< REFRACTORY SANDS & BINDERS
SODIUM SILICATE BONDED PROCESS (CO2)
Sodium silicate bonded sand is perhaps the most common sand moulding system used in art foundries. A finely graded dry SILICA refractory sand is normally selected for this process, with particle sizes typically in the region of 200µm diameter. Before use the sand is mixed with a pre-determined quantity of a sodium silicate binder. Sodium silicate (also known as WATERGLASS) is a water soluble vitreous material suspended in SODA (a strong alkaline), this results in a thick, viscous syrup like substance.
The most efficient means of mixing sand with this binder is via a MILL, which automatically injects appropriate levels of the binder into the free running sand as it passes along a WORM (a revolving screw). Ejected from the mill’s outlet, the mixed sand has a moist, plastic like consistency, which is easily moulded and compacted around a MASTER PATTERN.
Though sodium silicate bonded sand will eventually harden through exposure to air (the water component in the binder evaporates, setting the sand), waiting for this reaction to complete in ambient conditions would be quite unsatisfactory given the extended time lapse between moulding and hardening. To contract moulding times, a CATALYIST is introduced into the sand which both accelerates and controls the hardening process. The catalyst for sodium silicate sands may be in the form of an added ESTER or powder (FERROSILICON), which is injected into the sand during milling. The addition of the ester creates a thermo-reactive SELF SETTING moulding medium with a relatively short hardening time [ref]. However, by far the most popular catalyst for the hardening of sodium silicate bonded sand is pressurised CARBON DIOXIDE gas or CO2.
After the foundry moulder has completed a section of the mould, a needle like probe is inserted into the still plastic sand piece to create a vent or series of vents. Carbon dioxide gas (the same gas used for carbonating soft drinks and shielded MIG welding operations), is supplied to a GAS GUN via a cylinder and regulator system – broadly similar to the flow control system used in gas welding processes. The gas gun is then inserted into each vent on the sand piece and carbon dioxide released at a specified pressure for a pre-determined period of time – variable according to the volume of the sand in the mould. Large sand volumes are sometimes needle vented all over, then fitted with a flask cover plate, enabling the enclosed mass to be gassed off by a single CO2 injection.
By repeating the gassing off process for each moulded section, the individual elements of the mould can be hardened to a point that allows them to be removed or STRIPPED from around the pattern without breaking. The hardness of a fully set foundry sand can be close to that of some naturally occurring sand stones. As a consequence, large blocks of sand can be very robust, and difficult to break up. Indeed, such is the strength of many artificially bonded sands, that it is quite possible to make a FLASKLESS MOULD, where moulded sand pieces are simply glued together and/or reinforced with a steel pallet strapping, thus dispensing totally with an outer containing flask.
Sections of spent sand can be exceptionally difficult to remove from around a formed metal cast during the KNOCK OUT stage, this can can cause problems, especially when clearing sand from a CORE MASS. Potentially problematic sections like the core often have an added ingredient called a BREAKDOWN POWDER processed into the sand during milling operations. Breakdown powder (often coloured for easy identification), contains an organic compound, which in the case of sodium silicate bonded sands is a sugar (DEXTROSE) based substance. Breakdown powder reacts with the heat evolved in the mould after a metal pour to degrade the chemical bond between the sand and binder. This reaction makes the removal of refractory sand cores far easier than might otherwise be the case.
Whilst considered old fashioned and dated for most commercial founding operations, sodium silicate bonded system is a straightforward process to use, allowing the art founder to produce good quality metal casts at moderate materials expense. Both the materials used, and the process itself are regarded as being versatile and tolerant of operator error (for instance in a minor miscalculation of binder proportion, or injected catalyst times). Once hardened, the sand cannot be reused without expensive recycling equipment, so spent sand is often disposed of after casting.
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