Investment material

from Wikipedia, the free encyclopedia
Investment ring with investment material. Left: molded part that created the casting funnel in the investment.
The investment makes a decisive contribution to the accuracy of fit of dental workpieces (here: gold crown on plaster model).

Under an investment material ( English investment ) is understood in the dental technology a refractory material, into which dental work pieces such as crowns , inlays , bridges or partial denture consisting of wax or plastic , have been modeled to the casting to be embedded. After the wax has melted or the plastic has burned out, the workpiece is cast precisely in the cavity that has been created. For the casting result, it is crucial that, due to an expansion of the investment, the cavity is enlarged before casting by exactly the factor by which the metal shrinks when it cools. Depending on the preheating temperature required and the material to be cast, gypsum-bound, phosphate-bound, silicate-bound or acetate-bound investment materials are used. The investment material is the central material for the most important manufacturing process in the dental laboratory. The lost wax technique is known from the times of ancient Egypt for non-dental applications. Countless researchers have been involved in further improvements to the investment materials since the application of the method in dental technology at the end of the 19th century. Despite the introduction of CAD / CAM technology, in which dentures are milled from blocks or manufactured using the laser sintering process, traditional pressing and casting techniques using investment materials still dominate in the manufacture of cast dentures.

application

With dental casting, the smallest cast objects with dimensions in the millimeter range are achieved with minimal wall thicknesses in the range of 0.3 millimeters. Dental workpieces that are manufactured using the casting process are first modeled from organic wax or plastic that can be burned without leaving any residue . After casting channels have been attached to the model, the workpieces are encased in a muffle (steel ring) with the liquid investment material. For this purpose, the investment material is mixed under vacuum using a time-controlled mixer . The investment is applied to delicate parts with a brush and then the muffle is filled. After the investment has hardened, the muffle is first heated in a computer-controlled preheating furnace to 300 ° C for about 45 minutes, whereby the wax or plastic is melted out and burned ("wax expulsion"). A negative form is created . In a second step, it is then gradually heated to the required preheating temperature for casting. This procedure, which is also known in numerous other industries, for example in the jewelry industry, is known as the lost wax process. Dental precision casting requires perfect process quality with an initial yield of 100 percent. For example, whether a cast piece of jewelery turns out 1–2% larger or smaller after casting - in contrast to dental technology - hardly matters. However, a tooth crown would have to be discarded. If it were too small, it would not fit on the tooth. If it were too big, it would not seal tightly and would encourage tooth decay . In the complex process chain in the manufacture of cast dentures, the processing of the investment material plays a central role.

expansion

Bond angle of 144 ° of the β-quartz

Dental precision casting involves the production of a unique piece that is modeled in its original size and after the casting process has to fit exactly on the tooth . When the cast alloy cools, the workpiece shrinks. However, in order for it to be the same size as it was modeled in wax, the investment material must expand by the shrinkage factor, i.e. In other words, the mold must be larger before the casting process, namely so large that the cast object has the desired size after cooling. It must therefore be produced with an allowance ( shrinkage ). The thermal contraction is around 1.6% for gold casting and 2.2% for model casting alloys. If the liquid metal were to be poured into a cold casting mold, it would approach the solidus temperature again while it was flowing in and would partially change into the solid phase , i.e. become hard, which would only partially fill the casting mold. The enlargement happens once through the setting expansion, i.e. while the investment material hardens. It is around 1–1.5%. By interposing of fleeces are placed in the mold rings has the material opportunity to carry out this expansion unhindered.

On the other hand, the mold expands by 0.5–1.5% when heated to the preheating temperature, which is ensured by the refractory components of the investment ( quartz , cristobalite ). The thermal transformation of quartz and its modifications is related to the arrangement of the atoms. A conversion of β-quartz into α-quartz (deep quartz into high quartz) takes place at 573 ° C (± 10 ° C) by changing the bond angle of the SiO 2 molecules ( silicon dioxide ) from 144 ° to 147 °. The conversion of tetragonal α-cristobalite into cubic β-cristobalite (deep cristobalite in high cristobalite) takes place by such a process from 147 ° to 148 °, at a temperature of 240-275 ° C.

  • α-cristobalite

  • β-cristobalite

  • α-quartz

  • β-quartz

ButtonGray.svg= Silicon atom       Glass button red.svg= oxygen atom

execution

The mixing time and the initial temperature of the investment have an influence on the setting expansion . The ideal starting temperature of the liquid and the powder is 17 ° C, which is achieved by storing the liquid and the powder in a thermal cabinet . For this purpose, controllable thermal cabinets are used, which allow a temperature of +15 ° C to +28 ° C for the exact temperature control of the investment material. At an assumed room temperature of 23 ° C, which corresponds to the wax model, a temperature rise to 19 ° C to 23 ° C is reached shortly after mixing at an initial mixing temperature of 17 ° C. The investment material meets the wax model in a thermal equilibrium without the wax model being cooled down and thus shrinking or warping. The mixing ratio of the investment must be precisely adhered to in order to achieve an accuracy of fit of the dental workpiece. Dental laboratories work with slightly modified mixing ratios, depending on the impression material of the impression they receive from the dentist . It is also tailored to the master model of the dental technician, which may differ slightly from the original dentition due to its own setting expansion. The investment material is stirred under vacuum in order to avoid air bubbles and thus casting defects ( gas bubbles ). ( Holes can form during volume contraction when the cast part cools). The investment is poured into the muffle on a vibrator , which compresses the investment and expels any air bubbles. The vibrator must not be used for more than ten seconds, otherwise grains could settle.

Casting preparation

A molten and thus liquefied dental alloy can be introduced into the resulting hollow shape either by means of a centrifugal centrifugal force or in a vacuum pressure casting process. There is a temperature difference between the outer wall of the muffle and the core. The hotter outer wall reaches the desired final temperature, while the core can be up to 80 ° C colder, depending on the specific thermal conductivity of the respective investment material, which acts like an insulator. Quartz sintering of the investment takes place at 820 ° C to 870 ° C. The casting temperature should be around 150 ° C above the liquidus temperature . The latter is between 1200 ° C and 1500 ° C, depending on the alloy to be processed ( gold , chromium-cobalt-molybdenum ). Titanium is cast at temperatures between 1800 ° C and 2000 ° C. After casting, the mold is destroyed by devesting the workpiece.

Heating up the investment

An even and low-tension heating of the investment in a muffle must be achieved through very slow heating. Due to the granularity of the powder, water is trapped during mixing. Water changes to a gaseous state at 100 ° C. A certain amount of energy is required for this conversion , which causes the water to increase in volume by a factor of 1700 at normal pressure . This increase in volume generates gas pressure when trapped , which slowly pushes the water vapor in the investment out through the gaps. If the heat is supplied too quickly, the rapid rise in pressure would cause the finest cracks to form.

The heating is mainly generated by thermal radiation inside the furnace. The radiation hitting a body is partly absorbed and partly reflected by it. The absorbed heat is transported inside the body. The heat transport inside muffles is essentially generated by heat conduction . The speed depends on the thermal conductivity of the material. The thermal conductivity of cristobalite is 900 times worse than that of gold. When filling a furnace with a large number of muffles, isotherms can arise that can go straight through the muffles. The bottom of the furnace chamber should be covered with a corrugated plate in order to reduce the contact area with the muffle, which leads to a reduction in the flow of heat through conduction.

  • Wax modeling of tooth crowns / inlays

  • A casting muffle is placed over the wax modeled crowns. The inserted blotting paper allows the investment to expand when heated.

  • Pot and agitator for mixing the investment, which are clamped in a mixer

  • Vacuum mixer for the investment

  • The investment is poured into the muffle on a vibrator, free of air bubbles, and compacted.

  • After the investment has hardened, the wax is burned out in the preheating furnace.

  • Casting machine (muffle not yet inserted)

  • Here a model casting base is removed from the investment after casting.

  • Blank with sprues after casting

Types of investment materials

Depending on the required preheating temperature and the material to be cast, gypsum-bound, phosphate-bound (ammonium phosphate), silicate-bound or acetate-bound investment materials are used. In order to avoid microcracks in the investment material, preheating takes place in precisely specified temperature steps, which should be 10 ° C / min. The preheating temperature should be around 250-300 ° C below the solidus point of the alloy. The compressive strength of the investment material for precious metal casting must be at least 250  N / mm², with the model casting technique at least 1000 N / mm². The density of the investment determines the thermal conductivity, the expansion curve and the volume behavior.

Demarcation

In the manufacture of a one-piece cast prosthesis, both the shape and the model are destroyed. This is referred to as the lost model method, sometimes the lost form method . (A "model" is used to produce a "lost form" ( sand mold , chill mold , injection mold , die casting mold ) and is used to produce a casting that is destroyed after the casting process. A "lost model" is destroyed during the production of the mold). In industry, series of work pieces are cast using permanent molds made of iron or steel ( chill casting process ) or wood, plastics or metals. Permanent forms can also consist of chamottes in metal casting or concrete in concrete and artificial stone casting . In individual cases, investment materials from dental technology are also used in other production areas. The casting of blanks, which indicate the later intended use, but must be intensively further processed, is not a form casting . The term “ model cast prosthesis ” is derived from the fact that the prosthesis framework is cast on a duplicate model of the dentition from a special investment material, which is destroyed during devestment.

Plaster-bonded investment material

Gypsum-bonded investments consist of gypsum (CaSO 4 · ½ H 2 O, calcium sulfate-α-hemihydrate) as a binder and two high-temperature modifications of quartz, in particular tridymite , a crystalline form of silicon dioxide (SiO 2 ), and cristobalite . Other additives such as sodium chloride (NaCl), potassium chloride (KCl) and lithium chloride (LiCl) increase the thermal expansion. Borax (Na 2 B 4 O 2 · 10 H 2 O) reduces and sodium sulfate (Na 2 SO 4 ) increases the setting time. In the case of precious metal alloys, the addition of the reducing agent borax prevents oxidation of the melt. Plaster-bonded investments are only used when processing smaller workpieces made of gold alloys, where the preheating temperature is limited to 700 ° C. Above 750 ° C gypsum decomposes and damages the metal alloy by forming metal sulfides.

Solder investment

If two or more metal parts are to be soldered together, a soldering block is made from an investment material in which the metal parts are fixed. Solder investment materials must not expand as usual. It is a plaster-bonded investment material with a plaster-quartz ratio of 1: 3. A setting expansion would possibly shift the parts to be soldered against each other. A slight thermal expansion must be linear with the expansion of the parts to be soldered. A coarse-grained solder investment material ensures faster heating through due to the greater porosity . If crowns veneered with ceramic have to be soldered, especially after glaze firing, the veneer would be attacked by the liquid soldering investment material. The ceramic is therefore coated with wax to protect it from investing.

Phosphate-bound investment material

Magnesium oxide (MgO) and ammonium dihydrogen phosphate NH 4 H 2 PO 4 act as binders for cristobalite and tridymite and control the processing time. In addition to the selection of the binder, the expansion depends on the ratio of water to mixing liquid in the case of phosphate-bonded investment materials. The mixing liquid consists of water and silica sol , usually in a mixing ratio of 70%: 30%. The preheating temperature is limited by the stability of the binder. The decomposition temperature of phosphate-bound investment materials is 1300 ° C.

Chemical reaction

The chemical reaction when the binding agent of phosphate-bound investment material sets is as follows:

During preheating, the binding agent magnesium ammonium phosphate reacts again at 160 ° C , releasing part of the crystal water :

At approx. 250 ° C it becomes magnesium diphosphate (magnesium pyrophosphate), which is then fire-resistant:

The toxic ammonia (NH 3 ) produced in the preheating furnace, like the pyrolysis products of wax and plastics, must be discharged into the open.

The thickness of the muffle fleece limits the expansion, with the muffle itself also expanding when heated.

Investment material for press ceramic

Modified phosphate-bonded investment materials are used in the manufacture of press and full ceramics from lithium disilicate ceramic (LS2). They are based on high-purity, colorless crystal quartz, which guarantees a high degree of color security for the pressed objects. The total expansion can be precisely adapted to the press ceramic used by mixing the mixing liquid with demineralized water . The embedded wax model is left to rest for 19 minutes and then placed in a burnout oven preheated to 850 ° C. for 50 minutes. The filled muffle is placed in the press furnace and the liquid ceramic is pressed at a pressure of 4.5 bar at 940 ° C for about 30 minutes.

Silicate-bonded investment material

Silicate-bonded investment materials contain a liquid made from tetraethylorthosilicate [TEOS, Si (OC 2 H 5 ) 4 ] as a binder for quartz and tridymite , which reacts with water to form Si (OH) 4 ( orthosilicic acid ) and C 2 H 5 OH ( ethanol ). They are mainly used for the production of model casts, at a preheating temperature of 1000 ° C to 1100 ° C. Silicate-bonded investments show no setting expansion, but only a thermal expansion of around 1.8%. The conversion of the ethyl silicate during mold production is a sol-gel process . This can be divided into a hydrolysis reaction , a condensation reaction , and a subsequent drying and conversion into a ceramic. The investment is mixed by hand. The mass is then compacted on a vibrator for 20 minutes and air bubbles are completely expelled. This is followed by a hardening process at 180 ° C in a cold hardener based on synthetic resin .

  • Wax modeling of a model cast part prosthesis with casting channels and casting funnel

  • Filling in the investment

  • Investment material after casting the model cast prosthesis

  • Devest the model cast prosthesis

  • Blank after casting

  • Cast and finished framework of a model cast prosthesis (here with bite wall made of red wax)

Chemical reaction

Si (OC 2 H 5 ) 4 + 4 H 2 O → Si (OH) 4 + 4 C 2 H 5 OH

Fine investment

Prefabricated plastic molded parts for modeling model cast prostheses

Prefabricated plastic molded parts ( Flexiseal , Flexetten ) for modeling the model cast, such as clips or brackets, are loosened by the ethyl alcohol of the binding agent of silicate-bound investment materials. To protect them, they are first covered with special insulating materials and then with fine investment materials. The binding mechanism is mostly based on water glass ( colloidal sodium silicate ). The hardening takes place through the formation of silica gel through acidification when overlaying the fine investment with the main investment.

Acetate-bonded investment

Gypsum or phosphate-bonded quartz investments can not be used for titanium casting because of the high reactivity of liquid titanium with these substances. In the case of the special, acetate-bound ( esters of acetic acid ) titanium investment materials, expansion is not controlled by changing the mixing ratio, but by changing the holding time (30 min) at a maximum preheating temperature of 965 ° C or by changing the preheating temperature. An increase in the preheating temperature (e.g. by 10 ° C) leads to more expansion, a reduction in turn leads to less expansion within the permissible temperature range. A holding time that is longer (e.g. by 10 minutes) leads to more expansion, a holding time that is ten minutes shorter results in less expansion. These measures allow the accuracy of fit to be controlled for each laboratory. The casting temperature of the muffle is not identical to the maximum preheating temperature, but the muffle is cooled down again in the preheating furnace before casting to a temperature of 430 ° C, which is sufficient to ensure that the mold flows completely through the titanium alloy and at the same time to ensure a good surface quality guarantee. The titanium is melted with an arc under an argon shielding gas . The arc is then a tungsten - electrode ignited. The casting process itself is a die-casting process that uses the high pressure of the argon in the melting chamber and the negative vacuum pressure in the casting chamber. This enables the negative mold in the investment to be completely filled with the titanium melt. The melting point of titanium is 1668 ° C. Compared to alloys, titanium does not have a melting range but a defined melting point, which means that the melt solidifies very quickly and there are no solidification voids or porosities , as is often the case with alloys with a more or less wide melting range.

Speed ​​investment

Speed ​​or high-speed investment material for precious metal and model casting technology, which is also known as shock-heat material, sets faster and can be placed in the heated preheating furnace after 15 minutes from mixing. Whereas the preheating process for conventional investments from investing to pressing or casting took three to four hours, the time required for modern speed investments has been reduced to around 90 minutes. Speed ​​investment materials consist of the binder magnesium oxide and ammonium dihydrogen phosphate as well as quartz and cristobalite as fillers. They are mixed with a mixing liquid made from aqueous silica sol.

Material characteristics

The material parameters are given in accordance with DIN EN ISO 15912. This includes the material consistency , the flowability , the start of solidification ( Vicat time ), the compressive strength in megapascals (MPa) and the linear thermal expansion in percent.

Osh

When processing quartz and cristobalite-containing investment materials when investing, devesting and blasting dental workpieces, employees can be exposed to inhalation exposure. As part of the risk assessment , the hazardous substances occurring at the workplace must be determined and suitable protective measures established. The information "Mineral dusts when investing, devesting and blasting in dental laboratories" can be used in the risk assessment for activities with investment materials. It provides the company with practical information to ensure that occupational exposure limits and other assessment criteria are complied with or that state-of-the-art technology is otherwise achieved. Measures are described which ensure compliance with assessment criteria when investing and deflating manually and when blasting dental workpieces.

history

VMK crown on tooth 24 (in the picture: 4th tooth from left above). Cast metal framework not visible.

The lost wax technique is known from the times of ancient Egypt. Barnabas Frederick Philbrook from Council Bluffs ( Iowa ), described for the first time in 1897 a modern industrial process for producing crowns and inlays using investment materials. The development was driven by William H. Taggart from Chicago , who presented his findings to the professional world in 1907. The development of cristobalite-containing investment materials by RL Coleman and LJ Weinstein in 1929, who then received a US patent in 1933, and the introduction of hygroscopic technology by CH Scheu in 1932 were decisive for the most important improvement in the accuracy of fit of dental castings. Countless researchers have since been involved in further improvements to the investment materials.

The demand for tooth-colored crowns increased. When the composite metal ceramic (porcelain-veneered crowns) was introduced by Abraham Weinstein, M. Weinstein and S. Katz in 1952, the ceramic often flaked off. The coefficient of thermal expansion (CTE) of metal and ceramic differed greatly when they cooled down from the firing temperature of 880 ° C, which led to tension. In 1962 it was possible to adjust the CTE between metal and ceramic, thereby considerably reducing the risk of breakage. At the same time, the company developed Whip Mix Corporation , the phosphate-bonded investment, with the first refractory gold - platinum - alloys of JF Jelenko Company and J. Aderer Company were cast, which (as a scaffold for keramikverblendete crowns crowns PFM ) are used.

literature

Web links

Wiktionary: Investment material  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Hans H. Caesar, Klaus M. Lehmann: The partial prosthesis: Basics, construction and dental technology . Verlag Neuer Merkur, 2007, ISBN 978-3-937346-42-7 , p. 104-108 .
  2. ↑ Michael Rudolph: Edge gap measurement and strength testing of metal-ceramic crowns with laser-melted framework. Dissertation . 2006. Retrieved August 22, 2015.
  3. ^ A b c Heinrich F. Kappert, Karl Eichner: Dental materials and their processing 1. Basics and processing . Georg Thieme Verlag, 2005, ISBN 3-13-127148-5 , p. 34 ff .
  4. ↑ Horst Koinig: PFM . Verlag Neuer Merkur, 2003, ISBN 3-929360-90-X , p. 156 ff .
  5. ^ Andreas Hoffmann: Dental casting technique. ( Memento of the original from December 22, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: Quintessenz Zahntech. 36 (6), 2010, pp. 814-827. Retrieved August 22, 2015. @1@ 2Template: Webachiv / IABot / www.1dsz.de
  6. ^ Andreas Sabath: Casting in dental technology. P. 2.13.
  7. ↑ Hans H. Caesar: The training as a dental technician . Verlag Neuer Merkur, 1996, ISBN 3-929360-01-2 , p. 381 ff .
  8. ↑ Andreas Tilburg: The behavior of cristobalite-containing investments during preheating. Retrieved November 1, 2015.
  9. ^ A b c Arnold Hohmann, Werner Hielscher: Dental technology in questions and answers: questions about anatomy, prosthetics, orthodontics and materials science . Verlag Neuer Merkur, 1995, ISBN 3-921280-93-1 , p. 419-420 .
  10. ↑ The Metals: Materials science with its chemical and physical principles . Verlag Neuer Merkur, 1999, ISBN 3-929360-44-6 , p. 102- .
  11. ↑ W. Brämer, H. Kreutzer: Casting in dental technology - a casting error atlas. Heraeus, Hanau 1993.
  12. ↑ David Comiskey: Presskeramik. In: ZWL. 05 2003, Oemus Media. Retrieved August 19, 2015.
  13. ↑ Basics of precision casting technology . Karlsruhe Institute of Technology . Retrieved August 18, 2015.
  14. ↑ J. Lindigkeit, Th. Schneiderbanger, U. Schmitt, P. Ohnmacht: Model casting investment materials : a tough nut to crack for CAD / CAM. In: Zahntechnik Magazin. Spitta-Verlag, 11, 3, 2007, pp. 122-129. Retrieved September 5, 2015.
  15. ^ Siegfried Ernst, Hans H. Caesar: The non-metals . Verlag Neuer Merkur, 2007, ISBN 978-3-937346-31-1 , p. 137 ff .
  16. ↑ Jürgen Lindigkeit: The application of titanium for implant-supported suprastructures. Part 2: Processing in the dental laboratory. S. 2. Dentaurum. Retrieved August 18, 2015.
  17. ↑ J. Lindigkeit, Th. Schneiderbanger, U. Schmitt, P. Ohnmacht: Titanium investment materials: always ready for the biocompatible material. In: Zahntech Mag. 11, 6, 2007, pp. 374-380. Retrieved August 19, 2015.
  18. ↑ Bernhard Egger: Universal phosphate-bonded investment materials, economic and qualitative decision-making criteria. ( Memento of the original from March 5, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: the dental laboratory. LX, 10/2012. Retrieved August 22, 2015. @1@ 2Template: Webachiv / IABot / shofu.de
  19. ↑ DIN EN ISO 15912 Dentistry - High temperature-resistant investments and die materials (ISO / DIS 15912: 2016); German version prEN ISO 15912: 2016, German Institute for Standardization, Dental Standards Committee (NADENT). Retrieved April 28, 2016.
  20. ↑ Table of contents E DIN EN ISO 15912: 2014-08 (D) , NADENT. Retrieved August 15, 2015.
  21. ↑ German statutory accident insurance e. V. (DGUV): DGUV Information 213-730 - Mineral dusts when investing, devesting and blasting in dental laboratories - Recommendations for risk assessment by the accident insurance carriers (EGU) according to the Ordinance on Hazardous Substances. Retrieved October 17, 2019 .
  22. ↑ K. Asgar: Casting metals in dentistry: past – present – ​​future. , (English) In: Advances in dental research. Volume 2, Number 1, August 1988, pp. 33-43. PMID 3073783 (Review).