Abrading – Work holder
Reexamination Certificate
2002-10-02
2004-05-25
Shakeri, Hadi (Department: 3723)
Abrading
Work holder
C451S397000
Reexamination Certificate
active
06739959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an assembly for the manufacture of shaped parts from ceramics, in particular for medical or dental-medical purposes.
2. Description of the Related Art
In medical engineering, dental medicine or dental engineering, prosthetic parts have hitherto been manufactured mainly from high-quality precious metal alloys, cobalt chromium alloys and partially also from titanium. Due to the necessary bio-compatibility of such medical products, the metal surfaces of these prosthetic parts are usually coated with ceramic substrates. In dental medicine and dental engineering a veneering with dental porcelains, in particular in the anterior tooth region, often takes place for mainly aesthetic reasons.
For a fairly long time, efforts have been made to substitute these metal alloys with completely ceramic systems. However, this requires the use of high-performance ceramics, as are used in industrial ceramics partially with industrially manufactured serial products. In contrast with the industrially manufactured ceramic parts, however, the prosthetic workpieces manufactured in medical engineering or dental engineering are in each case unique, which is why, for economical, material-technical and industrial reasons, the known industrial production methods cannot be used. A further known problem of such high-performance ceramics is the partially great contraction of the ceramic pastes during sintering, which can amount to up to 20%. However, such dimensional changes cannot be tolerated with dental-technical shaped parts, because, for example with bridgework, the distances between the columns (stumps) or the height of the contact points to the antagonist, must be kept within the micrometer range.
For this reason, several attempts have been made to machine prosthetic parts from solid ceramic blocks, the sintering of which is complete (semifinished material), directly by way of machining with geometrically defined cutting edges, machining with geometrically undefined cutting edges or by way of erosion by means of ultrasound erosion or lasers.
The grinding-out or milling of ceramic parts with the aid of diamond tools, for example with the so-called CAD-CAM method, has become generally accepted in practice—although only to a limited extent. With this method, first of all a measurement is taken of the tooth stump, and subsequently of the crown provided thereon, which is available, for example, as a wax model. The data is then entered into a CAD program which controls a milling machine. This milling machine then automatically machines the sintered and high-strength ceramic block. However, expenditure for this is extremely high because the ceramic block which is already sintered is extremely hard. If, for example, high-performance ceramics such as aluminum oxide (Al
2
O
3
) or zirconium oxide (ZrO
2
) are ground, the diamond tools wear out very quickly, resulting in geometry tolerances on the workpiece because the geometry and the diameter of the tools change during the machining. Moreover, at critical points of the prosthesis parts, for example at crown edges, material eruptions or micro-tears can arise. A further problem is represented by the long grinding time, because work can only be done at low eroding rates and at reduced rate of feed, because otherwise great stresses in the material can arise, which can lead in turn to hairline cracks and the like. Moreover, a separating step is necessary, where, at the end of the machining, the milled-out crown is separated from the rest of the ceramic block. With this manual separating and grinding procedure, both geometry errors and material eruptions can arise, with the result that the expensively manufactured part can possibly no longer be used. Finally, the machining of high-performance ceramics requires expensive and automatically operating grinding or milling machines because the dental technician or machine operator can no longer manually control the machining parameters (feed, delivery) at all. Alternatively to the high-performance ceramics, modified dental ceramics can, admittedly, also be used, which permit a still economical grinding machining, but these modified ceramics then also only have reduced strength values.
Alternatively to the machining of ceramic blocks which are already sintered and which are high-strength, methods have therefore been developed, where the ceramic shaped parts are manufactured from a ceramic raw material which is not yet sintered or from presintered material. With two known methods, first of all an impression is made of the machined tooth stump and then a positive is formed, which for its part consists of fire-proof material, in particular ceramics. A tooth crown of wax is formed on to this positive stump, the tooth crown of wax simulating the final shape of the crown. Subsequently, the positive stump, with the wax crown located thereon, is placed upon a rubber base which forms the floor for a rubber ring, with this rubber ring surrounding the positive stump with the wax crown with clearance. A liquid or plastic embedding mass is then introduced into the muffle form formed in this way, the embedding mass surrounding the positive stump with the wax crown apart from a pouring channel. This pouring channel is formed, for example, by a wax cone which is connected to the wax crown.
After the hardening of the embedding mass, the rubber foundation and the rubber ring are removed, so that the hardened embedding mass with the filling wax cone is freely available. This unit is then put into an out-waxing and preheating furnace so that the wax of the wax crown is expelled by way of the filling vent. In this way a cavity corresponding to the wax crown is formed in the embedding mass.
The embedding mass with the cavity located therein and a sintered porcelain blank are then introduced together into a preheating furnace and heated to about 800° C. At this temperature the sintered porcelain blank becomes plastic, whereas the embedding mass itself hardens. After the removal of the embedding mass and the plastic porcelain blank from the furnace, the now plastic porcelain is introduced by way of the filling opening into the cavity by means of a pressing device. This pressing-in itself takes place in a special pressing-in furnace. After the cooling of the porcelain mass, the embedding mass is then destroyed so that the crown with the filling vent located therein becomes free. As a concluding step, finally the separation of the crown from the filling vent, which has arisen in the pouring channel, and a final external machining take place.
With this method also there is the danger that tears can arise in the crown upon separation of the porcelain crown from the filling vent. The use of an already sintered porcelain blank guarantees that no more shrinkage occurs with the crown if it is subsequently fired again in the baking oven. In contrast with ceramics, porcelain which is already sintered can again be plasticized upon heating to about 800° C., this no longer being possible with ceramics even at extremely high temperatures. However, compared with porcelain, ceramics have considerably greater bending strength. The manufacture of two or more crowns which are connected to each other by way of a connecting bar, is, for example, not possible with porcelain, because this connecting bar would break. Such complicated dental-technical shaped parts can therefore only be manufactured with ceramic material. This is the reason why porcelain is usually only used for inlays, onlays or single crowns.
The method currently most widespread in dental engineering for the manufacture of ceramic crowns is the so-called slip method. In this respect, an impression is first of all made of the machined tooth stump and then a metal frame, in particular of gold, titanium or the like, is prepared. This metal frame consists of a thin layer fitting the tooth stump and finally produces a cup-shaped part. Ceramic material is then applied to this frame in plastic form (slip) in several layers, with a fi
Bodenmiller Anton
Steinhauser Pius
Kaltenbach & Voigt GmbH & Co.
Shakeri Hadi
LandOfFree
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