Thermal Expansion of Investment Casting Shells
A review of the thermal expansion of typical shells used for investment casting is presented. Discussion of differences between types of shell systems and practical implications in the foundry is given. Data are presented for typical commercial foundry shells which are comprised of fused silica, alumino-silicate, and fused silica / aluminosilicate combinations. Data and discussion are also given for shells made of zirconalumina which are typically used for directionally solidified and single crystal applications. The thermal expansion curve for a zircon substitute is presented. Materials for ultra-high temperature shell systems are investigated.
Thermal expansion is a very important property of materials and must be considered for most applications. A common example is expansion joints in bridges and highways. The concrete and steel must be allowed to expand and contract or cracking and ultimately failure will occur in the structure or road. Relating to investment casting, everyone is familiar with the issue of wax thermal expansion causing cracks during dewax.
When considering the ceramic shell for investment casting, there are several reasons to be interested in and to understand its thermal expansion.
- Thermal cycling (heating and cooling) of the shell
- Final dimension of the casting
- Wax tooling from other shops
- Changing materials for the shell
- Shell distortion at moderate temperatures
- High temperature stability
Thermal expansion of ceramic shells is measured using an instrument called a dilatometer. One common instrument is made by The Orton Ceramic Foundation. See Figure 1. A prepared sample is placed in the alumina tube holder. An alumina rod is in contact with the sample and the movement of the rod is measured with a sensitive LVDT.
The sample is heated by a silicon carbide tube furnace. See Figure 2. Since the LVDT is measuring the movement of the entire system, an alumina standard is used and a correction factor is determined first.