JEWELLERY REFRACTORY MATERIALS & PROCESSES

1 THE BINDER & ADDITIVES

The most common binder for jewellery and dental refractories is calcium sulphate hemihydrate, the same basic compound found in traditional plaster of Paris block moulds for sculpture casting. Combined binder/refractory jewellery materials are generally supplied as a ‘single pack’ powder mix. The binder content (along with any modifying additives), usually comprises upwards of 30% of the total material, though this is variable according to casting alloy used. Reproductions formed in metal alloys with high melting points (nickel for instance), may be composed of refractories bonded with either sodium phosphate, or ammonium diacid phosphate. Both phosphates create a stronger and more detail sensitive (if expensive) mould, than one formed using a calcium sulphate binder. All the binders are activated by the addition of purified water, though sometimes the phosphate binder is mixed with a SILICA SOL, the same water based colloid used in CERAMIC SHELL systems [ref].

2 THE REFRACTORY

The refractory body component of a jewellery lost wax mould is usually based on crystobalite, which together with quartz and tridymite is a form of mineral silica. The refractory content for a mould of this type is in the region of 70% by weight. Supplied as a proprietary binder/refractory mix, the refractory requires no other preparation for use by the founder other than mixing with water or silica sol to wet and initiate a reaction and setting process. A range of refractory moulding products are available for use with various ALLOYS and casting techniques.

REFRACTORY MOULDING OF WAX PATTERNS

Designs destined for jewellery casting are typically very small in scale, but often reproduced in very large quantities. As a result, it makes sense where possible to mount large volumes of individual wax copies in a cluster on a single SPRUE (runner), this type of wax assembly known as a TREE. Cluster mounting  on a tree allows a great many copies of a given design to be densely distributed and cast within a single block mould, making this an efficient and economic use of otherwise expensive refractory materials.

The ‘painted on’ and ‘dipped’ fine refractory application techniques described previously for CERAMIC SHELL and PLASTER & GROG moulds are almost impossible to apply successfully to a sprue tree cluster (due to the very dense spacing of multiple wax patterns). Access to the wax pattern surfaces is often restricted and any air pockets between the mounted waxes would either expand and break up the mould block during kiln firing, or else cause molten metal to run between the pattern impressions to produce inferior reproductions. For both VACUUM and CENTRIFUGAL casting processes, it is vital that the prepared refractory sets as ‘cleanly’ as possible around the wax assembly; any repair work in metal to these small casts is usually problematic and rapidly becomes uneconomic. The following stages describe the basic methods used to create a jewellery type refractory mould for vacuum and centrifugal casting.

1 FLASKS

Reminiscent of the linoleum containers often used to form poured block moulds, the liquid refractory used in jewellery processes is similarly contained by a retaining wall around the sprue tree. In this case though, the restraining wall is a metal container, known to as a FLASK. The flask is an open ended tube, sometimes with a flange around the top section for accurate location into casting apparatus. Unlike the temporary linoleum containers used for forming traditional plaster & grog moulds, the metal flask remains in place around the jewellery refractory until the mould is broken out after casting. The flask may also be perforated to allow a more even distribution of vacuum force over the mould during casting. Exposed to high temperatures and thermal shock, the flask is robustly constructed from a heat resistant stainless steel, allowing it to be reused on numerous occasions.

2 INVESTMENT PREPARATION

The wax tree is mounted on a BUTTON former, which later automatically impresses a cup entrance (button)into the refractory mould. The tree is then degreased with alcohol to assist refractory WETTING. An appropriate sized flask is selected to surround the wax assembly, leaving a small air gap between the outermost wax patterns and flask wall. The flask is sleeved over the inverted sprue tree and located onto the button former, which also acts as a cover on the  base of flask to prevent the escape of fluid refractory. Dry refractory powder is carefully measured out to a quantity that will adequately fill the flask without too much wastage. A quantity of water or silica sol is then added to the refractory powder in a mixing bowl to activate the preparation. The proportion of refractory powder to water or sol is critical if the mould is to achieve it’s full strength and refractory value. Typical proportions for jewellery moulds are in the region of 40% water/sol to 60% refractory/binder powder. The fluid refractory is then mechanically mixed for a short period of time to ensure an even distribution of the powder throughout.

3 DEGASSING THE INVESTMENT

To ensure that no air bubbles are present in the refractory preparation, the liquid mixture is transferred to a hermetically sealed chamber and exposed to a vacuum atmosphere for some 30 seconds or so. This forces any trapped air out of the refractory mix.

4 INVESTING THE WAX

Once degassed, the refractory can be carefully decanted into the flask containing the wax tree assembly. Perforated flasks (if used) are sleeved with a temporary sheath to prevent loss of fluid refractory through the holes. The flask may then either be vacuum degassed again, placed on a vibrating table, or else gently tapped to assist in the dispersion of any remaining air bubbles. Once the refractory is set, the flask is permitted to stand for some hours. Standing allows the refractory to obtain it’s maximum GREEN strength before progressing to the KILN and BURNOUT stages.

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