JPS6045978B2 - Coating agent for molds - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a mold coating agent that is coated on a required surface of a mold such as a mold for centrifugal force casting.

For example, in centrifugal casting, it has been common to use sand resin (Sand resin) coating, but
In recent years, the use of wet spray coating (diatomaceous earth + ten bottles of bentonite) has become popular due to its quality and economy.

However, the biggest problem with this wet spray coating method is that it is easy to cause casting defects due to the gas (water vapor) generated from the coating during casting.
The gases generated from the coating are caused by crystalline water in bentonite and mainly by water adsorbed by diatoms, but in any case, the presence of these moistures, although necessary for the coating, There are various obstacles during casting.

The adsorbed water is heated in a high temperature mold (150-2500C)
Immediately after wet spray coating, the material is completely dispersed, but after coating or when stored at room temperature, the amount of water absorbed increases over time because diatomaceous earth has hygroscopic properties.

When this adsorbed water increases, a large amount of gas (water vapor) is generated, which leads to casting defects. Furthermore, if the mold is left in the atmosphere, oxidation progresses on the inner surface of the mold in contact with the coating, reacts with the adsorbed water in the coating layer, and produces iron hydroxide.
Half of the n-thick coating layer turned red, and since this iron hydroxide contains crystal water, in addition to defects caused by adsorbed water, it also caused defects caused by crystal water (outer dent defects). It comes to that. Therefore, it is necessary to reduce the moisture in the coating mold as much as possible, which causes casting problems, especially when storing it (in actual operations, it is often used after storage). ), it is important to prevent moisture absorption.

Therefore, as measures to prevent the paint film from absorbing moisture, there are two methods: one is to prevent contact between the moisture-containing atmosphere and the paint film, and the other is to improve the properties of the paint film so that it does not easily absorb moisture. Possible chemical means to do so.

Of these, the present invention relies on the latter means. The present invention is an improvement of diatomaceous earth, which is used as the main material for paint coatings for casting, in particular, by modifying its moisture adsorption property so that it does not absorb moisture even during long-term storage.

That is, the mold coating agent of the present invention is a mold coating agent consisting of a base material, bentonite, and water, in which the base material contains 1% or less of sodium carbonate, borax, and nitric acid based on diatomaceous earth. It is characterized by the use of glass-coated diatomaceous earth which has been calcined with the addition of a sodium-containing substance such as sodium or an alkaline earth chloride flux. The present invention will be explained in detail below.

It is well known that diatomaceous earth is produced as a natural product in Japan and abroad, and is generally widely used as a filter aid. It has a high content of SiO2, which is its main component, and has excellent heat resistance and heat insulation due to its crystal nucleus structure, making it suitable as a casting material. However, due to the moisture content of the raw clay and the high loss on ignition (crystalline water), it cannot be used for casting purposes. It is common to perform heat treatment to adjust the ingredients for heat drying.

As a result, the ignition loss is maintained at a constant value, but on the other hand, when the intended casting material is coated as a coating film material, moisture is absorbed as adsorbed water (free water.min) due to subsequent changes over time. The amount tends to increase gradually. For example, if you heat dry at 100°C or higher,
Immediately after that, the moisture content is almost zero, but if left in the atmosphere, the moisture content will increase until it is saturated with absolute humidity. Therefore, as a result of considering a method for maintaining the initial low level of free moisture content of diatomaceous earth without absorbing moisture over time, the present inventors discovered that dry diatomaceous earth particles could be coated with a glass-like coating. I came to know the means to do this. As an example, when sodium carbonate (Na2CO3) is used as a coated material, the mixing ratio of the coated material and the change in the gasification component (free water) over time, as well as the results of mixing it with the coating material and casting it. The relationship is shown in the table below.

As is clear from the table, when sodium carbonate is added as a fluxing agent (coated material), a remarkable effect is observed in suppressing free water (adsorbed water), and a correspondingly significant improvement effect on casting results is observed. It is being demonstrated.

The mold coating agent and execution conditions used to obtain the above casting results are as follows.

1 Mixing ratio of mold coating agent (weight ratio) Base material: Bentonite water = 12:0.6:2 For the cardstock,
Glass-coated diatomaceous earth or uncoated (Na2CO
Diatomaceous earth with no blending ratio of 3) was used.

2 Mold temperature...160℃ (during wet spray coating) 3 Coating mold coating thickness...27 WL 4 Casting product diameter φ1500-200 (G) Length 4-6 m1 Weight 3
800 to 6,500 kg/piece of cast iron pipe In addition, in the same results, casting defects refer to defects such as pinholes and dents on the outer surface of the cast product, and if even one of these defects occurs in one pipe, it is considered a defective product. did.

That is, a value such as 90% means a product defect rate. The improvement effect caused by the addition of a flux can be explained phenomenologically as follows.

In other words, when sodium carbonate is added to diatomaceous earth,
Its main component. The following chemical reaction with SjO2, which is a
2SiO3) is produced.

stop. This glass-like film prevents moisture absorption, and at the same time smoothes the particle surface and reduces the specific surface area, and the synergistic effect of both prevents moisture absorption into the diatomaceous earth particles. By the way, from the perspective of ensuring the quality of cast products, in addition to preventing this adsorbed water, the loss on ignition (crystalline water) of diatomaceous earth cannot be ignored. It is clearly understood by the difference.

In other words, when looking at the melting point of sodium carbonate, 849°C, even when the addition rate of the fluxing agent is low,
At a heating temperature of 800 to 850°C, the loss on ignition increases and casting defects occur.

This is because some unfired sodium carbonate remains, and in order to remove this, it is necessary to uniformly heat the product to a temperature of 850° C. or higher. In other words, when using sodium carbonate as a fluxing agent, it must be added and mixed in advance before the firing process of diatomaceous earth, and the firing temperature must be 850°C.
It is necessary to heat it more than that. Regarding the amount of sodium carbonate mixed, the practical upper limit is about 10%.
If this exceeds 10% of diatomaceous earth, it will increase to 85%.
This is because the effect of so-called firing unevenness becomes noticeable at temperatures between 0 and 900'C, and rather induces casting defects.This can also be seen from the results in the previous table. In addition to the above-mentioned sodium carbonate, borax,
Substances containing sodium such as sodium nitrate and alkaline earth chlorides can be used and can exhibit similar moisture absorption prevention effects.

As shown in the attached drawing, the method for producing glass-coated diatomaceous earth according to the present invention requires only adding a fluxing agent addition step (indicated by a two-dot chain line) to the existing diatomaceous earth production process.

The firing method is generally a rotary kiln, but it is preferable to add a flux to the diatomaceous earth by kneading it with a mixer or the like immediately before firing after crushing and drying the raw earth to achieve a more uniform mixing state. As described above, the present invention relates to a hygroscopic coating mold applied to the surface of a mold such as a centrifugal casting mold, which contains sodium of 1% or less by weight based on diatomaceous earth as a base material, which is the main compounding material. Since we use crinkle-coated diatomaceous earth that has been calcined with a fluxing agent of substance or alkaline earth chloride, moisture absorption phenomenon is suppressed as much as possible even when the coating mold is left in the air, and moisture in the coating mold does not evaporate. The occurrence of casting defects caused by this can be significantly reduced.

Therefore, the use of the mold coating agent of the present invention has the effect of improving casting operations, casting costs, and quality of cast products.

[Brief explanation of the drawing]

The attached drawings are diagrams showing the manufacturing process of glass-coated diatomaceous earth according to the present invention.

Claims (1)

[Claims] 1. In a mold coating agent consisting of a base material, bentonite, and water, the base material is a substance containing sodium such as sodium carbonate, borax, sodium nitrate, or an alkaline earth chloride in an amount of 10% by weight or less based on diatomaceous earth. A coating agent for a mold, characterized in that it uses glass-coated diatomaceous earth which has been calcined with the addition of a fluxing agent.