JPH0469017B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field] The present invention relates to a method for producing resin coated sand for shell molds. [Background Art] A shell mold is produced by bonding molding sand such as silica sand 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. The binder for this shell mold is generally a novolak type phenolic resin made by adjusting the molar ratio of phenols and aldehydes to 1:0.6 to 0.9 and reacting them under an acidic catalyst, or a phenol and aldehyde resin. A solid resol type phenol resin is used, which is prepared by adjusting the molar ratio of 1:1 to 3 and reacting it under an alkali catalyst.For novolak type phenol resin, hexamethylenetetramine is blended as a curing agent, and resol type phenol resin is used. Leave the resin as it is,
A resin-coated sand coated with a coating layer of phenolic resin binder is created by mixing it with heated molding sand, and this resin-coated sand is sprinkled or filled into a heated mold to form a sticky mold. A shell mold is created by melting and hardening the binder. However, shell molds using phenolic resin as a binder generally have a problem in that cracks are likely to occur during pouring. This is thought to be due to the rapid thermal expansion of the resin or sand due to the high heat during pouring. In order to solve this problem, we added a substance with a cushioning effect to phenolic resin or resin-coated sand to make the shell mold more flexible and to reduce the coefficient of thermal expansion of the shell mold, thereby preventing the occurrence of cracks. Attempts are being made to do so. Bisphenol A, bisphenol A,
Petroleum-based resins, rosin, etc. are used, but although these are effective to some extent in reducing the coefficient of thermal expansion of the shell mold and preventing the occurrence of cracks, they cause thermal decomposition and volatilization during pouring, resulting in bad odors. However, there are disadvantages in that the shell mold has poor disintegration properties after pouring. Also, although this product is effective when casting aluminum or cast iron, when casting stainless steel or high manganese steel, which have high pouring temperatures, cracks are inevitable due to rapid thermal expansion during pouring. In addition, the decomposition and volatilization described above occur rapidly due to rapid heating, and decomposition gas is generated due to the oxidation of the phenol resin itself. It also has the disadvantage of causing defects in the casting surface. In order to suppress the thermal expansion of shell molds, attempts have also been made to modify the phenolic resin with bisphenol A, residues, or alkylphenols, to reduce the crosslinking density, or to mix iron oxide, etc. Although it has been shown to be effective to some extent, there are limits and high temperature hot water cannot be expected to be very effective. If a phenolic resin modified to suppress the crosslink density is used, the hot strength of the shell mold will be lowered, resulting in cracks due to insufficient strength, or the amount of gas generated due to carbonization during use of the resin will increase. It also has the disadvantage of worsening the casting surface. In order to improve this casting surface, conventional coating agents containing graphite, zircon, aluminum oxide powder, etc. have been applied to the surface of the shell mold in order to improve surface precision and strength. Since the agent contains some resin, the coefficient of thermal expansion of the surface of the shell mold that comes into contact with the hot water increases, making cracks more likely to occur. Since it is used after being dispersed, it has the disadvantage that it requires complicated operations such as coating, dipping, and drying. As mentioned above, in the past, it has not yet been possible to prevent shell mold cracks without deteriorating the casting surface of the casting. [Object of the Invention] The present invention has been made in view of the above points, and provides for the production of resin-coated sand for shell molds that can prevent cracks in shell molds without deteriorating the casting surface of castings. The purpose is to provide law. [Disclosure of the Invention] According to the method for producing resin-coated sand for shell molds according to the present invention, a phenolic resin binder and a metal powder are respectively added to molding sand and mixed. The present invention is characterized in that the surface of the sand is coated with a coating layer made of a phenolic resin containing metal powder, and the present invention will be described in detail below. As the phenolic resin as a binder, either a novolak type phenolic resin or a resol type phenolic resin can be used. Metal powders that have a high affinity for oxygen and are easily oxidized, such as Al, Mg, Si, Fe, Ni, Cr, Ti,
Nb, V, or alloys of these metals such as Al
-Mg, Al-Si, etc., and one or more of these can be used in combination. The particle size of the metal powder is preferably 10 μm or less from the viewpoint of uniform mixing and increasing the surface area to facilitate oxidation. Then, phenolic resin and metal powder are added to molding sand such as silica sand and mixed, and the sand is coated with a coating layer of phenolic resin binder containing metal powder to obtain resin coated sand. .
Coating the sand containing metal powder with the phenolic resin binder can be carried out by a dry hot coating method, a rolled coating method, a semi-hot coating method, a powder solvent method, or the like. In the dry hot coating method, metal powder and solid phenolic resin are added to sand heated to 130 to 160°C and mixed, the solid phenolic resin is melted by the heating by the sand, and the metal powder is mixed into the molten phenolic resin. In this state, the surface of the sand is wetted with molten phenolic resin to coat it as a phenolic resin coating layer containing metal powder, and then this mixture is cooled while maintaining it to obtain granular and smooth resin-coated sand. . In the cold coating method, phenolic resin is dissolved in a solvent such as methanol to form a liquid, and this and metal powder are added to sand and mixed, and the solvent is evaporated to obtain resin-coated sand. be. In the semi-hot coating method, resin coated sand is obtained by adding and mixing liquid phenolic resin dissolved in the above solvent and metal powder to sand heated to 50 to 90°C. In the powder solvent method, a solid phenolic resin is crushed, the crushed resin and metal powder are added to sand, a solvent such as methanol is added, and the mixture is mixed to obtain resin-coated sand. Although granular and smooth resin-coated sand can be obtained using any of the above methods, the dry hot coating method is preferred from the viewpoint of workability. The mixing ratio of sand, phenolic resin, and metal powder varies depending on the required performance of the shell mold, but in general, the mixing ratio of phenolic resin to 100 parts by weight of sand is 1 part by weight in terms of resin solid content.
It is preferable to use the metal powder in an amount of about 0.01 to 1 part by weight. Further, during this mixing, a curing agent and a silane coupling agent for making the sand or metal powder compatible with the phenol resin binder and the like may be added as necessary. The resin coated sand thus obtained is sprinkled or filled into a heated mold according to a conventional method to melt and harden the phenolic resin binder. It is molded into a shell mold by the binding action of sand. Since this product contains metal powder, the low coefficient of thermal expansion of the metal powder suppresses thermal expansion during pouring into the shell mold and prevents cracks from occurring in the shell mold. In addition, metal powder easily reacts with oxygen and becomes oxidized, and the phenolic resin binder is oxidized during rapid heating during pouring, meaning that oxygen is consumed by the metal powder. This can be prevented by doing so. Next, the present invention will be specifically explained using examples. Example 1 Novolac type phenolic resin 1300 with a softening point of 90°C
Transfer g to a four-necked flask and add methanol to it.
700g was added and dissolved well. The viscosity of the obtained novolak type phenolic resin varnish at 25°C was 8 poise. On the other hand, 30 kg of flattery silica sand heated to 80°C was placed in a Whirl mixer, 75 g of metal aluminum powder with a particle size of 5 μm was added thereto, and mixed for 30 seconds. The mixture was further added and kneaded until the sand grains disintegrated, and then discharged and aerated to obtain resin-coated sand. Example 2 30 kg of flattery silica sand heated to 140°C was charged into a Whirl mixer, and 700 g of novolac type phenolic resin with a softening point of 90°C and 35 g of metal aluminum powder similar to Example 1 were added simultaneously and kneaded for 30 seconds. 300g of hexamethylenetetramine 110g
g of water and kneaded until the clumps of sand particles collapsed. Next, 15 g of calcium stearate was further added to the mixture, kneaded for 30 seconds, and then discharged for aeration to obtain resin coated sand. Example 3 Instead of the novolak type phenolic resin and metal aluminum powder in Example 2, a material with a softening point of 85°C was used.
700g of resol type phenolic resin and particle size of 6μm
A resin-coated sand was obtained in the same manner as in Example 2, except that 35 g of metallic silicon powder was used and hexamethylenetetramine was not used. Example 4 Solid resol type phenolic resin with a softening point of 78°C
1,300 g was placed in a four-necked flask, and 700 g of methanol was added thereto to dissolve it well. A resin coated sand was obtained in the same manner as in Example 1, except that this resol type phenolic resin varnish was used in place of the novolak type phenolic resin varnish of Example 1, and hexamethylenetetramine was not used. Comparative Example 1 A resin-coated sand was obtained in the same manner as in Example 2, except that the metal aluminum powder was not used. Comparative Example 2 A resin-coated sand was obtained in the same manner as in Example 3, except that no metal silicon powder was used. Various tests were conducted on the resin coated sand obtained in Examples 1 to 4 and Comparative Examples 1 and 2 above. The results are shown in the table below. In the following table, the melting point (°C) is according to JACT test method SM-1.
Room temperature bending strength (Kg/cm ² ) is based on JACT test method C-1, and rapid thermal expansion coefficient (%) is measured at a measurement temperature of 1000℃ in N ₂ gas based on JACT test method SM-7. Tests were conducted on each. Hot bending strength (Kg/cm ² ) was determined by placing a test piece made according to JACT test method SM-1 into an electric furnace set at 1000°C, and measuring the bending strength at 1000°C after processing for 1 minute. I turned it over and finished it. Oxidation resistance is JACT
Test pieces (20φ x 50mm) prepared according to test method SM-7 were placed in an electric furnace set at 1000°C for 5 minutes, then taken out, cooled, placed on a vibrating sieve, vibrated for 1 minute, and then placed under a heavy weight. The weight percent of the residue was calculated using the following formula and evaluated. Weight after treatment / Weight before treatment × 100 (%) To judge the casting surface, a crucible with an inner diameter of 100φ, a height of 100 mm, and a wall thickness of 20 mm was created using resin-coated sand, and the crucible was heated to 1650℃. This was done by visually observing the casting surface of the finished casting after pouring hot water and cooling it.

[Table] ◎〓Very good ○〓Good △〓Fair The results in the table show that the rapid thermal expansion coefficients of each example are smaller than those of the comparative example, and are effective in preventing cracks from occurring in the Schieremold. It is also confirmed that the products of each example have excellent oxidation resistance and can be cast into castings with good casting surfaces. [Effects of the Invention] As described above, according to the present invention, by adding and mixing a phenolic resin and a metal powder to molding sand, the surface of the molding sand is coated with a phenolic resin containing metal powder. Since the layer is coated, the low coefficient of thermal expansion of the metal powder contained suppresses thermal expansion during pouring of the shell mold and prevents cracks from occurring in the shell mold. Powder easily reacts with oxygen and becomes oxidized, and the fact that oxygen is consumed by metal powder means that the phenolic resin used as a binder is oxidized during rapid heating during pouring. This can prevent deterioration of the casting surface of the casting due to the generation of gas components due to oxidative decomposition of the phenolic resin. In addition, the metal powder is contained in the molding sand in a state where it is dispersed in the phenolic resin, and the metal powder can be dispersed uniformly, making it possible to stably obtain the effects of adding the metal powder. be.

Claims (1)

[Claims] 1. A resin coat for shell molds, characterized in that a coating layer of phenolic resin containing metal powder is coated on the surface of molding sand by adding and mixing phenolic resin and metal powder to molding sand, respectively. How to make Tsudo Sandwich.