Characterization of Expanded Polypropylene ... - ACS Publications

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Characterization of Expanded Polypropylene Bead Foams with Modified Steam-Chest Molding Nemat Hossieny, Aboutaleb Ameli, and Chul B. Park* Microcellular Plastics Manufacturing Laboratory Department of Mechanical & Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8 ABSTRACT: Steam is commonly used in polymer bead foam sintering due to its high heat transfer capacity. However, the pressure loss unavoidable during flow through the beads causes a temperature decrease and thereby negatively affects the sintering behavior. In order to reduce the sensitivity of the temperature to the pressure variation, hot air was added to the steam line. The effects of the flow rate, the pressure, and the temperature of the hot air on the surface roughness, thermal properties, and mechanical properties of the molded products were studied. The results showed that the introduction of hot air at a flow rate of 120 L/min decreased the total steaming time by about 32%. Further, a decrease of 16 °C in the processing temperature was observed, which decreased the surface roughness by 50%. Hot air improved the heat flow and thereby enhanced the uniformity of the tensile properties across the molded part.

1. INTRODUCTION Expanded polymeric bead foams are popular materials used in packaging and thermal and sound insulation applications.1,2 Expandable polystyrene (EPS), expanded polyethylene (EPE), and expanded polypropylene (EPP) are widely used modern moldable bead foams. The successful commercialization of EPP has led to the application of polymeric bead foams into more advanced applications in areas such as automotive production.3 Currently, there is increasing interest in investigating the processing behavior and mechanical properties of EPP,4−8 because it has a higher service temperature and better mechanical properties compared to those of EPS and EPE. In addition, EPP has some other advantages such as excellent impact resistance, energy absorption, insulation, heat resistance, and flotation. Furthermore, it is lightweight and recyclable and exhibits good surface protection and high resistance to oil, chemicals, and water. Due to these advantages, the use of EPP is gaining increased momentum in the automotive, packaging, and construction industries.2,4−8 For instance, EPP molded foams are utilized as bumper cores, providing significantly higher energy absorption upon impact as opposed to conventional systems.3 EPP bead foams have also been moving into more complex applications in such areas as energy management, acoustic preference, and structural support.2−9 For all applications of EPP, the physical and mechanical properties of EPP bead foams are influenced mainly by interbead bonding, because the bead boundaries usually develop into fracture paths when a force is applied.10 Interbead bonding is highly dependent on the temperature of the medium transferring heat, and interbead bonding management is essential for quality control.11 Steam-chest molding technology is a commercially available and utilized high-temperature steam to cause sintering of EPP beads. The processing steam temperature in a steam-chest molding machine is coupled with the steam pressure.12 The EPP bead foam has a high melting peak of about 150−170 °C, and hence high steam temperatures and pressures are required for processing, which causes a higher operating cost. The final © XXXX American Chemical Society

physical and mechanical properties of EPP molded products depend on the strength of the interbead bonding, which is significantly affected by the molding conditions such as the steam pressure, steam temperature, and molding time. During processing, however, the steam pressure varies because of the resistance of the flow through the beads, which makes it difficult to determine the actual temperature in the mold. Moreover, considering the large volume and complicated shape of the mold cavity, the temperature distribution across the mold cavity is not uniform. Nakai et al.13 reported reduced heat conduction to the core area of the mold caused by a decrease in steam temperature due to a decrease in steam pressure. Zhai et al.14,15 also showed that both the degree of interbead bonding and the tensile strength had a direct relationship with the steam pressure/temperature. Other studies also reported that interbead bonding strength normally increased with the molding pressure and time,11,16 and that improved the tensile and compressive strengths and fracture toughness.17 However, if beads are steamed for too long a time, their cell structure might collapse.18 Furthermore, a higher operating steam pressure relates to higher temperature leading to an increase in localized temperature near the steam entry, and hence beads exposed to this high temperature may melt resulting in shrinkage at the surface of the product. This dramatically deteriorates the surface property of the molded product. In this study, the existing steam-chest molding machine was modified with the introduction of hot air in an attempt to reduce the sensitivity of the decrease in the steam temperature with a pressure drop. The hot air conditions were optimized using different critical parameters such as the hot air flow rate, hot air temperature, and hot air pressure. Also, the effects of adding hot air on the process heating time, surface quality, Received: March 6, 2013 Revised: May 8, 2013 Accepted: May 17, 2013

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dx.doi.org/10.1021/ie400734j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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low pressure, the superheated steam changes to saturated steam and finally condenses to saturated water. The decrease in the steam temperature with the pressure decline can be estimated by considering a throttling process caused by the resistance of the passage among the closely packed beads. Both the fixed and moving molds in the steam-chest molding machine were insulated, and hence for simplicity we can consider the process to be adiabatic. The decline in steam temperature with a decrease in pressure can be described using the Joule− Thompson coefficient (μJ).23 Considering a steady-state throttling process with a steady flow across a restrictor, the Joule−Thompson coefficient is given by

thermal property, and tensile properties of the molded EPP products are thoroughly investigated.

2. THEORETICAL BACKGROUND A double-peak melting behavior (Figure 1) is required for EPP beads to have good sintering during steam-chest molding. The

μJ =

⎛ ∂T ⎞ ⎜ ⎟ ⎝ ∂P ⎠h

(1)

where T is the temperature, P is the pressure, and (∂T∂P)h denotes a partial derivative at constant enthalpy. Goodenough24 computed values of μJ for superheated steam covering a wide range of pressure and temperatures. For a range of temperatures between 121 and 176 °C and a pressure of 2.44 atm, the μJ of superheated steam was 13 °C/atm.25 This range of pressure and temperature is very similar to the actual condition of steam used for melting the Tm‑low crystals and creates sintering of EPP bead foams in a steam-chest molding machine. As observed from the μJ of superheated steam, the steam temperature is very sensitive to the steam pressure and decreases significantly with a decline in pressure. The actual steam temperature during sintering of EPP beads in a steamchest molding machine varies over a broad range between 120 and 167 °C (Figure 1) due to a decrease in the pressure, which is discussed in detail in section 5.2. Due to the broad temperature range, proper heat transfer would not occur in the core area of the molded EPP part, which results in poor sintering of EPP beads and hence leads to poor mechanical properties. In order to maintain a high temperature in the core area, an undesirable high steam pressure (i.e., high temperature) will therefore be required on the surface. This will melt high temperature peak crystals (Tm‑high) on the surface and thereby causes a nonuniform morphology with a high operating cost. In our new design, we propose to use a mixture of steam and hot air to reduce the resultant μJ. Compared to superheated steam, the μJ of hot air at the same pressure and temperature is significantly low at 0.01 °C/atm.25 Hot air can potentially present a very attractive, cost-effective method to fundamentally reduce the sensitivity of a decrease in the steam temperature with a drop in pressure. However, hot air is a very poor heat conductor and has a thermal conductivity value of 36.6 × 10−3 W/m·C.26 On the other hand, steam has a very high thermal conductivity of 32.1 × 103 W/m·C.27 Hence, using a mixture of hot air and steam can provide a synergistic effect of the low μJ of hot air and the high thermal conductivity of saturated steam. Overall, much superior heat transfer can be achieved at the core of the mold by supplying a mixture of hot air and steam during the steam-chest molding of EPP beads. The introduction of hot air in the steam-chest molding process may result in EPP bead products with improved surface quality, enhanced mechanical properties, and shortened cycle time resulting in a reduced operating cost.

Figure 1. A typical double-peak melting behavior of foamed beads.

hatched area in Figure 1 represents the desirable steam temperature range between the low and high melting peaks of EPP within the steam-chest molding machine. When the foamed beads are processed in the steam-chest molding machine, crystals associated with the low melting temperature (Tm‑low) melt and contribute to the fusing and sintering of individual beads. Meanwhile, the unmelted high melting temperature (Tm‑high) crystals help to preserve the overall cellular morphology of the bead foams.19 A very narrow processing window between the two melting peaks poses a significant challenge in setting the processing steam temperature. The steam temperature is sensitive and depends on the corresponding steam pressure. The slight variation in steam temperature may cause the Tm‑high crystals to get affected and destroy the cellular morphology of the beads and hence cause shrinkage of the molded product. The ratio between the low and high melting peaks is thus crucial in determining the surface quality and mechanical properties of bead foam products.20 The phenomenon of creating multiple crystal melting peaks for semicrystalline polymers was reported in earlier studies.21,22 The appearance of a new peak can be attributed to various crystal structures, crystal sizes, and their arrangement and perfection during the heating or annealing treatments. In the case of EPP, the Tm‑high peak originates from the perfection of crystals during the gas-saturation stage in an autoclave at an elevated temperature around the melting point (Tm) of polypropylene (PP).14,19 The less perfect PP crystals are allowed to partially melt and restack. During the prolonged gas impregnation stage, the remaining crystals behave as crystallization nuclei that grow and become more perfect crystals due to the rearrangement of the polymer molecular chains. The Tm‑high melting peak is typically 15−20 °C above the annealing temperatures.14,19 The Tm‑low melting peak is created during the subsequent foaming and rapid cooling stage. The steam supplied for the molding of EPP beads in a steamchest molding machine is in the superheated state, which flows via small ports into the mold cavity. As the steam flows from the surface toward the core of the cavity, its pressure decreases due to resistance from the EPP beads. Overall, the steam temperature decreases with a decline in the pressure. Thus, at B



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3. MODIFICATIONS ON STEAM-CHEST MOLDING MACHINE TO INCORPORATE HOT AIR The steam chest molding machine was modified to accomplish the following main functions: (i) preventing steam condensate from entering the mold during the heating cycle, (ii) supplying hot air into the steam injection pipe during the heating cycle, and (iii) monitoring the processing temperature and pressure of steam and hot air mixture entering both the moving and fixed mold channels. Figure 2 shows a schematic of the

4. EXPERIMENTATION 4.1. Materials. The EPP beads, APPRO 5415, were supplied by JSP International. The beads have an expansion ratio of 15 with a bulk density of 60.9 g/L. The melting behavior of the EPP beads was examined by DSC (TA Instruments, Q2000). The melting behavior of the beads showed double peak melting characteristics with low melting (Tm‑low) and high melting (Tm‑high) peaks at 141.2 and 160.9 °C, respectively (Figure 1). 4.2. Steam-Chest Molding Setup and Experimental Design. Laboratory-scale steam-chest molding equipment (DABO Precision, Korea) was used in this study. The dimensions of the mold cavity were 15 cm × 6 cm × 5 cm. The steaming process in the steam-chest molding included: (1) steam injection from the fixed side of the mold (first steaming cycle), (2) steam injection from the moving side of the mold (second steaming cycle), and (3) steam injection from both sides of the mold (third steaming cycle). The first and second steaming cycles were conducted to create the fusion between the EPP beads. The third steaming cycle was used to remove pores on the surface of the molded EPP part. In the first steaming cycle, the steam was flushed from the fixed side, passed through the bed of EPP beads in the mold cavity, and exited from the moving side. During the second steaming cycle, the process is reversed, and the steam was flushed from the moving mold side. For the third steaming cycle, the steam was flushed from both the fixed and moving sides of the mold. The hot air was introduced during all three steam injection cycles. For each set of experiments, the sample cooling time remained unchanged. To investigate and optimize the effect of hot air on the surface quality and the tensile properties of the molded EPP, the temperatures of the air and air flow rate were varied at three levels, as shown in Table 1. Since the interbead bonding usually

Figure 2. A schematic of a modified steam chest molding machine with a hot air supply.

modified steam chest molding machine. To facilitate the first function, i.e., preventing the steam condensate from entering the mold, the steam supply piping was redirected to enter from the bottom of both the fixed and moving molds. To maintain the steam above its condensation temperature, band heaters with proportional-integral-derivation (PID) feedback control (Omega, CN7833) were located on the steam supply piping, and special heaters were inserted on the mold surface. Furthermore, all the exposed steam supply piping and the metal surface of the fixed and moving molds were insulated. The steam could be supplied in a wide range of pressures from 0 MPa to a maximum working steam pressure of 0.4 MPa. To supply hot air into the steam injection line, special heaters using coiled copper piping were designed to heat the supplied compressed air. The compressed air could be heated to approximately 200 °C (T2 and T4 in Figure 2). The compressed pressure could be controlled over a wide range from 0 to 0.69 MPa using a pressure controller as seen in Figure 2. The flow rate of the supplied hot air could be varied from 0 to 120 L/min using a flow control valve as seen in Figure 2. To achieve a good mixing of hot air and steam, hot air was introduced into the center of the steam supply line to create annular flow of the steam into the hot air (marked with a circle in Figure 2). To monitor the processing temperature of steam and hot air mixtures, thermocouples were located inside both the molds (T1 and T3 in Figure 1). Similarly, pressure gauges were located to monitor the pressure during processing (P1−P4 in Figure 2).

Table 1. Experimental Parameters and Design Variables fixed parameters steam pressure (MPa)

steam temperature (°C)

0.38

151

variables air flow rate (liters/min)

air temperature (°C)

80 100 120

110 160 200

air pressure (MPa) 0.41 0.69

increases with the steam pressure and the heating time,11,14,16 the steam pressure was kept constant at 0.38 MPa (gauge pressure). The unit of steam pressure/gauge pressure used in this study is the relative pressure in MPa, which is 0.1 MPa lower than the absolute pressure. The corresponding steam temperature at a gauge pressure of 0.38 MPa was 151 °C from the steam table. The hot air pressure was also kept constant at 0.41 MPa. Table 2 shows the complete experimental matrix. To investigate the effect of air pressure on the surface quality and the tensile properties of the molded EPP, the air pressure was varied at two levels of 0.41 and 0.69 MPa as seen in Table 1. At the lower air pressure, the air heaters have a higher possibility of getting damaged and hence only two pressures could be investigated. The hot air temperature and the flow rate were kept constant at 160 °C and 80 L/min, respectively. 4.3. Surface Quality Characterization. Line scans were performed over 10 mm at six different locations on the fixed and moving mold sides of each sample (Figure 3) using an C



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calculated using the same equation as for Ra (eq 2) but by using data over a wider sampling length (i.e., 20 μm).29 The morphologies of the molded EPP samples were also observed by SEM (JEOL JMS 6060). 4.4. Tensile Property Characterization. The tensile strength of molded EPP samples was measured using a Micro tester (Instron 5858) at a crosshead speed of 5 mm/min. Rectangular specimens were cut from three different locations across the thickness of the molded samples as shown in Figure 4. Typical dimensions of the specimens were as follows: thickness = 14 mm, width = 19 mm, and height = 155 mm. At least five specimens were tested under each condition.

Table 2. Experimental Matrix run

air temperature (°C)

air flow rate (L/min)

steam pressure (MPa)

hot air pressure (MPa)

1 2 3 4 5 6 7 8 9

110 110 110 160 160 160 200 200 200

80 100 120 80 100 120 80 100 120

0.38

0.41

Figure 3. Rectangular area showing the location of line scans to characterize the surface property on fixed mold and moving mold surface of molded EPP sample.

Figure 4. Schematic of specimen preparation for tensile tests.

optical profilometer (Nanovea ST 400, Microphotonics Inc., Irvine, CA, U. S. A.) to measure the surface profile and, thereby, the surface roughness of the samples. The locations on the moving mold side are designated by M1 to M6. The sampling rate was 500 data points per millimeter in all the scans. The ISO 4287 standard was adapted in the calculations. The surface quality was characterized by the roughness values of Ra and Rz, the waviness value of Wa, and the surface roughness profile. The roughness values of Ra and Rz are calculated using eqs 2 and 3:28

4.5. Thermal Property Characterization. The thermal history of molded EPP samples was analyzed by using a Differential Scanning Calorimeter, DSC (Q2000, TA Instruments), calibrated against characterized indium. The thermal behavior was investigated at three different locations of the fix mold and moving mold surfaces of the molded EPP samples. A temperature ramp process from 20 to 230 °C at a heating rate of 10 °C/min was carried out to investigate the melting behavior of the EPP samples. The degree of crystallinity was calculated from the integration of the DSC melting peaks by using 290 J/g as the heat of fusion (ΔHm) of 100% crystallized PP.30

Ra =

1 n

n

∑ |yi | i=1

5. RESULTS AND DISCUSSION 5.1. Effect of Hot Air on the Steaming Time. In order to increase the productivity and reduce the operating cost, it is necessary to shorten the processing time. One of the major impediments to shortening the processing time is the time required to build up the steam pressure to flow through the EPP beads in the mold during the steaming cycles. The introduction of hot air may affect the build-up time of the steam pressure and hence the overall steaming time, which will ultimately affect the processing time. The steam pressure supplied to the equipment was 0.45 MPa. However, the desired processing steam pressure (0.38 MPa) during the individual steaming cycle was controlled by using a compound gauge. The gauge measured the pressure inside the mold cavity during the steaming cycle using a

(2)

where yi is the vertical distance from the mean line to the ith data point. The roughness profile contains n ordered, equally spaced points along the trace. Rz =

1 s

s

∑ R ti i=1

(3)

Rz is the average distance between the highest peak and lowest valley in each sampling length. s is the number of sampling lengths, and Rti is Rt for the ith sampling length. Surface profiles were measured using similar line scans with 5 μm intervals between each line scan. In order to capture the surface irregularities with spacing greater than the roughness sampling length (2 μm), the waviness value (Wa) was used. Wa was D



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to 0.69 MPa showed similar results to those observed at a hot air flow rate of 120 L/min. 5.2. Effect of Hot Air on the Total Processing Temperature. Figure 6a and b compare the final processing temperatures after the completion of the first and second steaming cycles for pure steam and steam mixed with hot air having various flow rates. The processing temperature was measured at locations T1 and T3 as shown in Figure 6c. Two important observations can be made from the actual temperatures measured at T1 and T3 and from the difference between the temperatures of T1 and T3 (ΔTcycle1 and ΔTcycle2 in Figure 7a and b). First, the introduction of hot air resulted in a decrease of ΔTcycle1 and ΔTcycle2. The ΔTcycle1 and ΔTcycle2 values for the EPP part molded with pure steam was 47 and 48 °C, respectively. The introduction of hot air at a low flow rate of 80 L/min resulted in ΔTcycle1 and ΔTcycle2 values of 31 and 22 °C, respectively. By increasing the hot air flow rate, ΔTcycle1 and ΔTcycle2 further decreased, and at the highest flow rate of 120 L/min, ΔTcycle1 and ΔTcycle2 reached 3 and 4 °C, respectively, which corresponded to roughly 94% and 92% reduction, compared to those of pure steam. The high ΔTcycle1 and ΔTcycle2 values of pure steam suggest that the process of expansion and sintering of EPP beads restricted the flow of steam and caused a decrease in its pressure. Based on the discussion in section 3, due to the high Joule−Thompson coefficient of steam, there was a significant decrease in the steam temperature leading to very poor heat transfer across the mold. With the introduction of hot air, however, the heat transfer across the mold improved significantly and thus resulted in a more uniform temperature profile. The second observation is that the localized source temperatures (T1 and T3) at the end of the steaming cycles decreased with the introduction of hot air. It can be observed

pressure transducer and signaled to start the subsequent cycle after the desired processing pressure was achieved. As discussed in section 4.2, the first and second steaming cycles were crucial for the overall sintering of the EPP part, and hence, the time required to complete these cycles with pure steam and steam mixed with hot air was recorded. The third steaming cycle was set for 10 s for all the experiments. The total steaming time to complete the molding of one EPP part was calculated by adding the times for the three steaming cycles. Figure 5 depicts the

Figure 5. Effect of hot air and its flow rate on the total steaming time.

total steaming time for pure steam and steam mixed with hot air with various flow rates and a fixed temperature and pressure of 160 °C and 0.41 MPa, respectively. Overall, the total steaming time decreased with the increase of the hot air flow rate, and the highest air flow rate of 120 L/min resulted in a time decrease of approximately 32%. The reduction in the total steaming time shows the effective use of hot air to reduce the overall processing time. The total steaming time to complete the molding of one EPP part by increasing the hot air pressure

Figure 6. Effect of hot air and its flow rate on the processing temperature during (a) the first steaming cycle and (b) second steaming cycle. (c) A schematic illustrating the locations where the processing temperatures of T1 and T3 were measured. E



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Figure 7. Effect of hot air and its pressure on the processing temperature during (a) first steaming and (b) second steaming cycles.

Figure 8. Comparison between actual line profile values measured over the scan length of EPP parts molded with (a) pure steam and (b) steam mixed with hot air at 120 L/min.

Figure 9. Effect of hot air and its flow rate on (a) Ra and (b) Rz surface roughness parameters.

that after the completion of the first and second steaming cycles with pure steam, the temperatures at T1 and T3 were 167 and 166 °C, respectively, which were approximately 11.5 and 10.5 °C higher than the supplied steam temperature of 155.5 °C at 0.45 MPa. This can be understood considering the quick sintering of EPP beads on the surface exposed to the high temperature steam. The sintered beads started restricting the flow of steam and created a plugging behavior. Consequently, the latent heat caused an increase of the steam temperature, and this further aggravated the temperature gradient, i.e., further overheating on the surface whereas the core has not received enough heat to sinter each other because of the lowered temperature of the steam flowing at a lower pressure. But by introducing hot air, the temperature in the core could be maintained high to be able to cause more uniform sintering across the thickness. So instead of causing a premature sintering

on the surface and an increase in the source temperatures (T1 and T3), the temperature of the beads became more uniform. At a low hot air flow rate of 80 L/min, the temperatures at T1 and T3 decreased slightly to 164 and 162 °C, respectively. By increasing the hot air flow rate to 120 L/min, the temperatures further decreased to 151 and 155 °C, respectively. Thus with the introduction of hot air, the flow of steam is improved significantly and the source temperature increase (i.e., T1 and T3) due to the blockage of the flow could be successfully decreased. Controlling the pressure of the hot air resulted in a trend similar to the case of controlling the flow rate of hot air. Figure 7a and b compare the final processing temperatures after completion of the first and second steaming cycles for pure steam and steam mixed with hot air at two pressures of 0.41 and 0.69 MPa. The introduction of hot air at a pressure of 0.41 F



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MPa resulted in the decrease of ΔTcycle1 and ΔTcycle2 values by 53% and 58% to 31 and 27 °C, respectively. By further increasing the hot air pressure to 0.69 MPa, the ΔTcycle1 and ΔTcycle2 values significantly decreased and reached only 2 °C, which accounted for about a 95% reduction. The variation in the hot air temperature did not significantly change the processing temperatures and hence is not discussed. 5.3. Effect of Hot Air Flow Rate on Surface Properties. Figure 8 compares the actual profile data from the line scans performed using the optical profilometer on the EPP parts molded using pure steam and steam mixed with hot air. The data were measured at six different locations (shown in Figure 3) on the surface of the molded EPP part on the moving mold side and designated from M1 to M6. As seen in Figure 8a, for the EPP part molded with pure steam, the variation of the line profile over the scan length of 10 mm spanned a range of 195 μm between −120 and 75 μm. However, by introducing hot air, the variation of line profile decreased significantly and spanned within a range of 75 μm between −50 and 25 μm (Figure 8b). To obtain more quantitative information on surface quality, the surface roughness (Ra and Rz) and waviness values (Wa) were calculated, and the results are discussed. Figure 9 compares the surface roughness values (Ra and Rz) of EPP parts molded using pure steam and steam mixed with hot air. The roughness was measured for the surfaces of the molded EPP part on the fixed and moving mold sides, since these surfaces are exposed to the steam and hot air entrance ports on the molds. To investigate the effect of the hot air flow rate, the temperature and pressure were kept constant at 160 °C and 0.41 MPa, respectively. The flow rate was varied from 80 L/min to 120 L/min. Overall, the introduction of hot air improved the surface quality. It is observed that at a low hot air flow rate of 80 L/min, the surface roughness was not significantly improved compared to the pure steam case with similar standard deviations. But by increasing the hot air flow rate to 120 L/min, the surface roughness values decreased by approximately 50%, reaching an Ra value of only about 1 μm, which is considered a soft touch finish and thus a significant improvement. Furthermore, the high hot air flow rate resulted in very similar surface roughness values on both the surfaces, indicating improved uniformity in the surface quality. Both Ra and Rz roughness values showed similar dependency on the hot air flow rate. Figure 10 shows the waviness values (Wa) of the EPP parts molded using pure steam and steam mixed with hot air. Similar to the roughness values, the EPP parts molded with a mixture

of steam and hot air possessed lower waviness values. The waviness values decreased proportionally with an increase in the hot air flow rate. The improvement in waviness is visualized in Figure 11 by the surface profiles. In both Figure 11a and b, the solid arrows show the surface topography of a single bead. The surface height within a single bead of the samples molded with steam spanned a range of 150 μm as shown in Figure 11a. But with the introduction of hot air, the surface height varied within a range of only 50 μm (Figure 11b). Molded EPP samples produced using pure steam and steam mixed with hot air at different flow rates were cut directly with a sharp knife. SEM micrographs of the cut surfaces of the samples are shown in Figure 12. The samples were prepared from the fixed mold side of the molded part. The sample molded with pure steam showed a high degree of cell collapse in the structure of the EPP beads at the surface, which caused formation of a thick skin (marked by arrow) as shown in Figure 12a. Overall, the introduction of hot air reduced the cell collapse of EPP beads. It is observed that at a low hot air flow rate of 80 L/min (Figure 12b), the cell collapse improved slightly compared to the pure steam case. But by increasing the hot air flow rate to 120 L/min (Figure 12c), the cell collapse of EPP beads decreased significantly. 5.4. Effect of Hot Air Temperature on Surface Properties. To investigate the effect of hot air temperature, the air pressure and flow rate were kept constant at 0.41 MPa and 100 L/min, respectively, and three temperatures of 110 °C, 160 °C, and 200 °C were investigated. Figure 13a and b show the Ra and Rz roughness values measured for the surfaces of the molded EPP part on the fixed and moving mold sides. It can be noted that varying the hot air temperature did not cause any significant change in the surface quality of the molded parts. Compared to the roughness values of the EPP molded part with pure steam, the molded part with a mixture of steam and hot air at 110 and 160 °C showed slight improvement in the overall surface property. However, at a higher temperature of 200 °C, the overall Ra value became more inconsistent. 5.5. Effect of Hot Air Pressure on Surface Properties. To investigate the effect of the hot air pressure, the air temperature and flow rate were kept constant at 160 °C and 80 L/min, respectively, and two pressures of 0.41 and 0.69 MPa were investigated. Figure 14a and b show the Ra and Rz roughness values measured on the fixed and moving mold side surfaces of the molded EPP part. It is seen that by introducing hot air at a low flow rate of 80 L/min and a pressure of 0.41 MPa, the surface roughness became more inconsistent with an increase by about 9% on the fixed mold surface. However, by increasing the hot air pressure to 0.69 MPa, the surface roughness decreased by approximately 50%, reaching an Ra value of about only 1.12 μm and thus a significant improvement. Furthermore, a hot air pressure of 0.69 MPa resulted in very similar surface roughness values on both the surfaces, indicating an improved uniformity in the surface quality. Both Ra and Rz roughness values showed similar dependency on the hot air pressure. Figure 15 shows the waviness values (Wa) of the EPP parts molded using pure steam and steam mixed with hot air at two different air pressures. It is seen that by introducing hot air at a lower pressure of 0.41 MPa, the waviness value (Wa) decreased by 9% and 45% at the fixed and moving molds surfaces, respectively. By increasing the hot air pressure to 0.69 MPa, the waviness value (Wa) showed a further decrease by 28% and 55% at the fixed and moving molds surfaces, respectively.

Figure 10. Effect of hot air and its flow rate on the waviness (Wa) values of molded EPP’s surface. G



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Figure 11. Fixed mold surface microtopography of EPP bead molded products using (a) pure steam and (b) steam mixed with hot air with an air flow rate of 100 L/min.

Figure 12. SEM micrographs of the cut surfaces of the fixed mold surface of EPP samples produced using steam and steam mixed with hot air at different flow rates (a) pure steam, (b) 80 L/min, and (c) 120 L/min.

constant under all the processing conditions. The lowest melting peak, Tmc in the DSC curve (marked with arrow in Figure 16) was reported to be the melting peak of crystals formed during the cooling process.15 Generally, this temperature was reported to be slightly lower than the original low melting temperature of EPP beads (Tm‑low = 140.6 °C),14 and a similar behavior was observed in the melting endotherm of samples from the surface of the molded EPP part on the fixed mold side in the presence of pure steam and steam mixed with hot air under different conditions. The total crystallinity (XT) of the EPP samples molded with pure steam and steam mixed

Furthermore, at both hot air pressures, both the surfaces indicated significant uniformity in the surface quality. 5.6. Thermal Properties of Molded EPP Samples. Figure 16 shows the DSC thermographs for the surfaces of the molded EPP part on the fixed and moving sides of the mold. Table 3 also lists the melting behavior and crystallinity of the molded EPP samples under different conditions. As seen in Figure 16, the molded EPP samples exhibited three melting peaks from high to low temperatures, denoted as Tm‑high, Tmi, and Tmc, respectively. The Tm‑high was the original high melting peak of the EPP beads (Tm‑high = 160.4 °C), which remains H



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Figure 13. Effect of hot air temperature on (a) Ra and (b) Rz surface roughness parameters.

Figure 14. Effect of hot air pressure on (a) Ra and (b) Rz surface roughness parameters.

The melting peak Tmi in the DSC curve (dashed line in Figure 16) was reported to be created by melting of the crystals that had possibly been induced by the fast heating and annealing treatment that followed.14 As seen in Table 3, Tmi decreased from 151.2 °C for pure steam to 148.4 °C with the introduction of hot air at a flow rate of 120 L/min. Another important observation is that the Tmi peak was the weakest in the case of pure steam and became more pronounced with the increase in the hot air flow rate for the surfaces of the EPP part, on both the fixed and moving mold sides. This further confirms the decrease in the processing temperatures with hot air which reduces the annealing temperature on the surface of the molded EPP part. Zhai et al.14 also found and reported that Tmi tends to become weak or even disappears at higher processing temperatures. They also reported that the Tmi was very sensitive and increased linearly with increased treatment temperature. This strongly confirms that the improved surface quality seen with an increase of hot air flow rate was due to the reduced local surface temperature, which ultimately caused less melting of original crystals in the EPP beads. The EPP beads molded at different hot air temperatures showed similar correlation between their thermal behaviors and surface properties. As also discussed earlier, the hot air temperature does not cause any significant effect on the surface properties, and hence the thermal behaviors at different hot air temperatures are not discussed in detail. 5.7. Effect of Hot Air on Tensile Properties. As discussed earlier, the surface roughness and thermal property of the molded EPP parts showed high sensitivity to the flow rate and the pressure of the hot air. The hot air temperature

Figure 15. Effect of hot air pressure on the waviness (Wa) values of molded EPP’s surface.

with hot air is also shown in Table 3. It appears that the XT value decreased with an increase in the hot air flow rate. The samples of the EPP part on the moving mold side showed a similar decreasing trend in XT after the introduction of hot air. The increase in XT is caused by treatment at higher temperatures causing melting of the original crystals and subsequent formation during cooling.14 The formation of crystals during cooling is related to Tmc.14 The crystallinity during cooling was estimated by the shaded area from the DSC plots (Figure 16), and their corresponding values (Xc) are listed in Table 3. Overall, Xc decreased with the introduction of hot air with various flow rates. This further indicated that the melting of original crystals was lower on the surface of samples molded using steam mixed with hot air. I



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Figure 16. DSC thermographs of molded EPP samples, (a) fixed mold surface and (b) moving mold surface.

Table 3. Melting Points and Crystallinity of Molded EPP Samples at Fixed and Moving Mold Surfaces under Different Processing Conditions of Pure Steam and Steam with Hot Air fixed mold surface Tmc (°C) Tmi (°C) Tm‑high (°C) XC (%) [cooling] XT (%) [total]

moving mold surface

pure steam

80 L/min

100 L/min

120 L/min

pure steam

80 L/min

100 L/min

120 L/min

139.3 151.2 159.7 25.7 38.0

139.9 151.1 159.4 28.2 37.2

138.9 149.1 159.3 21.5 35.5

139.1 148.4 159.2 20.1 34.6

142.4 152.7 159.6 29.0 38.0

141.9 152.2 159.1 27.1 38.1

138.2 150.3 159.4 24.8 34.6

138.6 149.7 159.4 19.7 31.1

compared to those of the part molded with pure steam. However, the tensile strength in the center improved by approximately 20%. By increasing the hot air flow rate to 100 L/min and 120 L/min, the tensile strength at the center increased by 14% and 16%, respectively. Figure 18 compares the tensile strength of EPP molded parts using pure steam and steam mixed with hot air at different air

however did not cause any significant change in the properties of the molded EPP part. Figure 17 compares the tensile strength of EPP molded parts using pure steam and steam mixed with hot air at different flow

Figure 17. Tensile strengths of molded EPP samples produced with pure steam and steam mixed with hot air at different flow rates.

rates. The tensile strength was measured for samples from the surfaces of the molded EPP part on the fixed mold side, center, and moving mold side. The tensile strength measured at the center of the sample molded with pure steam was approximately 12% and 20% lower than the corresponding values of samples of the molded EPP part on the fixed and moving mold sides. This was caused due to the reduced heat flow to the core of the sample caused by a decrease in the steam pressure. Overall, the introduction of hot air improved the tensile strength in the center of the molded part. The tensile strength became very uniform over the entire molded part with the introduction of hot air. At a hot air flow rate of 80 L/min, the tensile strength of the samples from the surface of the EPP part at the fixed and moving mold side did not change much as

Figure 18. Tensile strengths of molded EPP samples produced with pure steam and steam mixed with hot air at different temperatures.

temperatures. Three temperatures of 110 °C, 160 °C, and 200 °C were investigated. Overall, the tensile strength was seen to be consistent over the entire molded part at all the investigated air temperatures. It can be seen that by introduction of hot air at a temperature of 100 °C, the tensile property of the samples from surfaces of the molded EPP part on the fixed mold and moving mold side increased by 16% and 8%, respectively, as compared to those of the parts molded with pure steam. With an increase of the air temperature to 160 °C, the tensile strength remained approximately unchanged, compared to the case of pure steam. Further increase in the hot air temperature J



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to 200 °C did not change the tensile strength at the fixed mold surface, but it decreased the tensile strength of the moving mold surface by approximately 10%, compared to the pure steam case. On the other hand, at all three hot air temperatures of 110, 160, and 200 °C, the tensile property of the center of the molded sample increased by 27%, 14% and 20%, respectively, compared to corresponding values of the pure steam samples. As discussed earlier, the steam temperature did not play a major role in the overall quality of the molded EPP parts. However, the observed improvement in the tensile property of the molded samples in the center originated from the improved heat flow caused by the hot air flow rate. Figure 19 compares the tensile strength of EPP molded parts using pure steam and steam mixed with hot air at different

The use of hot air with steam showed significant improvement in the overall consistency of the tensile property across the molded EPP part as compared to the samples molded with pure steam. The corresponding consistency was achieved due to an improved heat flow to the core of the molded sample. This is possible due to the synergistic effect of the high thermal conductivity of steam and the low Joule−Thompson coefficient of hot air. With either an increase in the hot air flow rate or an increase in the pressure, the heat flow is improved, leading to an overall improvement in the tensile property. Hence, the results of this work reveal the potential application of hot air in the steam-chest molding process to produce EPP bead products with improved surface quality, enhanced mechanical properties, and a shortened cycle time resulting in a reduced operating cost.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 416-978-3053. Fax: 416-978-0947. E-mail: park@mie. utoronto.ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the JSP (Japan) for generously donating the EPP used in this study and members of the Consortium of Cellular and Micro-Cellular Plastics (CCMCP) for their financial support of this project. The authors would like to also acknowledge Prof. Marcello Papini for providing the optical profilometer (Nanovea ST 400, Microphotonics Inc., Irvine, CA, U. S. A.) at Ryerson University.

Figure 19. Tensile strengths of molded EPP samples produced with pure steam and steam mixed with hot air at different pressures.

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pressures. Overall, the uniformity of the tensile strength across the molded sample increased with the increase of air pressure. By the introduction of hot air at a pressure of 0.41 MPa, the tensile strength at the fixed mold surface, center, and moving mold surface increased by 4%, 6%, and 20%, respectively, compared to their corresponding values in pure steam case. A further increase in hot air pressure to 0.69 MPa resulted in further improvement in tensile strength. The tensile strength at the fixed mold surface, center, and moving mold surface increased by 19%, 12%, and 34%, respectively, compared to their corresponding values in pure steam case.

REFERENCES

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6. CONCLUSIONS In this study, the existing steam-chest molding machine was modified to accommodate the application of hot air in an attempt to reduce the sensitivity of the steam temperature decrease with a pressure drop. The introduction of hot air was optimized to investigate the effect of different parameters such as the hot air flow rate, the hot air temperature, and the hot air pressure, while the steam pressure was kept constant. The steaming time decreased by 32%, and the local temperature at the entry port decreased by 8% at the highest available air flow rate of 120 L/min. The overall heat transfer improved significantly with an increase in the hot air flow rate. The surface roughness values (Ra and Rz) decreased by approximately 50% at the hot air flow rate of 120 L/min. An increase in the hot air pressure also showed a decrease in the surface waviness (Wa) by approximately 50%. However, varying the hot air temperature did not cause any significant change on the surface property. K



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