JPH0811273B2 - Mold making method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a mold making method, and more specifically, to a mold having excellent productivity, good sand removal after pouring molten metal, and excellent appearance of casting surface. Regarding the molding method.

[Conventional technology]

There are mold making processes that use organic binders to improve the productivity of castings (shell mold method, cold box method, etc.), but the molds made by these molding methods are especially used when used as a core. However, there is a problem that gas defects are likely to occur due to organic substances. On the other hand, if a mold made of an inorganic binder is used, there is a problem that productivity is very poor, and sand dropping after pouring the molten metal is very poor.

Therefore, it is desired to develop a mold molding method which has good productivity, generates little gas from the mold, and has good sand removal property after pouring molten metal.

By the way, a method using both an organic binder and an inorganic binder has already been proposed (Japanese Patent Publication No. 51-312).
09, JP-A-63-242439 and JP-A-1-148438). In particular, JP-B-51-31209 and JP-A-1-148438 disclose that a colloidal silica solution is added to a porous core substrate which is formed by adding an organic binder to refractory sand and then fired (JP-B-51-51).
-31209) or a binder for colloidal silica and an amine-based organic substance (JP-A-1-148438). However, these cores are precision casting cores used for precision casting by the lost wax method or the like, and aim at obtaining a high bending strength when casting at a high temperature by casting. It is. Therefore, although it is rich in maintaining the bond strength at high temperatures,
The transverse rupture strength during cooling is as low as ² to 12 kg / cm ² as reported in the examples of JP-A-1-148438. This is because it is impregnated with colloidal silica, and it cannot be used as a normal mold that requires a bending strength of 30 to 100 kg / cm ^{2 at} room temperature.

[Problems to be Solved by the Invention]

The present inventor, unlike precision casting by the lost wax method or the like, has sufficient strength at room temperature, has good productivity, generates little gas from the mold, and has good sand removal properties after pouring molten metal. We have developed a molding method and have already applied for a patent (Japanese Patent Application No. 2-40953). That is, this mold molding method is a mold formed by adding an organic binder to foundry sand, impregnated with an inorganic binder, dried, and then burned with an organic material under high temperature in the atmosphere to mold with the inorganic binder. It is a mold molding method that maintains the shape, and the drying is performed by vacuum microwave drying, or by silicic acid (SiO ₂ ) and alkali (Na
It is characterized by using an inorganic binder having a molar ratio of ₂ O, K ₂ O, Li ₂ O) of 4.5 to 6.0.

Although the effect as described above can be obtained by this mold making method, the casting surface of the casting produced by this method is similar to the casting surface by the mold using the conventional organic binder or the inorganic binder, 50 to It is about 40S.

Therefore, an object of the present invention is to provide a method capable of further improving the casting surface of a casting produced by the above-described mold making method developed by the present inventor.

[Means for solving the problem]

According to the present invention, in order to achieve the above-mentioned object, after impregnating an inorganic binder into a mold produced by adding an organic binder to foundry sand, fine particles of a size that enters the sand grain gap on the mold surface. Mold coating characterized by applying a surface coating material containing aggregate refractory as an aggregate, drying it, then burning organic matter in the atmosphere at high temperature and holding the mold shape with an inorganic binder Law is provided.

[Operation and Mode of Invention]

First, the mold molding method of the present invention will be briefly described. Molding is carried out by kneading a molding sand and an organic binder (binder) by a conventionally known molding method such as a shell mold method, a cold box method and a furan method. Hereinafter, the mold thus formed is referred to as an original mold because it is a base of a mold to be fired. Then, after impregnating with an inorganic binder, a surface coating material containing fine particulate refractory having a size to enter the sand grain gap on the surface of the original mold as an aggregate is dipped, brushed, applied by a method such as spraying. After drying, the organic matter is burned at a high temperature in the air, and the mold shape is maintained by the inorganic binder. That is, "former mold making" → "inorganic binder impregnation" → "surface coating" → "drying" →
It consists of a process called "firing".

As described above, by adopting the surface coating step between the inorganic binder impregnation step and the drying step, as shown in FIG. 1, the surface coating material is provided in the gap between the sand grains 1, 1, ... of the original mold. The fine-grained refractory material 2, 2, ... contained therein enters and the surface portion becomes smooth, and as a result, the casting surface of the manufactured casting can be improved. Also, by the subsequent firing,
Since organic substances are thermally decomposed and burned off, gas defects can be prevented, and SiO _{2 as} an inorganic binder is crystallized to improve fire resistance. Although the air permeability of the original mold is reduced to about 1/100 by applying the surface coating material, the air permeability is restored to about 1/10 (10 times) by the subsequent firing. No problem.

As is clear from the above description of the operation, the casting surface of the casting to be produced changes depending on the grain size of the original sand of the original mold, but it is improved by the inclusion of fine grain refractory having a small grain size in the sand grain gaps of the surface portion. Therefore, the size of the fine particulate refractory to be used is appropriately selected according to the grain size of the original sand of the original mold, but it is preferable to use the fine particulate refractory having a particle size of 44 μ or more and 5% or less. . As the binder contained in the surface coating material, either an organic binder or an inorganic binder can be used, but in terms of adhesion to the surface of the original mold, an inorganic binder to be impregnated in the original mold and It is desirable to use the same one.

In a preferred embodiment of the present invention, as the inorganic binder to be impregnated, silicic acid (SiO ₂ ) and alkali (Na ₂ O, K ₂ O, Li ₂
An inorganic binder having a molar ratio of O) (hereinafter typically referred to as SiO ₂ / Na ₂ O molar ratio) of 4.5 to 6.0 is used. According to the study of the present inventor, if the SiO ₂ / Na ₂ O molar ratio of the inorganic binder used is set in the range of 4.5 to 6.0, it is 30 to 30 at room temperature.
It was found that a bending strength of 100 kg / cm ² was obtained and that it was also excellent in sand removal. Conventionally used colloidal silica (commercially available as Snowtex)
Has strong penetrability and gels when dried to act as an inorganic polymer binder, but due to its extremely high SiO ₂ content,
As a result, even if an attempt is made to maintain the shape of the mold, the transverse rupture strength at room temperature is extremely low and it cannot be used as a mold. On the other hand, in the case of using water glass (SiO ₂ / Na ₂ O molar ratio of about 4.3) and sodium silicate (about 2.4 to 3.3), too high transverse rupture strength, there is a problem in terms of disintegration properties.
In the first aspect of the present invention, by setting the SiO ₂ / Na ₂ O molar ratio of the inorganic binder in the range of 4.5 to 6.0, both the transverse rupture strength and the sand removal property at room temperature are satisfied. It is a thing.

Such an inorganic binder having a specific SiO ₂ / Na ₂ O molar ratio can be easily produced by a method of adding impurities to colloidal silica or adding colloidal silica to water glass. In the mold making method according to the first aspect of the present invention, the drying after applying the surface coating agent may be any of natural drying (leaving dry), low temperature drying at 50 to 150 ° C, or reduced pressure microwave drying. .

In the mold making method according to the second aspect of the present invention, the drying after applying the surface coating agent is performed by vacuum microwave drying. In this case, as the inorganic binder, the above specific SiO ₂ / Na ₂ O mol is used. In addition to the ratio of inorganic binders, other inorganic binders such as colloidal silica can also be used. That is, by performing the reduced pressure microwave drying after impregnation of the inorganic binder and application of the surface coating agent, the impregnated inorganic binder oozes out near the surface layer and only the water is blown off, and the binder remains near the surface. Since a large amount is deposited, the surface strength is increased after firing, and the central part has a small bonding force, so that the sand removal property is also good. Further, since the boiling point of the aqueous binder is lowered by the reduced pressure, the advantage that a short treatment is sufficient is also obtained. Further, since the reduced pressure microwave drying is performed, there is no possibility that bubbles are generated in the case of high temperature drying and the surface becomes fluffy.

When colloidal silica is used as the inorganic binder, the strength of the mold is difficult to come out at room temperature as described above, so baking after drying should be performed at a higher temperature than when using the special inorganic binder. Is desirable.

Further, in any of the above embodiments, the thickness of the impregnated layer of the inorganic binder is such that the impregnation of the inorganic binder is performed under reduced pressure in a vacuum,
The thickness of the impregnated layer can be adjusted by adjusting the pressure and impregnation time, or by using an inorganic binder with a penetrant (surfactant) added to adjust the thickness of the impregnated layer. it can.

〔Example〕

Hereinafter, the present invention will be specifically described with reference to examples, but it goes without saying that the present invention is not limited to the examples.

Examples 1 and 2 and Comparative Examples 1 to 3 Using the original sand refractory mullite having the particle size distribution shown in FIG. 2, test pieces of φ28 × 50 ^H mm were manufactured by the shell molding method, and shown in Table 1. Treatments such as impregnation with an inorganic binder, surface coating, drying and firing were performed under the conditions. The composition of the surface coating material is as shown in Table 2.

Each test piece 4 (φ28 × 50 ^H ) obtained as described above
Set 4 pieces on the bottom of CO ₂ type 5 as shown in Fig. 3 and Fig. 4, and inject cast steel from runner 6 (injection at 1580 ℃, 35k
g).

The casting surface of the obtained casting is the case of Comparative Example 1 (grain size of A).
It was 60S, and in the case of Comparative Example 2 (particle size of B), it was 40S. On the other hand, the casting surface of the castings produced by applying the surface coating material containing the fine-grained refractory as an aggregate was 20S in both Examples 1 and 2, which was a considerable improvement. Further, the results obtained by measuring the casting surface of each of the obtained castings with a stylus type roughness tester are shown in FIG.

In addition, as a result of investigating the state of sand falling by a method of spraying air at a pressure of 4 kg / cm ² from a nozzle arranged at a distance of about 120 mm from the test piece, there was no seizure in all cases, and good sand It showed a falling property.

〔The invention's effect〕

As described above, the mold molding method of the present invention, the original mold molding,
After impregnating with an inorganic binder, a surface coating material containing a fine-grained refractory having a size that enters the sand grain gap of the original mold as an aggregate is applied, and then dried and fired,
The surface of the obtained mold becomes smooth, and the roughness of the casting surface can be greatly improved. Further, in the present invention,
In the mold making method consisting of the above series of steps, as an inorganic binder, SiO ₂ / Na ₂ O molar ratio of 4.5 to 6.0 is used, or by performing the drying by reduced pressure microwave drying, The obtained mold has sufficient strength at room temperature, has no seizure, and has good sand removal property after pouring the melt. In addition, there is little gas generation from the mold as in the case of using an organic binder, mold molding can be performed with good productivity by the above series of steps, and drying time can also be shortened by performing reduced pressure microwave drying. .

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a mold surface portion by a molding method of the present invention, FIG. 2 is a graph showing a particle size distribution of original sand used in Examples and Comparative Examples, and FIG. 3 is Examples and Comparative Examples. FIG. 4 is a plan view of the apparatus for the injection test performed, FIG.
The figure is a graph showing the surface roughness measurement results of casting surfaces obtained in Examples and Comparative Examples. 1 is a sand grain of the original mold, 2 is a fine refractory material.

Claims (5)

[Claims] 1. A surface containing, as an aggregate, a fine-grained refractory having a size capable of entering a sand grain gap on the surface of the mold after impregnating a mold made by adding an organic binder to foundry sand and impregnating the mold with the inorganic binder. A mold making method, characterized in that a coating material is applied and then dried, and then an organic substance is burned at a high temperature in the atmosphere to maintain the shape of the mold with an inorganic binder. 2. The mold making method according to claim 1, wherein the inorganic binder is a water glass-based inorganic polymer binder. 3. The mold making method according to claim 1, wherein the surface coating material is applied by dipping, brushing or spraying. 4. Drying is carried out by natural drying, low temperature drying or reduced pressure microwave drying.
The molding method according to any one of the above. 5. The inorganic binder to be impregnated is an inorganic binder having a molar ratio of silicic acid (SiO ₂ ) and alkali (Na ₂ O, K ₂ O, Li ₂ O) of 4.5 to 6.0. The mold making method according to any one of claims 1 to 4.