Technical Article
Aluminosilicate-Fused Silica Blended Shell Solutions for PIC Applications
by Thomas Krumrei and Michael Weissenbacher/Imerys CARRD, Villach Austria – Danilo Frulli, Imerys Refractory Minerals, Paris
geared toward increasing the efficiency of Precision Investment Casting (PIC) foundries. These have obviously been focused on solving specific problems that foundries have been having. Good examples of these are the need for increased throughput in the shell- room, the need for better shell strength during de-waxing, and also the need for better shell removal. Imerys, a global organization, understands that the needs of PIC foundries are often very much dependent on the prevailing shell systems used in different areas of the world, as well on the different metals being poured into these shells. Imerys has recently developed a new range of blended products, Teco-Sil®
I Hybrid that is based
on a mixture of Imerys aluminosilicate and Teco-Sil fused silica. Based on the properties of these blends, the company has found that these products can be used, for varying reasons, in many different PIC backup applications.
Shell Knock-Out
Of course, this is a bit of an overgeneralization, but it is safe to say that the majority of PIC foundries in Europe and Asia are heavily focused on using aluminosilicates (almost completely) in their back-up shell systems. We believe the main reason for this to be proximity to raw materials and cost. Obviously, aluminosilicates are very effectively used in backup systems, but there are some “issues,” still, that are inherent to their use. Generally speaking, a 100% aluminosilicate shell back-up system can form very high strength during casting, thus causing problems during knockout. This is due to the “other” (besides alumina and silica) oxides found in these products…most specifically ferric oxide, titania, and alkali/alkali earth oxides. Imerys has determined that a
22 ❘ April 2017 ®
n recent years, several companies, including Imerys, have successfully developed engineered shell solutions
Figure 1: Shell strength data, measured at room temperature. The green strength of the shell should be high for stability during de-waxing and handling of the shell after de-wax. In contrast, CMOR values after the casting should be low to facilitate the knock out.
Figure 2: Thermal expansion of various shell system. Fused silica has a very low thermal expansion. The more fused silica is used to build the shell, the lower is its thermal ex- pansion. The expansion of the stucco material impacts the overall shell expansion much stronger than the expansion of the flour material.
“hybrid” system helps to overcome this problem. The addition of fused silica to an aluminosilicate-based back-up system yields the formation of B-cristobalite during
the casting process.
weakening the shell, and helping to ease shell removal (Figure 1)
During
cooling, the phase transition from Beta to alpha cristobalite is quite disruptive, thus
Dimensional Accuracy & Shell Strength Another issue that can be encountered using a high percentage of aluminosilicate in shell back-up systems is problematic
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