Silica sol in the Investment casting foundry industry
Narcol INV30 Aqueous Colloidal silica is a popular binder used in the investment casting foundry industry today. It is safe, economical, and easy to use. WHAT IS COLLOIDAL SILICA? Colloidal silica is a stable dispersion of millimicron size SiO2 particles in water. The silica particles are non-agglomerated, spherical in shape, negatively charged, and stabilized with a counter ion. Refractory flours and colloidal silica are mixed to form ceramic slurries for producing shells. These slurries are stable mixtures and have been used six months or more on instance. The slurries are destabilized by impurities and conditions leading to form an irreversible gel. Shells made from unstable slurry leads to a weak shell and causes casting quality problems. Stability of Silica Sols: The discrete silica particles in a sol begin to form agglomerates when destabilized. Such an unstable sol loses its ability to form a complete bond. Slurries of unstable sols results in weak shells and surface imperfections on the casting. Slurry with agglomerated sol cannot always be detected because the gel is continuously broken during mixing. Once the slurry has begun to gel, the rate is unpredictable and cannot be controlled. A simple test to determine if slurry has gelled is to let a sample stand undisturbed for 8 to 12 hours in a sealed bottle @ 80C in a oven. The slurry should be discarded if the total sample becomes solid. Another method is to centrifuge a sample of decanted slurry to separate the refractory from the sol. The sol density is checked using a sp.gr. bottle. If the sp. Gr. is lower than what originally was then binder system may be gelling and must be discarded.
Colloidal silica sols preferred for Investment Casting generally have the following properties: Sio2 30% Specific gravity 1.20 @ 250 C Particle size 9-12 millimicrons Surface area 300 to 350 sq.m/g pH 9.5 to 10.2 @ 250 C Viscosity 12 -13 seconds Ford B-4 cup @ 250 C. Alkali, as Na2 0 0.35 to 0.5 % Chlorides 0.01 % The following greatly affect the stability of silica sols: • Ph modifiers • Soluble salts • Alcohol • Temperature Ph modifiers Silica sols are most stable in a ph range of 8.5 – 10.5. When the pH is slowly raised or lowered to or through the neutral range, the sol. becomes unstable and eventually gels. The base or acid used to raise or lower the pH of a sol will usually form a soluble salt which will also affect stability. Soluble salts Soluble salts ionize in water and affect the negative charge on silica particles in suspension. This reduces the stability of the binder. The effect becomes more pronounced when the pH of the binder is close to the neutral range. Wetting agents and inoculants usually destabilise slurries. Wetting agents are anionic, cationic or nonionic. Anionic and cationic materials dissociate to ionic form when dissolved in an aqueous media. These may neutralize the charges on the silica particles and start a gelling action. Careful selection and use of these products is required. Inoculants, used as grain refiners, generally carry impurities which affect the stability of the slurry.
A safe practice is to use products of the highest purity. Alcohols The effect of alcohol on the stability of a sol is important for those foundries that use alcohol in their pattern cleaning process. Pattern cleaned in any alcohol cleaning system must be thoroughly dried before applying slurry. Alcohols such as Isopropanol, Methanol, and Ethanol cannot be tolerated by alkaline water based sols. Temperature Effects Cooling silica sols to freezing temperatures causes irreversible coalescence of silica from the suspension. The freezing water concentrates the colloidal particles in such a way that causes them to gel. Slurry Controls Colloidal silica is used to bond many refractory systems for the manufacture of ceramic shell molds. Control of slurry properties is required for optimum results. The most widely used methods for controlling slurries are a) Viscosity b) Specific gravity. When new slurry is made, the correct amount of colloidal silica and refractory flours are blended and mixed until the system reaches equilibrium. At this point, the viscosity has stabilized. As time passes and the slurry is used, the original properties change. Water in the binder is lost because of evaporation increasing the solid : liquid ratio and the viscosity of the slurry will increase. If binder is added to the slurry to lower viscosity, the dispersed silica content of the binder phase increases even further. It becomes obvious at this point that distilled or deionized water must be added to the system in order to bring it back to its original state.
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.