DE102022105961A1 - Process for producing a mold core or riser for creating cavities in castings - Google Patents

Abstract

A method for producing a mold core or riser for producing cavities in castings is proposed, in which the molded body is made from glass, with starting materials of the glass being first mixed and then melted, then cast in a mold and then heated. This creates cores that have a high degree of dimensional stability at high temperatures and are not prone to degassing at these temperatures.

Description

The invention relates to a method for producing a mold core or riser for producing cavities in castings, the mold core or riser being made from glass.

Mold cores are usually made from sand or salt and are used to create a cavity in a casting. These cores are removed either mechanically, for example by shaking, or chemically by dissolving.

Risers are usually hollow cores into which the liquid metal penetrates during casting, so that during solidification and the resulting shrinkage process, especially in areas with large wall thicknesses, liquid metal can flow from the risers into the cavities via these cavities.

These mold cores and risers usually contain binders to hold the salts and sand together. This binder evaporates during casting, which means that degassing from the core to the outside must be provided, otherwise the gas will settle in the casting and lead to bubbles and increased condensation formation. For this purpose, channels are usually provided through which the gas can be removed. However, there are positions in castings for which degassing is difficult or impossible to achieve.

An attempt is therefore made to expose such cores to microwaves immediately before casting, which significantly reduces degassing during casting. However, this requires an additional manufacturing step.

Furthermore, it is known to use glass as a material to produce the cores. Glass cores are primarily used to create small, delicate channels.

Such a glass core is made, for example DE 1 132 295 A known. The core described there is made of borosilicate glass, which is highly insensitive to temperature fluctuations and has low thermal conductivity, which means that very precise cavities can be achieved in the castings.

Even if good dimensional accuracy can be achieved with such a core, the problem often arises of removing the glass core from the casting, which is therefore often done chemically. In particular, the glass should only have a low thermal conductivity and a low heat capacity in order not to extract too much heat from it when it comes into contact with the melt, as this would lead to accelerated solidification, which is particularly undesirable when used as a feeder. Above all, however, it is important that the coefficient of thermal expansion of the glass should be lower than that of the material to be cast, especially aluminum, to allow mechanical removal from the casting and to prevent dimensional inaccuracies from arising.

The task is therefore to create a method for producing a mold core or riser for creating cavities in castings, with which very precisely dimensioned cavities can be created, which should also have the lowest possible surface roughness, and which can simply be done mechanically by shaking or hammering are to be removed. It should be possible to avoid degassing the cores. In addition, the thermal conductivity should, if possible, be below 0.00580 W/(cm*K) at 527 °C. The shape stability of the mold core or riser should also be maintained for around 5 minutes at 700°C. The coefficient of thermal expansion should be less than that of aluminum to allow mechanical release after casting.

This object is achieved by a method for producing a mold core or riser for producing cavities in castings with the features of claim 1.

The mold core or riser is made from a glass, whereby this glass core is designed to be hollow when used as a riser, and when used as a mold core forming a cavity it is designed as a solid body. The starting materials for the glass are first mixed according to specifications and then melted. The molten glass is then poured into a mold that can depict the desired outline. After casting, the mold core or riser is heated. The temperature must be adapted to the manufacturing process.

Mold cores or risers produced in this way almost do not degas during a subsequent casting process, which significantly improves the casting quality because gas inclusions are reduced and condensate build-up is reduced without the need for degassing channels. The resulting glass bodies have lower thermal conductivity and lower heat capacity compared to conventional glass bodies. In addition, such a body has a very low coefficient of thermal expansion, which allows the core to be removed mechanically by shaking or hammering, as it expands less during subsequent casting than the surrounding material. Nevertheless, sufficient thermal stability can be achieved.

Preferably, the cast glass is first crushed into a powder and mixed with a blowing agent. Such a mixture makes it possible to produce a foam glass that is very porous, which significantly reduces the thermal conductivity and the heat storage number.

The glass powder mixed with the blowing agent is heated to 1000 ° C, which creates foam glass, as the heating causes glass bubbles to form due to the blowing agent, which results in a highly porous body being created, which accordingly has a low thermal conductivity coefficient with good thermal stability.

In a further embodiment of the method according to the invention, the starting materials of the glass are at least silicon dioxide and boron trioxide. Such glasses have a relatively low thermal conductivity.

Preferably the glass is a borosilicate glass which has 70-80% silicon dioxide, 7-13% boron trioxide, 4-8% alkali oxides, 2-7% aluminum oxide and 0-5% alkaline earth oxides as starting materials and which has physical properties suitable for use as Core are particularly suitable if subsequent tempering is carried out to reduce stress and thus the temperature stability of the glass is sufficiently increased.

The glass can also contain barium oxide as a starting material. Such a glass can, for example, contain 35-45% barium oxide, 20-30% boron trioxide and 25-40% silicon dioxide as starting materials. Such a glass has similar physical properties to those of borosilicate glass.

For such a glass, the heating is preferably carried out as a heat treatment in the form of annealing at a temperature of 400 to 700 ° C. The tempering of the glass is therefore carried out in the area of the transformation temperature and below the melting temperature of the glass. The temperature during tempering is correspondingly below the melting temperature of the glass and usually also below the melting temperature of the material that will later enclose the mold core or riser during casting. In this way, stresses in the glass core can be reduced, which means that the core has greater high-temperature stability than is the case with glasses manufactured differently.

Furthermore, the porosity of such a glass can be increased to reduce the thermal conductivity by etching the surface after tempering and then washing, then carrying out an extraction process, then carrying out washing and drying and finally carrying out sintering. Accordingly, a phase of the mixture is removed from the cast glass core and the subsequent sintering results in further strengthening and simultaneous shrinkage. Such a process is called the Vycor process.

The extraction process is carried out using acids or inorganic salt solutions at 90 to 100°C. In this way, the phase to be removed can be removed in sufficient quantities to increase the porosity.

The sintering process advantageously takes place at 1000 to 1300°C. The porous body is significantly solidified by melting the glass particles and thus sufficient temperature stability is achieved.

The molded body is manufactured as a hollow body or solid body, so that it can be used for both solid mold cores and risers.

A method is thus provided with which dimensionally accurate cavities with desired surface roughness can be produced. The arrangement of degassing channels can be completely dispensed with, since a core produced in this way has almost no gas development during casting, so that no condensates are formed during casting. The mold core can be easily removed by shaking or hammering because it has lower thermal expansion than the aluminum of the casting. It is also particularly suitable for cores because it has a very low heat capacity and low thermal conductivity. Furthermore, a mold core or riser produced in this way has sufficient dimensional stability at around 700 ° C and thus at the usual casting temperature for aluminum castings.

Two exemplary embodiments of the method according to the invention for producing a mold core or riser for producing cavities in castings are described below.

In a first process according to the invention, 70-80% silicon dioxide, 7-13% boron trioxide, 4-8% alkali metal oxides, such as sodium oxide or potassium oxide, 2-7% aluminum oxide and 0-5% alkaline earth metal oxides, such as calcium oxide or magnesium oxide, are mixed as starting materials and melted, producing a borosilicate glass.

Alternatively, for example, 35-45% barium oxide, 20-30% boron trioxide and 25-40% silicon dioxide can be mixed as starting materials and then melted.

In both cases, this glass is then poured into a mold. This mold can be used to create a hollow body, which is then used as a feeder, or a solid body, which can be used as a mold core when casting aluminum.

Due to its composition, such a core has a relatively low thermal conductivity and is therefore well suited as a core for casting. However, the problem is that it may not have sufficient dimensional stability at the usual aluminum casting temperatures of around 700 ° C. This should remain for at least 5 minutes during casting.

For this reason, a correspondingly manufactured core is then subjected to a tempering process in which the mold core or riser is heated to around 600°C. This leads to a significant reduction in stress in the glass after casting and, as a result, to significantly increased dimensional stability under thermal stress.

To increase the porosity and/or surface roughness and thus reduce the thermal conductivity, a phase can then be extracted from a mold core or feeder produced in this way by removing the alkaline earth metal oxides, such as barium oxide, or the alkali metal oxides present, such as sodium oxide or potassium oxide, from the mold core become. This is done by first etching the surface with an appropriate acid, such as hydrofluoric acid, or alkali, such as caustic soda, and then washing it with alcohol, water or a dilute soda solution. The actual extraction process then takes place by exposing the mold core or riser to an acid or inorganic salt solution at 90°C to 100°C, whereby the alkaline phase is removed from the mold core. The acid or salt solution used must be adjusted accordingly to the alkali metal oxide to be removed. A washing process and drying are then carried out, creating an amorphous porous body.

A sintering process can then be carried out, which leads to volume shrinkage, which gives the body greater strength and reduces the proportion of micropores.

A glass produced in this way has a high chemical, thermal and mechanical stability and can be used as a mold core or riser, which has almost no degassing during subsequent casting, has the required dimensional stability and, due to the porosity, a low thermal conductivity coefficient and increased surface roughness in comparison to usual glass cores.

According to an alternative second method according to the invention, a foam glass can also be produced, which is made from recycled waste glass and can have chemically different compositions.

When producing this foam glass, the starting materials are mixed again and then melted and cast. The resulting body is then ground into a powder and a blowing agent such as carbon is added. The following heat treatment, in which the powder melts again and is to be shaped accordingly in a mold, is carried out at a temperature of 1000 ° C, which causes oxidation, which creates gas bubbles in the glass. The resulting foam body has a very high porosity, which makes it particularly suitable as a riser or mold core when casting aluminum, because the porosity significantly reduces the thermal conductivity and heat storage capacity compared to solid glass. Accordingly, less heat is removed from the liquid aluminum. At the same time, however, such a foam glass body has a sufficiently high high-temperature stability so that the mold core or riser remains in shape for a sufficiently long time during a subsequent casting process. Such a core also has a lower coefficient of thermal expansion than aluminum, so that the mold core or riser can easily be removed mechanically from the later casting by hammering or shaking.

The mold cores or risers produced by one of these two processes can be used advantageously in aluminum casting, as they are dimensionally stable for a sufficiently long time at a casting temperature of around 700 ° C. The thermal conductivity of mold cores or risers produced in this way is less than 0.58 W/(cm*K) at 527 °C and less than 0.593 W/(cm*K) at 727 °C and is therefore in the range of a usual sand or salt core, whereby a small amount of heat is removed from the material to be cast and the core remains dimensionally stable for longer. The specific heat storage number is less than 2.732 J/(cm ³ *K) at 527 °C and 2.980 J/(cm ³ *K) at 727 °C and is therefore also in the range of a sand core. This is achieved either, as with foam glass or with glasses treated using the Vycor process, through the high porosity that can be achieved or through the use of special glass suitable glass mixtures and subsequent chip reduction through tempering. Above all, these mold cores or risers ensure that when casting a metal casting, almost no degassing occurs when the core or riser comes into contact with the molten metal, which significantly improves the casting quality and makes it possible to dispense with the provision of degassing channels. This also reduces the formation of condensation in the mold.

It should be clear that different chemical compositions of the mold cores or risers are possible. The values described for thermal conductivity, heat capacity or temperatures are also guide values and are subject to a certain range and depend on the starting materials and values.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 1132295 A [0007]

Claims (11)

Method for producing a mold core or riser for producing cavities in castings, the molded body being made from glass, characterized in that starting materials of the glass are first mixed and then melted, then poured into a mold and subsequently heated. Method for producing a mold core or riser for creating cavities in castings Claim 1 , characterized in that the cast glass is first crushed into a powder and mixed with a blowing agent. Method for producing a mold core or riser for creating cavities in castings Claim 2 , characterized in that the glass powder mixed with the blowing agent is heated to 1000°C, resulting in foam glass. Method for producing a mold core or riser for creating cavities in castings Claim 1 , characterized in that the starting materials of the glass are at least silicon dioxide and boron trioxide. Method for producing a mold core or riser for creating cavities in castings Claim 3 , characterized in that the glass is a borosilicate glass, which has 70-80% silicon dioxide, 7-13% boron trioxide, 4-8% alkali oxides, 2-7% aluminum oxide and 0-5% alkaline earth metal oxides as starting materials. Method for producing a mold core or riser for creating cavities in castings Claim 3 , characterized in that the glass also contains barium oxide as a starting material. Method for producing a mold core or riser for producing cavities in castings according to one of Claims 4 until 6 , characterized in that the heating is carried out as annealing at a temperature of 400 to 700°C. Method for producing a mold core or riser for creating cavities in castings Claim 7 , characterized in that after tempering the surface is etched and then washed, an extraction process is then carried out, washing and drying is then carried out and finally sintering is carried out. Method for producing a mold core or riser for creating cavities in castings Claim 8 , characterized in that the extraction process is carried out using acids or inorganic salt solutions at 90 to 100°C. Method for producing a mold core or riser for creating cavities in castings Claim 8 or 9 , characterized in that the sintering is carried out at 1000 to 1300°C. Method for producing a mold core or riser for producing cavities in castings according to one of the preceding claims, characterized in that the molded body is produced as a hollow body or solid body.