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 |
|---|---|
| dentine-like | dentinähnlich |
Focus text of the month
Implant superstructures Implant superstructures
A superstructure can be purely implant-supported or supported both on teeth and implants. Particularly with bridges the term used is hybrid or tooth/implant-borne.
Cemented superstructures are differentiated between provisional (temporary), definitive (permanent) and semi-permanent cementation. The latter should enable secure retention and at the same time removal of the superstructure by the dentist, if necessary. This is referred to as an operator-removable (i.e. fixed for the patient) superstructure and also includes screw-retained superstructures. Both types of retention have advantages and disadvantages: Screw retention creates gaps, which can be colonised by bacteria. To prevent this special gels, intended to remain effective in the long-term, are supplied for applying in the interior of the implant. If superstructures are screw-retained, they can also fail due to loosening, overloading or fracturing. As implants are not self-mobile and are anchored rigidly in the alveolar bone the aim is always to ensure a passive fit of the superstructure. With screw-retained superstructures the passive fit can be checked on a minimum of two abutments using the Sheffield test (gap-free fit when tightening any individual screw). Procedures for intraoral bonding (such as adhesive bonding) of components of the superstructure and/or digital fabrication (e.g. milling, sintering) are used to fabricate stress-free frameworks. To avoid inaccessible excess cement, which could result in peri-implantitis and implant failure, the restoration margin of cemented superstructures should always terminate in the region of the gingival margin. This allows fabrication of implant platforms mainly at bone level with corresponding (if necessary, custom-fabricated) abutments. Abutments are used as a connection between implants and superstructure. Angled abutments enable parallelisation to produce a common path of insertion with superstructures on several abutments. Abutments can either copy the form of a tooth preparation or include a component of a connector (e.g. press-stud systems, ball abutment, bars, magnets). In these cases the superstructure incorporates the respective complementary components. |
Full upper and lower dentures (fitting surface with ball sockets)
Implant-borne metal-ceramic crowns