Part Buyers Authority Issue #1

TEST GEOMETRY In this case, a double suction impeller roughly 16 inches in diameter was chosen, pictured in Figure 2 as the test geometry. The conventional wax injection tooling for this design cost $40,000 and had a lead time of eight to 10 weeks. 9 Figure 2: Test Geometry TEST & CASTING PROCEDURE Tech Cast processed the SLA pattern in the using the standard investment casting process. This involves dipping the pattern into a resin slurry, and building up enough layers to create a shell that is durable. The shell is then heated to 1500° F, causing the SLA pattern to burn and leave a void in the shell. The shell is then washed out to ensure no debris remains, and the molten steel is then poured into the shell mold. Once the metal has cooled, the shell is broken up and the actual part is ready to be inspected. Tech Cast documented labor hours at each step of the process and compared the casting results between SLA patterns and conventional wax patterns. RESULTS / Dimensional Accuracy Tech Cast measured critical dimensions on both the patterns and their respective castings. Figure 3 shows the location of the four dimensions measured on each pattern or casting. Table 1 shows measurements of the patterns which are scaled to compensate for shrinkage. The accuracy of the additive manufactured pattern was comparable to the wax pattern. The largest deviation from an individual measurement for the SLA pattern was .004”. All deviations were less than one tenth of a percent compared to the nominal value and insignificant when compared to the tolerances of the casting requirements. This is significantly better than can be produced in either green sand or no bake methods. Figure 3: Measurement Locations Figure 4: Casting from Additive Manufacturing/SLA Pattern Table 1: Additive Manufacturing /SLA Pattern Comparison Table 2: Casting Comparison Table 3: Surface Finish Comparison Dimension ID Target Actual Deviation %Deviation 1 15.955 15.959 0.004 0.03% 2 7.782 7.779 -0.003 -0.04% 3 3.030 3.032 0.002 0.07% 4 9.745 9.747 0.002 0.02% Average Absolute Percent Deviation 0.04% Dimension Casting Casting from wax pattern Casting from SLA pattern ID Target Actual Deviation %Deviation Actual Deviation %Deviation 1 15.75 15.880 0.130 0.83% 15.728 -0.022 -0.14% 2 7.63 7.560 -0.070 -0.92% 7.615 -0.015 -0.20% 3 2.93 2.933 0.003 0.10% 2.949 0.019 0.65% 4 9.62 9.648 0.028 0.29% 9.581 -0.039 -0.41% Average Absolute Percent Deviation 0.53% 0.35% Pattern Surface Roughness pattern ( in Ra) Wax 122 SLA 159 Dimensional inspection of the castings is shown in Table 2 below. Both castings showed similar deviations to the intended casting target with an average dimensional error less than one percent. From a dimensional accuracy viewpoint, the SLA pattern demonstrated the capability of producing a predictable, precise casting. Click here to SUBSCRIBE or CONTRIBUTE as an author.