JPH04147741A - Resin coated sand for shell mold - Google Patents

Abstract

PURPOSE:To manufacture a mold having high strength with little binder resin quantity by coating binder resin on molding sand having the specific value of porocity. CONSTITUTION:This invention is related to coating of the binder resins on the molding sand having <=44% porocity and the molding sand having >=44%, respectively. What the binder resin quantity is much, is the easiest method to raise the strength of mold, but this is economically disadvantageous and gas quantity generated by decomposing the binder resin, is much and it is fear that this gas develops blow hole or bad casting surface by involving into the casting. Mixed ratio of the sand and the binder is not particularly limited and to 100 wt parts of the molding sand, the binder resin can be made to <=1 wt parts in terms of resin solid content.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to resin coated sand for shell molds.

[Conventional technology]

A shell mold is obtained by molding foundry sand such as silica sand bound with a binder resin. This shell mold is widely used because it has excellent features such as a smooth casting surface and good dimensional accuracy. In this shell mold, the type of foundry sand and binder used are changed depending on the type, temperature, shape, etc. of the metal to be cast.For example, in the case of foundry sand, Foundry sand with a high melting point is used to prevent seizure at high temperatures, and foundry sand with low thermal expansion at high temperatures, such as S, is used to suppress expansion due to the heat of molten metal.
sand with a low content, or recovered sand that has undergone thermal history. In the case of binders, we use high-strength and low-expansion binders as appropriate. In order to increase the strength of the mold, it is easiest to increase the amount of binder resin, and conventionally, the strength of the mold is ensured by the amount of binder resin.

[Problem to be solved by the invention]

However, increasing the amount of binder resin is not only economically disadvantageous, but also increases the amount of gas generated when the binder resin is decomposed by the heat of the molten metal, and this gas enters the casting and forms cavities. There is a problem that there is a risk that the casting surface may deteriorate. The present invention has been made in view of the above points, and an object of the present invention is to provide a resin-coated sand for shell molds that allows a mold with high strength to be obtained with a small amount of binder resin.

[Means to solve the problem]

The resin-coated sand for shell molds according to the present invention is characterized in that foundry sand with a porosity of 44% or less and foundry sand with a porosity of 44% or more are each coated with a binder resin. It is. The present invention will be explained in detail below. Although there are no particular limitations on the potable river sand, silica sand is generally used, and recovered sand recovered after collapsing the mold can also be used. The present invention is characterized by using a mixture of foundry sand having a porosity of 44% or less and sand having a porosity of 44% or more. Here, porosity refers to the ratio of the volume of voids between sand particles to the apparent volume of foundry sand, and in the present invention, porosity is defined as a value measured by the following method. . First, put 100 mff1 of a mixed solution of water: methanol = 7:3 (weight ratio) into a 20011 measuring cylinder, gradually add 100 mN of foundry sand measured in another measuring cylinder, and then seal it until no air bubbles appear. After confirming that this is the case, read the liquid level in the graduated cylinder, and take the difference between this value (Mm/) and the 200 μl scale as the porosity. Therefore, it is defined as porosity (%)=200-M. In addition to the above-mentioned mixed solution of water and methanol, the solution to be used may be a mixture of water and a surfactant or other liquids.
This means that the volume of the voids is small and the filling rate is high, and it is thought that each particle of foundry sand can be bound together with a small amount of binder resin. As a result of studying the relationship between this porosity and the strength of the mold, the present invention found that molding sand with a porosity of 44% or more cannot provide high strength of the mold, and found that molding sand with a porosity of 44% or less cannot be used. It was discovered and completed by the discovery that the strength of molds can be dramatically increased by mixing them together. Further, the size of the foundry sand is not particularly limited, but it is preferably in the range of about 28 mesh to 200 mesh. By coating the surface of the foundry sand with a binder resin, resin-coated sand for shell mode can be prepared. Binder resins include, but are not particularly limited to, novolac type phenolic resins,
It is preferable to use a phenolic resin such as a resol type phenolic resin or a mixture of a novolac type phenolic resin and a resol type phenolic resin. Coating foundry sand with a binder resin to prepare resin-coated sand can be carried out by a dry hot coating method, a cold coating method, a semi-hot coating method, a powder solvent method, or the like. In the dry hot coating method, a solid binder resin such as a solid phenol resin is added to foundry sand heated to 130 to 180°C and mixed, and the solid binder resin is melted by heating with the foundry sand to form a molten binder resin. In this method, the surface of foundry sand is coated and then cooled while maintaining this mixture to obtain granular and smooth resin coated sand. The cold coating method is a method in which a binder resin is dissolved in a solvent such as methanol to form a liquid, which is added to potted river sand, mixed, and the solvent is evaporated to obtain resin-coated sand. In the semi-hot coating method, 50% of the liquid binder resin dissolved in the above solvent is applied.
This is a method of obtaining resin-coated sand by adding and mixing it to foundry sand heated to ~90°C. The powder solvent method is a method in which solid binder resin is pulverized, this powder resin is added to foundry sand, a solvent such as methanol is added, and this is mixed to obtain resin coated sand. Granular and smooth resin-coated sand can also be obtained using the method described above, but the dry hot coating method or semi-hot coating method is preferable in terms of workability, etc. The mixing ratio of sand and binder resin depends on the requirements of shell molding. Although it varies depending on the desired performance, and is not particularly limited, it is possible to reduce the amount of binder resin to 1 part by weight or less in terms of resin solid content per 100 parts by weight of foundry sand. Further, during this mixing, a curing agent, various coupling agents such as a silane coupling agent for making the foundry sand and binder resin compatible, wax, etc. may be added as necessary. By sprinkling or filling the resin-coated sand obtained in this way into a heated mold according to a conventional method, the binder resin is melted and hardened, and the binder resin binds the foundry sand to form a shell mold. It is for molding. If the amount of binder resin in the resin-coated sand can be reduced, the amount of gas generated when the binder resin is carbonized when pouring the molten metal into the formed mold can be reduced. This makes it possible to reduce the occurrence of cavities that get into the casting and deteriorate the casting surface, and also because the binder resin is easily decomposed by the heat of the molten metal, it improves the separation of sand from the casting and makes the casting surface beautiful. Furthermore, even when casting is performed using a low melting point alloy, the binder resin can be easily decomposed, and the mold can be easily disintegrated and removed.

【Example】

Next, the present invention will be specifically explained using examples. Large Jll silica sand (sand A) with a porosity of 41.3% and a porosity of 45.2
This is shown in Table 1 using % silica sand (sand B) rl-I
They were mixed at a blending ratio of J to r1-5J. The overall porosity of the silica sand thus mixed is shown in the same table. Furthermore, sand A
and the particle size distribution of sand B are shown in Table 3. And this silica sand 3
Heat 0kg to 150℃ and put it into a whirl mixer.
In addition to this, novolak type phenolic resin 60 with a softening point of 90°C
After adding 0 g of the mixture and kneading for 30 seconds, 90 g of hexamethylenetetramine (15% of the phenol resin) dissolved in 450 g of water was added, and the mixture was kneaded until the clumps of sand grains collapsed. Next, add 30g of calcium stearate to this.
was added, kneaded for 30 seconds, and then discharged for aeration to obtain resin coated sand having a resin content of 2.0% by weight. Resin-coated sand with a resin content of 2.0% by weight was obtained in the same manner as in Example 1 except that only silica sand (Sand B) having a porosity of 45.2% was used. X silica sand (sand A) with a porosity of 41.3% and a porosity of 48.0
% of recycled sand (sand C) as shown in Table 2.
1” to r2-5. They were mixed at a blending ratio of The overall porosity of the silica sand thus mixed is shown in the same table, and the particle size distribution of sand C is shown in Table 3. Then, using this sand, the rest is the same as Example 1.
V-small 1 bar;-no-coated sand was obtained. L Comparison Resin-coated sand with a resin content of 260% by weight was obtained in the same manner as in Example 2 except that only silica sand (Sand C) with a porosity of 48.0% was used. Molds were made using the resin coated sand obtained in Example 1.2 and Comparative Example 1.2, and the room temperature bending strength of the molds was measured in accordance with JACT Test Method 5M-1. Furthermore, the fusion point and angle of repose of the resin coated sand obtained in Example 12 and Comparative Example 1.2 were measured. The fusion point was measured in accordance with JACT test method C-1, and the angle of repose was measured in accordance with the HM method. These results are shown in Tables 1 and 2. Further, the relationship between the porosity of the silica sand and the strength of the mold is shown in the graphs of FIG. 1 (Example 1 and Comparative Example 1) and FIG. 2 (Example 2 and Comparative Example 2). As seen in Tables 1 and 2 and the graphs in Figures 1 and 2, by mixing sand A with a porosity of less than 44% with sand B and sand C, It is confirmed that the bending strength increases rapidly. Therefore, even if the amount of binder resin was reduced, strength could be ensured.

【Effect of the invention】

As mentioned above, the resin-coated sand for shell mode of the present invention is composed of foundry sand with a porosity of 44% or less and a resin-coated sand with a porosity of 4.
It is formed by coating molding sand of 4% or more with binder resin, and by mixing foundry sand with a porosity of 44% or less, the mold can be formed even if the amount of binder resin is small. It is possible to ensure high strength and to sufficiently reduce the amount of binder resin coated on foundry sand.

[Brief explanation of drawings]

FIGS. 1 and 2 are graphs showing the relationship between the porosity of foundry sand and the bending strength of a mold.

Claims (1)

[Claims] (1) Foundry sand with a porosity of 44% or less and a porosity of 44%
A resin-coated sand for shell molds, characterized in that the above foundry sand is coated with a binder resin.