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.