DE4002815C2 - High temperature molding material and its use for the production of precision molds for high temperature molding processes - Google Patents

Description

The invention relates to a high-temperature molding material made of metal oxides for fired Precision forms that expand by a desired amount when fired, as well as one under Precision mold for high temperatures produced using such a molding material tur molding process, which allow precision at high temperatures products with reduced or non-shrinking metal (e.g. titanium, nickel, cobalt and chrome), ceramic materials and glass to manufacture ceramic materials that are difficult to form at low temperatures are.

Metals, ceramic materials and glass-ceramic materials can be cast, powder metallurgical processes, high temperature pressing processes and superplastic processes can be brought into complicated shapes. Because this procedure However, if they are carried out at high mold temperatures, they will disappear Products obtained process due to solidification, sintering, phases conversion and cooling when they are cooled at ordinary temperature. As a result, the products obtained differ in size and shape from the target products. To solve this problem, the original form should ver have larger dimensions to compensate for the shrinkage. When casting who the following measures are used to compensate for such shrinkage to worry about:

• (1) The dimensions of wax or wood models are adjusted by appropriate amounts made larger in order to compensate for a freezing down.
• (2) The molds are expanded by an amount to compensate for the shrinkage of the molten metal when the molds are fired in the course of their manufacture.
  The following alternatives can be considered for procedure (2):
• (2) -a) The shapes are expanded when the molten metal solidifies by is used as a binder for the forms of phosphate.  
• (2) -b) There is the expansion of christobalite (SiO₂) at about 300 ° used, whereby Christobalit is used as aggregate.
• (2) -c) The oxidation expansion of metal powder (the one Molding material is added) during the firing of the mold uses.

The above procedures have the following shortcomings.

In the process (1), the dimensions of the wax and wood models must be in Depends on the type of metal or kerami used in casting material. This is extremely troublesome.

In the procedure (2) -a), phosphate in the form can be used at high Temperatures react with molten metal, causing the surfaces the castings become rough. When an active metal, like titanium, is shed, the mold must be fired at 1200 ° or higher to break down phosphates set, after which the mold must be cooled to room temperature. This The process requires a special high-temperature furnace, and it is not economical.

In the case of process (2) -b), christobalite has a low thermal resistance Stability, and christobalite reacts strongly with metals. If active me cast talle (titanium, zirconium and alloys containing titanium or zirconium) the castings obtained have rough surfaces. Will the shape cooled during casting to the problem of forming a rough To meet the surface, the casting temperature is below the transformation temperature (about 300 °), which compensates for solidification shrinkage is rendered ineffective.

Even if additional warming and stretching is provided, that is Process (2) -a) and (2) -b) share the disadvantage that the elongation rate at is just over 2%. For materials with strong solidification shrinkage, like Ni-Cr and Co-Cr, the compensation is insufficient.

With the procedure according to (2) -c) the size of the stretch depends significantly on the grain size of the molding material. There are also differences  the degree of oxidation of the metal powder depending on the respective position of the Powder in the form. This inevitably reduces accuracy. To one To achieve uniform expansion, the heating rate must be extremely slow be set. This results in a reduction in efficiency. In addition, a extremely expensive material, such as zirconium oxide, can be used as aggregate powder, when active metal is formed at high temperatures. This makes the procedure ren less economical. DE 35 42 921 A1 describes a high-temperature molding material and a method for casting known from pure titanium or titanium alloys. The known molding material has according to one embodiment, a semimetal in the form of main components of SiO₂ and alumina, the corresponding mixture when heated the structure of mullite spinel or cordierite is said to form. Correspondingly one rejected The main components are mullite or spinel. In one case as in the other, the molding material also contains a curing agent with a Contains a phosphate and a basic metal oxide. For example, a Mixture containing 55 to 95% by weight of mullite or spinel, 2.5 to 15% by weight Phosphate and 2.5 to 30 wt .-% magnesium oxide with colloidal silicon dioxide with egg nem SiO₂ content of 20 to 40 wt .-% or water. With the ground A wax model is encased in a container in a mixture. Then there is one spontaneous hardening. The spinel is one of the raw materials of the mold fabric; that is, the molding material contains the finished spinel before it is molded of the wax model and the subsequent hardening and firing of the mold. On Shrinkage compensation through solidification expansion of the shape is intended in both versions shapes in accordance with the procedure described in section (2) -a) above can be achieved by adding phosphate.

The invention has for its object, especially for forming active Metal-suitable high-temperature molding material with large shrinkage compensation ons range as well as a precision mold made from it, which can also be used at ho hen temperatures do not react with the molten metal and can be produced in an economical and efficient manner.

A high-temperature molding material made of metal oxides for fired precision molds expands by a desired amount when fired, according to the invention characterized in that the molding material at least one oxide of the general formula A O and B₂O₃ contains an oxide when the mold is expanded under volume expansion form with spinel structure of the general formula AB₂O₄, the 5 to 85 wt .-% of the form makes up, where A and B are each a metallic element.  

The high-temperature molding material according to the invention does not contain finished spinel, son a mixture of metal oxides, which only forms an oxide with Spi when the mold is fired Form nellstruktur, this spinel structure formation to that for the fading leads to the desired expansion.

Another object of the invention is the use of the high-temperature molding material according to the invention for producing a precision onsform for high-temperature molding processes, which is characterized in that a high-temperature molding material according to the invention mixed with a binder, a Mixed liquid added and the mixture according to a desired model is cast and cured as well as the cured form at a temperature of 1300 ° or less is burned, with the burning process extending the Form an oxide with a spinel structure of the general formula AB₂O₄ is formed, the 5th makes up to 85 wt .-% of the form, wherein A and B is each a metallic element.  

The oxides with spinel structure include in particular CuFe₂O₄, MgAl₂O₄, CoAl₂O₄, CoCr₂O₄, CoFe₂O₄, CuAl₂O₄, CuCo₂O₄, CuCr₂O₄, FeAl₂O₄, FeCo₂O₄, MgCr₂O₄, MgFe₂O₄, MgTi₂O₄, MnAl₂O₄, MnCr₂O₄, MnFe₂O₄, MnTi₂O₄, NiAl₂O₄, NiCo₂O₄, NiV₂O₄, NiCr₂O₄, NiFe₂O₄, NiMn₂O₄, SnAl₂O₄, ZnAl₂O₄, ZnCo₂O₄, ZnCr₂O₄, ZnFe₂O₄, ZnMn₂O₄, ZnV₂O₄, MnCo₂O₄, SnCr₂O₄, TiCo₂O₄, TiFe₂O₄, SnMn₂O₄, TiMn₂O₄, SnZn₂O₄, TiZn₂O₄ and the like. Is preferably worked with MgAl₂O₄. The starting materials (metal oxide expansion materials) for producing such Oxides with a spinel structure are represented by AxOy and BmOn. To the AxOy group include in particular CuO, MgO, CoO, CuO, FeO, MnO, NiO, SnO, ZnO, TiO and the like. The BmOn group includes Fe₂O₃, Al₂O₃, Cr₂O₃, Mn₂O₃, Co₂O₃, Ti₂O₃, V₂O₃, Zn₂O₃ and the like. The Oxides with spinel structure mentioned above are obtained by at least two oxides are combined together to form the A O and B₂O₃ group pen belong. When the two groups entered in the same molar ratio are set, they all lead to oxides with spinel structure. Will they be at  groups with different molar ratios will be used Amount of oxide with spinel structure corresponding to the amount of oxide with generated smaller molar ratio. The additional amount that does not add to the Reaction contributes remains unchanged. As a result, it is not necessary that to use both groups in the same molar ratio.

The fired form contains 5 to 85 wt .-% oxides with spinel structure. If the If the proportion of oxides with a spinel structure is below 5% by weight, the shape expands not sufficient to cause the molten metal to shrink retire. In contrast, the content of oxides with a spinel structure is above 85 % By weight, the spinel causes excessive expansion. This will the binding forces of the form are reduced; the shape can easily break.

The grain size of the metal oxide expansion materials (especially MgO and Al₂O₃) which are used as the starting material for the above-mentioned oxides To obtain the spinel structure is set to 80 µm or less. At a The spinel cannot be produced uniformly with a grain size of more than 80 μm, and the shape can easily deform.

The temperature at which the mold is fired is raised to 1300 ° C or we niger set. When the firing temperature exceeds 1300 ° C, spinel becomes in excessive amount is generated, and the shape can easily break.

For the molding material, MgO, which is used as a metal oxide expansion material, in the form of electro-magnesia or magnesia for the following reasons clinker used. When the mold is made, the expansion material gets together men with the aggregates mixed with water or the like. Too general Magnesium oxides available include so-called lightly burned magnesia. The magnesium oxide reacts with water and water contained in the mixed liquid produces magnesium hydroxide, which expands the molding material. The out curing speed of the mold and the amounts of hydrates produced differ depending on the temperature and the water content of the rivers liquid, the size of the mold and the storage temperature of the mold. This ent speaking differences in the size of the extent, and it is difficult to ensure high accuracy on a constant basis. Elec troschmelz magnesia and magnesia clinker do not react with water,  and hardening and stretching during grinding are greatly reduced different. Only the spinel production during firing determines the expansion the form. This facilitates effective control. Although Al₂O₃ with what reacts, its reaction rate is far below that of MgO. The Hydration reaction of Al₂O₃ can consequently be prevented by the Concentration of binder is increased by using a curing accelerator is used or by a rapidly curing phosphate or a small amount of low water ethyl silicate is used. This can prevent expansion due to the hydration reaction will.

Silicon oxide, mullite, zirco are particularly suitable as oxide aggregate materials nium oxide, magnesium oxide, aluminum oxide, calcium oxide, spinel, zircon sand, Dolomite and the like.

In the binder used in the context of the use according to the invention can be aluminum oxide cement, zirconium oxide cement, Magnesium oxide cement, colloidal silicon dioxide, ethyl silicate compounds and Act phosphate compounds. When using phosphate compounds must the mold is fired at 1100 ° C or higher. Otherwise they react Connections with high temperature metal (molten metal), whereby the metal surface is affected.

The fired form according to the invention contains oxides with a spinel structure (AB₂O₄), which is formed by a reaction with oxides (A O- and B₂O₄ during volume of burning. Therefore, once the shape of the predetermined original model (which matches the molded product is correct) and the mold is fired, when molding high-temperature raturwerkstoffe a shrinkage due to the cooling of the molding material to let. As a result, the molded product has almost the same shape as the original model. If a binder with the molding material according to the invention mixed, reacts when mixing liquid is added to the molding material, when pouring the molding material according to the desired model and when Hardening of the molding material MgO hardly with that contained in the liquid Water, and there is no expansion. Then when the molding material  Fired at 1300 ° C or less to produce the fired mold ren MgO and Al₂O₃ with a grain size of 80 microns or less with formation of spinel. Because only the production of spinel is the extent of the burned Shape affected, it is possible to use a fired shape with the desired ex get rate of expansion by the ratio between MgO and Al₂O₃ is appropriately set with a grain size of 80 µm or less. The one setting the quantitative ratio between MgO and Al₂O₃ with a grain size of 80 µm or less takes place in such a way that the spinel produced 5 to 85% by weight of the total fired form. This will break up due to excessive expansion as well as problems due to insufficient the expansion avoided.

In the use according to the invention, a shape is maintained by mixing metal oxide aggregates, metal oxide expansion substances with a grain size of 80 μm or less and a binder. After mixing, liquid is added. The molding material is then poured into a mold (for example a plaster mold or a casting mold ring with a wax model in the case of the melting process), using an original model of the desired shape. Then the mold is hardened. The shape of the cured mold is almost the same as that of the original model, although the mold expands slightly when hydrates are formed due to the reaction between the expansion materials and water. Then, when the mold is fired at 1300 ° C or less, the aggregate and expansion materials are fired to form a rigid fired mold while simultaneously ensuring a mutual reaction of the expansion materials. It oxide with spinel structure (AB₂O₄) is formed in the fired form due to the reaction of the expansion substances. The lattice constant of spinel (AB₂O₄) is about 0.8 nm. The volume of the spinel is much larger than that of the starting material, whose lattice constant is about 0.4 to 0.6 nm. The fired shape is stretched in this way. Because the spinel remains as a new phase, the expanded state of the fired mold is maintained even after the mold has cooled. This is confirmed by the results shown in FIGS. 6 and 7. FIGS. 6 and 7 here illustrate the relationship between the firing temperature and the extent of, appropriately set mold material discussed above. After the temperature has been raised to the desired value and then lowered again to normal temperature, as can be seen from FIGS. 6 and 7, the shape is expanded by up to a few percent, although a shrinkage occurs by the amount due to the thermal expansion is due.

Embodiments of the invention are below with reference to the accompanying Drawings explained in more detail. Show it:

Fig. 1 shows an X-ray diffraction pattern of a calcined form of the present invention,

Fig. 2 is a side view of a Standardkokille for the preparation of he drive inventive fired molded in Wachsausschmelzver,

Fig. 3 is a vertical cross-section of the presently provided wax pattern molding ring assembly,

Fig. 4 is a vertical section of an embodiment of an arc-differential-pressure casting apparatus in which the ge burned shape is used according to the invention,

Fig. 5 is a view for explaining a method for determining the rate of expansion,

FIGS. 6 and 7 Temperature / Stretch characteristics for the erfindungsge MAESSEN molding material, and

Fig. 8 (I) and 8 (II) are sectional views of a modified form of apparatus.

A molding material with a composition according to Table 1 was lightly ground with water and poured into a suitable mold. This shape was then fired at 850 ° for one hour, followed by X-ray diffraction measurements. The X-ray diffraction diagram illustrated in Fig. 1 shows clear diffraction peaks, as are typical for spinel (MgAl₂O₄). It follows that the fired form contains a considerable amount of spinel (MgAl₂O₄).

Table 1

A wax model 2 was produced on a metal mold 1 according to FIG. 2 and inserted into a casting ring 3 according to FIG. 3. Molding material with a composition according to Table 2 was lightly ground with water and poured into the casting ring 3 , the wax model 2 being enclosed by the molding material. The molding material was then cured. The inner surface of the casting ring 3 was lined with ceramic wool 31 to absorb the expansion of the material in the radial direction during the firing. The wax model 2 was placed on a rubber base 32 , which was arranged at the lower end of the casting ring 3 . After curing, the rubber base 32 was removed and the wax model 2 was fired under the conditions shown in Table 3 (temperature and burning time). A fired mold 4 with a cavity 41 corresponding to the wax model 2 was placed in an arc differential pressure casting unit 5 shown in FIG. 4, and titanium was cast. The casting was carried out under two different conditions. In one case, the casting mold was kept at normal temperature; otherwise the mold was held at 800 °. A casting 6 was put over the mold (metal mold) 1 , and the gaps h and g shown in Fig. 5 were measured. The measurement results are summarized in Table 3.

Table 2

Table 3 shows that the molds can break if the mold firing temperature exceeds 1300 °. The gap h can often be zero at casting temperatures of 900 to 1300 °. In the temperature range concerned, the molds expand in a suitable manner. A small gap g can occur if the gap h is zero. In this case, the casting 6 sits on the mold 1 with a certain play. This condition is desirable in some types of castings. The gap h remains relatively small even when casting at normal temperature. The shrinkage compensation which is achieved in the case of high-temperature casting of metals, such as titanium, can therefore also be achieved when casting at normal temperature. In this case, inexpensive silicon dioxide or the like, which can react with titanium at high temperatures, can then be used as the aggregate.

The relationship between the amount of spinel produced in the molding material and Columns g and h are explained below with reference to Table 3. The Columns g and h are zero in the case of molded materials in which 19% and 34% spinel are generated. These substances therefore lead to extreme high dimensional accuracy when the shapes expand. At a Molding material with a spinel production rate of 91% breaks the mold. The molding material, in which 5% spinel is produced, leads to a gap g of Zero and a gap h of 0.4 mm, while the molding material with a Spinel generation rate of 85%, a gap h of zero and a gap g of 1 mm or more without breaking the mold. These Molded materials are therefore within the practical area of application.

The molding materials with the content ratios according to Tables 1 and 4 were fired, and their elongation values were measured at the temperatures lying in the measuring range. The results are shown in FIG. 6 (for the molding material with the content ratio according to Table 1) and in FIG. 7 (for the material with the content ratio according to Table 4). From FIGS. Shows 6 and 7 that decreases when lowering the temperature of the internal Tempera ture to normal temperature the elongation value and a gradual shrinkage occurs, but that a relatively high elongation value is then th preserver even after the temperature to a normal value has returned. In particular, the molding material according to FIG. 7 has an elongation value of several percent because the expansion materials of the starting material have small grain sizes and a large amount of spinel is produced.

Table 4

Molding materials with aggregate materials and expansion materials with the content ratio nits according to Table 5 were fired at 900 ° and 1400 ° to make this possible to investigate the fracture of the molds. The results are clear from the column "Appearance after firing" in Table 5. "o" means none Fracture; "x" means break. The expansion fabrics each had a grain size of 80 µm or less, and they were 50% by weight. Commercially available Bare aggregate materials were used without further modification; they made 42 % By weight. In addition, binders of the type explained above were provided.  

Table 5

From Table 5 it follows that there is no break at 900 ° while all Forms broke at 1400 °. Burnt molds can be made len by the appropriate combinations of the above-mentioned aggregate and Expansion fabrics can be burned at 1300 ° or less. CaO and ZrO₂, which are used as aggregates, are combined with Al₂O₃ and MgO niert, which serve as expansion materials, since stable oxides ge as expansion materials must be chosen. CaO and ZrO₂ are oxides that are stable to metals are which are active at high temperatures. With the expansion fabrics and the aggregate substances can be the same components, provided that their grain sizes are not more than 80 µm. The stretch value of the burned Shape can be depending on the firing temperature, the firing temperature Holding time, the casting temperature, the grain size and the spinel content control the relevant oxides. The shapes are therefore excellent Way for precision molding of metals active at high temperature, ke ramischen materials and glass-ceramic materials.

Figures 8 (I) and 8 (II) show the use of superplasticity techniques on titanium alloys. In the case of the method of Fig. 8 (I) is a plate 7 hydraulically or pneumatically pressed (made of a titanium alloy or the like) which is heated to a high temperature, between a lower mold 7 a and an upper mold 7 b inserted and so that there is a plastic deformation corresponding to the shape of the upper and lower mold 7 a and 7 b. In the process illustrated in FIG. 8 (II), a plate 8 similar to the plate 7 mentioned above is placed on a lower mold 8 a. The plate 8 is immediately subjected to gas pressure or the like to plastically deform the plate until it takes the shape of the lower mold 8 a. The upper mold 7 b and the lower molds 7 and 8a can be prepared in the manner explained below. Models are detached from the original models using agar or silicone. Then the models are poured into the molding material and the molding material is cured. After removing the models made of agar or silicone, the molding material is fired. If the molds 7 a, 7 b and 8 a are produced in the manner described here, the molds after firing are somewhat larger than the original models, which is due to the production of spinel oxides explained above. Therefore, even if the plates 7 and 8 heated to high temperatures shrink after cooling after processing, the shrinkage is compensated for by the expansion explained above. The dimensional accuracy of the products obtained is excellent if the components, the grain size and the firing temperature of the molding material are set accordingly.

In the manner explained above, therefore, when active at high temperature Metals, ceramic materials and glass ceramic materials among users high temperatures, the shrinkage of these materials be compensated. The expansion value of the fired form can be exactly control by the proportion and grain size of the spinel producing Oxides, the firing temperature, the firing temperature holding period and the like Parameters can be set. The elongation value can be up to several percent be; this allows an extremely wide shrinkage compensation range. Accordingly, the respective requirements can be in a wide range be complied with. Because spinel production determines expansion, sufficient expansion can be obtained even if the Temperature of the mold is lowered. When molding one at high temperature active metal, such as titanium, can replace costly aggregates, such as Zirconium oxide and magnesium oxide, an inexpensive aggregate such as sili ziumdioxid be used.

The precision mold explained above, the precision molding material described and the explained mold manufacturing process are excellent for a Precision casting of dental crowns, dental fillings and dental prosthesis plates. In addition, titanium alloy plates can be particularly useful Be processed in the superplasticity process.

Claims (5)

1. High-temperature molding material made of metal oxides for fired precision molds, which expands to a desired extent during firing, characterized in that the molding material contains at least one oxide of the general formula AO and B₂O₃, which has an oxide with a spinel structure when the mold is fired with volume expansion form the general formula AB₂O₄, which makes up 5 to 85 wt .-% of the form, wherein A and B are each a metallic element. 2. High-temperature molding material according to claim 1, characterized in that the Oxides of the general formula A O and B₂O₃ have a grain size of 80 microns or have less. 3. High temperature molding material according to claim 1 or 2, characterized in that the oxides of the general formula A O and B₂O₃ in a stoichiometric Ver ratio are mixed. 4. High-temperature molding material according to one of the preceding claims, characterized characterized in that MgO as the oxide of the general formula A O and Al₂O₃ as Oxide of the general formula B₂O₃ are provided, and that the MgO as Elek troschmelz magnesia or magnesia clinker is present.   5. Use of the high-temperature molding material according to claim 1 to 4 for the manufacture a precision mold for high-temperature molding processes, characterized records that the high temperature molding material mixed with a binder, a Mixed liquid added and the mixture according to a desired Mo dell is poured and cured, as well as the cured form at a tempe temperature of 1300 ° C or less is burned, with the burning process under off expansion of the form an oxide with spinel structure of the general formula AB₂O₄ ge is formed, which makes up 5 to 85 wt .-% of the form, with A and B each a me is a metallic element.