JPH0255138B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to binder-coated sand grains for shell molds used for mold cores, etc., and particularly to binder-coated sand grains for shell molds suitable for copper alloy castings. [Prior Art] Conventionally, binder-coated sand grains for shell molding have been made by using a solid novolak resin as the binder and coating the sand grains by a hot marring method. [Problems to be Solved by the Invention] Conventional binder-coated sand grains for shell molds coated with solid novolac resin tend to be prone to gas defects. In particular, copper alloy castings have a much wider solidification temperature range during casting than cast iron castings, so the combustion gases generated from the core, especially hydrogen gas, have an adverse effect, causing gas defects such as blowholes and pinholes. is likely to occur. When using a green sand mold for the main mold and a shell mold mold that uses binder-coated sand grains for the core, the difference in cooling rate between the green sand mold and the shell mold causes molten metal to be deposited on the main mold surface and the core surface. There is an imbalance in the cooling rate, which tends to cause sinkholes. An object of the present invention is to provide binder-coated sand grains for shell molding that are free from the above-mentioned casting defects. [Means for solving the problem] In order to overcome the above-mentioned casting defects, the inventors
As a result of various studies, we have completed the present invention by confirming that casting defects can be greatly reduced by forming a shell mold using sand grains coated with a binder for shell molds that have a high cooling capacity. That is, the binder-coated sand grains of the present invention are binder-coated sand grains for shell molding that are composed of foundry sand and a binder coated on the surface of the foundry sand, wherein the binder is a methylolmelamine compound or a ureamethylolmelamine compound. It is characterized by being made of a solid ammonia aresol type phenolic resin mixed with a compound. In a shell mold made using the binder-coated sand grains of the present invention, the binder decomposes due to the heat of the molten metal, and a large amount of heat is absorbed. It is thought that this heat absorption speeds up the cooling of the molten metal and eliminates casting defects. The binder that characterizes the binder-coated sand grains of the present invention includes a solid ammonia resol type phenolic resin as a constituent component thereof. Solid ammonia aresol type phenolic resin, like novolak type phenolic resin, has the characteristic that it softens at a certain temperature and can be coated on the surface of sand grains such as foundry sand. Furthermore, compared to novolak type phenolic resins, solid ammonia resol type phenolic resins have methylol groups that form methylene ether bonds due to heating during molding, and the number of these bonds increases compared to novolaks, and when pouring Due to the high temperature, it decomposes instantaneously and absorbs heat. Therefore, the solid ammonia resol type phenolic resin itself exhibits a large endothermic reaction, which speeds up the cooling of the molten metal. Both the methylolmelamine compound and the urea methylolmelamine compound, which are other components of the above-mentioned binder, contain a large amount of methylol groups, and like the solid ammonia resol type phenolic resin, during molding, methylene ether bonds and methylene amine bonds are formed. When a large number of molten metals are poured, they decompose instantaneously and exhibit an endothermic reaction, removing a large amount of heat. Therefore, a shell mold formed from the binder-coated sand grains of the present invention is a shell mold with excellent cooling ability. Note that the solid ammonia aresol type phenolic resin herein refers to a resin obtained by polycondensing phenol and formalin using an alkaline ammonia catalyst.
The main component of this resin may be solid ammonia resol type phenol, or it may be a resol type phenol resin modified such as oak oil modification depending on the purpose. A methylolmelamine compound is a methylolmelamine obtained by reacting melamine and formaldehyde, such as monomethylolmelamine,
Dimethylolmelamine, trimethylolmelamine, tetramethylolmelamine, pentamethylolmelamine, hexamethylolmelamine, or an etherified compound of these methylolmelamine and alcohol. The urea methylol melamine compound refers to methylol urea obtained by reacting the above methylol melamine compound and urea, or urea and formaldehyde, or a mixture of these methylol ureas and alcohols etherified. Here, for 100 parts by weight of methylolmelamine, urea or
The methylol urea is preferably used in an amount of 0 to 100 parts by weight. As the methylol urea, monomethylol urea, dimethylol urea, trimethylol urea, tetramethylol urea, etc. can be used. The solid ammonia resol type phenolic resin and the methylol melamine compound or the urea methylol melamine compound that constitute the binder exist in a mixed state and may partially react, but it is best to exist as a mixture as much as possible. preferable.
Further, even when the binder-coated sand grains of the present invention are made into a shell mold by a conventional method, the solid ammonia resol type phenolic resin and the methylolmelamine compound or urea methylolmelamine compound constituting the binder in the shell mold are mixed as much as possible. , they do not react, and both exist in a mixed state. There are four typical methods for producing the binder-coated sand grains of the present invention: The first method is to mix a methylol melamine compound or a urea methylol melamine compound before the vacuum dehydration step during the production of a normal resol type phenolic resin, and then to perform vacuum dehydration and mix the methylol melamine compound or the urea methylol melamine compound. Prepare a solid ammonia aresol type phenolic resin,
This resin is coated on the surface of foundry sand using a conventional hot marring method to obtain binder-coated sand grains. The second method is to mix and disperse a methylol melamine compound or a urea methylol melamine compound in a solid state into a solid ammonia resol type phenolic resin obtained by a conventional method to form a binder. This method is obtained by coating the surface of foundry sand with a method. The third method is to separately produce a solid ammonia resol type phenolic resin and a methylolmelamine compound or a urea methylolmelamine compound by a normal method, and then produce binder-coated sand grains by a normal hot marling method. First, solid ammonia aresol type phenolic resin is added, and then,
This is a method for producing binder-coated sand grains by adding a methylolmelamine compound or a ureamethylolmelamine compound and then using a conventional method. The fourth method is to coat the surface of the foundry sand with a solid ammonia-resol-type phenolic resin using a normal hot mart ring method, and then add a methylol-melamine compound or urea-methylol-melamine compound to the cooling water. This method involves dissolving the compound, adding this cooling water, and then using a conventional method to obtain binder-coated sand grains. Incidentally, the binder-coated sand grains of the present invention may also be obtained by other methods such as cold marling and semi-hot marling. When manufacturing the binder-coated sand grains of the present invention, it is important to select conditions in which the solid ammonia resol type phenolic resin and the methylolmelamine compound or the urea methylolmelamine compound do not react as much as possible for the mixed coating. As a method for making a shell mold using the binder-coated sand grains of the present invention, a conventional shell mold manufacturing method can be used as is. That is, sand grains coated with a binder are blown into a heated mold, and the heat of the mold melts and hardens the binder, thereby bonding the sand grains together. Although the binder-coated sand grains of the present invention are excellent for use in shell molds for copper alloy castings,
It can also be used for shell molding of other metals such as cast iron. [Example] The following is an explanation based on an example. In addition, all examples were carried out by dry hot marling. However, it is not limited to dry hot marling. Example 1 100 parts by weight of phenol and 122 parts by weight of 37% formalin were reacted using an ammonia catalyst to obtain a solid ammonia resol type phenolic resin. 43 parts by weight of trimethylolmelamine was mixed with 100 parts by weight of this resol type phenolic resin to form a binder. This binder was added to foundry sand heated to about 150°C in an amount of 2% by weight, and the binder-coated sand grains of the present invention were obtained by a conventional hot marring method. Using these binder-coated sand grains,
Binder-coated sand grains were filled into a shell mold heated to 250° C. and hardened in the mold for about 1 minute to produce a casting core 1 for water fittings shown in the figure. The mold shown in the figure was made using the foundry core 1 and the main mold 2 made of green molding sand, and a copper alloy was cast at a casting temperature of 1200°C. In addition, water fittings were cast with 18 pieces per frame.
The resulting 18 water fittings were cut in the center, the core surface and outer surface of the castings were observed, and the number of sink holes was determined. Also, the melting point of coated sand grains (℃)
The room temperature strength (Kg/cm _N ) and the warm strength (Kg/cm _N ) were measured. Warm temperature here refers to the transverse rupture strength after a test piece with dimensions of 22 x 22 x 203 mm is fired for 40 seconds at a mold temperature of 250°C and released from the mold for 15 seconds. still,
The results are shown in the table. Example 2 A solid ammonia resol type phenolic resin was produced in the same manner as in Example 1. A binder was obtained by mixing 14 parts by weight of urea and 29 parts by weight of trimethylolmelamine with 100 parts by weight of this solid ammonia resol type phenolic resin. Using this binder, binder-coated sand grains of the present invention were formed in the same manner as in Example 1. Also, using the binder-coated sand grains, a foundry core 1 shown in the figure was made, and a copper alloy was cast in the same manner to cast water fittings. The casting defects of the obtained water fittings and the characteristics of the coated sand grains were investigated. These results are shown in the table.

[Table] Example 3 A reaction was carried out using 100 parts by weight of phenol, 122 parts by weight of 37% formalin and an ammonia catalyst. Next, 67 parts by weight of trimethylolmelamine was added to 100 parts by weight of the obtained initial condensation part, and after mixing,
Dehydration was carried out under reduced pressure to obtain a binder containing a solid ammonia resol type phenolic resin. Using this binder, binder-coated sand grains were formed in the same manner as in Example 1,
A foundry core 1 was made using the binder-coated sand grains, and a copper alloy was then cast in the same manner as in Example 1 to cast water fittings. The casting defects of the obtained water fittings and the characteristics of the coated sand grains were investigated. These results are shown in the table. Example 4 Using the same solid ammonia resol type phenolic resin as in Example 1, 100 parts by weight of silica sand heated to 150°C was put into a speed muller, 1.4 parts by weight of the above resol type phenolic resin was added, and 30 parts by weight of the above resol type phenolic resin was added. Kneaded for seconds. Then, 1.8 parts by weight of a 50% aqueous solution of trimethylolmelamine was added and kneaded.
Calcium stearate 0.1 when the sand disintegrates
After adding parts by weight and kneading for 30 seconds, the mixture was taken out from a speed muller and cooled to room temperature to obtain binder-coated sand grains of the present invention. Using this binder-coated sand grain,
A water fitting was cast in the same manner as in Example 1. The casting defects of the obtained water fittings were similarly investigated. In addition, the characteristics of the coated sand grains were investigated. These results are also shown in the table. Example 5 In the method of Example 4, instead of the 50% aqueous solution of trimethylolmelamine, 1 part by weight of urea was added.
50% of a mixture of 5 parts by weight of trimethylolmelamine
An aqueous solution was used. Otherwise, binder-coated sand grains and foundry cores were made in the same manner as in Example 4, and water fittings were manufactured. The table shows the results of investigating the casting defects of water fittings and the mold characteristics of the coated sand grains. Comparative Example 1 As a comparative example, only a solid ammonia aresol type phenolic resin similar to that used in Example 1 was used as a binder. Using this binder, binder-coated sand grains were made in the same manner as in Example 1, and further a foundry core was made and a copper alloy was cast to manufacture water fittings.
The casting results of the obtained metal fittings and the characteristics of the coated sand grains were investigated. The results are shown in the table as Comparative Example 1. Comparative Example 2 100 parts by weight of phenol and 74 parts by weight of 37% formalin were reacted using an oxalic acid catalyst to produce a normal novolak type phenolic resin. Using this novolak type phenolic resin as a binder, Example 1
2% by weight of silica sand in the same manner as
was added to produce binder-coated sand grains. A foundry core was similarly made using the binder-coated sand grains, and water fittings were cast. The casting defects of the obtained metal fittings and the characteristics of the coated sand grains were investigated. The results are shown in the table. As is clear from the results of Examples 1 to 5 and Comparative Examples 1 and 2, the melting point indicating mold characteristics was 98°C to 103°C in the Examples. On the other hand, the temperatures in the comparative example were 100°C and 101°C, and there was almost no difference in the melting point between the core using the binder-coated sand grains of the example and the core using the sand grains of the comparative example.
Also, the strength at room temperature of the example is 40.2Kg/cm.
_N ~ 48.6Kg/cm _N , 44.7 and 45.4 in the comparative example
There was no significant difference compared to Kg/cm _N. Furthermore, regarding the warm temperature, the mold of this example has a temperature of 10.5~
It was 12.3 Kg/cm _N , which was slightly smaller than Comparative Examples 13.0 and 13.2 Kg/cm _N. However, no particular obstacles were observed in the actual casting process. Regarding Examples 1 to 5, no casting defects such as pinholes, blowholes, or sinkholes occurred in any of the 18 water fittings, and all good castings were obtained. On the other hand, in Comparative Example 1, 11 out of 18 children had sinkholes, and in Comparative Example 2, 1 out of 18 children had sinkholes.
From these tables, it is clear that the binder-coated sand grains of the present invention have fewer casting defects. [Effects of the Invention] The shell mold obtained using the binder-coated sand grains of the present invention has methylene ether bonds and methylene amine bonds formed by solid ammonia resol type phenolic resin, methylol melamine compound, and urea methylol melamine compound constituting the binder. It is contained in a large amount, and as this bond is decomposed by the heat of the molten metal, a large amount of heat is absorbed, which promotes cooling of the molten metal. Therefore, by using the binder-coated sand grains of the present invention, blowholes, pinholes,
Casting defects such as sink holes can be effectively reduced.

[Brief explanation of the drawing]

The figure is a cross-sectional view of a mold using a foundry core 1 made using binder-coated sand grains shown in Examples and Comparative Examples of the present invention. 1... Core for casting, 2... Main mold.

Claims (1)

[Scope of Claims] 1 Binder-coated sand grains for shell molding consisting of foundry sand and a binder coated on the surface of the foundry sand, wherein the binder is mixed with a methylolmelamine compound or a ureamethylolmelamine compound. Sand grains coated with a binder for shell molding, characterized in that they are made of a solid ammonia aresol type phenolic resin. 2 The amount of methylolmelamine compound or ureamethylolmelamine compound mixed is 5 to 5 parts by weight per 100 parts by weight of solid ammonia resol type phenolic resin.
100 parts by weight of sand grains coated with a binder for shell mold according to claim 1. 3. The binder-coated sand grains for shell molding according to claim 2, wherein the methylolmelamine compound is at least one of methylolmelamine and an etherified product of methylolmelamine. 4. The binder-coated sand grains for shell molding according to claim 2, wherein the urea methylol melamine compound is a mixture of at least one of urea, methylol urea, and an etherified product of methylol urea and a methylol melamine compound.