JPH0468061B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a casting mold using green molding sand, and more specifically, to a method for hardening the surface of a mold that has been formed. [Technical Background] Molds used in the foundry industry are generally made of foundry sand, the main material of which is silica sand. That is, a so-called green sand mold is used, which is made by compression molding silica sand mixed with a binder such as bentonite into a desired shape. This method has many advantages and is effective and widely used for manufacturing small and medium-sized castings, but
If left unattended, the surface of the mold tends to become brittle, and once it becomes brittle, the sand grains on the mold surface will fall off due to the flow of molten metal, causing defects such as sand-eating, and if you touch it with your hands, more sand grains will fall off. It was so violent that even if it had been thoroughly pounded, it could not be used as a core. To prevent these drawbacks of green sand molds, it was common practice to take measures such as pouring hot water as soon as possible after molding the main mold, or applying a coating to the core. In particular, since the core is surrounded by molten metal, the conditions are harsh, and in recent years various mold manufacturing methods have been proposed to improve these problems. [Prior art and problems] Conventional mold manufacturing methods include the above-mentioned green sand mold method, as well as a method that improves the drawbacks of the green sand mold, such as using an inorganic substance, sodium silicate (water glass), in the molding sand. After molding, use a mixture of silica sand as a binder and solidify the entire mold by aerating carbon dioxide gas, or by blending organic substances such as phenol resin or furan resin with silica sand as a binder. How to solidify the entire mold,
Alternatively, a method is known in which the entire mold is solidified by passing an amine gas after molding using foundry sand mixed with Isokure resin. According to these methods, the strength is increased and the brittleness of the surface is improved compared to green sand molds, but the former method, which uses an inorganic binder to solidify the entire mold, has poor mold disintegration and prevents casting. It is difficult to reuse the sand, and because it dissolves in water and becomes alkaline, there are restrictions on how to dispose of it. The latter, which uses organic binders, is generally expensive and may use toxic gases.
Further, it has drawbacks such as generating a pungent odor when pouring molten metal or kneading sand, and having a short pot life of foundry sand after kneading. Furthermore, in a method in which the above-mentioned sodium silicate, phenolic resin, furan resin, or isokyure resin is added to the foundry sand as a binder,
If foundry sand contains fine powder such as bentonite, the binder will be absorbed and consumed by the fine powder, and its original function will not be fully exerted. In addition, these methods solidify the entire mold, making the mold too strong and, when used as a core, unable to absorb the shrinkage of the metal that occurs when the molten metal solidifies, resulting in cracks in the casting. Another known method is to apply dextrin as a mold coating agent to the surface of the mold and then dry and solidify it. It absorbs moisture in the atmosphere, tends to soften and become brittle, and in particular, the surface stability of the mold decreases over time, causing problems in mold storage and workability, making it impractical. For this reason, there has been a strong desire in the foundry industry to develop a more practical mold manufacturing method. The present inventors previously developed a method for manufacturing a mold having a hardened layer on its surface by applying a photosensitive resin liquid to the surface of the mold and then performing an exposure treatment (Japanese Patent Application No. 58-245189). ). However, this method is complicated because it uses light. [Object of the Invention] As a result of intensive research to obtain a cheaper and more practical mold, the present inventors applied an aqueous solution containing polyvinyl alcohol to the surface of the mold,
The inventors have discovered that the object can be achieved by using a mold obtained by hardening the surface of the mold by applying heat treatment, and have completed the present invention. [Structure of the Invention] That is, the present invention provides a mold having a degree of polymerization of 200 to 3000 on the surface of a mold formed using green molding sand,
and a polyvinyl alcohol aqueous solution containing 1 to 50% by weight of polyvinyl alcohol with a degree of saponification of 50 to 98 mol%, or a partially saponified polyvinyl alcohol or completely saponified polyvinyl in a proportion of 1 to 50% by weight to another resin aqueous solution. After applying the aqueous solution obtained by adding alcohol, heat treatment is performed,
This mold manufacturing method is characterized by hardening the surface of the mold. The present invention will be explained in detail below. (Molding Sand for Green Molds) As the molding sand for green molds used in the present invention, foundry sands generally used in the foundry industry that contain bentonite or the like as a binder can be used. In addition, this foundry sand does not necessarily have to be newly blended, and may be one obtained by adding water to the returned sand obtained by molding, pouring, and breaking the mold as molding sand for green molds, and then kneading it. As a method for forming a mold using such green molding sand, known mold forming methods such as a mechanical compression method and a manual compression method can be used. After a mold is formed into a desired mold by the method described above, an aqueous solution containing polyvinyl alcohol is applied to the surface of the mold. (Aqueous solution containing polyvinyl alcohol) The aqueous solution containing polyvinyl alcohol used in the present invention may be an aqueous solution of polyvinyl alcohol, or may be an aqueous solution of polyvinyl alcohol dissolved in another aqueous resin solution. (Polyvinyl Alcohol Aqueous Solution) As the polyvinyl alcohol used in the polyvinyl alcohol aqueous solution, commercially available polyvinyl alcohols having a degree of polymerization of 200 to 3000 can also be used, but preferably those having a degree of polymerization of about 300 to 2400 are used. If the degree of polymerization is too high, the resulting polyvinyl alcohol aqueous solution will have a high viscosity, making it difficult to uniformly coat the mold surface. On the other hand, if the degree of polymerization is low, the coating properties will be good, but the strength of the mold obtained by heating and curing will be insufficient.Also, since the viscosity is low, the amount of penetration into the mold will be large, resulting in a satisfactory coating layer. This is not preferable because it does not provide Furthermore, the degree of saponification of the polyvinyl alcohol used must also be considered. Whether the degree of saponification of polyvinyl alcohol is high or low, its solubility in water deteriorates, resulting in a highly viscous aqueous solution. Therefore, in order to prepare the polyvinyl alcohol aqueous solution used in the present invention, it is preferable to use polyvinyl alcohol having a degree of saponification of about 50 to 98 mol%. Further, the concentration of the polyvinyl alcohol aqueous solution can be 1 to 50% by weight, but in consideration of applicability and curing strength, a particularly preferable concentration is 2 to 30% by weight. If it is less than 1% by weight, the strength of the mold obtained by heating and curing will be insufficient, and if it exceeds 50% by weight, the strength of the polyvinyl alcohol obtained will depend on the degree of polymerization of the polyvinyl alcohol used. Since the viscosity of the solution increases, it becomes difficult to apply it uniformly to the mold surface, which is not preferable. (Polyvinyl alcohol aqueous solution containing other resins) Other resin aqueous solutions used in the present invention include acidic polymers such as polyacrylic acid, polymethacrylic acid, polyitaconic acid, polyvinyl acetic acid, polycrotonic acid, and polymaleic acid, polycrotyl alcohol, Alcohol-based polymers such as polymethylbutynol, polyallyl carbinol, polyhydroxypropyl methacrylate, polypropylene glycol monoacetate, amide or amine-based polymers such as polyacrylamide, polymethacrylamide, polyisopropylacrylamide, polyallylamine, hydroxyethyl cellulose, carboxy There are products in which water-soluble resins such as modified cellulose such as methyl cellulose, aldehyde polymers, ether polymers, and aqueous alkyd resins are dissolved in water. Also included in the present invention are aqueous solutions of water-soluble copolymer types of styrene-acrylamide, styrene-maleic anhydride, polyvinylpyrrolidone-methyl vinyl ether, and polyethylene oxide. The degree of polymerization and degree of saponification of the polyvinyl alcohol to be added to the aqueous solution of other resins mentioned above is not particularly limited, and partially saponified polyvinyl alcohol, completely saponified polyvinyl alcohol, etc. can be used. The concentration of polyvinyl alcohol relative to the aqueous solution of other resins is 1 to 50% by weight, particularly preferably 2 to 10% by weight. If it is less than 1% by weight, the strength of the mold obtained by heat curing will be insufficient, and if it exceeds 50% by weight, the resulting polyvinyl alcohol Since the viscosity of the solution increases, it becomes difficult to apply it uniformly to the mold surface, which is not preferable. (Special example of polyvinyl alcohol) Furthermore, the aqueous solution containing polyvinyl alcohol used in the present invention may be a water-soluble photosensitive resin liquid containing polyvinyl alcohol as a component. In this case, the concentration of polyvinyl alcohol in the water-soluble photosensitive resin liquid is 1 to 50% by weight,
Particularly preferred is 2 to 20% by weight. If the concentration of polyvinyl alcohol is lower than this range, the adhesion of the coating liquid to the mold surface will be reduced, and if it is higher, the viscosity will increase significantly, which is undesirable in terms of work. (Method of Coating and Hardening the Surface of the Mold) In the method of the present invention, an aqueous solution containing polyvinyl alcohol as described above is applied to the surface of the molded mold by a spray coating method, a brush coating method, or a roller coating method, and then a heat treatment is performed. This process hardens the surface of the mold. This heat treatment can be done by blowing with a burner, blowing hot air, placing it in a heater using electric heat or infrared rays as a heat source, putting it in a microwave heater, or heating the polyvinyl alcohol aqueous solution to be used. There is a method in which a flammable substance such as methanol or ethanol is added, the solution is applied to the mold surface, and the coated layer is immediately ignited to burn the flammable substance. . In the method of burning, after burning the combustible substance, if the hardness of the surface of the mold is insufficient, the other heat treatment described above may be further performed. The amount of the flammable substance added is preferably at least 50% by volume or more based on the aqueous solution containing polyvinyl alcohol, and less than this is not preferable because the mold surface tends to become insufficiently hardened due to combustion of the flammable substance. Further, although such heat treatment can be carried out alone, it is practically preferable to use the heat treatment in combination, since it is possible to shorten the heat treatment time. In addition, if there is sufficient time after molding a mold before it is used, either leave the mold as it is, or heat it for a short time immediately after molding, so that the surface of the mold can be carried. It is also possible to harden the mold, store it in a suitable place, let it air dry, further harden the surface of the mold, and then take it out and use it when desired. This method is a useful method that helps level out molding operations and save thermal energy. The method of the present invention can be effectively used not only for the core but also for the main mold. (Effect of boron compound) In the method of the present invention, by using molding sand for green molds containing a boron compound, a mold having a hard shell can be obtained. That is,
Polyvinyl alcohol reacts with the boron compound mixed on the mold surface and gels on the mold surface, making it difficult for the coating liquid to penetrate into the mold, making it possible to form a relatively thick and stable coating layer. This is because a mold having a highly hard outer skin can be formed by the heat treatment, which is extremely effective in the method of the present invention. In this case, as a method for preparing molding sand for green molds containing boron compounds, an aqueous solution containing about 1 to 10% by weight of boron compounds such as boric acid or borax is added to molding sand for green molds at a concentration of 0.5 to 10% by weight. wt% added,
It can be kneaded using a sand mill or the like. By doing so, a gelled layer of polyvinyl alcohol can be efficiently formed over the entire surface of the mold. (Other Compounds) If a conventionally used mold coating agent such as dextrin is blended with an aqueous solution containing polyvinyl alcohol, it has the effect of accelerating the hardening of the mold surface and also strengthening it. [Example] The present invention will be specifically explained by describing Examples and Comparative Examples below. Note that the following examples are merely illustrative and do not limit the method of the present invention in any way. Example 1 Molding sand for green molds prepared using silica sand equivalent to No. 6 currently used in ductile iron casting factories as aggregate and sodium bentonite as the main binder was pounded and hardened three times, and the diameter
A test piece measuring 50 mm and 50 mm in height was prepared using a standard method. Next, a 2.5% by weight aqueous solution of polyvinyl alcohol with a degree of polymerization of 1000 and a degree of saponification of 90 mol% was applied to the surface of the test piece by spraying at a coating amount of approximately 0.1 g/cm ² , and then a burner was used to spray the solution. 150～
Heat treatment was performed at 180°C for 15 minutes to harden the surface of the test piece. After cooling, the compressive strength, surface stability and hardness were measured by standard methods and the results are summarized in Table 1. For comparison, measurements were also made on test pieces that were not coated.

[Table] Example 2 A test piece was prepared using the same molding sand for green molds as in Example 1. Next, as described in Japanese Patent Publication No. 54-3796, 0.025 parts by weight of methylhydroquinone was dissolved in 10 parts by weight of water, and 74 parts by weight of dimethylol urea dimethyl ether, 202 parts by weight of N-methylol acrylamide, and 2 parts by weight of ammonium chloride were added. The reaction mixture was poured into 1000 parts by weight of acetone, the precipitate was removed by filtration, and the filtrate was distilled to remove the acetone. 10 parts by weight of a condensation polymer, 10 parts by weight of polyvinyl alcohol with a degree of polymerization of 500 and a degree of saponification of 86 to 89 mol%, and anthraquinone.
An aqueous solution of 5% by weight (approximately 2.5% by weight of polyvinyl alcohol) of a water-soluble photosensitive resin obtained by heating and dissolving 0.135 parts by weight of sodium 2,7-disulfonate in a 90°C water bath and stirring; A 30% by weight aqueous solution (approximately 15% by weight of polyvinyl alcohol) was applied to the surface of the test piece by spraying at a coating amount of approximately 0.1g/ ^cm2 , and heated at 150 to 180℃ for 15 minutes using a burner. Heat treatment is performed for 1 minute to harden the surface of the test piece, and after cooling, the compressive strength is determined by the standard method.
Surface stability and hardness were measured. The results are summarized in Table 2.

[Table] Example 3 2% by weight boric acid aqueous solution in 5 kg of return sand from the current green mold molding line of a ductile iron casting factory
A standard test piece was made in the same manner as in Example 1 using foundry sand that had been mixed with 150g of sand for 5 minutes in a small test mill, and the polyvinyl alcohol used in Example 1 was adjusted to 5% by weight on the surface of the test piece. After applying the aqueous solution at a coating amount of approximately 0.1 g/cm ² , the test piece was placed in a hot air dryer at 150 to 180°C.
The surface of the test piece was hardened by heat treatment for about 15 minutes. Using this test piece and green molding sand containing no boron compound, test pieces similar to those described above were prepared, and the hardness of each piece was measured over time using a green mold hardness meter. The results are shown in Table 3.

[Table] Example 4 The 5% by weight polyvinyl alcohol aqueous solution used in Example 3 was applied to the surface of an automobile differential case mold made using the green molding sand mixed with the boron compound used in Example 3. , coating amount is approximately 0.1g/cm ²
Approximately 150 to 180℃ after applying to the ratio of
The surface of the mold was hardened by heat treatment for about 15 minutes in a hot air dryer, and molten ductile cast iron was poured into the resulting mold.As a result, no casting defects were observed in the product, and the casting surface was smooth. It was fully practical. Example 5 A standard test piece was prepared in the same manner as in Example 1 using the green molding sand mixed with the boron compound used in Example 3, and the 30% of the material used in Example 2 was coated on its surface.
% by weight water-soluble photosensitive resin liquid and industrial ethyl alcohol (Neocol, manufactured by Nippon Kaseihin Kogyo Co., Ltd.).
A mixture of 35:65 was applied by spraying so that the coating amount was approximately 0.1 g/cm ² .
Next, the surface of the test piece was immediately ignited to burn the ethyl alcohol to solidify the surface of the test piece, and after cooling, the surface stability and compressive strength were measured over time using standard methods. In addition, after igniting and burning, the burner will give 150~
Surface stability and compressive strength were also measured over time for the samples that were heat treated at 180°C for 5 minutes and 10 minutes, respectively. The results are summarized in Table 4.

[Table] Spray the above coating liquid onto the surface of the core of an automobile parts differential case so that the coating amount is approximately 0.1 g/cm ² , and after igniting and burning, heat the mixture at 150 to 180℃ using a burner. When molten ductile cast iron was cast into a core that had been heat-treated for 5 minutes, a core that had been heat-treated for 5 minutes was completely usable as a product without any defects. Example 6 A test piece was prepared using the same green molding sand as in Example 1 according to the standard method, and 2.5% by weight of Sunrose (trade name, modified cellulose manufactured by Sanyo Kokusaku Pulp Co., Ltd.) was added to it. Add 200 parts by weight of the aqueous solution to Kuraray Poval (trade name, manufactured by Kuraray Co., Ltd., polymerization degree of 1700,
Polyvinyl alcohol with saponification degree of 98 mol%)5
By adding and mixing parts by weight, a water-soluble resin liquid containing 2.5% by weight of polyvinyl alcohol was obtained. Next, this water-soluble resin liquid was applied to the surface of the test piece by spraying at a coating amount of about 0.1 g/cm ² , and then the following three heating methods were performed. (1) Heated with a burner at 150-180℃ for 15 minutes, (2) After heated with a burner at 150-180℃ for 3 minutes,
Leave at room temperature for 5 hours, (3) After heating at 150-180℃ for 3 minutes with a burner,
Furthermore, microwave heating was performed for 5 minutes using a microwave heater, a so-called commercially available household microwave oven. The compressive strength, surface stability and hardness of the test pieces subjected to the above three types of heat treatments were measured by standard methods, and the results are summarized in Table 5. For comparison, measurements were also conducted on test pieces that were as-molded without applying the water-soluble resin liquid.

[Table] Based on Table 5, the following facts were confirmed. (1) A mold that has just been heated in a burner for a short period of 3 minutes is not very practical. (2) After a short heat treatment of 3 minutes with a burner, a practical mold can be obtained by leaving it for another 5 hours at room temperature. (3) By using a combination of heat treatment for 3 minutes with a burner and microwave heat treatment, the heat treatment time can be reduced to approximately half that of heat treatment using only a burner. Comparative Example Two test pieces similar to those in Example 1 were prepared, and one of them was sprayed with the 2.5% by weight polyvinyl alcohol aqueous solution used in Example 1, so that the coating amount was approx.
Apply 0.1g/ ^cm2 to the surface of the test piece, and spray a 10% by weight dextrin aqueous solution onto the other side.
It was applied to the surface of the test piece at a concentration of 0.3 g/cm ² . Next, both test pieces were heat-treated with a burner at 150 to 180°C for 15 minutes to harden the test piece surfaces. After cooling, each test piece was left in a desiccator with water in the bottom, and the surface stability was measured over time. The results are shown in FIG. 1st
In the figure, the line connecting the points marked with ○ is the result of the test piece obtained by the method of the present invention, and the line connecting the points marked with △ is the result of the test piece obtained using the dextrin aqueous solution. Indicates the results of As is clear from FIG. 1, the surface stability of the test pieces prepared by the method of the present invention did not change, whereas the surface stability of the test pieces using the dextrin aqueous solution deteriorated rapidly. Effects of the Invention When an aqueous solution containing polyvinyl alcohol is applied to the surface of a mold according to the present invention, the aqueous solution forms a coating layer on the surface of the mold and also permeates into the mold. And by applying heat treatment,
Water and other solutions are evaporated from the coating layer, the polyvinyl alcohol is solidified, and the mold surface is continuously bonded and solidified, forming a strong mold surface several millimeters thick, with a relatively flexible casting inside. The sand creates a mold. This mold has a non-fragile surface, is easy to core, carry by hand, and set in the mold, and can withstand the pressure and heat when molten metal is poured. In addition, since the inside of the mold is flexible, it has enough flexibility to absorb the shrinkage that occurs when the metal solidifies, which prevents cracks in the casting that would occur if conventional molds were too strong. It also has the effect of improving the disintegration of the mold. Furthermore, since the mold surface according to the method of the present invention is coated with a hardened layer, any conventionally known foundry sand can be used as the molding sand as long as it can form a mold. There is no problem in using returned sand obtained by collapsing it after use. Therefore, an inexpensive and highly practical mold can be easily formed.

[Brief explanation of the drawing]

FIG. 1 is a graph showing changes in mold surface stability over time.

Claims (1)

[Claims] 1. On the surface of the mold made with green molding sand,
Polymerization degree is 200 to 3000 and saponification degree is 50
It is obtained by adding partially saponified polyvinyl alcohol or fully saponified polyvinyl alcohol at a ratio of 1 to 50% by weight to an aqueous polyvinyl alcohol solution containing 1 to 50% by weight of polyvinyl alcohol, which is ~98 mol%, or an aqueous solution of other resins. A method for manufacturing a mold, which comprises applying an aqueous solution to the mold and then applying heat treatment to harden the surface of the mold.