Moisture Adsorption

All photopolymer resins are subject to moisture adsorption on some level. Given this hygroscopic nature, the Perfactory, Viper, and InVision materials could have had dimensional expansion introduced during shipping, storage, and processing of the patterns.

However, in our evaluation, the amount of time that elapsed between pattern measurement, assembly, and the commencement of investing was relatively short, so the exposure to pre-investment humidity, although present, was minimal.

The investing process, on the other hand, took place over several hours and introduced varying levels of moisture, first via the face coats, and then through phosphate-bonded investment procedures. Nevertheless, as demonstrated in the section that follows, our findings indicate that moisture alone is not likely to have caused all of the expansion shown in Table 3.

Thermal Expansion

All materials expand with heat, and this thermal expansion can result in cracking of the investment, core failures in the setting areas, finning, ceramic inclusions, and malformed features. Thermal expansion in RP patterns is well documented in available literature.

In our study, evidence of thermal expansion during high temperature burn-out was minimal in terms of shell cracking (where one would expect to see it), with only a minor amount of finning around the gate areas and some evidence of core failures in the setting areas, specifically on the Viper castings. Given that significant expansion was experienced, but shell cracking and pre-investing pattern growth were not factors, our investigations focused on the investing/de-wax stage of the process.

Although dimensional growth was not evaluated originally during investing and de-wax/pre-burnout, we now believe that the bulk of the expansion took place during this phase. We performed an experiment to approximate the conditions that photopolymer patterns are subjected to in these processes, using an electric kiln set initially at the peak temperature experienced during the setting of the investment (29°C/84°F), and secondarily set at the de-wax/pre-burnout temperature (150°C/302°F). A moist environment designed to emulate the solid mold investing process was created by wrapping each photopolymer pattern in a paper towel moistened with water.

Tables 4, 5 & 6: click to enlarge

Tables 4, 5, and 6 depict the results of this experiment. They point to the combination of temperature and moisture as the cause of the highest rate of dimensional growth during the investing/de-wax stage of the process.

Given the strength and low ductility of the face coats after setting, one would expect cracking of the shell upon thermal expansion of the patterns, but not overall growth without cracking, as was observed in the experimental group. This finding indicates that expansion of the face coats after drying occurs as a result of moisture introduced by the solid investment. Therefore, we think that the face coats are softening upon contact with the solid investment and allowing the thermal expansion of the patterns to occur without fracturing shell surfaces.

This softening of the face coats has been physically observed in previous experience at TechForm, although documentation of dimensional impact was not evaluated. This hypothesis should be confirmed with additional testing.