Porcelain furnaces
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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Word of the day
| English | German |
|---|---|
| symphyseal | symphysisch |
Focus text of the month
Spray mist ejection Spray mist ejection
When compressed air is added and the water collides with rapidly rotating instruments, so-called "spray mist", i.e. (mostly contaminated) aerosols are created. They consist of finely distributed water droplets ("mist") which surround the patient's mouth up to a radius of 1.5 m during, and for some time following, the treatment. Spray mist also contains a large number of further constituents such as solid particles from the various materials employed in and reduced from the mouth and tooth structure as well as liquids such as saliva or blood containing all types of pathogen (bacteria, viruses, fungi). Although no longer visible, when atomised to form a "cloud", the minute single droplets are especially apt at contaminating surfaces of objects in the surgery, the skin of persons in the vicinity or for being inhaled ("aspirated"), swallowed or absorbed via membranes, e.g. the eyes. Where spray mist is concerned, the most important objective of dental hygiene, i.e. avoiding contamination and preventing cross-contamination, is achieved by providing surgery staff with passive protection via a mouth/nose mask and protective glasses. The stream of air/water is guided accurately and extracted which ensures that all constituents are removed as completely as possible from the mouth immediately after being released, which shrinks the "spray mist cloud" considerably but without compromising the desired effects. Within a few years of high speed, water-cooled dental handpieces being invented in the mid-20th century, suitable suction devices such as a mobile suction pump (1955) and a suction unit specifically designed for extracting spray mist when treating reclined patients (1961/1964) were developed.
Modern spray mist extractors include integral or external modules for separating air, water and solids (such as amalgam particles). The flow rate ranges from several hundred to one thousand l/min. Innovative technology (for example, the radial motor or electronic controller) greatly reduces the required space, energy consumption and noise level. Whereas previously "suction machines" were installed in the basement, nowadays they can be used in close proximity to the treatment zone. The suction effect generated in dental units is applied via flexible suction hoses (some can be adjusted or closed off with slider valves) with adapters for fitting rigid, standard diameter (mostly 16 mm for spray mist) saliva ejectors which are normally for multiple-use and autoclavable. The ejectors (available in different sizes for children and adults) include gripping surfaces and their ends are usually formed like a flat pelotte for holding back soft structures. To prevent suck-back of liquids if the ejector becomes totally blocked it may include permanently open side air-inlets. Suction hoses for surgical purposes (often with sterile, disposable cannula) or saliva ejection (often with non-sterile disposable cannula) usually have smaller diameters. Regular rinsing, cleaning and disinfection of the entire suction system using purpose-made, non-corrosive solutions is an important measure for avoiding blockage, damage and formation of contaminated biofilms (germ colonisation). |
Spray mist ejector
Spray mist ejector viewed from above