Bonding sodium silicate and sand: the most typical of all sand-casting or moulding systems used by art foundries. The process uses finely-graded and dry silica sand as a refractory agent, with the particles typically being 200µm in the diameter department. A predetermined amount of some sodium-silicate binder is added to the sand mixture before use. Sodium silicate, or water glass, is water-soluble and is a vitreous material kept in soda, i.e., a strong alkaline. This blend results in a viscous, syrup-esque substance.
A mill is the most effective method of mixing the sand and binder together. It automatically injects the binder in appropriate amounts into the sand as it revolves around a worm, i.e., a rotary screw. After this process and out the outlet, the mixture becomes a moist, plastic-esque substance, making it easier to mould with the maquette or master pattern.
Waiting for the mixture to harden through evaporating the water in the binder under ambient conditions is something you could do. However, it is prohibitively slow, so to shorten the time considerably, we introduce a catalyst into it, which performs two functions: it controls and accelerates hardening. So the mill also injects an ester or a powder, e.g., Ferrosilicon or any other catalyst, into the sandy mixture. This addition forms a thermo-reactive and self-setting mould medium that is comparatively quick to harden.
However, the predominant hardening catalyst is not an ester, but pressurised CO2, also known as Carbon Dioxide.
After the moulder completes a part, they insert a needle-esque probe into the plastic sand section, creating a group of vents, or even vent singular. Moulders fill a ‘gas gun’ with CO2 using a cylinder and regulator system. They use the now loaded gas gun to release carbon dioxide into each and every vent at a precise pressure for a pre-planned unit of time according to the variable of sand’s volume when finally with the mould. When dealing with large sand sections, the moulder needle vents everywhere, then fits with a casting flask, enabling the gassing of this enclosed entity with a single carbon dioxide injection.
By gassing off each section, all the mould’s individual ingredients can harden to the point that facilitates their removal or stripping from the pattern without tearing or otherwise breaking. The mixture can get as hard as some sandstones, so consequently, big blocks of it can be pretty robust and a tad hard to break up. With such strength, it is quite possible to have flaskless moulds, where the pieces are simply glued together and possibly reinforced with steel-pallet straps.
Removing the sand can be excruciatingly tricky to accomplish during the inevitable knockout stage, especially from the core mass. During milling, an extra ingredient is explicitly added to the core called: Breakdown powder, which is often coloured for ease of identification. Such powder has an organic compound, in this case, a sugar, i.e., Dextrose, based substance. The powder reacts with heat, which stems from the metal pour, which degrades the chemical bonds between the binder and the sand. The reaction acts as a catalyst for removing the refractory sand, making the procedure an easier one.
This process is not a new thing and is even considered a bit too old for commercial founding purposes. However, this is the most uncomplicated process, allowing founders to produce high-quality metal casts at a moderate material expense. These substances and the procedures are versatile and quite tolerant of user/operator error, e.g., minor miscalculations. However, after hardening, the sand mixture cannot be recycled without expensive equipment, meaning most dispose of the spent sand after casting.