Paper 10-087.pdf, Page 1 of 7 AFS Proceedings 2010 © American Society, Schaumburg, IL USA

Improving Investment Mold Permeability Using Graphite Particles

D.M. Kline, S.N. Lekakh, V.L. Richards Missouri S&T, Rolla, MO

Copyright 2010 American Foundry Society

ABSTRACT of permeability is not as high in the ceramic mold as for most filters and the method of creation is different. A Experiments and calculations were performed to increase compaction and sintering method creates a purely the permeability of an mold through monolithic structure, whereas investment casting molds the use of sacrificial pore formers. Graphite particles are created through a stucco layering process. When were added to the fused-silica-based slurry and they were lower viscosity slurries are used for coatings, a near incorporated throughout the shell. The resulting shells monolithic structure is formed, but it still has some were fired to remove the graphite and produce pores layered structure characteristics. within the shell. Tests were performed to measure increases in permeability using a digital permmeter. The remainder of this paper will focus on the simulation Possible reduction in strength was evaluated through of thin layer ceramic coatings with relatively large-scale flexural testing. The formation of tunnels in three- pores, and their experimental results from laboratory dimensional layers by connection of randomly distributed testing. Since the corresponding simulations assume a spherical pores was modeled by Monte Carlo simulation. monolithic structure and random particle dispersion, it The permeability of layers formed was modeled using was limited to single mold layers. This is because the use Fluent CFD. The experimental data was compared to the of stucco between coating layers disrupts the assumption theoretical model and practical recommendations for of random graphite dispersion. To make up for this and to increasing investment shell permeability were proposed. correlate the simulated results with complete molds, multiple layer test samples were created and tested for INTRODUCTION comparison.

The permeability of a molding material is of high DESIGN OF EXPERIMENTS importance to the metal casting process. The mold must be capable of allowing gases to pass through with at least MATERIALS AND SAMPLE DESIGN the same rate as molten metal enters the mold. If gases Experimental Test Factors cannot exit the mold faster than it is filled, then air For this experiment, percentage of sacrificial graphite pressure increases within the mold. This build-up of within an investment slurry, layer thickness to graphite pressure can result in incomplete mold filling, shell particles size ratio, and coating number were investigated. cracking and gas defects in . Unless stated otherwise, weight percent graphite is in reference to the weight of graphite particles in the slurry The key reason for low permeability is the use of fine compared to the weight of all solids in the slurry. particle-size material with high packing density. While the dense structure gives high strength at low thicknesses, Material Selection it also restricts the flow of gases. In industry, the issue of The ceramic slurry used to produce the test samples low permeability of investment shell molds is generally constituting +325mesh fused silica flour suspended in a handled by adding vents to risers. Another, though less colloidal silica binder. High-purity carbon particles were exact, method used by a few is to rely on imperfections added to the slurry and kept in suspension with the fused and micro-cracks forming during mold processing to act silica. The graphite particles used were angular in shape as vents. The experiment presented in this paper took and ranged in size from 0.007-0.031in (180-800um). The generic silica slurry and added graphite particles as a size of the graphite particles was selected such that only sacrificial fugitive. By using graphite, sample molds were two to three graphite particles would be required to align generated with controlled and repeatable levels of to for a complete channel through the roughly 0.039in permeability throughout all areas. (1mm) thick coating layers. The stucco applied to the test samples following each dip coating consisted of 0.012- The effect of ceramic porosity on permeability and 0.024in (300-600 μm) fused silica particles. 1-3 mechanical properties has been studied intensively .The majority of previous investigations were done for filters Sample Building and compacted ceramics. These studies have limited To reduce the number of confounding variables in the comparison to investment casting molds, since the level samples, all coating parameters were targeted to be the Paper 10-087.pdf, Page 2 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

same for all coatings. The dynamic slurry viscosity was maintained at 300cP with an allowed variation of 25cP (roughly equivalent to 7-8sec with a #5 Zahn cup). The size of stucco was -30+50 mesh (300-600um) for all coatings. Commonly the first (prime) coat of an investment mold has a different slurry and stucco than the subsequent (back-up) coats. For this study, the first (a) coating used the same materials and parameters as the other coatings. This created a more uniform structure within the sample and eliminated the presence of a smooth, dense packed internal surface, which could limit permeability. In addition, a seal coat (which does not contain stucco) was omitted since it would impede airflow out of the test samples and limit permeability4.

All of the test samples used 0.91x0.39x6.9in (23x10x175mm) foam strips as coating patterns. The foam strips were lightly abraded using 320-grit sandpaper to remove surface texture differences between cut and uncut surfaces. The abrading of the foam surface also (b) created thicker and less smooth first coats, which assisted in making it more like a back-up coat instead of a prime coat.

The dipping process used for this experiment followed the Fig. 1. (a) Sample connected to a permmeter for same steps as previous experiments by the author5. permeability measurement. (b) Sample located between upper and lower supports for 3-pt bend Following firing at 1650ºF (900ºC), approximately 1.5in testing. (38mm) of the closed end of the shell was cut off for use in permeability testing, while the remaining shell had the SIMULATION DESIGN sides cut off to produce two samples for flexural testing. Model Generation Three-dimensional Monte Carlo simulations were TESTING EQUIPMENT AND PROCESSES generated using physical properties taken from the mold Porosity samples created for this experiment. This simulation was The percentage of open porosity within the test samples used to determine the average number of through and sample density were tested using ASTM standard C- channels in each layer. A pseudo-random number 20. The process involved suspending strips of the test generator utilizing the Mersenne Twister algorithm6 was samples in water and comparing suspended, wet, and incorporated into the simulation model. The simulated dried weights following water removal by heating to porous structure was modeled as a cuboid with 300°F (150°C) using Archimedes equation to calculate dimensions of H×L×L, where the L×L faces were defined weight and outer and internal volumes. as being open to air and the H×L faces had closed airflow boundary conditions. The parameters H and L were set in Permeability relation to the mean pore diameter, D, where H was Permeability tests were done following the same process varied between 1.5 to 5 times D for different simulation as in previous experiments by the author5. Measurements runs. For the simulation, L was arbitrary, but to ease were taken using a digital permmeter and hose attachment simulation, L was adjusted so only 100 porous spheres shown in Fig.1a. would be generated within the cuboid for each pore density value. Modulus of Rupture Flexural testing was performed using a ceramic, large Pores with diameters within the range of D±dD were radius, 3-point bend fixture shown in Fig.1b. The lower randomly assigned coordinates within the cuboid, with D support radii were 0.64in (16.25mm) and the upper being the mean diameter of the pores and dD being half loading radius was 1.24in (31.5mm). Samples were the range of possible pore diameters. The number of loading in compression at a rate of 0.12in of central pores generated within the cuboid was governed by the displacement per minute. fractional density, ρ. The three-dimensional coordinates for the pore centers were randomly generated under the condition that two pore centers must be at least a distance of D-dD apart and can not be within a distance of (D- dD)/2 of the outer edges of the cuboid. In the event that the coordinates of a pore center would violate one of Paper 10-087.pdf, Page 3 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

those two conditions, and the two centers in conflict could not be shifted to fit the center in, such a center would be rejected and a new set of coordinates would be generated instead.

Air Flow Simulation Models consisting of only 3, 4 and 6 pore channels were generated for use with CFD to calculate individual channel permeability. The pores for these models were based on the average graphite particle diameter, 0.014in (0.36mm), and were to connect two surfaces 0.039in (1mm) apart, which was the average distance between stucco layers of the test samples.. The structure of the pore channels were randomly generated through the same (a) process as described above, but only single channels were 1000

evaluated. The airflow through the channels was varied 2 100 D for this portion of the experiment. A steady state solver 4 was applied to calculate the pressure drop across the 10 channel. 1

H=1.5D RESULTS 0.1 H=2D H=3D 0.01 MODELING PREDICTIONS H=5D Channel Formation 0.001 The Monte Carlo generated cuboid models were Number of formed channels, of formed n/10 Number evaluated to determine the number of channels connecting 0.0001 the base to the top. The simulations were run 1-10 0.00001 million times for each value of the height and pore density 0.01 0.10 1.00 parameters. The mean number of through channels Pore Density formed for each thickness is displayed in Fig.2 as a (b) function of pore density. As the relative thickness of the Fig. 2. (a) Example of formed interconnected modeled volume increased, the number of through channel and (b) number of channels (100Dx100D channels randomly formed decreased exponentially. As area) versus pore density in layers with different H/D the number of pores within the sample volumes increased, ratios the number of channels that connected through the

volume also increased exponentially. For volume thicknesses, three or more times the pore diameters, it was 70

necessary to have a minimum of 5% of the modeled 60 volume being pores. 50 Single Channel Air Flow 3 spheres The predicted geometry on channels created by 3, 4 or 6 40 4 spheres interconnected spheres was used for CFD modeling of 6 spheres airflow in individual channel. The inlet conditions for 30 each model were changed to get results over a range of darcy Permeability, 20 superficial air velocities. The calculated Darcy 7 permeability is shown in relation to the superficial air 10 velocity in Fig.3 for all three models. The viscous permeability for the models selected ranged from 50-60 0 0.0001 0.001 0.01 0.1 1 10 darcy for superficial air velocities below 3cm/s. As the Superficial velocity, m/sec superficial air velocity became faster than 3cm/s, the airflow became increasingly less linear and more Fig. 3. Permeability through the generated turbulent because of fluctuating channel widths. These channels for different superficial air velocities. changes in the airflow cause an increase in the inertia The constant values at low velocities are the Darcy permeability of the channel. energy losses and creation of gas vortices within the pores. The numbers of pores making up a channel were determined to have little effect on the resulting permeability.

Paper 10-087.pdf, Page 4 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

Combined Channel Permeability The results of the Monte Carlo and CFD simulations were used to estimate the overall permeability of a layer. The number of pores predicted through the Monte Carlo simulations was multiplied by the airflow rates through the CFD models to determine an average airflow rate through the entire cuboid. The results from these calculations were used to determine curves relating permeability to pore density and layer height. These results are compared with the experimental results later in this paper.

EXPERIMENTAL TESTS Shell Thickness The thickness of the test samples, as shown in Fig. 4a, had (a) a near linear relationship to the number of coatings applied. It is also seen that there was a significant difference in thickness between the samples with 5.8 and 10.9wt% graphite. The rate of sample thickening per coating in Fig. 4b shows that the use of 10.9wt% graphite or more gave the samples on average an additional 0.003- 0.007in (10-20um) per layer. The downward trend displayed in Fig. 4b for all graphite concentrations leads to the conclusion that at least one of the first three layers was larger than the remaining layers. Since only the first layer’s coating surface was different, it is inferred that the first layer was thicker and made more so by higher graphite concentrations. Since the surface was relatively smooth in comparison with the later stucco (b) covered surfaces, the graphite particles at the bottom of Fig. 4. (a)Average thickness with error bars of the layer all rested on the same plane instead of possibly test samples following different numbers of dip ending up in the stucco portion of a previous layer. This coatings and (b) the average thickness for each causes an increase in the average slurry height for that layer. first layer making the stucco applied to it sit higher. The increase from 5.8 to 10.9wt% , but not from 10.9 to Graphite Pick-up 15.5wt% can be explained by there being a limit to how Table 1 shows the calculated weight percent of graphite in many graphite particles can rest on the pattern surface, 0.5oz (14g) test samples assuming that all weight loss and that limit being reached for a concentration between during firing was because of graphite removal. Although 5.8 and 10.9wt%. steps were taken to maintain a uniform distribution of graphite particles within the slurry, the concentration of graphite in the samples did not match that of the slurry. A Table 1. Experimental weight and calculated volume data for test samples

wt% Graphite of # of Coating wt% Graphite of vol% Graphite in Δ Open Graphite Slurry Solids Layers Samples Samples Porosity (%) Effectiveness (%) 0.00 3 0.00 0.00 -2.43 --- 0.00 4 0.00 0.00 -0.63 --- 0.00 5 0.00 0.00 -0.11 --- 5.77 3 2.92 7.42 2.66 35.81 5.77 4 3.48 8.75 3.16 36.06 5.77 5 2.62 6.68 1.46 21.87 10.91 3 4.81 11.85 5.13 43.26 10.91 4 5.02 12.33 4.17 33.80 10.91 5 4.25 10.56 3.40 32.20 15.52 3 6.28 15.14 6.12 40.41 15.52 4 5.51 13.43 5.64 41.97 15.52 5 5.29 12.95 5.60 43.21 Paper 10-087.pdf, Page 5 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

(a) (b) Fig. 5. The calculated volume percent of open porosity within the test samples shown before (a) and after (b) graphite removal by firing. possible reason for this occurrence would be differences pronounced. As expected, the amount of porosity in the in surface tension between the graphite and the liquid samples increased with the addition of more graphite in slurry, pattern, previous mold layer, and air. Since the the slurry. study of this occurrence is outside the scope of this paper, no formal investigation into this was done at the current Ideally, the volume increase in open porosity would have time. Although the amount of graphite in the test samples equaled the volume of graphite removed, but this was not increased with higher graphite concentrations in the the case. The effectiveness of the graphite at creating slurry, the ratio between the two dropped. This difference open porosity is shown in Table 1 where the percentage in the ratio presents the possibility that there is a limit to given is the amount of resulting pores, which were open the amount of graphite particles, which can adhere per dip to the surfaces. As the percentage of graphite within the coating. samples increased, the percentage of resulting pores that were open to the surfaces also increased. For the lower Porosity and Permeability Improvements two tested levels of graphite concentration, a drop in The amount of open porosity in the tested samples is effectiveness was seen as layers increased. This was not shown in Fig.5 for both the unfired and fired conditions. the case though for test samples made from the 15.52wt% Prior to graphite removal during firing, the samples slurry. Test samples created from that source showed showed only a slight difference in porosity ranging from nearly the same percentage of graphite created pores 17 to 26%. As the number of coating layers increased, being open for all test sample thicknesses. the porosity lowered because not all pore channels through the newest layers aligned with those of previous layers. The drop in unfired open porosity with the addition of 5.8wt% graphite to the base slurry is caused by the presence of large graphite particles. Possible explanations for this are that the presence of graphite changed the normal surface tension, wetting behavior, and deterred the retention of air bubbles within the samples. The slight increase in unfired open porosity with increasing graphite levels can be accredited to lower density of the purely silica portion of the slurry.

The decrease in open porosity for the baseline samples after firing is assumed to be from small pores and channel necks sintering shut during firing. This same occurrence would be present in the other samples, but is counter- Fig. 6. Specific permeability for the different parameters tested. acted by the large pores generated by the removed graphite. The specific permeability of the samples showed a large Following graphite removal by firing the test samples, a improvement with the addition of higher concentrations noticeable increase in open porosity was recorded and is of graphite as shown in Fig.6. There was an only slight shown in Table 1. The drop in open porosity with difference in permeability for samples made from the increasing coating layers became slightly more 5.77wt% graphite slurry, but the differences in permeability became greater as more graphite was Paper 10-087.pdf, Page 6 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

included in the slurry. The permeability of the test Another parameter of interest for flexural testing of samples was divided by their respective thicknesses to ceramics is adjusted fracture load (AFL)8, which is determine the air flux through them and is displayed in presented below: Fig.7. As the concentration of graphite increased, the allowable air flux through the test samples also increased. AFL = f * MOR*t 2 Equation 1 At five coating layers, the 5.77% and 10.91% graphite samples appear to converge to the same air flux. where: f is constant factor, t is thickness.

Since the thickness (t) of an investment casting mold is a product of slurry and pattern parameters, number of coatings, and how the stucco was randomly orientated, the thickness is not a convenient parameter for strength calculations. By multiplying the flexural stress of the test samples by their thickness squared, the load capacity for a standardized width and length is calculated. The averages of the measured AFL values were adjusted using the constant factor f to set the lowest value to 1. The adjusted data presented in Fig. 9 shows that the samples with graphite additions were able to support 1.3-1.6 times as much load as the baseline samples when only 3 layers were present.

Fig. 7. Air flux through the test samples during The calculated AFL values for the samples showed results permeability testing. similar to flexural stress but with less difference between graphite concentrations. Mechanical Testing While the addition of graphite was beneficial to the gas permeability of the test samples, it was not beneficial for the strength. In Fig.8, a drop in the maximum flexural stress was experienced whenever graphite concentration rose above 5.8wt%. It was noticed that for the range of coating layers tested, the flexural stress of failure generally had a linear correlation with the number of dip coats. Although all the samples showed more strength with more layers, the rate of strengthening was not the same for all. As the percentage of graphite caused pores increased, the strength benefit of additional coatings decreased. The presence of more porosity in the baseline samples compared to the 5.8wt% samples is noticed here still by the consistently higher strength of the low graphite samples. Fig. 9. Adjusted fracture load of test samples standardized so that the lowest value is 1lb.

COMPARISON OF RESULTS

A comparison of specific permeability values derived through simulation and laboratory testing is presented in Fig.10. The predicted trend curves were generated through the CFD simulations and represent ideal results using uniform pore sizes. The experimental data points were determined by combining Table 1 and Fig.6 and adjusted for the molds natural permeability without graphite additions. The experimental data with 4 % open porosity or more all resided between the 2*D and 3*D curves generated through simulation. Since the size range for the graphite particles was originally calculated to Fig. 8. Flexural stress of the test samples upon require 2-3 particles to form a one layer high channel, this failure. result matches the simulation. Test samples with less than

4 % open porosity all had nearly the same permeability as for the base slurry. Paper 10-087.pdf, Page 7 of 7 AFS Proceedings 2010 © American Foundry Society, Schaumburg, IL USA

shell permeability with controlled consideration of decreasing mold load bearing capacity.

ACKNOWLEDGMENTS

We would like to thank U.S. Army Benet Labs for the funding of this research. The conclusions and opinions expressed are those of the authors and not of Benet Laboratories under contract number W15QKN-07-2- 0004.

We would like to thank Dr. Oleg Neroslavsky for development of the three-dimensional Monte Carlo

simulations program used for this research. Fig. 10. Data points from experimental testing are compared to predicted trend curves generated through simulation. We would like to thank Asbury Carbon for donating the graphite used for this research. The conclusions and Since the addition of graphite particles to an investment opinions expressed are those of the authors and not of casting mold has opposing effects for permeability and Asbury Carbon. flexural strength, there is a limit to its benefit. Fig.11 gives a comparison of air flux to adjusted fracture load, both of which are independent of thickness. If graphite REFERENCES particles are to be used, one should first determine the minimum requirement for shell strength to guarantee safe 1. Isobe, T., , “Gas Permeability and Mechanical mold handling, pattern removal, and mold filling. The Properties of Porous Alumina Ceramics With strength requirements of the mold will dictate the Unidirectionally aligned pores,” Journal of the maximum concentration of graphite particles allowable in European Ceramic Society, vol 27, pp. 53-59 (2007) the slurry and the resulting maximum air flux attainable 2. Ding, S., Zeng, Y., Jiang, D., “Gas Permeability through this method alone. Behavior of Mullite-Bonded Porous Silicon Carbide Ceramics,” J Mater Sci, vol. 42, pp. 7171-7175 (2007) 3. Guzman, I.Y., “Certain Principles of Formation of Porous Ceramic Structures. Properties and Applications (A Review),” Glass and Ceramics, vol. 60, pp. 280-283 (2003) 4. Hendricks, M.J., Peter Wang, M.J., Filbrun, R.A., Well, D.K., “The Effect of Seal Dips on Ceramic Shell Properties and Performance,” ICI, vol. 50, pp. 18:1-13 (2002) 5. Kline, D.M., Lekakh, S.N., Richards, V.L., Chandrashekhara, K., Chen, J., “Stucco Size Comparison and Simulated Pressure Loading of Investment Casting Shells,” AFS Transactions, vol. Fig. 11. Comparison of trade between gas removal and 117, pp. 341-353 (2009) strength for the tested graphite concentrations. 6. Matsumoto, M., Nishimura, T., “Mersenne Twister:

A 623-dimensionally equidistributed uniform CONCLUSION pseudorandom number generator,” ACM Trans. Modeling and Computer Simulations, vol. 8, pp. 3-30 The computer simulation methods generated a good (1998) representation of the experimental results. The 7. Poirier, D.R., Geiger, G.H., “Transport Phenomena in permeability of an investment casting mold increases Materials Processing,” TMS, Warrendale, PA, pp. exponentially when the concentration of pores increases. 90-101 (1994) As more pores are generated within a mold the probability 8. Hendricks, M.J., Wang, P.M.J., “Ceramic Shell of them contributing to open porosity greatly increases. Green Strength: Why Is It Measured And What Does The addition of relatively large particles to a slurry can It Mean?” ICI, vol. 47, pp. 7:1-11 (1999) affect its coating behavior and how smaller particles align during drying. The proposed process allows developing