JPS588938B2 - Manufacturing method of resin-coated sand for molds - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing molding sand used as a casting mold, particularly resin-coated sand for molding whose surface is coated with a resin.

Generally, foundry sand is preferably one that maintains the necessary strength before pouring and has good disintegration properties after pouring.

For this reason, resin-coated sand for molds has been proposed, which uses phenolic resin as a binder and coats the surface of the sand with phenolic resin to maintain the necessary strength during casting. has been done.

However, although this type of resin-coated sand shows relatively good disintegration properties when used with cast iron, etc., which has a high casting temperature,
When casting materials such as aluminum, which have a low casting temperature, there are problems in that the disintegrability is insufficient and it takes time to remove sand from the casting after casting, especially to eject the core.

In order to solve this problem, Special Publication No. 54-120229
The publication states that ``phenol formaldehyde resin contains 2~
It has been proposed that the sand be coated with a resin in which 20% by weight of polybutadiene and/or polyvinyl alcohol are mixed.

However, in such resin-coated sand, polybutadiene and polyvinyl alcohol, which improve disintegration properties, are present in the resin coating, so while the disintegration properties are improved, the strength required during casting is There is a problem that the necessary hot strength cannot be obtained, and it is difficult to say that it has necessary and sufficient properties as foundry sand.

Furthermore, JP-A-52-53723 discloses coated sand in which "the surface of refractory particles is first coated with a vinyl resin, and then further coated with a thermosetting resin." With the structure, when the vinyl resin layer is further coated with a thermosetting resin such as phenolic resin, the two become compatible or mixed, so the intended purpose cannot be effectively achieved, similar to the above. .

The present invention has been made in view of such problems, and includes:
The resin coatings that cover the sand are not made to be compatible with each other; they are made into two layers so as to maintain their independence from each other, and the surface layer guarantees caking properties, while the inner layer is made to have disintegrability. The present invention proposes a method for producing resin-coated sand for molds, which can guarantee hot strength and also ensure good disintegration properties after casting due to its inner layer.

In the method for producing resin-coated sand for molds according to the present invention,
First, prepare a polyvinyl acetate solution with an average degree of polymerization of 400 or less, mix it with silica sand at a solid content of 0.1% by weight or more, and apply polyvinyl alcohol to the surface of the silica sand. Coat polyvinyl acetate.

This polyvinyl alcohol or polyvinyl acetate is
A polymer having a low average polymerization degree of 400 or less, preferably 300 or less is used.

Those with a low degree of polymerization are effective in preventing a decrease in hot strength.

Commercially available polyvinyl alcohol and polyvinyl acetate have a high average degree of polymerization, and when used as is, the viscosity of the solution increases, making it difficult to uniformly coat the surface of silica sand, resulting in poor coating, and as a result, disintegration. becomes worse.

Therefore, commercially available polyvinyl alcohol and polyvinyl acetate are depolymerized before being kneaded with silica sand to reduce their viscosity to the required level.

In order to obtain good coverage, the degree of polymerization is preferably such that the viscosity is in the range of 50 to 3000 cp in a 50% by weight solution.

As a depolymerization agent, oxygen, ozone, hydrogen peroxide, potassium nitrate, sulfuric acid, potassium dichromate, potassium permanganate, potassium chlorate, saran powder, etc. can be used, but hydrogen peroxide should be used for handling reasons. is preferred.

Although polyvinyl alcohol and polyvinyl acetate can be used alone, a partially saponified product of polyvinyl acetate can also be used.

The polyvinyl alcohol or polyvinyl acetate to be mixed with the silica sand has a solid content of 0.1% by weight or more based on the silica sand,
Preferably it is added in an amount of 0.2 to 1.0% by weight.

As described above, after coating silica sand with polyvinyl alcohol or polyvinyl acetate, the coating is gelled by adding a boron compound and an alkaline solution.

This gelling treatment is effective in preventing compatibility between this layer and the phenol resin layer coated on this layer and improving the hot strength of the coating.

As the gelling promoter, boron compounds such as boric acid, sodium borate, perboric acid, and zinc borate can be used.

It has been found that by adding an alkaline solution to the boron compound during this gelation treatment, the amount of boron compound added can be reduced, as is clear from the experimental data shown in Table 1.

The silica sand with a gelled coating is coated with a phenol resin by a conventional method to finally obtain resin-coated sand for molds.

In this case, since the coating as the inner layer is gelled, the phenol resin will not be compatible with the inner layer.

Therefore, the resin-coated sand for molds according to the method of the present invention has a coating of gelled polyvinyl alcohol with a low degree of polymerization or polyvinyl acetate as an inner layer, and is not compatible with the gelled inner layer and is independent from the inner layer. The surface phenolic resin coating provides the necessary caking properties, and the inner gelled layer guarantees the necessary hot strength and prevents casting. When exposed to high heat, there is no caking and exhibits extremely good disintegration properties.

Because of its good collapsibility, the resin-coated sand according to the method of the present invention can be advantageously applied to cores with complex shapes and relatively small thermal effects, and can also be advantageously applied to molds of light alloys whose casting temperature is relatively low. It can be advantageously applied to molten metal, and sand can be easily removed without requiring any treatment after pouring.

Furthermore, according to the method of the present invention, the inner layer is gelled before coating the surface layer with the phenolic resin, so the phenolic resin is compatible with the inner layer (the functions of the inner layer and the surface layer are maintained independently). be able to.

Furthermore, in the method of the present invention, since an alkaline solution is used during the gelling treatment, the amount of boron compound used can be reduced.

Next, examples of the present invention will be described.

To 1000 g of partially saponified polyvinyl acetate with a degree of saponification of 88% and an average degree of polymerization of 600, 500 g of 35% hydrogen peroxide and 1675 g of water were added, and the mixture was stirred at 95°C for 6 hours in a three-necked beaker. The viscosity at 20°C is 2.
A partially saponified product of 20 CP polyvinyl acetate was obtained.

Next, the polyvinyl acetate iooy was added to 10 kg of silica sand heated to 150°C, and after kneading for 60 seconds with a kneader, 5 g of boric acid and 8 ru of 6N-NaOH were added.
l and 150ml of water! After which mixture was added and kneaded for an additional 60 seconds, liquid phenol resin 3007 for shell molding with a solid content of 65% was added, and after stirring, 10 g of calcium stearate was added to obtain resin-coated sand.

The above phenol resin for shell molds was produced by adding 776 grams of formalin and 52 grams of oxalic acid to phenol iooooy, reacting at 95° C. for 4.5 hours, dehydrating under reduced pressure, and adding an appropriate amount of methanol.

Table 2 shows the properties of the resin-coated sand obtained by the above method in comparison with conventional resin-coated sand.

Claims (1)

[Claims] 1. Polyvinyl alcohol with an average degree of polymerization of 400 or less for silica sand;
A solution of 0.1% by weight or more of cp in terms of solid content is added and kneaded, the surface of the silica sand is coated with polyvinyl alcohol or polyvinyl acetate, and then a boron compound and an alkaline solution are added to gel the above coating. A method for producing resin-coated sand for molds, which comprises: adding and kneading a phenol resin to coat the surface with the phenol resin, and producing sand grains with the surface of the silica sand coated with polyvinyl alcohol or polyvinyl acetate and the phenol resin. 2 Polyvinyl alcohol Moshi《Polyvinyl acetate 10
2. The method for producing resin-coated sand for molds according to claim 1, wherein 20 to 60 parts of hydrogen peroxide are added to 0 parts to cause depolymerization. 3. The method for producing resin-coated sand for molds according to claim 1, wherein polyvinyl alcohol or polyvinyl acetate is partially saponified.