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SILICA FAQ

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1. Dimensional Stability
   Fused Silica shells have much lower thermal expansion than shells based on alumino-silicates, zircon or alumina over all temperature ranges. In addition, Fused Silica shells are much better with respect to sintering shrinkage and high temperature creep. Some foundries use Fused Silica shells with preheat temperatures as high as 2,600 degrees Fahrenheit.
   
   NOTE: A Fused Silica shell will produce a somewhat smaller casting from a given pattern than the alternatives. However, many foundries that have converted to Fused Silica have found that the dimensional change has been minimal and well within normal tolerances. The geometry of the castings and the temperature of the preheat furnace also affect dimensions. In general, the wax and secondarily the metal, have more of an effect on dimensions than the shell. If a smaller castings problem is encountered, a modest change in the wax is one way to compensate.
   
   
2. Ease of Shell Removal
   With a Fused Silica shell, you want to transform as much of the Fused Silica to cristobalite as possible during preheat and casting operations, and the higher preheat temperatures create more cristobalite. When the shell is cooled below 500 degrees Fahrenheit, the high-to-low cristobalite transformation takes place, which causes the shell to rupture. This makes the shell much easier to remove than if alternative materials were used. This statement holds true whether shell removal is mechanical, shot blast, water blast or caustic leaching.
   
   
3. Immunity from Thermal Shock
   The thermal expansion coefficient of Fused Silica is very low. As a consequence, a Fused Silica shell can be loaded directly into a preheat furnace at any temperature. As the shell slowly transforms to cristobalite, there is very little volume change and the thermal expansion coefficient remains very low.
   
   
4. Reduced Shell Weight
   Fused Silica has half the density of zircon, 55% of alumina and 83% of alumino-silicates. It is sometimes possible to use a thinner shell when using Fused Silica which lightens it further. Hand dippers appreciate this difference. In the case of robots, it is often possible to increase the number of trees on the fixture, or get by with a lower capacity, less expensive robot.
   
   
5. Increased Feeding Distance and Less Shrink Porosity
   In a Fused Silica shell of 6 coats, as an example, you can actually observe the metal rise in the shell cavity during the pour. This is because Fused Silica is 95% transparent in the visible spectrum and this continues into near infrared. The net result is that heat is removed from the casting much faster, resulting in greater temperature gradients in the metal. All things being equal, the larger the temperature gradient in the solidifying metal, the better the results. The casting will be more sound with better mechanical properties.
   
   
6. Refractoriness
   Fused Silica shells are much more refractory then any other shells. As long as enough temperature and time is available to transform a fair amount of the shell to cristobalite, it can be used at very high temperatures without experiencing mold bulge or excessive sintering shrinkage.
   
   
7. Reduced Hot Tearing
   In the casting of hot tear prone alloys, the hot crushing strength of the shell should be low. Tests have shown that all Fused Silica shells have roughly half the hot crushing strength of their alimino-silicate counterparts. This is partly because Fused Silica grains are very angular so the shell has considerable porosity and crushes easily.
   
   
8. Below OSHA Limits for Crystalline Silica and No Radioactivity
   Unlike zircon, Fused Silica is NOT radioactive at all. Fused Silica also contains NO measurable amount of the known carcinogen, crystalline silica. Alumino-silicates on the other hand, contain 0.3% to 28% of crystalline materials depending on the one selected. Consequently, using Fused Silica makes exposure of personnel to dust during slurry makeup less of a problem and less of a hazard. During shell knockout however, as ALL shell materials contain substantial amounts of cristobalite, strict compliance to OSHA Limits and requirements for personal protective equipment (PPE) are recommended.
   
   
9. Slurry Stability and with No Leach Out
   Zircon, alumina and alumino-silicates all contain multi-valent cations that can leach into the slurry and cause micro-gelling of the colloidal silica binder. This results in weak shells and reduces the useful life of the slurry. There is nothing in Fused Silica to leach out.
   
   
10. Expansion Matches Ceramic Cores
    Fused Silica shells are essentially the same composition as ceramic cores. Both start out as Fused Silica and partially transform to cristobalite. Consequently, you need not be concerned about floating ceramic cores with slip joints, which is always a questionable proposition.
    
    
11. Lower Heat Capacity and Better Fill of Thin Cross-Sections
    The heat capacity of Fused Silica is half that of zircon and considerably less than that of alumina or alumino-silicates. As the molten metal moves through the shell, less heat is lost to the shell and thinner cavities can be successfully filled without increasing the pour temperature.
    
    
12. Lower Stresses on the Shell During Autoclave or Flash Dewaxing
    Many alumino-silicate shelling materials actually have a very high thermal expansion in the 250 to 375 degrees Fahrenheit temperature range. This is because they contain considerable cristobalite, which undergoes a rapid phase transformation in this range. In autoclave dewaxing, the temperature difference between the outside of the shell and the wax/shell interface peaks at about 45 to 60 seconds after introduction of the steam. This temperature gradient is enough to cause shell cracks to develop in the inside of the shell cavity if a high expansion alumino-silicate is used. These shell cracks will normally propagate through only a few dips layers, but the later expansion of the bulk of the wax pattern will push the cracks further through the shell wall. With Fused Silica and its very low thermal expansion, these stresses are negligible.
    
    
13. Increased Permeability for Liquid Wax during Dewax and Later for Air during Casting
    Tests have shown that Fused Silica shells have markedly high permeability. This is due to the more angular nature of the Fused Silica grain and flour particles. The Fused Silica shell structure is less densely packed and the porosity is higher. A more permeable Fused Silica shell allows the melted wax to penetrate the shell during autoclave or flash dewaxing, thus reducing the stress on the shell. In filling the shell cavity during casting, the air must have a means of egress or the backpressure will slow mold filing enough that non-fill results. A higher permeability shell is always a good thing.
    
    
14. Consistent Gap Formation between Shell and Solidifying Casting
    As the shell temperature increases after casting, shell walls will bow toward the hot side (inside of cavity). This makes castings with thin walls relative to print. As the temperature gradients through the shell wall levels outward, the shell will move outward. This opens a gap between the still very hot, but solid metal and the shell, markedly reducing heat transfer. With a Fused Silica/ Cristobalite shell, the thermal expansion is much less so thin walls and air gaps do not develop. The result is a more consistent microstructure (sometimes a deleterious phase is avoided), less micro-porosity and improved mechanical properties.
    
    
15. Increased Shell Toughness in the Hot-Wet Environment of the Autoclave
    Tests have shown that all Fused Silica shells have better toughness (Kic) and reduced slow crack growth rates in a simulated autoclave environment (hot-wet) than do alumino-silicate shells.
    
    
16. Improved Leachability in Molten Caustic or Hot Caustic Solutions
    Alumino-silicates and zircon do not dissolve in caustic (molten or solutions) at a significant rate. Consequently, a shell containing substantial, if not all Fused Silica, is necessary for satisfactory shell removal by these methods.