JPS5811292B2 - How to manufacture casting molds - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a graphite master mold for molding thermosetting molds, room temperature hardening molds, exothermic self-hardening molds, and the like.

Known molds for metal casting include sand molds, metal molds, shell molds, plaster molds, and refractory molds using ethyl silicate as a binder (Shaw process and lost wax method).

Among these molds, sand molds, shell molds, plaster molds, etc. are made using masters such as wooden molds, metal molds, or rubber molds.

Sand molds and plaster molds can be molded at room temperature, so using a wooden mold master will not cause any operational defects, but in the case of shell molds, thermosetting molding methods are common, so it is difficult to use a mold master. It is molded using.

In addition, heat-generating self-hardening molds, which are a type of sand molding method, are made using a wooden mold master, but since heat generation can damage the wooden mold, the surface of the wooden mold is coated with a refractory material, etc. It is often used.

The purpose of the present invention is to find a master material that can replace metal molds and wooden molds and is easy to handle, and to put it into practical use as a master mold.

In particular, the main purpose is to produce master molds for a wide variety of small quantity products that are stable to heat and have excellent dimensional accuracy.

Therefore, the conditions for use as a master mold must be that it is easy to process, has excellent heat resistance, and is stable against thermal shock.

Wooden molds have excellent workability but are weak against heat, and metal molds are strong against heat but have the drawback of requiring extremely long processing times.

For example, the main methods for producing shell molds include the shell mold method, which uses phenolic resin as a binder, and the thermosetting shell mold method, which uses furan resin as a binder. There are two methods: the hot box method, which uses phenolic resin and incyanate as a binder, and the cold box method, which uses phenolic resin and incyanate as a binder, blows amine gas into it, and cures it at room temperature.

In these shell mold manufacturing methods, the shell mold method and the hot box method are thermosetting molding methods, so a master mold that is stable to heat must be used.

Since the cold box method is a molding method that hardens at room temperature, the master mold can be a wooden mold, but for mass production, a metal mold is used.

Therefore, in addition to shell molds, as well as other molding methods, a master mold that is stable against thermal shock, which is a substitute for a mold, is used in thermosetting or exothermic self-hardening molding methods, and a wooden mold is used in cold-curing molds. If we can find a durable master mold to replace it, it will surely be a blessing in terms of molding.

Based on the above reasons, the inventor investigated several types of materials.

First, we investigated sintered crystals of alumina, which is a material for refractories, but found that once alumina has been sintered, secondary processing such as cutting is extremely difficult.

This is not suitable for master mold conditions.

I also learned that it is unstable against thermal shock and is not worth using.

The same results were obtained with sintered silicon carbide.

Next, we hardened the alumina with a phosphoric acid binder to form a master mold of a certain shape, and experimented with it, but it was still unstable to the heat shock machine.

We also fabricated a master mold made of heat-resistant fiber-reinforced plastic made of graphite fiber into a certain shape and conducted experiments.

However, in the case of the thermosetting molding method, the temperature rises to 200 to 400°C, which deteriorates the plastic and makes it impossible to use it repeatedly, but with the room temperature curing molding method, it can be used satisfactorily as a master mold. found.

Master molds made of fiber-reinforced plastics have another disadvantage compared to wooden molds because secondary processing such as cutting is difficult.

The same can be said for other plastics, such as epoxy master molds.

Therefore, the inventor focused on graphite, which is stable against thermal shock, and produced a master mold and conducted experiments.

Graphite is roughly divided into two types: natural and artificial. When further categorized by material, it can be divided into artificial graphite, natural graphite, impermeable graphite, graphite carbon, metallic graphite, and clay graphite. .

We investigated the processability, thermal shock resistance, and handling aspects of these types as master molds.

First, two aspects of workability were investigated: lathe processing and engraving using a wood chisel.

As cutting tools for the lathe, a 5th grade (SK5) carbon steel tool and a tungsten carbide tool were used, and a 1st grade carbon steel (SKI) chisel was used for the woodworking chisel.

Among the types of graphite, graphite-carbonaceous materials were excluded because they are paste-like, and all other materials were prepared in advance with the dimensions of 200 square pieces and tested.

As a result, no problems occurred during processing in the processing experiments.

During lathe processing, a 200mm square piece was turned into a 150z piece, but no damage to the tool was observed.

In addition, even when using a wood chisel for carving, it was possible to carve the carving with the same level of difficulty as the wooden pattern.

Being able to process graphite with a wood chisel is extremely useful when making master molds.

Processing time was reduced by 50% compared to molds. Next, for thermal shock experiments, bottomed containers with an inner diameter of 100 da and an inner height of 150 da were made of each material, and sand heated to 300°C was poured into the graphite container, which was then placed in a water tank. We repeated the burying experiment.

The reason why sand at 300°C was injected into the graphite container was to create a difference in the cooling time between the outside of the container and the sand-filled inside when it was buried in a water tank.

The results of this experiment also revealed that it is stable against thermal shock.

As a result of these experiments, it was found that graphite was fully suitable as a material for the master, so we finally considered the handling aspect.

As mentioned above, graphite is an important element of the master mold in terms of dimensional accuracy and mechanical strength.

In particular, dimensional accuracy is important in terms of the volume of soil to be handled, and graphite has a coefficient of linear expansion of about 1 to 3×10 −6 , and its value does not change much even at high temperatures.

In particular, it can be said that artificial graphite is not affected by expansion due to heat.

In terms of mechanical strength, artificial graphite has a bending strength of 50
~250kg/cm2 Metal graphite 200~900kg/
The range of cm2 and its numbers is wide.

In addition, the elastic modulus is 400 to 1000 kg/mm for artificial graphite.
2. The numerical range is large, as in natural graphite, 500 to 1.129 kg/mm2.

This means that there are many variations in the material, but if you check the material thoroughly, there should be no problem in use.

Also, recently it has become possible to produce isotropic graphite products, which reduces the variation in strength.

However, the most problematic defect with graphite is the high porosity of the product.

The density ratio of normal graphite products is 70 to 85, which means that the porosity is 15 to 30. ).

In particular, it goes without saying that if there are cavities on the surface, it is not desirable as a master mold.

Furthermore, high-density graphite has a high density ratio, has a fairly fine structure, and has few cavities, but it cannot be said to be completely void-free.

When molding a shell mold using thermosetting resin, the resin instantly becomes liquid, causing the resin to sink into the cavities, resulting in a shell mold with a rough skin.

Therefore, it is necessary to use graphite that is free of cavities or that has been treated with cavities.

The true specific gravity of graphite is 1.9 to 22, but the apparent specific gravity of the product is only 1.45 to 1.85.

Therefore, it is an important condition for use that the surface be treated by some method to contain the nests.

In other words, surface treatment is performed so that at least the density of the graphite surface is higher than the compacted powder density of the graphite molded body,
It is necessary to eliminate the nest.

We investigated crimping and impregnating treatments as a method to increase the density of the powder on the surface of graphite, and in other words, to eliminate cavities.

As a pressure bonding process, we experimented with a method in which carbon paste was embedded inside the cavity, but because the surface of the graphite product became dirty and the strength of the paste was weaker than that of the graphite base material, very favorable results were not obtained.

Therefore, we conducted an experiment on a method of curing molybdenum silicide (MoSi2), a type of metal silicon compound whose properties are similar to graphite, by reacting it with sodium silicate.

A mixture of molybdenum silicide and fine graphite in a ratio of 1:1 is mixed with sodium silicate to form a slurry with an inner diameter of 1:1.
00φ and an inner height of 150 mm, it was applied to the surface of graphite and allowed to self-harden at room temperature. After self-hardening, the graphite surface was finished and dried at high temperature.

This test was repeated using the same method as the thermal shock experiment, but
No changes were observed in the nested areas, which were very firmly attached.

The same results were obtained by using fine alumina instead of fine graphite, but the finish of the adhesion layer was better when fine graphite was used.

For impregnation treatment, there are three methods: molten metal, inorganic solution, and resin.
I experimented with types.

All types were impregnated in a vacuum to obtain pieces, and the test was repeated in the same manner as the surface treatment test.

Although there was no significant difference in the results, the surface became rough most quickly when impregnated with resin, while the surface did not become rough when using molten metal and inorganic solutions.

As impregnating agents, zinc was used for molten metals, phosphoric acid solutions were used for inorganic solutions, and thermoplastic resins were used for resins that had the best impregnating properties.

Since zinc has the property of corroding graphite, it is of course excellent in impregnating properties, but impregnating other metals such as aluminum has been difficult.

In the case of resin, we also tested thermosetting resins such as epoxy and unsaturated polyester, but the work was difficult because the resin hardened during the impregnation process.

The density ratio of the graphite product, which had a density ratio of 70 to 85% before the improvement treatment, was improved to 95% or higher by the compression treatment and the impregnation treatment.

As a result of the above studies of graphite in terms of workability, thermal shock resistance, density in terms of handling, that is, the relationship between porosity and voids, we found that if the apparent specific gravity of graphite is 1.45 or more, it can be used as a master mold. There was found.

Next, the present invention will be explained by examples.

Example 1 Thermosetting shell mold, apparent specific gravity 1.65, density ratio 8 as master mold material for molding shell mold
A master mold was manufactured using artificial graphite of 0 ratio.

Processing was carried out by cutting with a lathe and milling machine and engraving by hand.The density ratio of the graphite surface was improved (the purpose of nesting) by mixing graphite powder using the reaction of molybdenum silicide and sodium silicate as a binder. It was carried out using a pressure bonding method.

The composition of the surface treatment agent is as follows.

A preparation was prepared by mixing a small amount of sodium silicate and water in a weight ratio of 6 to the above formulation.

The surface-treated product was fired at 400 to 600°C for 10 hours to obtain a master mold.

Using the graphite master mold produced in this way,
A thermosetting shell mold was created using phenolic resin as a binder.

The shell mold material uses resin-coated sand that has been coated with resin in advance, and is molded by blow injection.
The mold was molded by pouring into a master mold that had been preheated to 00°C.

Although the molding process was repeated 300 times, no damage occurred to the graphite master mold.

The surface condition of the shell mold was also good, and the gloss was particularly good.

Furthermore, the number of molds per hour was 1.5 times that of molds.

As the mold release agent, a dry (solvent type) modified silicone was used.

Example 2 As in Example 1, a master mold was manufactured using artificial graphite graphite having an apparent specific gravity of 1.50 and a density ratio of 70% as a master mold material for making a thermosetting shell mold.

The processing was the same as in Example 1, but in order to improve the density ratio of the graphite surface, polystyrene, which is a thermoplastic resin, was dissolved in trichloroethane to prepare a polystyrene solution with a weight ratio of 15%, and this was used as an impregnating liquid under reduced pressure. It was carried out using the impregnation treatment method.

The vacuum tank was maintained at a vacuum level of 60 mmHg, and the master mold was submerged for 10 hours.

Drying was carried out at room temperature for 4 hours and at 150°C for 1 hour.

Example 1 Using the graphite master mold thus obtained
A phenolic shell mold was molded by applying a modified silicone mold release agent in the same manner as above, but after about 200 times the impregnated resin joint became rough.

Example 3 Master molds were manufactured using artificial graphite and natural graphite having an apparent specific gravity of 1.60 and a density ratio of 75.

The processing was carried out in the same manner as in Example 1, and the density ratio of the graphite green compact and the density ratio of the surface were improved by using a combination of an impregnation method and a compression treatment method.

An aluminum phosphate solution was used as the impregnating agent, and the sample was submerged at a vacuum degree of 60 mmH9 for 10 hours.

After pre-drying at a low temperature after dipping, the surface treatment agent used in Example 1 was applied and baked at 400° C. for 10 hours.

Each master mold thus obtained had a greatly improved density, and no change in the surface was observed even after 600 shell mold moldings.

Example 4 Each master mold was manufactured using the same method as in Examples 1 to 3, and an experiment was conducted using the cold box method, which is a molding method that hardens at room temperature.

The cold box method is a method in which phenolic resin and isocyanate are used as a binder, and conversion is carried out at room temperature using amine gas.However, since a mold is used for mass production, we conducted an experiment using a graphite master mold as a substitute for the mold. did.

Although the master molds of Examples 1 to 3 were repeatedly molded 300 times, no roughness was observed on the surface.

It has been found that the master mold impregnated with the resin of Example 2 can also be used satisfactorily in the case of a cold-curing molding method.

As described above in Examples 1 to 4, a casting mold with both thermosetting and room temperature curability could be produced using a graphite master mold.

As for the longest life, the master mold using artificial graphite described in Example 3 has a lifespan of 800 times.
It withstood well enough.

Therefore, although graphite master molds are not suitable for mass production using thermosetting molding methods, they have been found to be advantageous in terms of delivery time and price for producing a wide variety of small quantities and as a durable alternative to wooden molds.

Claims (1)

[Claims] 1 Pressing,
A method of manufacturing casting molds using a graphite master mold that has been surface-treated by impregnation, etc.