Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C3/00—Selection of compositions for coating the surfaces of moulds, cores, or patterns

Definitions

• the invention relates to a casting investment mixture on a ceramic basis for producing a casting investment compound for the production of casting molds for metal or metal alloy parts, in particular titanium or titanium alloy parts in dental technology.
• the invention further relates to a casting investment in the form of a slurry obtainable from the above-described casting investment mixture and to a casting mold (casting investment model) and a process for its production.
• shaping by casting offers the possibility of also producing geometrically complex small parts.
• the main difference in shaping by casting compared to other shaping processes is that the material only acquires its shape, material structure and quality after cooling with a sometimes considerable shrinkage in the liquid state and during solidification as well as considerable shrinkage in the solid state. The shrinkage must be taken into account by taking an appropriate allowance.
• Centrifugal casting has proven to be a particularly suitable casting method, in which the casting metal is filled into a mold rotating about its axis, in which it is shaped into the casting under the action of centrifugal force.
• titanium or titanium alloys are increasingly used as the casting material.
• Titanium has the advantage of being biocompatible, so that the material costs for dental prostheses can be significantly reduced compared to the relatively expensive gold alloys. Titanium still has good strength properties, one relative low coefficient of expansion and low thermal conductivity and is ultimately corrosion and cavitation resistant. In addition, titanium can also be coated well.
• the mold which determines in particular the surface quality of the cast product, but also the undesired formation of voids and pores, is of crucial importance with regard to the quality of the cast part.
• a typical negative example in model casting is the dissolving of flexible plastic molded parts, from which clips, brackets, basic framework parts and other are modeled on an investment material model, using the binder investment material.
• the modeling plastic mentioned is not compatible with ethyl alcohol in the binder liquid, which leads to the surface becoming detached.
• flexible plastic molded parts flexible seals
• it has therefore already started to be covered with a special fine investment material before the model and the model are coated with the binder investment material. This disadvantageously increases the manufacturing outlay.
• oxide-ceramic molds In dental technology, investment materials based on silicon dioxide are used, which, depending on the use of the binders, can be divided into gypsum-bound and phosphate-bound or silicate-bound compositions.
• oxide-ceramic molds often have a tendency to react with molding materials with the casting metal, which can lead to the casting being unusable.
• the casting investment mixture described in claim 1 is proposed.
• this has 40 to 60% by mass of zirconium dioxide. Below 40% by mass of zirconium dioxide content, voids and pores were increasingly formed in the casting, whereas, with more than 60% zirconium dioxide content, no reliable setting of the cast investment material could be achieved.
• the casting investment mixture in powder form should not have any zirconium dioxide particles whose diameter exceeds 150 ⁇ m. At least 50% of the zirconium dioxide particles contained in the mixture should preferably have a diameter of less than 50 ⁇ m, and the remaining zirconium dioxide particles should furthermore preferably have a diameter of up to a maximum of 120 ⁇ m.
• the best experiences were made with a casting investment mixture, each half of which consisted of zirconium dioxide particles with a diameter of up to 50 ⁇ m and the rest of such zirconium dioxide particles with a diameter of up to 120 ⁇ m, preferably 100 ⁇ m.
• the zirconium dioxide can be both unstabilized and in a stabilized or partially stabilized form.
• the casting investment mixtures are preferably based on refractory constituents, such as cristobalite, tridymite and quartz, ie on quartz (SiO2) and its modifications. Quartz is the hexagonal crystal form, which is converted to tridymite when heated to 867 ° C. Upon further heating to 1470 ° C, a transformation into cristobalite occurs. If tridymite and cristobalite are cooled relatively quickly, the reversible conversion down to the quartz is prevented, so that cristobalite and tridymite can also be present at room temperature. In order to obtain a castable investment material from this mixture, monoammonium phosphate (NH4H2PO4), magnesium oxide (MgO) and water are added as binders.
• NH4H2PO4 monoammonium phosphate
• MgO magnesium oxide
• the setting and the production of an investment material model or a casting mold takes place in that the monoammonium phosphate reacts with the excess magnesite when the casting investment material is mixed with water-containing liquids to form magnesium-ammonium-phosphate-water complexes.
• the magnesium ammonium phosphate that forms when mixed is exothermic and is difficult to dissolve. Due to its crystalline morphology, it combines the ceramic powder components into a solid form. By heating the mold, it is dewatered and sintered, the magnesium ammonium phosphate gradually being converted into the compound magnesium pyrophosphate.
• MgO-Mg2P2O7-SiO2 particles Due to the stoichiometric predominance of magnesite in the investment materials, MgO-Mg2P2O7-SiO2 particles are distributed heterogeneously in the sintered casting mold.
• the additionally contained zirconium dioxide inhibits undesired reactions of the casting investment material with the casting material or prevents them almost completely if zirconium dioxide is essentially contained in the layers of the casting mold near the surface.
• the setting reaction is associated with an expansion of the investment, which can be increased if the mixing liquid used is not water but an aqueous silica sol in which silicon dioxide particles are colloidally dissolved.
• the colloidal silica sol contains alkaline additives in the form of sodium hydroxide.
• the invention is further characterized by the cast molding compound in slurry form described in claim 8, which, as described above, is obtained by adding solutions containing binders. From this, a mold is obtained by dewatering and heating, the zirconium dioxide content of which is greater on or towards the surface facing the casting space than in the interior of the casting investment model. To produce this casting mold, the casting investment slurry placed in a duplicating mold is stored before setting so that the surface facing the subsequent casting space is directed downward, so that, due to gravity, the specifically heavier zirconium dioxide increasingly diffuses into the layers near the surface. A casting mold is thus obtained which essentially consists of zirconium dioxide on the surface in question, while the other casting material components serve practically only as a “backfill”.
• the slurry is preferably shaken under vacuum or in a largely evacuated atmosphere, and further preferably also shaken after pouring into the casting muffle or the duplicating mold before setting.
• 100 g of investment powder consisting of 40% SiO2, 5% MgO and 5% NH4H2PO4, 50% ZrO2, each with half of particle diameters up to 50 microns and up to 120 microns and mixed with about 15 ml of a mixing liquid mixed in a porridge and placed in a silicone mold. After the investment model had dried out or set, the bottom layers contained a large majority of the zirconium dioxide particles, which had dropped due to their comparatively larger specific weight.
• the surface that is accordingly rich in zirconium dioxide has the advantage in the subsequent titanium centrifugal casting that reactions with the titanium are prevented, furthermore zirconium dioxide in a first approximation has the same coefficient of thermal expansion as titanium or titanium alloys, so that a great fit of the castings is ensured.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Mold Materials And Core Materials (AREA)
• Dental Prosthetics (AREA)

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Citations (2)

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• 1991
  • 1991-03-12 DE DE19914107919 patent/DE4107919C1/de not_active Expired - Fee Related
• 1992
  • 1992-03-12 EP EP19920104283 patent/EP0503626A1/fr not_active Withdrawn

Patent Citations (2)

Non-Patent Citations (1)

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