< HYBRID INVESTMENT
JEWELLERY INVESTMENT MATERIALS
1 THE BINDER & ADDITIVES
The most common binder in jewellery and dental refractories is calcium sulphate hemihydrate, the same basic compound found in traditional plaster of Paris block moulds for sculpture casting. However, unlike most investment systems used in art foundries, the material supplied to the founder for use in this process is already combined with the main refractory component as a ‘single pack’ powder mix. The binder content (along with any modifying additives), usually comprises upwards of 30% of the total investment material, though this is variable according to casting alloy used. Reproductions formed in metal alloys with high melting points (nickel for instance), may be produced from investments 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 mentioned are activated through the addition of purified water, though sometimes the phosphate binder is mixed with a SILICA SOL, the water based colloid used in ceramic shell systems [ref].
2 THE REFRACTORY
The refractory body component of a jewellery investment is usually based on crystobalite, which together with quartz and tridymite is a form of mineral silica. The refractory content of a typical investment mix is normally 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 investment products are available for use with various alloys and casting techniques.
INVESTING THE WAX PATTERN
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), creating a 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 investment block mould, making this an efficient and economic use of otherwise expensive investment materials.
The ‘painted on’ and ‘dipped’ techniques described previously for ceramic shell and plaster/grog moulds, are almost impossible to apply successfully when investing a sprue tree cluster due to the very dense spacing of multiple wax patterns. Access for coating the wax pattern surfaces is often restricted and any resulting air pockets between the mounted waxes would either expand and break up the investment block during the kiln burn out, or else allow cast metal to run between the pattern impressions to produce inferior reproductions. For both VACUUM and CENTRIFUGAL casting processes, it is vital that the prepared investment sets as ‘cleanly’ as possible around the wax assembly as any repair work to these small casts is usually problematic and rapidly becomes uneconomic. The following stages describe the basic methods used to create a jewellery type investment mould for vacuum and centrifugal casting.
1 FLASKS
Reminiscent of the linoleum containers often used to form poured block moulds, the liquid investment 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 when making make traditional investments, the metal flask remains in place around the jewellery investment 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 investment mould. The tree is then degreased with alcohol to assist investment 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 investment. Dry investment 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 investment powder in a mixing bowl to activate the preparation. The proportion of investment powder to water or sol is critical if the mould is to achieve it’s full strength and refractory value. Typical proportions for jewellery investments are in the region of 40% water/sol to 60% refractory/binder powder. The fluid investment 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 investment 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 investment.
4 INVESTING THE WAX
Once degassed, the investment 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 investment 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 investment is set, the flask is permitted to stand for some hours. Standing allows the investment to obtain it’s maximum GREEN strength before progressing to the kiln and burnout stages.
INVESTMENT – A SUMMARY >
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