The lost foam casting process offers several advantages over traditional green sand casting, most notably the ability to reuse casting sand, thus eliminating the ...
An Investigation into the Effect of Process Parameter Settings on Air Emission Characteristics in the Lost Foam Casting Process S. U. Behm K. L. Gunter J. W. Sutherland Michigan Technological University, Houghton, Michigan Copyright 2003 American Foundry Society ABSTRACT The lost foam casting process offers several advantages over traditional green sand casting, most notably the ability to reuse casting sand, thus eliminating the need for costly disposal of contaminated casting sand. The process does, however, produce significant quantities of airborne emissions that can be a danger to both the environment and to the health of workers. This research examines these emissions and considers how several process parameters (e.g., pouring temperature, presence of pattern coating, and pattern surface area to volume ratio) affect emission characteristics. The experimental results provide insight into how the process variables influence air quality measures. This knowledge may be useful for specifying process settings that minimize the environmental impact and also satisfy such traditional performance measures as casting quality and cycle time. INTRODUCTION The lost foam casting process (LFC) involves investing an expanded polystyrene (EPS) foam pattern, which has been coated with a refractory coating, in sand and then pouring molten metal onto the pattern. The pattern is converted into liquid and then gaseous degradation products (LEPS, liquid expanded polystyrene, and GEPS, gaseous expanded polystyrene, respectively). As polystyrene vaporizes and escapes through the refractory coating, the molten metal takes on the shape of the original pattern. LFC is a relatively new process that offers several advantages over traditional green sand casting, including high dimensional accuracy, good surface finish, greater design flexibility, and reduced skilled labor requirements (Berg et al., 1967). From an environmental (and economic) perspective one of the principal advantages of LFC is the fact that the casting sand can be reused. Traditional green sand casting requires a chemically treated sand to maintain the part shape inside the casting flask. Disposal of this contaminated sand is the greatest environmental obstacle associated with traditional green sand casting. In contrast, with LFC the foam pattern maintains the integrity of the part shape during pouring and therefore loose sand can be used to fill the casting flask. Unlike chemically treated sand, the loose sand is relatively clean and can be reused many times before disposal. This has led to the perception that LFC is more “environmentally friendly” than green sand casting. However, the process does produce significant quantities of airborne emissions that are known to be toxic to humans. Many researchers have investigated the process phenomena related to mold filling and pattern degradation (Butler and Pope, 1964; Capadona and Albright, 1978; Fu et al., 1991, 1995, and 1996; Gallois et al., 1987; Goria et al., 1986; Lee, 1976; Mehta et al., 1995; Shivkumar, 1993; Shivkumar and Gallois, 1987; Shivkumar and Gallois, 1987; Shivkumar et al., 1995; Sikora, 1978; Tsai and Chen, 1988; Tseng and Askeland, 1991; Walling and Dantzig, 1994), but little work has been done to characterize the emissions from this process (Madorsky, 1959; Berg et al., 1967; Kobzar and Ivanyuk, 1975; Gressel et al., 1987). Surprisingly, little is known about how individual process variables impact the quantity and makeup of these emissions. It is not only necessary to understand what the process waste streams are, but also how adjusting process settings can reduce (and perhaps minimize) the negative impacts of these waste streams. This fundamental knowledge may lead to a more environmentally responsible process by reducing the airborne emissions from lost foam casting operations. METHODS AND MATERIALS The composition of emissions from an LFC process is directly dependent on the degradation of the EPS pattern, which in turn depends on two main factors: 1) the amount of energy added to the system, and 2) the length of time the energy remains within the system. As more energy is transferred to the polystyrene pattern, more depolymerization occurs causing specific compounds to be emitted. Similarly, exposing the polystyrene pattern to the heat source for a longer period also allows for further depolymerization. With this rationale, three variables were chosen for study that are likely to have an impact on the airborne emissions from the LFC process: pouring temperature, pattern coating, and pattern thickness (impacts pattern surface area to volume, SA/V, ratio). Pouring temperature provides a direct indication of the total energy that enters the
system, while pattern coating and SA/V ratio are related to the length of time the energy remains within the system by impacting the ability of “energy-containing” decomposition products to escape the mold. The presence of a refractory coating is important since its permeability to decomposition products can control the rate at which decomposition products escape. The SA/V ratio dictates the amount of decomposition products that must pass through a unit area on the surface of the pattern (assuming constant pattern density). Design of Experiments (DOE) techniques were used to investigate the effects of these three process variables on LFC airborne emissions. An unreplicated 23 full factorial experiment design, consisting of eight total tests, was used to obtain the singular and joint effects of each of the variables. The variable settings for each of the eight tests are shown in Table 1. The tests were performed in a randomized order. The thickness of the EPS pattern used was adjusted to change the surface area to volume (SA/V) ratio parameter – an increase in pattern thickness reduces the SA/V ratio. With this exception, all the patterns used were the same shape as shown in Figure 1. Patterns consisted of a rectangular plate connected to a simple gating system, constructed completely out of EPS. Each pattern was coated with Ashland 530, a refractory coating made for LFC. 3
Table 1 - 2 Experimental Design for Aluminum Casting Experiments Test Number 1 2 3 4 5 6 7 8
Run 7 4 5 3 2 6 8 1
A: Pouring Temperature (°C) 700 750 700 750 700 750 700 750
B: Coating uncoated uncoated coated coated uncoated uncoated coated coated
C: Pattern Thickness (mm) 8 8 8 8 24 24 24 24
Peak Pouring Temperature (°C) 70.57 60.96 74.21 68.39 50.20 48.70 59.40 65.20
540 mm 203 mm 154 mm
Figure 1 - Plate-shaped patterns and gating systems used in all experiments (from left to right: thin/uncoated, thick/uncoated, thick/coated)
Patterns were invested in a casting flask using 50-60 GFN casting sand. The filled casting flask was placed inside a “casting box”, on top of which a fumehood was placed. This “casting box” had a small door and a Plexiglas window built into it. A “pouring tube” was constructed out of chemically bonded sand such that it would fit through the door in the casting box. The “pouring tube” allowed the castings to be poured outside the casting box, while still capturing all the air emissions inside the “casting box”/fumehood enclosure as shown in Figure 2. Pouring could be monitored through the Plexiglas window to assure that the pattern was completely filled, but not overfilled. Once pouring was completed, the “pouring tube” was removed and the door to the “casting box” was closed. All castings were poured with Aluminum 319. Once the casting had cooled, shakeout was performed by re-opening the door in the “casting box” and physically shaking the walls of the casting flask free from its base. This allowed the casting sand to flow out the bottom of the flask, releasing a second wave of air emissions that had been trapped in the sand. An exhaust fan was used to draw emissions up through the fumehood, which was equipped with several ports for temperature and pressure measurements along with several ports for emissions sampling. Upon pouring, emissions were sampled continuously for 25 minutes and then again for the 20 minutes following shakeout. Several different types of emissions
samples were taken. First, aerosol emissions were monitored in the fumehood using the DustTrak Aerosol Monitor. The DustTrak provided data about the quantity of aerosols emitted from the process. Secondly, a Aerodynamic Particle Sizer Spectrometer was used to sample aerosols in the ambient air where the fumehood exhaust was released. The Aerodynamic Particle Sizer (APS) provided information regarding the size of the aerosols emitted from the process. Unfortunately, the APS was incapable of handling the high aerosol concentrations found inside the fumehood, but it was assumed that the size distribution of aerosols directly outside the fumehood was the same as the distribution inside. Background samples were taken to assure that measurements were not tainted by ambient aerosol concentrations.
Fumehood
Pouring Tube
Casting Box
Figure 2 - Arrangement of box, fumehood, and “pouring tube” used to pour the casting from outside the enclosure
Lastly, air samples were also taken from the fumehood to monitor for the presence of organics produced during the degradation of the EPS pattern. Organics were captured on an adsorbent resin, which was packed inside of sampling tubes. Two sampling tubes were used during each casting, one during the 25 minutes following pouring and one during the 20 minutes following shakeout. Analysis of these samples via gas chromatography and mass spectrometry (GC/MS) provided quantitative results for BTEX compounds (benzene, toluene, ethyl benzene, and xylenes) and semi-quantitative results for various other compounds of interest (e.g. styrene). CASTING RESULTS A total of eight aluminum castings were produced during these experiments, as shown in Figure 3. Although it is difficult to see in the figure, the four coated-pattern castings possessed dramatically better surface finishes than the four uncoated-pattern castings. SA/V ratio and pouring temperature did not visibly affect the appearance or surface finish of the castings. With the exception of two castings, run #5 and run #7, all patterns filled completely. The castings produced during these two runs were poured with 8 mm (0.325 in) patterns and at the lower temperature of 700 °C (1292 °F). It is probable that the fluidity of the molten metal was hindered by the combination of the lower melt temperature and the thin pattern thickness. TEMPERATURE RESULTS The temperature of the emissions inside the stack was recorded every minute during each run approximately 1.4 m (4.59 ft) above the top of the casting flask. The temperature profiles during each of the eight runs show very similar behavior. Upon pouring, there is a sharp increase in the emissions temperature of at least 40°C (104 °F) above the initial ambient temperature measurement. The values for this peak temperature are shown in Table 1. After this first peak, the temperature decays in an exponential fashion over the next 25 minutes. Using the peak temperature as the response the singular and joint effects of the three process variables were calculated using standard DOE procedures (DeVor et al., 1992). To determine which of the effects are statistically significant, the effects may be plotted on a normal probability plot. In interpreting a normal probability plot, many of the points lying near an effect value of zero and a cumulative probability of 0.5 will be well described by a straight line. These unimportant points (effects) are said to arise from a normally distributed error distribution with a mean of
zero. Effects not well described by the straight line are judged to be important (significant) – also arising from a normal distribution, but with a non-zero mean. The normal probability plot for the peak temperature effects is shown in Figure 4. An ANOVA (analysis of variance) was also performed on the data, and the results are shown in Table 2. In principle, the normal probability plot and the ANOVA should reveal the same significant effects.
Figure 3 - Eight Castings Produced During Experiments (top row from left to right: run #1, #2, #3, and #4; bottom row from left to right: run #5, #6, #7, and #8)
Coating
SA/V
Figure 4 – Normal Probability Plot for Peak Temperature Response Effects
Table 2 - Analysis of Variance with Peak Pouring Emissions Temperature as the Response Variable (A = Pouring Temperature; B = Pattern Coating; C = SA/V ratio) Source of Variation A B C AB AC BC Error Total
Sum of Squares 15.47 168.89 320.32 15.35 48.62 26.80 1.55 597.01
Degrees of Freedom 1 1 1 1 1 1 1 7
Mean Square 15.47 168.89 320.32 15.35 48.62 26.80 1.55
F0
P-Value
10.00 109.23 207.17 9.93 31.45 17.33
0.195 0.061 0.044 0.196 0.112 0.150
Effect -2.78 9.19 -12.66 2.77 4.93 3.66
The normal probability plot of Figure 4 and the ANOVA results of Table 2 indicate that pattern thickness (i.e., the SA/V ratio) is the dominant factor in controlling the peak temperature of the emissions during pouring. It is statistically significant at a 5% level. If an empirical model is established based on this significant variable, the model residuals suggest that pattern coating may also be an important variable. The pattern coating effect is positive (i.e., when the pattern is coated, an increase in peak pouring temperature results), whereas the pattern thickness demonstrated a negative effect (i.e., a thicker pattern results in a decrease in the peak pouring temperature – or a smaller SA/V ratio reduces the peak emission temperature). Counter intuitively, there was no evidence that the actual pouring temperature had any significant effect on the peak emissions temperature. The results from the temperature effects are suggesting that the presence of GEPS is responsible for the temperature peak. When the polystyrene pattern decomposes, first LEPS is produced. When this LEPS is heated further, it is converted to GEPS. It is likely that the pattern coating does not allow LEPS to escape the mold as readily as GEPS. Therefore, LEPS is kept in the mold longer when coating is present, making it more likely to be converted to GEPS. Hence, the presence of a coating may induce a greater fraction of the original foam pattern to be converted to GEPS. The greater quantity of hot gases escaping the mold would subsequently cause the emissions temperature to be higher. This theory is further supported by observations made while cleaning up the castings. When LEPS is deposited in the lightcolored casting sand, it discolors it to a dark brown and bonds the sand together once it has cooled. These small dark brown clumps of sand were found in disproportionate numbers when cleaning up the sand from the uncoated-pattern castings. In addition, the dark brown clumps were significantly hotter than the casting sand around them, suggesting an unusually high concentration of the hot decomposition products. These dark brown clumps were especially prominent in the casting sand from runs #5 and #7, the two molds that did not completely fill. If the molds did not fill, this means that the unfilled portion of the EPS pattern was not exposed to as much thermal energy as the rest of the pattern. These portions of the patterns are therefore more likely to either not decompose at all or to decompose to LEPS, but not all the way to GEPS. It therefore makes intuitive sense that more dark brown clumps would be found in the casting sand from these two castings. The SA/V ratio also demonstrated a significant effect in controlling the peak emissions temperature. Thin patterns have a relatively large surface area when compared to the thick patterns, making it easier for degradation products to escape the “hot zone” within the mold. It follows that GEPS emitted from thin patterns is likely to be cooler than GEPS emitted from thick patterns, resulting in a lower peak emissions temperature. AEROSOL RESULTS At the start of each experimental trial, when the molten metal was poured into the flask, the pattern would initially combust for a short period of time. The polystyrene would burn readily in the oxygen-rich environment near the top of the casting flask, but the flames would die out quickly as the pattern filled. Although this combustion of the pattern did not last very long, it did produce considerable quantities of aerosol fumes. The aerosol measurements taken indicated that the shear quantity of fumes produced overshadowed any underlying effect of the process variables on aerosol emissions. No further conclusions have been drawn from the recorded aerosol data. ORGANIC RESULTS The casting experiment was designed as a 23 full factorial experiment (eight total tests). Two organic air samples were taken during each of the eight casting experiments, resulting in 16 total samples. However, for the purposes of statistical analysis, the sample type: pouring vs. shakeout air sample, was treated as a fourth variable making it a 24 full factorial design (16 total samples). It should be noted that the introduction of this fourth variable introduces some concerns about the statistical independence of the experimental trials, which should inform subsequent statistical analysis, and interpretation of the data. Before discussing the organic emissions results, it is necessary to understand the nature of EPS degradation. The complexity of the chemical phenomena associated with the thermal breakdown of EPS during the lost foam casting process makes it difficult to draw conclusions from the ANOVA of any one chemical compound. Both the LEPS and GEPS emissions will always be a mixture of various chemical species. The initial breakdown of the polystyrene pattern occurs through “random scission” of carbon-carbon bonds in the polymer chain, a method of degradation that produces large quantities of LEPS (Shivkumar et al., 1995). This LEPS can persist in the mold up to temperatures around 400°C (Shivkumar and Gallois, 1987). At higher temperatures EPS is almost completely volatilized and emissions are dominated by the monomer (i.e., styrene). Several researchers have shown that styrene yield reaches a maximum at a point between 400-700°C (752-1292 °F) (Shivkumar and Gallois, 1987). As temperature increases, the yield of styrene decreases as numerous depolymerized fragments (e.g., benzene and toluene) appear in greater quantities. In summary, there are essentially two general forms in which degraded EPS can escape the casting mold: 1) as GEPS consisting primarily of styrene (produced generally at lower
temperatures); 2) as GEPS consisting primarily of other more highly depolymerized fragments (produced generally at higher temperatures). As a result, the results for the individual BTEX compounds were summed as one quantity. For this investigation, the specific type of BTEX compound emitted was not as important as the fact that these compounds are produced only at higher temperatures. The responses from the 24 experimental design for the BTEX and styrene emission responses are provided in Table 3. For each of these organic emissions, the singular and joint effects of the four variables were calculated using standard DOE procedures. Normal probability plots were constructed for each response to identify the statistically significant effects. The initial pair of normal probability plots displayed an odd appearance; effects having values near zero were not well described by a straight line. Empirical models based on the largest effects produced residuals that increased in magnitude as a function of the response. Such behavior indicates that the experimental error is a function of the response (this is true for example if the percentage error is constant). The method to deal with data such is this is to apply a logarithmic transformation to the data (models developed using this procedure are of power function form). Natural logs of the emission data were taken and the effects recalculated. The final normal probability plots associated with the BTEX and styrene effects are shown in Figure 5 and Figure 6, respectively. The associated ANOVAs for the logarithm of the BTEX and styrene emission responses are displayed in Table 4 and Table 5, respectively. 4
Table 3 – 2 Experimental Design for Aluminum Casting Experiments A: Pouring Temp. (°C) 700 750 700 750 700 750 700 750 700 750 700 750 700 750 700 750
B: Coating uncoated uncoated coated coated uncoated uncoated coated coated uncoated uncoated coated coated uncoated uncoated coated coated
C: Pattern Thickness (mm) 8 8 8 8 24 24 24 24 8 8 8 8 24 24 24 24
D: Sample Type pour pour pour pour pour pour pour pour shakeout shakeout shakeout shakeout shakeout shakeout shakeout shakeout
BTEX mass (ng) 1907.23 1988.22 3174.74 614.66 1563.13 1444.25 301.37 4493.90 588.26 2311.91 3576.23 5184.23 1785.64 508.63 798.51 209.97
Temperature x SA/V x Sample Type
Figure 5 – Normal Probability Plot for ln BTEX Response Effects
Styrene mass (ng) 839.7 709.3 859.1 850.0 541.1 653.7 547.5 593.3 2994.8 2658.7 3483.5 3697.9 2368.2 2506.4 1786.1 507.1
Sample Type
SA/V
Figure 6 – Normal Probability Plot for ln Styrene Response Effects Table 4 - Analysis of Variance with ln(Total BTEX Concentration) as the Response (A = Pouring Temperature; B = Pattern Coating; C = SA/V ratio; D = Sample Type) Source of Sum of Degrees of Mean Variation Squares Freedom Square C 2.245 1 2.245 BC 1.076 1 1.076 CD 1.116 1 1.116 ABC 1.810 1 1.810 ACD 4.574 1 4.574 BCD 1.241 1 1.241 ABCD 0.786 1 0.786 Error 0.994 8 0.124 Total 13.80 15 Note: 8 smallest effects used for error estimation.
F0
P-Value
18.08 8.66 8.99 14.57 36.83 9.99 6.33
0.003 0.019 0.017 0.005 0.0003 0.013 0.036
Effect -0.749 -0.519 -0.528 0.673 -1.07 -0.557 -0.443
Table 5 - Analysis of Variance with ln(Styrene Concentration) as the Response (A = Pouring Temperature; B = Pattern Coating; C = SA/V ratio; D= Sample Type) Source of Variation A B C D AB AC AD BC BD CD Error Total
Sum of Degrees of Mean P-Value F0 Squares Freedom Square 0.0858 1 0.086 0.535 0.498 0.1025 1 0.102 0.638 0.461 1.1448 1 1.145 7.130 0.044 5.4158 1 5.416 33.729 0.002 0.0739 1 0.074 0.460 0.528 0.0301 1 0.030 0.188 0.683 0.1142 1 0.114 0.711 0.438 0.4389 1 0.439 2.733 0.159 0.1439 1 0.144 0.896 0.387 0.1635 1 0.164 1.018 0.359 0.8028 5 0.161 9.3192 15 Note: 3- and 4-factor interactions used for error estimation.
Effect -0.146 -0.160 -0.535 1.164 -0.136 -0.087 -0.169 -0.331 -0.190 -0.202
The BTEX responses and associated variable effects display behavior not often found from planned experimental designs. Contrary to conventional DOE wisdom, many of the 3-factor and higher order interactions are larger than the main and twofactor interaction effects (usually not the case). The eight smallest effects in absolute value were used for the estimate of the
experimental error in the ANOVA table for BTEX shown in Table 4. Using this approach, many effects are judged to be statistically significant, but the effects that are the most significant are ACD (temperature x SA/V x sample type interaction), C (SA/V), and ABC (temperature x coating x SA/V interaction). The normal probability plot of effects is shown in Figure 5 for BTEX, and the ACD interaction clearly stands out as the most important effect. Each of the three most important effects includes the SA/V variable, indicating its overall importance. In general, a three-factor interaction is difficult to interpret, and the fact that multiple three-factor interactions have demonstrated significance further complicates the interpretation of the experimental results. It appears that the significant three-factor interactions have resulted from several of the tests having exceedingly large BTEX responses, and a high order interaction is then required to describe the complex response surface. Interpretation of the styrene response is more straightforward than the BTEX response. Figure 6 shows the normal probability plot for the log styrene response, and Table 5 provides the ANOVA results. The normal probability plot clearly shows that the SA/V ratio and Sample Type variables are significant. The same conclusion is drawn from the ANOVA table. The fact that the SA/V ratio has a negative effect indicates that increasing the pattern thickness reduces the styrene emissions (a lower SA/V ratio produces less emissions). Sample type has a very strong positive effect indicating that emissions increase during shakeout. The peak temperatures within the fumehood were generally 25-30 °C (77-86 °F) higher during pouring than during shakeout. It is likely that much of the styrene produced during pouring by pattern degradation is further heated and depolymerized before escaping the mold. However, the lower temperatures present in the mold during shakeout are less likely to cause styrene to degrade further. As the entire mold heats in the minutes following pouring, LEPS trapped throughout the casting sand is converted to GEPS. Temperatures in the casting sand surrounding the pattern are likely high enough to produce GEPS in the form of styrene, but not high enough to produce other fragmented compounds. Therefore, the vast majority of GEPS trapped in the casting sand during cooling is in the form of styrene. Hence, shakeout emissions are dominated by styrene. For both styrene and BTEX emissions, increasing the pattern thickness, or alternatively reducing the SA/V ratio, reduces the level of emissions. As the SA/V ratio decreases, it should be more difficult for compounds to escape the mold because there is less surface area to transport across. Styrene emissions should decrease since degradation products would be less likely to escape the mold before being heated to BTEX-producing temperatures. By the same logic, BTEX emissions would be expected to increase. Surprisingly, the results given in Table 4 display the opposite effect, on the average BTEX emissions decrease when the pattern thickness is increased (i.e., SA/V ratio is decreased). However the raw data shown in Table 3 for BTEX clearly shows several instances when an increase in pattern thickness produces a large increase in BTEX emissions – exactly the sort of behavior that can produce a significant high order interaction. The phenomena that produce these results for BTEX are not clear. One possibility is that the lower SA/V ratio causes BTEX compounds to be heated and fragmented even further into other compounds not considered by this analysis. It is not possible, however, to investigate this theory further with the available data. CONCLUSIONS In general, this work has led to a clearer understanding of airborne emissions from the lost foam casting process. In addition several more specific conclusions can be drawn. First off, the results of this study indicate that process variables do play a significant role in determining the characteristics of airborne emissions from the lost foam casting process. The relationship between process conditions and the makeup of process emissions is rather complex. Consequently, the details of this relationship have not been fully revealed, but several findings have improved on the current understanding of airborne emissions from the lost foam casting process. Counter-intuitively, pouring temperature appear to have a minimal effect on the characteristics of the process emissions (it shows up in a three-factor interaction for the BTEX response). Information collected on aerosol emission revealed that most of the fumes are produced at the outset of the operation, with the fumes produced as a result of EPS combustion. The presence of a refractory coating on the foam patterns slows the release of degradation products from the mold. Gaseous degradation products are therefore heated to a higher temperature before escaping the mold, increasing the likelihood of producing highly depolymerized compounds. Results also indicate that an increase in the thickness (i.e., a decrease in the surface area to volume ratio) of the foam pattern causes an overall reduction in organic emissions. Preliminarily this would indicate that simply decreasing the pattern surface area to volume ratio would decrease the environmental burden associated with the process. However, due to the limitations of this investigation (only two levels of SA/V ratio, only styrene and BTEX organics examined, etc.), further work is required to fully understand the complex phenomena at work. In addition, it should be noted that part design often might not allow flexibility for changing the SA/V ratio.
Lastly, styrene emissions were found to increase dramatically between pouring and shakeout. This may also indicate that a greater percentage yield of the pattern is converted to BTEX and other fragmented polymers during pouring rather than during shakeout. This result is noteworthy because it illustrates the fact that emission characteristics do vary significantly at different points in the process. Any countermeasures implemented to alleviate the impacts of hazardous process emissions must take this factor into account. In summary, process variables do have a significant impact on the emissions from the lost foam casting process. This indicates that there is clearly a potential for minimizing the environmental impact of the process by manipulating process variables. However, at this point much more work is needed to fully understand the relationship between process variables and environmental impact.
ACKNOWLEDGEMENTS This work was partially supported by a Graduate Assistance in Areas of National Need (GAANN) fellowship and a grant from the Presidential Faculty Fellows/Presidential Early Career Award for Scientists and Engineers (PFF/PECASE).
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