Porcelain furnaces
Modern, 21st century porcelain furnaces are technically sophisticated, electronically-controlled devices with programmable cycles for firing dental porcelains. These include metal-ceramics for firing onto metal frameworks (classic precious or non-precious alloys, titanium) or all-ceramics such as zirconia or lithium disilicate. All-ceramic inlays or laminate veneers can be fired directly onto refractory model dies.
The principle unit of a porcelain furnace is its refractory firing chamber. Once the porcelain has been built up, the restorations can be placed onto mesh, cones, pins or firing pads for firing.
The heating coils are usually located in the upper housing of the furnace and arranged concentrically around the restoration. A motor-driven mechanism closes the firing chamber with the restoration inside, either by raising the firing platform or lowering the upper housing of the furnace. The firing cycle settings depend on the material being fired/procedures and run according to pre-set, standardised or custom programmes.
Many settings can be programmed precisely and independently of each other, for example times can be set to the split second (preheating/drying, heat-rate, hold-time, cooling) and firing temperatures for various materials such as opaquer, shoulder and dentine porcelains as well as glaze firings programmed accurately.
As the only way of preventing undesirable opacity in the porcelain is to evacuate the firing chamber during firing (vacuum phase), a built-in powerful vacuum pump is an essential part of a porcelain furnace.
Porcelain furnace
Combined firing/pressing furnaces are used for fabricating pressed-ceramic restorations (pressing procedure resembling casting which makes use of pressure and heat to liquefy ceramic blocks and force them into lost, refractory investment moulds) using special firing chambers and pressure plungers.
Whereas glass infiltration firing of presintered ceramic is possible in a porcelain furnace ("infiltration firing"), special high temperature sintering furnaces are required for the actual sintering process (such as for zirconia).
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dentine shade | Dentinfarbe |
Wax build-up technique Wax build-up technique The various anatomical structures (such as cusp tips and slopes as well as marginal ridges) are usually built up one after another by adding small portions of wax (often using differently coloured waxes for didactic purposes). The firm, special waxes first have to be melted at room temperature. This can be carried out by warming small portions on differently shaped working tips of hand instruments in an open flame (such as a gas burner) or using electrically heated instruments which provide for more accurate temperature control and avoid contamination (e.g. electric wax-knife, induction heaters, wax dipping units). The wax is applied drop-by-drop to ensure that the warmer molten wax added last fuses seamlessly with the firm, cooler material. After hardening, the wax pattern can be reduced by sculpting, milling guidance surfaces or drilling to add retainers. Modern procedures include flexible, occlusal preforms for adding contours to soft wax. In addition, wax preforms, such as for occlusal surfaces or bridge pontics, are available in various shapes and sizes. Recently, irreversible, light-curing materials have been introduced for use instead of reversible thermoplastic waxes. Wax preforms To ensure that the wax pattern can be released without being damaged, model surfaces, opposing dentition and preparations must be hardened/sealed with special lacquer (applied by spraying, brushing or dipping). These waxes are mostly relatively rigid/elastic after cooling. Attaching wax sprues to a removable framework supported on double crowns using a hand instrument When employing the lost wax technique, prefabricated wax sprues, bars and reservoirs are attached to the patterns. Once the pattern has been released and its sprues waxed onto the crucible former, it is invested in a casting ring with refractory investment material. The wax can then be burnt out residue-free and casting completed. Unlike standard wax build-up techniques, a diagnostic wax-up is not intended for fabricating an indirect restoration, but rather for simulating the appearance and/or external contouring for producing orientation templates. |