JPS61176439A - Production of ceramic core - Google Patents

Abstract

PURPOSE:To manufacture ceramic cores which have high initial strength, permit easy handling and have good dimensional accuracy even with a small number of or large sized patterns by packing a kneaded material composed of a powdery and granular refractory material and binder into a horizontal mold and parting from the pattern drying and calcining the molding after freeze curing. CONSTITUTION:The kneaded material prepd. by mixing a refractory powdery and granular material such as molten silica powder and zircon flour and a binder such as colloidal silica and adding an additive thereto according to need is packed into the pattern coated with an antifreezing liquid such as grease and is frozen at a low temp. of about -20 deg.C. The molded material is put into an ordinary temp. furnace after parting from the pattern and is dried by low temp. heating at 100-400 deg.C. The core permits easy handling at a room temp., does not shrink and has good dimensional accuracy as the core is produced by freezing. The use of the wooden pattern is possible and the manufacture of a small number of or large-sized cores at a low cost is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method of manufacturing a ceramic core for large castings.

(Conventional technology) Ceramic cores for precision casting are made of refractory powder (fused silica, alumina, zircon, etc.) and a binder (ethyl silicate hydrolyzate, colloidal silica, thermoplastic resin, thermosetting resin, etc.). After mixing and molding various auxiliaries,
It is manufactured by drying (degreasing) and firing. As the molding method, injection molding, extrusion molding, and casting molding are mainly used. Regarding these various molding methods,
This will be explained below.

In the injection molding method, refractory material powder is mixed with a thermoplastic resin or thermosetting resin as a binder, and various auxiliary agents, and the mixture is press-fitted into a mold in the same way as in plastic molding. is cooled, and thermosetting resin is heated and molded.

In the extrusion molding method, refractory powder is mixed with various binders and auxiliary agents to impart plasticity, and the mixture is extruded through a die to continuously mold the product. This method is applied to objects with a constant cross-sectional shape, such as pipes and rods.

In the pour molding method, a refractory powder and a binder (mainly ethyl silicate hydrolyzate and colloidal silica are used) are mixed to form a slurry, which is poured into a mold and reacted with a gelling agent. It is molded by drying and hardening.

However, these conventional molding methods have drawbacks as shown below.

(1) Since a mold is used, it cannot be economically applied to small quantities of cores or single cores.

(2) Since a mold is used, there is a limit to the size of the core to be manufactured (especially in the case of injection molding).

(3) When poured and gelled and hardened, the strength of the green mold is low and it is difficult to apply it to large cores.

Furthermore, deformation is likely to occur during drying.

(4) Cracks due to shrinkage when dried and hardened in a mold.
The occurrence of cavities or pores due to moisture evaporation is observed.

(5) Wooden molds cannot be used with methods other than the gelation curing method because they cannot withstand the pressure and heat during molding and the heat during dry curing.

(Problems to be Solved by the Invention) The purpose of the present invention is to eliminate the drawbacks of the above-mentioned conventional methods, and to improve the quality of large castings (dimensional accuracy and casting surface) compared to conventional precision castings.
The object of the present invention is to provide a method for manufacturing a ceramic core for large-sized castings.

(Means for Solving the Problems) The present invention attempts to solve the above problems by utilizing a freeze compression molding method for ceramics that has recently been studied (see "Sokeiza" published in November 1984). and
We propose a method that can practically produce ceramic cores on an industrial scale using this molding method.

The present invention relates to the production of a ceramic core, which is characterized in that a powdery refractory material is kneaded with a binder, filled into a model, hardened by freezing, released from the mold, and dried and/or fired. Regarding the method.

That is, conventionally, initial strength was obtained by heating or gelling reaction, but the present invention is characterized in that initial strength is developed by freezing.

The materials are mixed by adding and mixing refractory powder such as fused silica powder or zircon flour, a binder such as colloidal silica, and additives as necessary. Any mixer can be used as the mixing device as long as the materials can be mixed uniformly. It is even better if the mixture can be degassed by performing the mixing under reduced pressure.

For molding, it is easiest to pour the mixture into a slurry and mold it, but it is also good to form a paste and mold it under low pressure. It is also desirable to use reduced pressure, increased pressure, centrifugal force, etc. during molding.

The freezing temperature for freeze hardening varies depending on the type of binder used, freezing rate, etc. Freezing can be done in a cold room, freezer room, or liquid N.
! Any method such as usage may be used.

It is necessary to select a mold release agent used for mold release that does not freeze even at low temperatures and that does not adhere to frozen binders. For example, grease, silicone oil (paste, liquid), Teflon (spray), and other commercially available mold release agents can be used.

Drying is carried out by taking out the frozen core from the model, immediately inserting it into a furnace at room temperature, and gradually increasing the temperature to a predetermined temperature, for example, at a low temperature of about 100 to 400°C. This is done to remove moisture and various auxiliary agents, and to impart strength to withstand sintering. Therefore, if a method is adopted in which the temperature is gradually increased in the next baking process, moisture and the like can be removed during the temperature rise, so that drying is not necessary.

Firing is performed to provide strength to withstand casting and handling, and to remove gasified substances so that gas is not generated during casting.

Therefore, if a substance that decomposes and gasifies at the firing temperature is not added, it is not necessarily necessary and only the drying step is sufficient. Although the firing temperature varies depending on the type and particle size of the refractory material, and the type and amount of the binder, firing is usually performed at 800°C or higher.

Incidentally, a typical thermal history pattern of drying and firing in the method of the present invention is shown in FIG. The heat history patterns (■ to ■) in FIG. 1 are as follows.

■ Temperature increase + hold + temperature increase + hold + furnace cooling (drying) (baking) ■ Temperature increase (slow) 10 temperature increases (high speed) 10 hold + furnace cooling (baking) ■ Temperature increase + holding + furnace cooling (drying) ) The binder that can be used in the method of the present invention may be any binder as long as it freezes when cooled; Aqueous solutions of water-soluble organic substances and the like are preferred.

The method of the present invention not only manufactures ceramic cores using refractory powder, but also manufactures conventional cores and molds (main molds) (zircon sand, chromite sand, etc.) for the purpose of improving dimensional accuracy and preventing casting defects. (using aggregate) can also be applied. In addition, in the method of the present invention, various additives (such as antifoaming agents, dispersants,
Sintering aids, etc.) can also be used as long as they do not inhibit freeze-hardening.

(Function) In the method of the present invention, by freezing the binder,
The following effects occur. That is, the solute in the binder (e.g., a gelling substance such as colloidal silica) and the solvent (e.g.,
water) separates and freezes, and the ice and frozen gelled colloidal silica give strength to the core. In addition, since water (solvent) is frozen and separated as ice, it is easily removed during drying and/or firing.

Moreover, since the surface of the core dries at low temperatures, it is possible to maintain its shape during drying.

(Effects of the invention) (1) Due to the initial strength hole, non-linking is easy.

(2) Since it expands during freeze hardening, it is possible to prevent the generation of voids and pores and improve dimensional accuracy. That is, in the conventional method of hardening a slurry by heating in a model, the slurry hardens even more when the water evaporates, so the core contracts when the water evaporates, and the size of the model also becomes smaller. In contrast,
In the freezing-hardening method of the present invention, the water (solvent) in the slurry expands when it freezes, so if the model has the strength to withstand this expansion pressure, the freeze-hardened core will be faithful to the model. This results in improved dimensional accuracy.

(3) Since there is no heating process during curing, wooden molds can be used for the model. Therefore, by applying the method to a single core, it is possible to realize a reduction in the unit cost of the core due to a reduction in model cost.

(4) High initial strength and no shrinkage cracking during curing.

Furthermore, since it is possible to use a wooden mold, it can be applied to large cores.

(5) Small shrinkage during firing. Therefore, the dimensional accuracy is good.

Examples of the present invention are shown below.

Example 1 The slurry was kneaded according to the formulation shown in Table 1, degassed under reduced pressure, poured into a wooden mold measuring 80 days long x 20 m wide x 30 deep and frozen at -20°C. After freezing for 3 hours, the mold was released and kept at -10℃ for 3 hours, and then heated to 2aO℃.
A test piece was prepared by drying and firing at 1000° C. for 2 hours (heating rate 20° C./hour) and cooling in a furnace for 2 hours (heating rate 20° C./hour). The strength and shrinkage rate of the obtained test piece were as shown in Table 1.

Table 1 Example 2 A slurry is made by mixing according to the composition of Table 2, and the size is 80 m long x 20 cm wide x 30 m deep and 100 m long x 100 cm wide x s o m x wide.
It was frozen and molded into a wooden mold with a depth of 30 mm in the same manner as in Example 1. After both molds were finished, the molds were not left in a low temperature environment, but were immediately dried and fired in the same manner as in Example 1. The strength and porosity of the obtained test pieces are shown in Table 2 regardless of which wooden mold was used.
It was as shown in

Table 2 Example 3 A slurry was prepared by kneading according to the formulation shown in Table 3, and using the same wooden mold as in Example 2, it was frozen and molded in the same manner as in Example 2. After demolding, drying and firing were performed under the following conditions.

Room temperature to 200℃ (heating rate 20℃) 200 to 10
Table 3 shows the bending strength of the test specimens obtained after 2 hours of holding at 1000°C and cooling in the furnace, regardless of the wooden mold used. It was on the street.

Table 3 Example 4 After omitting the firing step of Example 3 and holding at 200°C for 2 hours,
The test was carried out in the same manner as in Example 3, except that it was cooled in a furnace, and the bending strength was 40 k&/
It was possible to obtain a test piece with a diameter of 2 cm.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a typical thermal history pattern during drying and firing in the present invention. Sub-Agents 1) Meikoku Agent Ryo Hagiwara - Figure 1 Time-

Claims (1)

[Claims] A method for manufacturing a ceramic core, which comprises adding a binder to a powdery refractory material, kneading the mixture, filling it into a model, hardening it by freezing, releasing it from the mold, and drying and/or firing it.