Experimental Procedure

To attain high quality castings using RP patterns, we must look closely at the unique characteristics of these materials and the defects that can result from their use in the investment casting process. The two areas of focus that emerge through this analysis are dimensional expansion and burnout residue.

Expansion of RP patterns can be problematic, as there is an obvious need in jewelry manufacturing to predict dimensional outcomes; e.g., ring size or setting size. Moreover, expansion in the burnout phase can cause investment cracking and failure in the core areas of settings, resulting in a metallurgically unusable product. Lastly, burnout residue trapped in the mold can create pitting and non-metallic inclusions in the casting.

Table 2: click to enlarge

In our experiment, we evaluated these key casting characteristics in RP patterns supplied by the Solidscape, InVision, Viper, and Perfactory machine manufacturers. The ring design chosen for the experiment was courtesy of David Trout of Coffin & Trout Jewelers in Chandler, Arizona. The design was a medium-weight ring with flat shank sides for ease of caliper measurement and a large number of setting holes to enable a good evaluation of diameter behaviors through processing. The patterns were cast in 90/10 platinum/iridium using a standard phosphate-bonded investment with ceramic shell face coats. Each casting tree contained two rings, and all lots achieved a 100 percent yield in casting. All assembly, investing, firing, casting, and other parameters were held constant, with the exception of mold cleaning and burnout methods, as illustrated in Table 2. The only other variable was the patterns themselves. Following typical investment removal procedures, the castings were inspected for visual defects and a series of key dimensions were taken.

Dimensional Evaluation

Figure 1 click to enlarge

In evaluating the dimensional accuracy of the RP systems tested, we relied on a series of measurements that could be easily referenced in the original data file. In this manner, we could track variance from design specifications to RP pattern dimensions. Subsequent comparisons were made between pattern measurements and the resulting metal casting.

Dimensional variations have been expressed in both numeric values and in percentages. It is our hope that with these percentage values jewelry manufacturers will be better able to successfully predict the scaling of designs required to compensate for dimensional variations in these materials. Figure 1 shows the locations for measurement.

Table 3: click to enlarge

In Table 3, columns two through four indicate actual dimensions, columns five and six indicate the difference of file to pattern and pattern to casting, respectively. Column seven specifies whether shrinkage, growth, or zero movement in dimensional value was experienced between the pattern and casting. Column eight indicates the percentage of shrinkage or growth from pattern to casting—the number that should be evaluated when inputting the CAD file for pattern build when dimensions are critical. Shrink/growth rates must be correlated and then designated in the X, Y, and Z directions for these purposes.

It should be noted that for inner diameters, when the casting result is a larger measurement than the pattern, this indicates shrinkage of the metal (metal is moving away from the investment). On the other hand, if the dimension is smaller, this is due to expansion of the pattern and consequent enlarging of the mold cavity. This effect, which adds metal to the casting, can be seen in the measurements of the shank inner dimension (Dim B).