Here we shall talk about the so-called dental welding, which is still actual for individual teeth technicians and large dental laboratories. Dental laboratories often deal with delicate and precision metal constructions with usual thicknesses 0,2-3 mm. During producing or repairing they need casting, assembling, heating, form correction and some other operations connected with usage of welding or allied processes, coupled with heating. In industrial developed countries the majority of teeth prostheses are manufactured by precision casting method. Nevertheless, in many cases and especially when prostheses are rather long or have complex structures and geometric form, dental technicians have to use joining of smaller and simpler parts of these prostheses.Among the usually used joining methods for dental structures there are such as mechanical (by screw, pressing, etc.), gluing, electroforming, welding, brazing, or soldering. However, for electroforming restoration Dr Michael Hopp (Ernst-Moritz-Arndt Universuty of Greifswald) refers to the following methods: brazing, cast on, sintering, lasering, luting and gold deposition.

Hence, according to this and other information - from the Moscow State Medical-Stomatology University (www.go-s.ru) - welding, brazing and soldering are still needed in dental laboratories which frequently have problems with prostheses joining due to:

That is why, having the dental laboratory in 66 working places, the Hospital Ortopedic Dentistry Department of the above mentioned University has been interested in such devices as our cheap, safe and simple sets.

From the 70s the former USSR microplasma welding had a good reputation for manufacturing of teeth prostheses. Among pioneers who began to use microplasma welding for dental purposes were Liubchevskii A.A., Shnaider B.I., Danchenko A.N., whose article "Microplasma Welding in Denistry (Experimental Study) was published in "Voen.Med. Zu." , 1973, May, 33-5 (in Russian) - www.pubmed.gov; www.ncbi.nlm.nih.gov/gquery. This welding permits gaps between joining elements up to 15% of their minimal thickness – without a filler material usage. It has been shown that brazed stainless steel false teeth produce inside, under a saliva presence, in a mouth cavity the so-called galvanic effect. This occurs because the filler material (for brazing-soldering) consists of such elements as silver, copper, cobalt, magnium, etc, with a different and some times high electrode potential. That is why galvanic currents reached 30 MKA. In order to avoid this effect the conclusion had been made – instead of brazing welding with the same filler material to use, that material in parts of teeth prosthesis. The following tests further illustrated that the welded prostheses are 4-5 times more durable than the brazed ones. Their structure and appearance were also much better, and galvanic current for welded prostheses is no more than 3 MKA.

Later in the 80s some successful attempts were made in microplasma welding of gold teeth prostheses. However, since it was necessary for them (independent of the used materials) to join parts with different thicknesses, the dental technicians had asked for brazing usage (for artificial teeth that are made from thin foils and rather thick castings). In relation to this Dr Michael Hopp suggests: "Only objects, that have a uniform thickness can be produced with electroforming technology. Unequally fat objects are not consequently possible like bridges or clasp retained prostheses frameworks. The thickness of objects is limits with good quality in the dental area on approximately 1 mm". Due to his information there are problems with laser welding of such objects. This method has many advantages as could be seen in works, for example, of dentistry schools of Milan and Showa (Japan) Universities, and first of all - the minimal dimensions of heating zone. At the same time laser welding demands high quality assembling conditions of parts which have to be welded. Usually gaps between them have to be no less than 5% of thickness of the thickness of a thinner part (if a filler material is not used).

According to the opinion of the Heraeus Kulzer Company "in some cases, wider gaps and crevices have to be filled up or occlusal cusps have to be built up" (www.heraeus-kulzer.com). Perhaps for the solution of this problem Italian and American companies 'Jelrus' and 'Air Technics' respectively have developed a computerized laser soldering (INFODENT, July/Sept 2004, page2, www.jelrus.com). But, of course, this process has to be quite expensive.

Brazing is now mainly used for joining of teeth metal prosthesis and the gas flame is mostly applied for this process. This flame is usually made by traditional archaic equipment (consisting also of a gas torch) as a result of burning some gas mixtures: acetylene-oxygen, propane-oxygen, hydrogen-oxygen, etc. The latter mixture may be also made by water electrolysis in special devices that have been developed in Italy, Germany, the former USSR, Russia, etc. During the burning, such mixture generates the maximum temperature of about 2600 °C. The biggest operated metal thicknesses may be up to 2 mm. The prices of water electrolysis devices are $450-900. However, all of them provide less gas flame temperature than traditional gas devices (acetylene or propane + oxygen), which, moreover, have rather low prices. We have to note that the traditional gas flame devices bring about serious ecological hazard and may be dangerous in operation. All these gas devices usage creates a large dimension of heating zone as a result of which large post-joining deformations emerge.

Various furnaces and induction devices are also used for brazing-soldering in dental laboratories. The corresponding equipment has large ranges of costs, needs rather big electrical power and some additional procedures coupled with safety and ecology. At the same time, this equipment is not sufficiant universal, and often a schielding gas is required for its work. some times this gas, for example, ammonia, may be rather harmful for operator's health.

During argon (TIG) or, moreover, plasma welding, heating zone and post-joining deformations are essentially narrower. A new one among plasma equipment is the Russian set, named 'Multipluz-2500' ("The Second International Special Exhibition ROSSVARKA 2002",'Svarochnoye Proizvodstvo', No.1, 2003). This set uses a steam as a plasma forming environment (instead of the usually used argon) as well as shielding gas, this steam (created directly in the torch) is produced from the cooling liquid. The time of such torch uninterrupted work is up to 25 minutes. The required electric power is nearly 2.5 KVA (220V, 50 hz). It is very important that this set does not need gas cylinders and compressors. It has a weight of 6.5 kg and may be placed in a small shoulder bag. According to its authors, one of the main advantages of this set is its ecological cleanness. As has been described in the above mentioned source, the mentioned set torch has a weight of 0.7 kg, requires a cooling liquid 0.18 l/h and generates the temperature of approximately 8000 °C. We have no information as to the price of this set, but it is known that the set had been awarded the Gold Gran Prize – the highest price of the 26th World Saloon in Geneva – as the best invention of 2002. The usage of 'Multiplus-2500' by the 'Vologdaheatingservice' company has proven to be a positive experience. Nonetheless, this fact cannot assure us of the set's capabilities within the framework of dental and medical-biological cases. Since the minimum operated thickness of it is merely 0.5 mm, the appliance of such a set for the teeth prosthesis manufacturing may prove to be rather problematic. Furthermore, there is not enough experience of water or spirit evaporations usage as a shielding gas for complex alloyed materials welding and brazing. Such evaporations can bring problems for the shifting off oxide contaminations from the melting materials, contenting elements with high affinity to the oxygen (Mg, Mn, Mo, Co, etc). It must also be noted that the torch of the mentioned set is rather heavy for manual operations.

Refering to www.smithlab.net/docpage.cfm?doc_id=199, where is described the Smith dental laboratory experience in the field of "constructing and repairing the metal components, bridges and implants", we may certify - in other words - that the traditional solder joints are 25% less strong than the original cast alloy, while microplasma and laser welds are stronger 36% and 95% than the used base material, respectively.

In any cases dental industry has problems with connections of artificial teeth parts, and these problems are studied in schools, colleges and universities. For example, such studies take an essential part in the Florida College of Dentistry and other organizations.

Summarizing the information about technologies and equipment which may be relevant to dental laboratories, we have built the table which is given below:

View table

As we see, usual microplasma sets permit only welding (if not to use a special brazing filler material), demand medium operator's qualification and are rather expensive, while the new cheap, safe and simple sets, which may be operated by unskilled operators, have many more perspectives for their using in dental industry. Our prior experiments showed that new sets may be used for welding and brazing of protheses made from the majority metal materials including composite, chrome-nickel-cobalt alloys, noble material produced by usual and powder metallurgy methods. Simultaneously new sets give additional possibilities: to melt and re-melt small amounts of the mentioned metal materials, to produce form correction, local heat treatment, etc. New sets may be useful for dental jewelry, especially for the purposes of eclusive tooth-teeth ornamentations by noble and special metals inclusions, the so-called tooth jewelry. As an option, they may be applied for microplasma spraying which becomes rather attractive for molibdenum and hydroxyapative coating for biomedical and dental purposes (including dental implants) in different countries.

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