In lost wax casting, a process through which wax patterns are dipped in a ceramic slurry and then burned out to create a mold, precise dimensions of the wax pattern are extremely critical. If one side of the wax pattern is difficult or impossible to reach, ultrasonics may be the only method to measure its thickness. Wax patterns for cast turbine blades are good examples: the pattern thickness is crucial in order to produce cast blades that are within specifications.
With an ultrasonic gauge, the thickness of the wax can be measured without destroying it. Some wax patterns are formed around ceramic cores. This is an important distinction because sound that is reflected from a wax-ceramic boundary reacts differently than sound reflected from a wax-air boundary. Although both measurements can be made, they require separate setups. We should be informed which setup is needed; in general, samples are required to enable our office to recommend the correct instrumentation.

Equipment: A Panametrics Model 35DL Thickness Gage with a 20 MHz or 10 MHz contact transducer will make thickness measurements in many wax pattern applications. If the application includes wax over ceramic, accurate measurements rely on good bonding between wax and core without areas of disbond. Wax over ceramic requires a special gauge setup (echo polarity inversion); these setups will result in less accurate thickness measurements when the wax is partially or totally disbanded.

In making thickness measurements on concave sides, especially on small radiuses, we recommend the Model 35DL and a M208 transducer with a special soft tip delay line. Some complex geometries may require use of a focused immersion transducer and bubbler. In such cases the Model 38DL PLUS gage is usually recommended, since waveform monitoring is often necessary to insure proper sound beam alignment.