< 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 used for this process (particle size about 200µm dia). Before moulding, the sand is mixed with a proportional quantity of a sodium silicate binder. Sodium silicate [WATERGLASS] is a water soluble vitreous material suspended in SODA (a strong alkaline), resulting in a thick viscous syrup like substance.
The most efficient method of mixing sand with the binder is with a MILL. More sophisticated machines automatically inject the correct quantity of binder into free running sand as it is pushed along a WORM (revolving screw). Ejected from the mill outlet, mixed sand has a moist, plastic like consistency which can be easily moulded and compacted around a MASTER PATTERN.
Though sodium silicate bonded sand will eventually harden with 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. To contract moulding times, a CATALYIST is added into the sand which both accelerates and controls the hardening process. The catalyst for sodium silicate sands can 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 sand 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 given section of the mould, a needle like probe is inserted into the still pliable sand section to create one or more vent inlets. Carbon dioxide gas (the same gas used for carbonating soft drinks and shielded MIG welding operations), is fed to a GAS GUN via a gas 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 volumes of sand are sometimes needle vented all over, then enclosed 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 an internal CORE MASS. Potentially problematic sections like cores often have an added ingredient called a BREAKDOWN POWDER processed into the sand during milling operations. Breakdown powder (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 modern commercial foundry operations, sodium silicate bonded system is a straightforward process to use, allowing the art bronze 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 example, 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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