The procedure for determining and controlling silica content in colloidal silica slurry follows: Determination of soluble silica in slurries
1. Carefully decant the liquid portion from the sample of the slurry to be tested.
2. Centrifuge the decanted liquid until a relatively clear supernatant is obtained.
3. Determine the specific gravity of the supernatant using a specific gravity bottle.
4. The SiO2 content of the binder may then be determined by referring the specific gravity reading obtained in step 3 to a curve which shows the relationship of specific gravity to soluble silica content.
Results The results of this test may be used to control the slurry as follows:
a. Slurry should be discarded if the SiO2 concentration of the liquid is less than that of newly made slurry.
b. Distilled water should be added to the slurry if the soluble silica concentration is greater than starting concentration.
Obviously, water loss has occurred through evaporation. If only binder is added to the slurry at every adjustment period, the silica concentration would increase to the point of instability and cause weak shells. SHELL DRYING Some investment casters are not aware of problems that may occur when drying shell molds. The major concern is usually processing time. It is important to understand what happens to shell molds during drying and what factors influence drying times. The materials used in processing the shell are not adversely affected by temperature and humidity changes.
Wax patterns however are, wax expands when heated and shrinks when cooled. Because of this, a major concern during the drying cycle is a drop in shell temperature while the mold cluster requires absorption of heat which will cause a reduction in temperature of the remaining system. This causes the reduction in the temperature of the shell and wax pattern, and with it a reduction in the pattern size. This effect can be illustrated by use of sling psycho meter. The psycho meter has two thermometers – one wet the other dry. As water evaporates from the wet thermometer its temperature drops. This is the standard method used to measure the humidity. For example: If the room temperature was 240F and the relative humidity 30%, the wet bulb temperature would be 170 F. As soon as a wax cluster has been dipped and stuccoed, it becomes a wet bulb thermometer. This results in a drop in temperature for the wax pattern. When the coating has dried, the wax surface returns to ambient (room) temperature.
The cycle is then repeated during drying of second coat. In this instance, the first coat is relatively rigid and the following can happen:
1. The wax pattern pulls away from the first coat allowing the second slurry to flow underneath the first coat. The first shell layer will then lift off the wax pattern causing it to buckle or spall.
2. The wax pattern pulls the coating along while it shrinks during cooling but when it resumes ambient temperature, it expands. This may results in a shell rupture or crack causes fins for veins on the casting In either case, there will be dimensional and or surface variations in the final casting as well as potential for ceramic inclusions.
Therefore, temperature is the first variable that must be controlled in the shell making process. The dip room temperature is never too closely controlled; however for most foundries + 10C is sufficient. Relative humidity is the second variable to control. The ideal humidity level is about 50%. Rapid Shell Drying systems The most effective rapid drying systems accelerate drying without any temperature change at the wax shell interface, minimizing contraction and expansion of the wax and shell. It would require large volumes of rapidly moving air. The temperature and humidity of the air should be controlled so that as the moisture content of the shell effectively decreases, the air temperature would also decrease to ambient conditions. This creates a zero net temperature change for the shell and wax during dry cycle. Such rapid drying systems are also known as Shell Producer Cells. These Cells are available in small semi-auto and large fully auto Conveyorised systems. A Mini-Shell-Producer-Cell could produce upto 20 shells a day while a large Conveyorised system would yield upto 500 shells per day. Many small foundries prefer several mini cells to maximize thru-put in a compact but reliable shell room layout in order to keep capital costs low. The Mini-Shell-Producer-Cell is a proven effective rapid drying system. Summary Colloidal silica may be used to produce ceramic shells with fused silica, alumino-silicates, zircon, alumina and zirconia refractory’s.
They have good green and fired strengths. The following precautions for maintaining good slurry properties must be taken:
1. Maintain pH within the stable range of 9.0 – 10.2 for water based sols.
2. Do not use slurry additives that are cationic or anionic.
3. Ensure that pattern materials are dry and free of alcohol, or other cleaning solvents.
4. Establish and use process controls for slurries.
Do not allow the dispersed silica phase to concentrate to levels that approach gelling. 5. Use of rapid drying systems ensures higher and more reliable throughput.