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The Choice Between Aluminum Silicates and Fused Silica for Stucco


The use of Aluminum Silicate stucco as a replacement for Fused Silica stucco is investigated. Two different types of backup slurries and three different stuccoes were evaluated. A new test is described and used to measure the ability of the slurry and/or stucco to build a uniform shell thickness. Much of the economic advantage of Aluminum Silicate stucco is lost when other factors are considered.


The choice between aluminum silicates and fused silica for stucco

Many investment casting foundries use fused silica based slurries with either fused silica stucco or Aluminum Silicate stucco. Invariably, the reason given for using Aluminum Silicate stucco is that the price per pound is roughly one half the amount for fused silica sand. However, in addition to the price per pound, there are other issues to consider that affect the overall economics of this decision. These include:

  1. Specific Gravity of the Material
  2. Number of dips needed
  3. Ability to produce a crack free shell
  4. Probability of operator injury while dipping or casting
  5. Ease in shell remove

The specific gravity of a typical 47% alumina silica grain is 2.64 and that of fused silica is 2.20 grams/cc. So, before one even begins to dip, you will have to order 20% more pounds of aluminum-silicate grain because it’s heavier.

For the same specific gravity issue, fused silica shells are light in weight and easier to lift and manipulate than those using Aluminum silicate. There have been comments made by both investing personnel and foundry personnel, that they prefer the lighter weight fused silica shell molds.

While I don’t have any direct evidence, it is not too much of a leap in faith to assume that there will be fewer muscle injuries handling lighter weight shells.

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