EP1043094B1 - Process for making castings - Google Patents

Description

The invention relates to a method for producing cast pieces, in particular filigree precision castings in the medical field, in which a consisting of a ceramic investment material mold is selectively expanded before filling a casting material to compensate for occurring during cooling from the molten state volume contraction of the casting material, said Investment of a quartz-free oxide ceramic mixture, a binder and serving as a mixing liquid silica sol is formed with the addition of an organic carboxylic acid.

In the medical field, in particular in dental technology, individually manufactured prostheses are usually cast from metal alloys, for example based on titanium (Ti), palladium (Pd), nickel-chromium (NiCr), cobalt-chromium (CoCr), or glass-ceramic materials , The casting molds used for this purpose consist of a ceramic investment material, which is obtained by means of a wax model corresponding to the body part to be reconstructed. This is done by mixing a slurry consisting of a ceramic mixture and a binder with a mixing liquid and applying it to the wax model. After the slurry is set at room temperature after a short time, the solid mold formed in this way is heated to a temperature at which the wax of the wax model flows out without residue. To the required casting parameters, such as the Edge stability of the mold, to reach the mold is preheated to temperatures of up to 1000 ° C before the introduction of the casting material.

During cooling, the casting process is characterized by a volume contraction of the casting material. Already in the liquid state, a shrinkage occurs, which passes into a shrinkage during solidification. In order to ensure the smallest possible deviation of the casting from the wax model corresponding to the body part to be reconstructed, it is known to compensate the volume contraction of the casting material by an expansion of the investment material forming the casting mold. The overall expansion of the embedding compound is composed of a setting expansion during the setting of the slurry and a thermal expansion during heating of the casting mold. A high thermal expansion can be achieved when quartz-containing investments are used. This is due to the thermal conversion behavior of silica modifications. Thus, by a β-conversion, a sudden expansion of cristobalite at about 270 ° C and of quartz at about 570 ° C instead.

Although quartz-containing investment materials are characterized by high expansion values of up to 2%, which often make a setting expansion unnecessary, but in addition to a harmful formation of fine dusts with a particle size of <10 microns to the disadvantage of a likewise uneven cooling behavior. As a result, in the casting which is in contact with the embedding compound during cooling, stresses occur which can lead to unacceptable plastic deformation or cracking. A disadvantage of quartz-containing investment materials is also a relatively low softening temperature, which cause at high casting temperatures, such as those required when casting, for example, titanium alloys, slagging reactions that bring pass inaccuracies with it. In casting materials based on titanium (Ti) or zirconium (Zr), which have a high affinity for oxygen, carbon, nitrogen and hydrogen above a temperature of about 900 ° C, there is also an embrittlement formed by oxidation of the outer skin of the Gußwerkstückes , Although known under the name alpha-case-form, edge zone oxidation is accompanied by increased material hardness, which is desirable for certain applications Precision castings but due to the associated fit inaccuracies due to, for example, voids or gas inclusions to avoid.

From the EP 0 417 527 A a quartz-containing Gusseinbettmasse for producing molds for alloy parts is known. Particularly in dental technology, a molten metal alloy can be cast into a mold made from the cast investment. After cooling and hardening of the alloy, the mold is removed and the alloy casting removed. The fit of the tooth replacement part produced by casting therefore depends decisively on the casting mold and in particular on the surface quality of the casting mold. When the investment material hardens, the chemical reactions that occur can result in "efflorescence" that adversely affects the usability of the casting mold. Practical experience has shown that blooming occurs particularly when the setting of the investment materials is completed, but these are not yet dry due to the formation of moisture caused by setting. It is therefore proposed a quartz-containing Gusseinbettmasse that is mixed with a water-soluble acid to reduce such efflorescence.

In comparison, the quartz-free ceramic investment materials, which are less hazardous to health and less susceptible to alpha-case formation, have an almost constant thermal expansion behavior. Due to a up to about 900 ° C, that is, up to the preheating temperature of the mold, small expansion coefficient, can be achieved with quartz-free investment materials, however, only a small thermal expansion of 0.5% maximum. To compensate for the occurring during cooling volume contraction of the casting material therefore high values for the setting expansion are required.

With the European patent application 97 119 939.3 It has already been proposed to supply an organic carboxylic acid, for example citric acid, a quartz-free magnesium oxide (MgO) containing oxide ceramic mixture, a binder based on magnesium oxide (MgO) and monoammonium phosphate (NH ₄ H ₂ PO ₄ ) and silica sol used as mixing liquid to obtain a high setting expansion. Due to the content of magnesium oxide (MgO) in the oxide ceramic mixture and the nature and proportion of the carboxylic acid, the setting expansion required for the casting material to be processed can be set.

However, this has the disadvantage that there is a high number of investments to be made available according to the different casting materials. In addition to this economically unsatisfactory stockpiling in the known from the above-mentioned European patent application method, the complex handling for setting the required Abbindeexpansionswerte is disadvantageous.

The invention has for its object to provide a method for the production of Gußwerkstücken, with which a comparatively simple control of the occurring during setting expansion of a quartz-free investment material can be achieved.

This object is achieved in a method for the preparation of Gußwerkstücken with the above genannton features according to the invention characterized in that the occurring during the setting expansion of the embedding material by varying the content of colloidal amorphous silica (SiO ₂₎ in the silica sol is controlled in dependence on the used castable is targeted by the addition of colloidal, amorphous silica (SiO ₂ ).

The invention takes the surprising finding of its own that it is sufficient to set a sufficiently large setting expansion in quartz-free investment materials to vary the content of silica (SiO ₂ ) in serving as a mixing fluid silica corresponding to the respective casting materials. The reason for the achievable setting expansion values is primarily the interaction of the organic carboxylic acid with the silica sol. Silica sol, as an aqueous anionic solution of colloidal, amorphous silica (SiO ₂ ) with a pH above 10, has negatively charged and, as a result, mutually repulsive SiO ₂ particles, which sufficiently stabilize the silica sol and give it a relatively low viscosity. The carboxylic acid lowers the pH of the silica sol and results in an acid-base reaction which precipitates silica gel according to the following equation:

≡Si-O ^- _(Sol) + H ⁺ → ≡Si-OH _(Gel)

with ≡Si-O ^- : silicate anion on the surface of a SiO ₂ particle and
≡Si-OH: Undissociated silica on the surface of a SiO ₂ particle.

For example, since carboxylic acid is relatively weak compared to mineral acids and their derivatives, the pH is not reduced so much that charge reversal, such as absorption of H ⁺ ions, would occur, stabilizing the now unstable silica sol. During the setting of the embedding mass, hydrous, crystalline or semicrystalline silicates form from the precipitated silica gel, which lead to an increase in volume and thus to an increase in image index expansion. By varying the content of silicon dioxide (SiO ₂ ) in the silica sol, the volume fraction of expanding, crystallizing silicates can consequently be changed and thus the Steplessly control setting expansion, almost independent of the composition of the oxide ceramic mixture.

It is particularly advantageous if the silicon dioxide (SiO ₂ ) content in the silica sol is varied between 30% by weight and 40% by weight in order to set a certain expansion of the embedding compound. Experiments showed that commercially available 30% silica sol leads to a setting expansion of 0.8%, whereas with a proportion of silicon dioxide (SiO ₂ ) in the silica sol of 40% by weight, a setting expansion of 6% can be achieved. The variation of the silica content in the above interval thus allows sufficiently large setting expansion values for most of the cast materials.

It has also been found to be advantageous that when 100 g of oxide ceramic mixture, binder and carboxylic acid are mixed with 15 ml to 40 ml of silica sol, a sufficiently high reduction of the pH can be achieved. It is expedient to use a silica sol in which the silicon dioxide piadicles have a BET surface area of 50 m ² / g to 350 m ² / g.

• Oxalsäure ((COOH)₂)
• Weinsäure (HOOC-(CHOH)₂-COOH)
• Citronensäure (HOOC-CH₂-C(OH)-(COOH)-CH₂-COOH)
• Apfelsäure (HOOC-CH₂-CH(OH)-COOH)
• Malonsäure (HOOC-CH₂-COOH)
• Maleinsäure (HOOC-CH₂=CH₂-COOH)

In a further development of the invention it is proposed that a di- or tricarboxylic acid is used as the carboxylic acid. Di- and tricarboxylic acids according to the chemical formula R (COOH) _x with x = 2.3 can be admixed as solids to the embedding compound, are very readily soluble in water and can be broken down without residue on heating. Particularly suitable are the following di- and tricarboxylic acids:

• Oxalic acid ((COOH) ₂ )
• Tartaric acid (HOOC- (CHOH) ₂ -COOH)
• Citric acid (HOOC-CH ₂ -C (OH) - (COOH) -CH ₂ -COOH)
• Malic acid (HOOC-CH ₂ -CH (OH) -COOH)
• Malonic acid (HOOC-CH ₂ -COOH)
• Maleic acid (HOOC-CH ₂ = CH ₂ -COOH)

According to a further feature of the invention, the quartz-free oxide ceramic mixture of oxides is preferably composed of aluminum silicates, zirconium silicate and stable silicates of the alkaline earth metals, as well as magnesium oxide (MgO) and corundum (α-Al ₂ O ₃ ). Such oxide ceramic mixtures are of high thermal and chemical stability, are generally high-melting and have a low thermal expansion coefficient. Compared to quartz-containing investments, they are much more stable and, in particular, more suitable for investment casting of titanium (Ti) or zirconium (Zr) based metal alloys. Moreover, the risk of slagging is comparatively low. The embedding composition according to the invention advantageously has a proportion of oxides of high thermal and chemical stability of 96 wt .-% to 30 wt .-%, a proportion of corundum (α-Al ₂ O ₃ ) of trace to 40 wt .-% and a proportion on magnesium oxide (MgO) of 4 wt .-% to 30 wt .-% to.

In a further development of the invention it is further proposed that a phosphate binder based on magnesium oxide (MgO) and monoammonium phosphate (NH ₄ H ₂ PO ₄ ) is used as the binder. The proportion of monoammonium phosphate (NH ₄ H ₂ PO ₄ ) in the binder is expediently between 5 wt .-% and 15 wt .-% based on the mass of the oxide ceramic mixture. If the oxide ceramic mixture contains magnesium oxide (MgO), the binding of the embedding compound converts the monoammonium phosphate (NH ₄ H ₂ PO ₄ ) of the binder into Mg ₂ P ₂ O ₇ = MgO · P ₂ O ₅ . Although the component P ₂ O ₅ is unstable to liquid metals, but passes at a temperature of about 1400 ° C in the thermochemically stable compound Mg ₂ P ₂ O ₇ . The relatively low proportion of monoammonium phosphate (NH ₄ H ₂ PO ₄ ) in the investment ensures that the P ₂ O ₅ content is less than 3% hereinafter. This is especially important when a titanium (Ti) based casting material is used because titanium (Ti) or titanium alloys are highly reducing.

Fig. 1

Die Zusammensetzung der Oxidkeramikmischung einer Einbettmasse in einem Dreiecksdiagramm und

Fig. 2

ein Diagramm, daß die Abbindeexpansion ε_A einer quarzfreien Einbettmasse in Abhängigkeit des Gehalts w an Siliciumdioxid (SiO₂) in einer aus Kieselsol bestehende Anmischflüssigkeit darstellt.

Details and further advantages of the subject matter of the present invention will become apparent from the following description of preferred embodiments. In the accompanying drawings show in detail:

Fig. 1

The composition of the oxide ceramic mixture of an investment material in a triangular diagram and

Fig. 2

a diagram that the setting expansion ε _{A represents} a quartz-free investment material as a function of the content w of silicon dioxide (SiO ₂ ) in a mixing liquid consisting of silica sol.

The triangular diagram according to Fig. 1 lets recognize the composition of the oxide ceramic mixture of an investment material. The corners of the equilateral triangle correspond to the pure components corundum (α-Al ₂ O ₃ ), magnesium oxide (MgO) and oxides of high thermal and chemical stability, such as aluminum silicates, a ternary system. On the triangular sides divided by mass parts, the binary edge systems are found, while points within the triangle represent the ternary mixture. Constant mass content lines w of one component are parallel to the triangle sides facing the respective corner of the pure component, and cut off the respective mass fractions on the edge scales. For a preferred investment comprising a proportion of oxides of high thermal and chemical stability of 96 wt .-% to 30 wt .-%, a proportion of corundum (α-Al ₂ O ₃ ) of trace to 40 wt .-% and a Proportion of magnesium oxide (MgO) of 4 wt .-% to 30 wt .-%, thus resulting in the area marked A.

The diagram in Fig. 2 shows the relationship between the size of the occurring when setting a quartz-free investment material expansion ε _A and the content w of silica (SiO ₂ ) in a serving as a mixing silica sol. It can be seen that with a content w of silicon dioxide (SiO ₂ ) of 30 wt.%, A setting expansion of about 0.8% occurs, while with a content w of silicon dioxide (SiO ₂ ) of 40 wt Be achieved curing expansion of nearly 6%. By dilution of an approximately 40% silica sol to a value between 30 wt .-% and 40 wt .-% can thus be adjusted specifically a setting expansion between 0.8% and 6.0%, without causing a change in the composition of other components of the investment, as was customary, needs. The setting expansion ε _A is understood as meaning the change in length ΔL of a particle of the embedding compound after setting in relation to the initial length L _{0 of} the particle in a coordinate direction.

• Oxidkeramikmischung (M₁): 19 Gew.-% Korund (α- Al₂O₃) mit einem mittleren Komdurchmesser von 250 µm
  5,4 Gew.-% Magnesiumoxid (MgO) mit verschiedenen Kornfraktionen von 10 µm bis 70 µm
  63 Gew.-% Schmelzmullit mit einem mittleren Korndurchmesser von 70 µm
  5,8 Gew.-% Zirkoniumsilicat (ZrSiO₄) mit einem mittleren Komdurchmesser von 30 µm
• Bindemittel: 6,2 Gew.-% Monoammoniumphosphat (NH₄H₂PO₄) in wasserfreier Qualität
• Organische Säure: 0,6 Gew.-% Citronensäure
• Anmischflüssigkeit: 30% bis 40%-iges Kieselsol; 16 ml für 100 g Einbettmasse

The according to the diagram Fig. 2 The underlying investment material is composed of the following components, each based on the total weight of the investment:

• Oxide ceramic mixture (M ₁ ): 19% by weight of corundum (α-Al ₂ O ₃ ) with a mean grain diameter of 250 μm
  5.4% by weight of magnesium oxide (MgO) with different particle fractions of 10 μm to 70 μm
  63 wt .-% enamel mullite with a mean grain diameter of 70 microns
  5.8 wt .-% zirconium silicate (ZrSiO ₄ ) with a mean diameter of 30 microns
• Binder: 6.2% by weight monoammonium phosphate (NH ₄ H ₂ PO ₄ ) in anhydrous quality
• Organic acid: 0.6% by weight of citric acid
• Mixing liquid: 30% to 40% silica sol; 16 ml for 100 g investment

Such an embedding compound can be processed in the mushy state to form a casting mold, wherein after about 10 minutes, the setting reaction occurs at room temperature. After about 1 h, when the mold is completely set and firm, the in Fig. 2 for different contents w of silicon dioxide (SiO ₂ ) in the setting solution ε _A shown in the silica sol serving as mixing liquid. In interest for the production of filigree precision castings temperature range up to 900 ° C, such investment has a low, almost linear thermal expansion ε _th of 0.35%. Thus, the investment material is particularly suitable for applications in the medical field, in particular in dental technology, for example for the production of implants.

Thus, for dental alloys based on cobalt, chromium and molybdenum in the model casting technique, a setting expansion on the order of 1.7% to 2.3% is required on a regular basis. In the case of the embedding compound described above, this can be achieved with a silica sol which has a content w of silicon dioxide (SiO ₂ ) of from 34% by weight to 36% by weight. For non-precious metal alloys (NEM), for example dental alloys based on cobalt, chromium and molybdenum, cobalt, chromium and tungsten or nickel, chromium and molybdenum, which replace expensive precious metal alloys, setting expansion values of 2.8% are generally to 3.5%, which can be achieved with the above investment with a 36% to 38% silica sol. In the crown and bridge technique, which mainly uses dental alloys based on cobalt, chromium and molybdenum or titanium, the setting expansion values must be between 2.3% and 3.8%. A silica sol whose content w of silicon dioxide (SiO ₂ ) is between 34% by weight and 36% by weight is suitable for this purpose. For alloys with a high shrinkage value, a setting expansion of up to 6% can also be achieved, as previously discussed. This opens up the possibility of new types of alloys or glass-ceramics for producing biocompatible To use implants that can not be produced even with a quartz-containing investment material.

The embedding compound described above is characterized in that the main constituents of the oxide ceramic mixture are refractory oxides, namely mullite and zirconium silicate (ZrSiO ₄ ). Alternatively, other aluminum silicates, spinel or magnesium aluminates, calcium aluminates or other silicates of the alkaline earth metals, and stabilized and unstabilized zirconia (zirconium (IV) oxide) may be used. To improve the Ausbettung of the casting, the likewise refractory corundum (α-Al ₂ O ₃ ) is present, which is relatively coarse-grained at a grain fraction of more than 100 microns and therefore forms predetermined breaking points in the matrix of the investment. In contrast, the zirconium silicate has a fine grain fraction of less than 40 microns and thus serves to improve the surface quality of the casting.

• Oxidkeramikmischung (M₂): 10 Gew.-% Korund (α-Al₂O₃) mit einem mittleren Komdurchmesser von 250 µm
  5,3 Gew.-% Magnesiumoxid (MgO) mit verschiedenen Kornfraktionen von 10 µm bis 70 µm
  78 Gew.-% Zirkoniumsilicat (ZrSiO₄) mit einem mittleren Komdurchmesser von 30 µm
• Bindemittel: 6,1 Gew.-% Monoammoniumphophat (NH₄H₂PO₄) von wasserfreier Qualität
• Organsiche Säure: 0,6 Gew.-% Citronensäure
• Anmischfllüssigkeit: 37%-iges Kieselsol; 17 ml für 100 g Einbettmasse

A further preferred embedding compound is composed of the following constituents, the content data again referring to the total mass of the embedding compound:

• Oxide ceramic mixture (M ₂ ): 10 wt .-% corundum (α-Al ₂ O ₃ ) with a mean grain diameter of 250 microns
  5.3% by weight of magnesium oxide (MgO) with different particle fractions of 10 μm to 70 μm
  78 wt .-% zirconium silicate (ZrSiO ₄ ) with a mean diameter of 30 microns
• Binder: 6.1% by weight monoammonium phophophate (NH ₄ H ₂ PO ₄ ) of anhydrous quality
• Organic acid: 0.6% by weight of citric acid
• Mixing liquid: 37% silica sol; 17 ml for 100 g investment

In the case of an embedding compound thus composed, the setting reaction likewise occurs after about 10 minutes at room temperature. After about 1 h, the finished casting mold is completely set and a setting expansion ε _A of about 2.5% occurs. The investment has up to 900 ° C an approximately linear thermal expansion of ε _th = 0.3% and is therefore universally applicable in the model casting, but especially for glass-ceramics. With regard to the latter comes advantageously to bear that zirconium silicate (ZrSiO ₄ ) with viscous glass ceramic almost no adhesive compound. Glass ceramics, which are used to produce biocompatible implants, for example as a substitute for tooth parts, cartilage or small bone parts in the jaw or ear area, are generally based on the semi-crystalline basic system SiO ₂ -Al ₂ O ₃ -MgO-CaO-K ₂ O / Na ₂ O. / NaF / P ₂ O ₅ . The glass ceramic is usually pressed at temperatures of about 900 ° C in the mold. Precisely fitting precision parts can therefore be produced because the volume contraction that occurs when the glass ceramic cools is compensated for by the setting expansion of the investment material forming the casting mold. In contrast to a quartz-containing investment, due to the quartz transformation at 570 ° C and the cristobalite conversion at 270 ° C have a discontinuous expansion behavior during cooling with the result that in a nearly constant expansion coefficient having glass ceramic stresses occur, the an impermissible deformation or even lead to cracking, the above-described quartz-free investment material characterized by an approximately constant coefficient of thermal expansion, which thus takes into account a high accuracy of fit.

The investment materials described above are particularly suitable for a titanium investment casting, in which the use temperature of the casting mold, however, should be in about 500 ° C to prevent the formation of an alpha-case shape. In addition, the above investments are characterized by their use in a process for producing castings in which the expansion occurring during setting is controlled in a simple manner. Simple handling is ensured not least by the fact that the organic acid required to achieve high setting expansion values either already contained in the investment material before the mixing with the silica sol, or is added only during the mixing process together with the silica sol in solid or already dissolved form.

LIST OF REFERENCE NUMBERS

ε _A

Setting expansion

ε _th

thermal expansion

w

matter content

A

Range of preferred mixtures

M ₁

Oxidkeramikmischung

M ₂

Oxidkeramikmischung

.DELTA.L

change in length

L ₀

initial length

Claims (9)

1. A process for making castings, in particular filigree precision castings in the medical area, in which a casting mould consisting of a ceramic embedding mass is specifically expanded before pouring in a casting material to compensate for volume contraction of the casting material which occurs during cooling from the molten state, wherein the embedding mass consists of a quartz-free oxide ceramic mixture (M₁, M₂), a binder and a silica sol serving as a pre-mixing liquid with the addition of an organic carboxylic acid,
   characterised in
   that the expansion (ε_A) of the embedding mass which occurs during setting can be specifically controlled by varying the content (w) of colloidal, amorphous silicon dioxide (SiO₂) as a function of the casting material used by means of the addition of colloidal amorphous silicon dioxide (SiO₂).
2. The process according to claim 1, characterised in that the content of silicon dioxide (SiO₂) in the silica sol is varied between 30 wt.% and 40 wt. % for adjusting a specific expansion (ε_A) of the embedding mass.
3. The process according to claim 2, characterised in that 100 g of oxide ceramic mixture, binder and carboxylic acid is pre-mixed with 15 ml to 40 ml of silica sol.
4. The process according to one of claims 1 to 3, characterised in that a silica sol have BET surface areas of the silicon dioxide particles of 50 m²/g to 350 m²/g is used.
5. The process according to any of the preceding claims, characterised in that a di- or tricarboxylic acid, preferably oxalic, malonic, maleic, tartaric, citric or malic acid is used as the carboxylic acid.
6. The process according to any of the preceding claims, characterised in that the quartz-free oxide ceramic mixture is composed of oxides, preferably aluminium silicates, zirconium silicate and stable silicates of alkaline earth metals as well as magnesium oxide (MgO) and corundum (α-Al₂O₃).
7. The process according to claim 6, characterised by a fraction of oxides of 96 wt.% to 30 wt.%, a fraction of corundum (α-Al₂O₃) of trace to 40 wt.% and a fraction of magnesium oxide (MgO) of 4 wt.% to 30 wt.%.
8. The process according to any of the preceding claims, characterised in that a phosphate binder based on magnesium oxide (MgO) and monoammonium phosphate (NH₄H₂PO₄) is used as binder.
9. The process according to any of the preceding claims, characterised in that the fraction of monoammonium phosphate (NH₄H₂PO₄) in the binder is selected to be 5 wt.% to 15 wt.% relative to the mass of oxide ceramic mixture.

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