraction Effect of Processing Parameters on the Degree of Mold Filling Effect of Gas Content on the Cell Density and Cell Size Uniformity Effect of Shot Size (Injection Stroke) on Cell Density and Cell Size Uniformity.. 18

5 iii Effect of Gate Size on Cell Nucleation in a Mold Cavity Injection Speed and Uniform Void Fraction Distribution Effect of Injection Speed on Flow Length of Foam Melt and Void Fraction Uniformity Effect of Variable Injection Speed Profile on Void Fraction Conclusion Reference.. 28

6 iv Table of Figures Figure 1. 4 Foaming processes in conventional and advanced structural foam molding Figure 2. 6 Solubility pressure of N 2 for 0.05, 0.10, and 0.15 wt% Figure 3. 7 Non-uniform cell size distribution due to varying pressure drop rate Figure 4. 8 Schematic of advanced structural foam molding machine [3] Figure 5. 9 Gate geometries and dimensions Figure Constant injection speed profile (A1-A9) Figure Variable injection speed profile (B1-B5) Figure Effect of processing parameters on the degree of mold filling Figure Effect of gas content and injection speed on the foam morphology Figure Cavity pressure profile near the gate area and the corresponding cell density

7 v Figure Effect of shot size on foam morphology Figure SEM pictures of the three gates Figure Effect of injection on (a) flow length of foam melt and (b) void fraction uniformity. Figure Effect of variable injection speed profile on void fraction uniformity. Figure Effect of speed change stroke on structural foams

8 vi Table of Tables Table 1. Summary of Experiments. 10 Table 2. Summary of Experiments.. 12

9 i Acknowledgements I wish to thank all those who helped me. Without them, I could not have completed this project. Professor Park, C.B. Ph. D. John W. S. Lee

10 1 1. Introduction The conventional injection molding is the most widely used technology in polymer production nowadays. Advanced from the conventional injection molding, foam injection introduces gas into polymer, which derived several advantages and strengthens positive characteristics of plastic. However, the conventional structural foam molding limits itself in producing uniform cell structure with uneven void fractions. Achieving uniformity is crucial since finer cell size and more uniform distribution exhibit better mechanical and thermal properties. The nonuniform cell density distribution along the melt flow direction comes from the change in pressure drop and pressure drop rate, which is inevitable in injection molding process during mold filling operation. The pressure drop and pressure drop rate at the beginning of the mold filling operation is the largest and decreases over time. It leads to high cell density near the end of the part and lowest near the gate. Accordingly, the void fraction is also highest near the end and lowest near the gate [1] as it is a function of cell density in high-density foams [2]. In this study, the experiments are carried out using an advanced structural foam molding technology was developed in University of Toronto [3,4]. It allows achieving a one-phase polymer solution in preparation stage, which is a starting to point to achieve a uniform cell structure in final

11 2 product. The study is focused on understanding the key parameters of the processing technologies, injection speed, blowing agent content, melt temperature and gate geometry, and their effect on the quality of mold product. 2. Background Low-pressure preplasticating-type structural foam molding machines are commonly used because a small molding system, with low pressure in the cavity, is required for producing large products [5]. Since the generated cells compensate for the shrinkage of injection-molded parts during cooling, structural foams typically have outstanding geometric accuracy. Advantages of foam injection molding include the absence of sink marks on the part surface, low weight, low back pressure, faster cycle time, and a high stiffness-to-weight ratio. The advantages of foam injection molding initiated a number of researchers to study and improve the structural foam molding technology. The representative cases will be microcellular injection molding technology (MuCell Technology) by Trexel Inc [6,7], an alternative microcellular foam process using preplasticating-type injection molding machine by Shimbo [8], and another foam injection molding process in IKV, Germany [9].

12 3 In 2006, Park et al. presented an advanced structural foam molding technology based on a preplasticating-type injection molding machine [3,4,10]. This technology was used for this study and its mechanism is explained in the following section. 3. Fundamentals 3.1 Structure of the Advanced Structural Foam Molding Machine In conventional structural foam molding, the main reason for large voids is from uneven blowing agent concentration in the polymer matrix; a complete dissolve of blowing agent is not possible for whole region of polymer melt. The existing conditions in conventional structural foam molding such as low barrel pressure, fluctuation of barrel pressure, inconsistent material flow rate, and screw stoppage during the injection stage are the cause of uneven blowing agent concentration. In order to overcome the problem, a new technology has been developed [3,4] at University of Toronto. In a modified machine, an additional material accumulator (i.e., a hydraulic piston) combined with a gear pump was installed between the extrusion barrel and the shut-off valve (before the main accumulator) to completely decouple the gas dissolution operation from the injection and molding operations. It is from the understanding the behavior of conventional structural foam molding; the stop-and-flow

13 4 molding behavior causes inconsistent gas dosing. The newly attached material accumulator allows nonstop screw rotation by accommodating polymer/gas solution during the injection stage. Also, the gear pump prevents backflow into the barrel; therefore, the pressure in the extrusion barrel can be relatively well maintained and consistent gas dosing can be attained to achieve a uniform polymer/gas mixture regardless of pressure fluctuations caused by the injection and molding operations. Maintaining constant pressure in the barrel is critical in an accurate control of blowing agent content because the injection of the blowing agent is driven by the difference between the gas pump and barrel pressure. poor diffusion and dissolution pressure drop Conventional Structural Foam Molding 2 Systems 2 phase gas- non-uniform polymer mixture bubble structure complete diffusion and dissolution pressure drop Advanced Structural Foam Molding Figure 1. Foaming processes in conventional and advanced structural foam molding

14 5 3.2 Use of a Very Small Amount of Blowing Agent With existing foam molding technologies, it is difficult to completely eliminate the swirl pattern on the surface of molded foam products. The swirl pattern is formed because the bubble nucleated at the melt front is pushed and smeared on the mold cavity wall because of the fountain effect. In order to improve the surface quality of foamed parts, several processing technologies have been proposed. Gas counter pressure molding [14-17] and co-injection molding [18-20] are used most commonly. However, these technologies require additional cost for the installation of extra devices. In this research, efforts will be made to improve surface quality by using a very small amount of blowing agent. The perfect fit for this purpose is N 2 because it can produce high cell density with less gas, compared to other blowing agents such as carbon dioxide (CO 2 ), butane, etc. [21, 22]. Figure 2 shows the solubility pressure of N 2 for 0.05, 0.10, and 0.15 wt%, which will be the amounts used for advanced structural foam molding. Since the amount of N 2 is very low, it is expected that the cell nucleation rate at the melt front will be lower. In addition, because the pressure required to keep N 2 inside the polymer matrix is very low, the pressure increase in the mold cavity resulting from sudden injection is expected to act like a gas counter pressure, thereby preventing cell nucleation at the melt

15 Solubility Pressure (psi) 6 front wt% N wt% N wt% N o Temperature ( C) Figure 2. Solubility pressure of N 2 for 0.05, 0.10, and 0.15 wt% 3.3 Use of an Effective Nucleating Agent Unlike the case of extrusion where the pressure drop rate at the die exit is constant, in injection molding, the pressure drop rate at the gate of the mold changes with time. The pressure drop rate is highest in the beginning, and it tends to decrease with time. Therefore, it would be very difficult to obtain uniform cell structures because cell nucleation is very sensitive to the pressure drop rate [23, 24]. A higher number of cells are nucleated in the beginning because of a higher pressure drop rate, but the driving force for cell nucleation will decrease as the pressure drop rate decreases with time. Figure 3 demonstrates this

16 7 outcome. One way of resolving this problem would be to use an effective nucleating agent such as talc. Previously, Park et al. revealed that the sensitivity of cell density to pressure drop rate decreases with increasing amount of talc [45]. The use of a nucleating agent is, therefore, one of the strategies which will be employed in advanced structural foam molding to produce fine-celled foams. Mold Cavity At t 1 dp/dt 1 At t 2 dp/dt 2 dp/dt 1 > dp/dt 2 > dp/dt 3 At t 3 dp/dt 3 Non-Uniform Cell Size Distribution Figure 3. Non-uniform cell size distribution due to varying pressure drop rate 4. Experimental 4.1 Materials The polymer material used in this study was HDPE of grade SCLAIR 2710, with an average melt flow index of 17 dg/min and a density of g/cm 3. The talc was of grade Cimpact CB710, with a density of 2.8 g/cm 3 and an average particle size of 1.7 µm.

17 8 The blowing agent used in this study was N 2 from BOC Gas. 4.2 Preparation of HDPE/Talc Compounds A 20 wt% talc masterbatch was prepared using an intermeshing and co-rotating twin-screw extruder with a screw diameter of 30 mm (Werner & Pfleiderer ZSK-30, L/D 38:1). The HDPE was then dry-blended with the talc masterbatch in a one-to-one ratio, to produce HDPE/talc compounds with talc content of 10 wt%. 4.3 Experimental Setup An 80-ton injection molding machine (TR80EH) from Sodick Plustech Inc. was modified into the advanced structural foam molding machine [3] and [10]. Figure 4 shows a schematic of the advanced structural foam molding machine. Gas Injection Port Gas 3Cylinder 5 4 Gas Pump 15 Hydraulic 16 Systems P2 P1 11 Secondary Accumulator 10 Accumulator Nozzle 13 Shut-Off Valve Extrusion Barrel 1 Screw 2 Shut-Off Valve (or Non-Returnable Check Valve) Gear Pump 7 9 Mold Molded 14 Part 12 Figure 4. Schematic of advanced structural foam molding machine [3]

18 9 4.4 Mold Geometry The mold contained a rectangular cavity, and a fan gate was located at one end. The cavity dimensions were mm mm 3.2 mm. The cavity and the gate (including dimensions) are shown in Fig. 5. Gate 1 Gate 2 Gate 3 3 Different Gate Geometry 1 mm 0.5 mm 1.5 mm 3.2 mm 6 x 4 x (3.2mm) dp/dt 4 MPa/s dp/dt 0.2 MPa/s dp/dt 0.04 MPa/s Figure 5. Gate geometries and dimensions 5. Experiment 5.1 Processing Parameters Affecting High Void Fraction The following table summarizes the experiments carried out in this study. While the talc content and mold temperature were fixed at 10 wt% and 30 C, respectively, factors such as gate size, gas content, melt temperature, shot size and injection speed are varied. Three different sets of experiments are conducted (A, B and C) while keeping some parameters constant and varying others to investigate their effect on mold filling and cell

19 10 size uniformity. Table 1. Summary of experiments Run # Gate Gas Content [wt%] Melt Temperature [ C] Shot Size [cc] Injection Speed [mm/s] A ~450 A ~450 A ~450 A ~450 A ~450 A ~450 B , 200, 350 B , 200, 350 B , 200, 350 C C C Characterization of Foams The void fractions were determined by the shot size (injection stroke) using Eq 1: Void Fraction 1 m m foam solid 1 melt melt ( r ( r plunger plunger 2 ) ( injection stroke) 2 ) ( injection stroke) foam solid (1) 1 ( injection stroke) ( injection stroke) foam solid The fractured cross-section of a sample with platinum coating is examined under scanning electron microscopy (SEM) for determining the cell density. With the aid of Eq 2 the cell density calculation can be completed [11]:

20 11 2 nm 2 1 Cell Density ( ) 3/ ( ) (2) A 1 Void Fraction 5.2 Injection Speed and Uniform Void Fraction Distribution Another set of experiments is conducted to look into changing the injection speed profile as a means of resolving the nonuniformity in void fraction along the melt flow direction. Throughout the experiments, N 2 content, talc content, melt temperature, and mold temperature were fixed at 0.2 wt%, 10 wt%, 200 o C, and 30 o C, respectively. The summary of experiments is described in Table 2. The first nine runs (A1-A9) are designed to observe the effect of different injection speeds on flow length of the foam melts and the void fraction distribution by filling the mold partially. The other five runs (Run B1-B5) used variable injection speeds to completely fill the mold with foam expansion, using speedchange strokes to optimize the void fraction uniformity. Figure 6 and 7 illustrates the various speed profile conducted for each run in a graphical form.

21 12 Table 2. Summary of Experiments Injection Injection speedchange Run Stroke Speed # [mm] [mm/s] stroke [mm] A N/A A N/A A N/A A N/A A N/A A N/A A N/A A N/A A N/A B N/A B B B B Injection Speed Plunger Position (Injection Stroke) 450 mm/s 400 mm/s 350 mm/s 300 mm/s 250 mm/s 200 mm/s 150 mm/s 100 mm/s 50 mm/s time 0% time Figure 6. Constant injection speed profile (A1-A9)

22 13 Injection Speed 200 mm/s Plunger Position (Injection Stroke) 0% 8% 17% 25% 33% 3 mm/s time 100% time Figure 7. Variable injection speed profile (B1-B5) Characterization of Foams The void fraction was used to characterize the foam samples. The foam density was determined by the water displacement method (ASTM D792-00). Samples were taken from regions near the gate, at the center, and near the end of the injection-molded part. The expansion ratio Φ is equal to the ratio of the bulk density of HDPE/talc compounds, ρ o, to the measured density of the foam sample, ρ f. The void fraction was calculated as follows: 1 Void Fraction 1 100% (3) 6. Results and Discussions 6.1 Processing Parameters Affecting High Void Fraction Effect of Processing Parameters on the Degree of Mold Filling Figure 8 illustrates the results from experiment runs A1-A6; the effect of blowing

23 14 agent N 2 and temperature on the degree of mold-filling by the foam melt. In this experiment, for different amount of N 2, different injection strokes were employed as follows: for experiment runs with 0.1 wt% N 2, 0.3 wt% N 2, and 0.5 wt% N 2 have 40 mm, 50 mm, and 60 mm injection strokes respectively. These injection strokes account for void fractions of 17%, 31%, and 45%, respectively. (a) 0.1 wt% N 2 <= Melt T = 170 o C <= Melt T = 200 o C (b) 0.3 wt% N 2 <= Melt T = 170 o C <= Melt T = 200 o C (c) 0.5 wt% N 2 <= Melt T = 170 o C <= Melt T = 200 o C mm/s Figure 8. Effect of processing parameters on the degree of mold filling The figure clearly indicates a critical role of injection speed; it is found that the

24 15 degree of mold filling increased as injection speed was increased. As a result, the foam expansion has less space/distance to cover to fill the cavity and time for nucleation is extended due to small temperature drop from reduced filling time period. An extended investigation regarding injection speed will be conducted in the following section. Unlike injection speed or blowing agent content, melt temperature does not perform a significant role on the degree of mold filling. The difference between two temperature setups, 170 C and 200 C, are not noticeable. However, it is premature to confirm there is no relationship between melt temperature and mold filling since the temperature range covered in the experiment was rather narrow Effect of Gas Content on the Cell Density and Cell Size Uniformity In Figure 8, which is cross-sections of samples from run A1-A6, it is possible to observe the influence of blowing agent content on the cell density. With 0.1 wt% N 2 the samples low cell density except for the end of the flow where the foaming occurs most due to high-pressure drop and pressure drop rate. With the increasing amount of blowing agent, cell density increases accordingly as seen in Figure 9.

25 16 (a) 0.1 wt% N 2 Gate End 10 mm/s 100 mm/s 200 mm/s 300 mm/s 400 mm/s (b) 0.3 wt% N 2 Gate End 10 mm/s 100 mm/s 200 mm/s 300 mm/s 400 mm/s (c) 0.5 wt% N 2 Gate End 10 mm/s 100 mm/s 200 mm/s 300 mm/s 400 mm/s Figure 9. Effect of gas content and injection speed on the foam morphology Cell nucleation in a mold cavity is governed by two factors: (1) the pressure drop rate at the gate when the cavity pressure is lower than the solubility pressure [12] and (2) the cavity pressure drop rate when the cavity pressure is higher than the solubility pressure. Gate pressure drop rate is determined by the gate geometry, while the cavity pressure drop rate is determined mainly by the material s rheology. Our previous observations indicate that the gate pressure drop rate is usually much higher than the cavity pressure drop rate. Therefore, in order to have a high cell density, it is desirable to have a low cavity pressure so that the gate pressure drop rate governs the cell nucleation. Figure 10 shows the cavity pressure profiles recorded near the gate and the

26 Pressure [psi] Cell Density Pressure [psi] Pressure [psi] 17 corresponding solubility pressures. As shown in Figure 10(a), when 0.1 wt% N 2 was used, the cavity pressure was higher than the solubility pressure. Thus, cell nucleation was mainly governed by the cavity pressure drop rate, resulting in a very low cell density. When N 2 content was increased to 0.3 wt% and 0.5 wt% [Figures 10(b) and (c), respectively], the solubility pressure was higher than the cavity pressure in most cases; thus, cell nucleation was governed in most cases by the gate pressure drop rate. Much higher cell density was, therefore, observed for the samples with 0.3 wt% and 0.5 wt% N 2, compared to those with 0.1 wt% N 2 (Figure 10(d)) Time [sec] Injection Speed = 10 mm/s Injection Speed = 100 mm/s Injection Speed = 200 mm/s Injection Speed = 300 mm/s Injection Speed = 400 mm/s P solubility Injection Speed = 10 mm/s Injection Speed = 100 mm/s Injection Speed = 200 mm/s Injection Speed = 300 mm/s Injection Speed = 400 mm/s Time [sec] (a) 0.1 wt% N 2 (b) 0.3 wt% N 2 P solubility Injection Speed = 10 mm/s Injection Speed = 100 mm/s Injection Speed = 200 mm/s Injection Speed = 300 mm/s Injection Speed = 400 mm/s Time [sec] (c) 0.5 wt% N 2 P solubility Injection Speed (d) Cell Density 0.1 wt% N2 0.3 wt% N2 0.5 wt% N2 Figure 10. Cavity pressure profile near the gate area and the corresponding cell density

27 18 However, some large bubbles were observed in the foam when 0.5 wt% N 2 was used. One possible reason for this observation is that it is difficult to dissolve a large amount of N 2 [13], which could result in large bubbles formed by undissolved gas pockets Effect of Shot Size (Injection Stroke) on Cell Density and Cell Size Uniformity In Figure 9, it illustrated that the highest blowing agent content, 0.5 wt% N 2, among the three different amounts tested produces the highest cell density. However, it also showed undesirable large bubbles in the cross-sections of the samples. In order to investigate the cause of the large bubbles, the experiments (B1-B3) are conducted under fixed N 2 content with various injection stroke sizes (40 mm, 45 mm, and 50 mm). As the Figure 11 illustrates, the large bubble disappears when the injection stroke size increased from 40 mm to 45 mm. The injection speed seems it influences little for the existence of the large voids for the case of injection stroke of 40 mm, which corresponds to 45% of void fraction. However, for the injection stroke of 45 mm, which corresponds to a void fraction of 38%, low injection speed of 50 mm/s results in large voids. This experiment results indicates that to achieve a uniform cell structure without any large voids, an appropriate size of injection stroke should be selected. However, in depth research should be conducted in choosing the size of the

28 19 injection stroke as larger sizes such as 50 mm (corresponding to a void fraction of 31%) resulted only partial foam near the end of part even though the large voids disappeared. It is assumed that due to the large shot size the foam melt experienced a pressure that was high enough to prevent nucleation of the cells until it solidified; therefore, creating unfoamed section at the end of the part. (a) Injection Stroke = 40 mm Gate End 50 mm/s 200 mm/s 350 mm/s (b) Injection Stroke = 45 mm Gate End 50 mm/s 200 mm/s 350 mm/s (c) Injection Stroke = 50 mm Gate End 50 mm/s 200 mm/s 350 mm/s Figure 11. Effect of shot size on foam morphology

29 Effect of Gate Size on Cell Nucleation in a Mold Cavity In order to achieve a uniform cell structure, the stage when cell nucleation starts plays a significant role. The cells nucleated before they reach the gate will experience a significant amount of shear stress when passing through the gate resulting in severe cell coalescence. Therefore, cell nucleation should start only after the melt passes thought the gate; otherwise, uniform cell structure will not be obtained. The three different thicknesses of gate sizes are employed in the experiments (C1- C3) to observe the changes of cell nucleation by locations. While Figure 4 shows the geometry of the three different gate design, Figure 12 illustrates the cross-sections of the samples from the experiments under fixed values of 0.5 wt% of gas content, 170 C of melt temperature, shot size of 40 mm, and injection speed of 400 mm/s. From Figure 12, it is obvious that Gate 1 has the smallest sizes of bubbles, which are not severely elongated. It indicates that the bubbles are foamed at the last stage of foaming. It is assumed that the thin gate size initiated high resistance, which led to high pressure preventing cell nucleation during the injection stage. In contrary, Gate 2 and 3 results in apparent cell coalescence with thick layer of highly elongated cells. It indicates that the nucleation of cells occurred before the melt

30 21 reached the gate, experiencing shear stress while passing the gate, therefore resulting in deformation of cells. It is likely due to the reduced flow resistance, which lowered the pressure inside the gate accordingly. The results from this experiment suggest that the gate design also plays a vital role in producing high-quality structural foam molding. Gate 1 Gate 2 Gate 3 Figure 12. SEM pictures of the three gates 6.2 Injection Speed and Uniform Void Fraction Distribution Effect of Injection Speed on Flow Length of Foam Melt and Void Fraction Uniformity It was found from the experiments that the flow length increased as higher injection speeds were applied. Figure 13(a) illustrates the effect of different injection speeds

31 22 on the flow length of foam melts. This result can be explained by the relationship between aped and momentum; higher injection speed results in a higher momentum. While lowpressure structural foam molding relies on foam expansion to fill the mold cavity to compensate its short-shot injection, higher injection speed decreases the distance that the foam expansion must cover in order to completely fill the mold cavity. Also, the reduced filling time from high injection speed allowed lowering the melt temperature drop during mold filling, which eventually provided more time for cell nucleation and growth. It should be noted that cell nucleation and growth in the mold cavity will stop once the melt temperature decreases below the crystallization temperature. Figure 13(b) shows the void fractions at three different positions (near the gate, center, and end of the flow) in a mold. It illustrate that the influence of injection speed upon the void fraction in different positions is rather negligible; the void fraction at the end of flow was the highest and the void fraction near the gate was the lowest regardless of the injection speed. However, this effect can be explained in terms of change in pressure drop and pressure drop rate over filling time. The pressure drop and pressure drop rate are the highest at the beginning of the process, and decrease over time. It means that the pressure at the melt front is zero, whereas the region near the gate is under some pressure due to the

32 23 melt filling the cavity. Therefore, it is much easier for the foaming to occur at the melt front than at the region near the gate. Even though Figure 13(b) shows little effect from injection speed to void fraction in different locations, it does display a relationship between injection speed and void fraction at the end of flow. A linear relationship exists between injection speed and the void fraction at the end of flow up to 250 ~ 300 mm/s, but it tends to level off at higher injection speeds. This may be due to increasing shear force exerted on the foam melt resulted from an increasing injection speed. Having a very high shear force might have caused active cell coalescence and rupture, resulting in lower void fractions for very high injection speeds (350 ~ 450 mm/s). On the other hand, the void fraction near the gate continued to increase as the injection speed increased. This may have been caused by the reduced pressure near the gate region due to the high momentum caused by high injection speeds.

33 Flow Length [cm] Void Fraction [%] (a) (b) Near Gate Center End of Flow Injection Speed (mm/s) Injection speed [mm/s] Figure 13. Effect of injection on (a) flow length of foam melt and (b) void fraction uniformity Effect of Variable Injection Speed Profile on Void Fraction Beside the relationship between injection speed and other factors, Figure 13(b) indicates the problem of a constant injection speed profile upon the void fraction distribution; it shows that the nonuniform void fraction distribution along the melt flow direction is inevitable. As a result, a variable injection speed profile was introduced considering the pressure and pressure rate drop at the beginning and the end. Figure 14 compares the void fraction distributions obtained by constant injection speed profile and variable injection speed profiles. As shown in the figure, a significant improvement in void fraction

34 Void Fraction [%] 25 uniformity was observed when variable injection speed profiles were applied Near Gate Center End of Flow Near Gate Center 10 0 Constant Injection Variable Injection Injection Profile End of Flow Figure 14. Effect of variable injection speed profile on void fraction uniformity. Figure 15 shows another crucial factor in void fraction uniformity; it depends on when the speed change occurs. For example, a constant injection speed of 200mm/s results in only 50% of the foam throughout the part; however, when the speed-change stroke (from 200mm/s to 3mm/s) is set to 33% from the end of injection stroke, foaming occurred throughout the part. The experiments are conducted for speed changes occurring at 8%, 17%, 25% and 33% from the end of injection stroke; the speed- change stroke was further increased, the foamed region started near the gate increased in size.

35 26 Figure 15. Effect of speed change stroke on structural foams 7. Conclusion In the study of processing technology of advanced structural foam injection molding, experiments were conducted to observe the effects from various processing parameters. Injection speed was studied in depth by analyzing its effect in degree of filling the mold as well as its profile s influence on void fraction of the part. It is shown through the experiment that the higher injection speed is, the higher degree of filling is achieved in mold. Also, the variable injection speed profile effectively made the void fraction uniform along the melt flow direction in structural foam molding. Another processing parameter was the amount of blowing agent. Throughout the experiments it was found that only the minimum amount was required (i.e., 0.3 wt% N 2 ) in achieving high cell density (over 10 7 cells/cm 3 ). It is due to its relationship with pressure

36 27 drop and pressure drop rate. When the content of N 2 content was low (i.e., 0.1 wt%), the pressure required to keep N 2 inside polymer matrix is very low. Therefore, the pressure increase in mold cavity resulted from sudden injection is expected to act like a gas counter pressure, thereby preventing the cell nucleation at the melt front. On the other hand, when N 2 content was higher (i.e., 0.3 wt% and above), pressure drop rate at the gate governed cell nucleation, resulting in a high cell density. Even though a specific geometry or sizes are not determined, the results from the experiments were enough to understand that a proper shot size and gate design is required in achieving uniform cell structure and high void fractions. By optimizing all processing conditions, we achieved a uniform cell structure with a very high void fraction (close to 40%).

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