JP4925289B2 - Method for producing a gypsum mold for casting - Google Patents

Description

  The present invention relates to a method for producing a gypsum mold for casting used in a gypsum casting method which is one of precision casting methods. More specifically, when a gypsum mold is manufactured by reversing the casting with respect to the mother mold, there is insufficient filling of bubbles and gypsum material at the contact surface portion with the mother mold of the gypsum mold, and when the gypsum mold is removed. It relates to a technology that can minimize defects such as chipping.

Currently, there are the following three types of precision casting methods.
1) burnout model method (lost wax method), 2) ceramic mold method, and 3) gypsum casting method. Table 1 summarizes the characteristics of each casting method.

  The mold material slurry in each casting method is prepared by mixing the binder, solvent, and refractory material. In the case of gypsum, water becomes an agent that initiates curing (condensation), but in the case of ethyl silicate, self-hardening is manifested only by adding a curing agent (accelerator). In the case of colloidal silica, self-hardening cannot be imparted, and water in the slurry is naturally dried to make it apparently “solid”, which is vitrified by high-temperature firing.

  Mold materials are usually made by mixing various refractory powders in a liquid binder (binder) to form a slurry, which is then poured onto the surface of a rubber mold or brazing material, cured, and demolded or dewaxed. It will be. This can be said to be the greatest feature of the casting method. By precisely reversing a complicated shape with a liquefied mold material, it is possible to create a shape that cannot be handled by machining.

Therefore, it can be said that the characteristics of the binder for the mold material determine most of the characteristics of various casting methods. In other words, colloidal silica and ethyl silicate-based binders are vitrified after firing, and can be used at high temperatures depending on the choice of refractory materials. While it can be used as a casting mold for almost all alloys up to about 1000-1500 ° C., gypsum can only withstand up to about 800 ° C. because of its low fire resistance. This is because thermal decomposition of gypsum CaSO ₄ ; decomposition into calcium and sulfurous acid gas begins from around 800 ° C. For this reason, it can be said that only casting of alloys having relatively low melting points such as aluminum and zinc alloys can be performed. Therefore, in the case of the gypsum casting method, the fire-resistant material should not be expensive and expensive, but rather it should be fine, dimensionally stable and smooth in the casting surface. It tends to be used.

In addition, the gypsum mold is in a state where the gypsum molecule after firing (drying) is type 3 anhydrous gypsum (CaSO ₄ III), and it is easy to incorporate water in the form of crystal water in the molecule. Then, there is a drawback that it quickly absorbs moisture from the atmosphere and easily causes casting pinhole defects during casting. If the gypsum mold is also baked at a high temperature of 700 to 800 ° C, the state of the gypsum molecule can be brought to the type 2 anhydrous gypsum (CaSO4II) that does not resorb moisture, but in the transformation from type 3 to type 2. It is accompanied by a very large “transformation shrinkage”, and the strength is greatly reduced, so it often causes “mold cracking” and “mold deformation” during mold firing. The mold is dried at 150 to 200 ° C. For this reason, among the above-mentioned three types of precision casting methods, the most accurate mold dimensional accuracy is gypsum casting.

  Colloidal silica is not self-hardening, and has a weak point that it cannot be used in a slurry-like manner, poured into a mold, cured, and reversed. For this reason, it can be said that colloidal silica can be used only for the disappearance model method. Even in this case, it takes a long time to cure the slurry (remove moisture in the slurry), and it can be said that productivity is poor. (If the moisture removal process is carried out rapidly, the slurry may be peeled off or cracked from the disappeared model. After removing the moisture, the colloidal silica is vitrified by baking at 800 ° C to 1000 ° C to withstand casting. As a complete cure.)

  In the case of ethyl silicate, instead of imparting self-hardening properties, it is essential to perform “rapid heating (firing)” after curing, and there is a weak point that the dimensional variation of the mold easily occurs due to this. (The reason why rapid heating is necessary is to remove the solvent in the mold rapidly, stop the "hydrolysis" reaction and suppress the dimensional change of the mold, and at the same time generate "microcrazing", (There is also the purpose of improving the air permeability. After this rapid heating, the main baking is performed at 800 ° C. to 1000 ° C. to completely vitrify the mold.)

  In terms of cost, it can be said that the cost increases in the order of gypsum, colloidal silica, and ethyl silicate. For precision casting of aluminum alloys, zinc alloys, and magnesium alloys, the gypsum casting method has many advantages, and has been adopted as a casting method for tire molding dies, various functional parts, and some decorative products.

  The production of a gypsum mold is generally performed as shown in FIG. In the process of FIG. 1, there are cases where bubbles adhere to the surface of the mother mold or the gypsum slurry cannot be completely filled on the mother mold surface during the casting operation of 5). In particular, when the matrix material is “organic materials (materials containing oils and fats)”, the gypsum slurry uses “water” as a solvent, so “water repellency” is easy to work on, and the above shape transfer is possible. It often causes problems.

  As a countermeasure against this, there is a method of casting the mother layer after brushing or spraying “skin gypsum” on the surface of the mother mold using a casting gypsum slurry as shown in FIG. Yes. However, since these measures use a slurry for casting the mother layer as the skin gypsum material, they are strongly influenced by the properties (viscosity, etc.) of the mother layer slurry. In some cases, insufficient filling problems could not be resolved. In addition, when the matrix has a so-called undercut shape with a reverse draft or a fine uneven shape, the part may be lost during demolding in step 7) of FIG. . (Mold missing)

Cast gypsum mold materials are often composed of hemihydrate gypsum (α gypsum), various refractory materials, and additives. Curing after slurried (condensation) is provided by transformation to hemihydrate gypsum _{(CaSO 4 · 1 / 2H 2} O) of gypsum _{(CaSO 4 · 2H 2 O)} . Since the gypsum material is premised on being kneaded with water to form a slurry and casting, it is generally used by kneading with much more water than the crystallization water necessary for the above transformation. For this reason, the higher the gypsum content ratio in the mold material, the lower the water mixing ratio (% by weight of water kneaded per kg of gypsum material), and the higher the strength characteristics of the gypsum mold after curing.

A gypsum mold (green mold: CaSO ₄ · 2H ₂ O + free water) is generally used for casting after drying to remove crystal water and transforming it to type III anhydrous gypsum (CaSO ₄ III). This is to prevent the occurrence of hydrogen pinhole defects in the casting due to the release of crystal water by heat input during casting.
When a gypsum mold transformed to type 3 anhydrous gypsum (CaSO ₄ III) is used for casting, it transforms to type 2 anhydrous gypsum (CaSO ₄ II) by heat input during casting. During this transformation from type 3 to type 2, gypsum is accompanied by significant shrinkage. As shown in Patent Document 1, it can be said that the amount of shrinkage increases as the gypsum content ratio in the gypsum mold material increases and the water mixing ratio increases.

  Since the gypsum mold has a low thermal conductivity, the transformation shrinkage is likely to occur only near the molten metal contact surface of the gypsum mold during casting, which may cause cracks on the mold surface. In some cases, burrs are generated in the insert casting, or the mold is partially lost, and the shape is lost in the casting.

  From the above, the problems occurring in the gypsum mold formed using the same gypsum slurry for both the skin gypsum layer and the mother layer showed a tendency as shown in Table 2 below. It should be noted that as AA → A → B → C → D, the corresponding defect becomes severe (problem is likely to occur). (It can be said that α and β are problems related to skin plaster, and γ is a problem related to mother layer plaster.)

  The reason that the more the gypsum content in the gypsum powder tends to decrease the α bubbles and the insufficient filling failure is presumed to be because the basicity (alkalinity) of the slurry increases. The matrix used for reversing gypsum molds is usually composed of organic materials such as rubber materials (materials containing fats and oils), or a mold release agent made of organic materials is applied to improve mold release properties. It is often done. These materials have a strong “water repellency” and are likely to directly lead to defects in bubbles and insufficient filling. However, as the basicity of the gypsum slurry increases, the effect of the slurry on dissolving the oil increases. The familiarity of the slurry (skin gypsum) is considered to be improved. Moreover, it is thought that the reason which shows the tendency for (alpha) air bubbles and a filling insufficiency defect to reduce, so that the water mixing rate at the time of slurry preparation is high is based on the viscosity of skin gypsum slurry. It is considered that as the water mixing ratio increases, the viscosity of the slurry decreases, and the slurry easily turns to the fine part of the mother mold.

That is, trying to improve one of the problems is likely to lead to worsening the other problems, and there is no good method that can solve all the problems. Patent Document 1 discloses a method in which the surface of the obtained gypsum mold is impregnated with colloidal silica later, but cannot cope with chipping during demolding.
Japanese Patent No. 3761414

  The present invention has been devised under such circumstances, and an object of the present invention is to provide a technique for overcoming the problems related to the mother-layer gypsum while overcoming the problems related to the skin gypsum. More specifically, by adjusting only the gypsum slurry used for skin gypsum, using a powder with a higher gypsum content ratio than the mother layer gypsum slurry, and setting the water mixing ratio higher than that of the mother layer. It is to provide technology that can deal with defects.

In order to solve the above-mentioned problems, the invention according to claim 1 is characterized in that when the gypsum mold used in the gypsum casting method is manufactured by the casting reversal method for the mother mold, the surface of the gypsum mold in contact with the molten metal is formed. The skin gypsum layer to be formed is composed of gypsum powder having a gypsum content ratio of 50 to 100% by weight , and the mother layer forming the interior of the gypsum mold is made of gypsum powder having a gypsum content ratio of 30 to 40% by weight. It is characterized by using. The invention according to claim 2 is the method for producing a gypsum mold for casting according to claim 1, wherein the water mixing ratio of the gypsum mold material slurry used for the skin gypsum layer is higher than the water mixing ratio of the gypsum mold material slurry used for the mother layer. It is characterized by doing.

The invention according to claim 3 is the method for producing a gypsum mold for casting according to claim 1 or 2, wherein the gypsum mold material used for the skin gypsum layer after the skin gypsum layer is applied as a first layer on the surface of the mother mold Using the gypsum powder within the range of the gypsum content ratio of the gypsum powder of the slurry to the gypsum content ratio value of the powder for preparing the mother layer, the second layer is formulated with a lower water content than the slurry for the first layer It is characterized in that it is applied and a mother layer gypsum is cast.

  According to the invention of claim 1, only gypsum powder used for skin gypsum has a higher gypsum content ratio than gypsum powder used to constitute the mother layer, and the α and ( While maintaining the characteristics of gypsum bubble filling insufficient defect) and β (surface chipping defect during demolding), it is possible to minimize casting mold cracking and shape defect, which are problems of γ. According to the invention of claim 2, bubble defects on the mold surface can be more reliably prevented. Further, according to the invention of claim 3, by forming the skin gypsum into a two-layer structure, the ability to counteract bubbles and filling insufficiently as in claim 2 and the ability to prevent surface chipping at the time of demolding as in claim 1 are combined. I can do it.

<Invention of Claim 1>
FIG. 3 is a process explanatory view showing the invention of claim 1, and first, a mother layer slurry 1 and a skin gypsum slurry 2 are prepared. The mother layer slurry 1 is prepared by mixing the mother layer gypsum powder 3 and water, and the skin gypsum slurry 2 is prepared by mixing the skin gypsum powder 4 and water. Although the water mixing ratio is the same, gypsum powder having a higher gypsum content ratio than the gypsum powder 3 for the mother layer is used as the skin gypsum powder 4. That is, if the gypsum content of the gypsum powder 3 for the mother layer is A wt % and the gypsum content of the skin gypsum powder 4 is C wt %, then A <C. The value of A is 30 to 40% by weight , and the value of C is about 50 to 100% by weight .

  The skin gypsum slurry 2 is sprayed on the surface of the mother die 5 to form a skin gypsum layer 6 near the molten metal contact surface on the surface of the mother die 5, and then the mother layer slurry 1 is cast to mold An inner mother layer 7 is formed, cured and demolded. In this way, the gypsum mold 8 can be manufactured by the casting reversal method for the mother mold 5. The obtained gypsum mold 8 is composed of a skin gypsum layer 6 in the vicinity of the molten metal contact surface and a mother layer 7 inside the mold, and the skin gypsum layer 6 is composed of gypsum powder having a higher gypsum content ratio than the mother layer 7. ing. As a result, a dense gypsum mold 8 having a high strength only in the vicinity of the mother mold can be obtained.

  In Table 2, slurry blending condition I has excellent characteristics for α bubbles, insufficient filling failure, and surface chipping failure when β is removed, whereas γ casting mold cracking and shape dropout failure. Will have bad characteristics. The invention of claim 1 is a gypsum that is higher than gypsum powder used for constituting a mother layer only for gypsum powder used for skin gypsum in order to overcome the problems of γ while maintaining the favorable characteristics of α and β. It has a fractional ratio, maintains the characteristics of α and β, and minimizes the problem of γ.

  By doing so, it is possible to minimize the occurrence of defects such as mold defects at the time of demolding from the mother die and surface sag at the sharp tip of the protrusion. Further, by using this method, it is possible to minimize a mold cracking defect during casting (a defect caused by transformation shrinkage cracking of a gypsum mold due to casting heat input). This will be described below with reference to FIG.

As shown in FIG. 4, when the mold is manufactured using only the powder for the mother layer (gypsum content ratio A weight %), the surface of the gypsum mold after demolding from the mother mold is free of defects, bubbles, and surface sag. Therefore, defects such as defects, surface sag, and bubble shape generated when the mold is removed from the mother die are caused. However, the casting burr is extremely small. On the other hand, when the mold is manufactured using only the powder for skin plaster (gypsum content ratio C weight %), the shape transfer accuracy of the mother mold is good, but the frequency of defects due to casting burrs and mold dropout during casting is low. Get higher. On the other hand, according to the first aspect of the present invention, the shape transfer accuracy of the mother die is good, and the occurrence frequency of casting burrs and cast-shaped mold dropping is low. (The thinner the skin gypsum layer, the less likely these defects occur.)

  As described above, the invention of claim 1 overcomes the problem γ under the compounding condition I in Table 2 and the defect in the mold cracking shape missing during casting by using the slurry under the compounding condition I only for the “skin gypsum layer”. This is the gist. That is, as shown in FIG. 4, claim 1 is a technique for avoiding a fatal wound related to the problem γ by keeping the mold crack of the gypsum mold caused by heat input from casting only in the vicinity of the skin gypsum layer. If the invention of claim 1 is used, almost all the defects of the conventional method (α, β, γ in Table 2) can be overcome.

  Even if only the gypsum content ratio of the skin gypsum layer is increased as in claim 1, the adhesion between the skin gypsum layer and the mother layer is ensured and there are few practical problems. If the ratio is made lower than that of the mother layer), the gypsum layer is likely to “peel” when the gypsum mold is removed from the mother mold, during mold drying, or during casting. There are almost no cases where you want to use the opposite case, but it is virtually impossible to use it.

<Invention of Claim 2>
Even when the invention of claim 1 described above is used, there is still a risk of frequent occurrence of bubble defects on the mold surface when the “setting of water mixing ratio” is low. In order to overcome this problem, the invention of claim 2 makes use of claim 1 and only the slurry for skin gypsum has a higher water mixing ratio than the mother layer slurry so as not to generate bubble defects on the mold surface. It is a summary of the matter.

  As shown in Table 2 above, it can be seen that the blending condition V is the most effective as a countermeasure against bubble defects. However, since the blending condition V has the weak point that the mold crack shape drop-out failure easily occurs during casting, the gist of claim 2 is to overcome this by the method of claim 1.

  That is, claim 1 can be said to be a countermeasure focusing on countermeasures against defect β (surface chipping at the time of demolding), and claim 2 can be said to be countermeasures focusing on countermeasures against malfunction α (bubble and insufficient filling defect). Specifically, the water mixing rate for creating the skin gypsum slurry 2 shown in FIG. 3 may be higher than the water mixing rate for creating the mother layer slurry 1, and the difference in water mixing rate May be, for example, 5 to 15%.

<Invention of Claim 3>
Claim 3 is a technique for simultaneously developing the defect α countermeasure ability equivalent to claim 2 and the defect alpha countermeasure ability equivalent to claim 1. Specifically, the skin plaster can be made into a “two-layer structure”. It is a point. That is, the first layer of skin plaster is applied by the method according to claim 2, and the second layer of skin plaster used thereafter is
1) Use the gypsum powder in the gypsum powder used to have a gypsum content ratio in the gypsum powder used in the first layer that is greater than or equal to the gypsum content ratio in the first layer,
2) Use a water mixture ratio that is prepared so that the slurry is lower than the water mixture ratio of the slurry for the first layer.
The gist is that the skin gypsum has a two-layer structure. As a result, it is possible to have both the ability to cope with bubbles and insufficient filling as in claim 2 and the ability to cope with surface chipping during demolding as in claim 1.

  FIG. 5 shows an embodiment of the invention of claim 3, in which a first layer of gypsum slurry 11 and a second layer of gypsum slurry 12 are applied to the surface of the mother die 5. Gypsum that has been kneaded with water and slurried is cured (condensed) in about 15 to 60 minutes. During the slurry state, a “diffusion” phenomenon occurs between the first layer, the second layer of gypsum, and between the second layer of gypsum and the mother layer gypsum. Try to be. It can be said that the invention of claim 3 utilizes this characteristic. The first-layer skin gypsum slurry 11 has a high water mixing ratio and is excellent in applicability to the mother mold 5 and can minimize the generation of bubbles, but it has a weak point that it does not exhibit high strength characteristics after condensation. This weak point can be overcome by intermingling with the second-layer skin gypsum slurry 12 applied later.

In addition, when the invention of claim 3 is used, it is possible to uniformly apply the gypsum without bubbles to the part (standing wall surface, etc.) where the gypsum applied to the surface of the mother mold is likely to hang down. One of the advantages.
Examples of each invention are shown below.

The original mold, master mold, gypsum material, and castings aimed at production used throughout the examples are as follows.
The original shape is as shown in FIG. 6, and the material is an aluminum alloy 2017 (Si 0.6%, Mg 0.5%, Cu 3.8%, Fe 0.5%, remaining Al). The base mold (rubber mold) is smooth-on polysulfide rubber FMC201, and the structure is a gypsum-lined structure with a rubber layer thickness of 10 mm (the minimum thickness of the backing material is 80 mm).

  The gypsum powder used was Noritake Gypsum casting gypsum mold material: G-6 non-foamed gypsum, and α gypsum FT-2 was used in combination for adjusting the gypsum content ratio in the powder. The casting shape is as shown in FIG. 7, and the material is made of aluminum alloy AC4C (Si 7.0%, Mg 0.4%, Cu 0.02%, Fe 0.15%, remaining Al). The dimensions of the details reflect the amount of “cast shrinkage” that occurs when the master mold is cast and reversed.

<Example 1> Example of claim 1 Slurry for skin gypsum and mother layer are separately prepared under the blending conditions shown in the upper part of Table 3, and the slurry for skin gypsum is molded using a spray gun (rubber mold) ), And then the gypsum mold was inverted by casting the slurry for the mother layer. The lower part of Table 3 shows the results of the gypsum mold thus obtained and the castings cast using the same.

<Comparative example 1> Comparative example of Example 1 The slurry for skin gypsum and the mother layer was prepared under the blending conditions shown in the upper part of Table 4, and the slurry for skin gypsum was applied to the mother mold (rubber mold) using a spray gun. Then, the gypsum mold was inverted by casting the slurry for the mother layer. The lower part of Table 4 shows the results of the gypsum mold thus obtained and the castings cast using the same. Thus, the superiority of Example 1 using the invention of claim 1 was confirmed.

<Example 2> Example of Claim 2 The slurry for skin gypsum and the mother layer is prepared in a separate manner under the blending conditions shown in the upper part of Table 5, and the slurry for skin gypsum is molded into a mother mold (rubber mold) using a spray gun. ), And then the gypsum mold was inverted by casting the slurry for the mother layer. The lower part of Table 5 shows the results of the gypsum mold thus obtained and the castings cast using the same. Bubble defects were reduced as compared with Example 1, but chip defects were scattered. However, there is no mistake in being superior to Comparative Example 1.

<Example 3> Example of Claim 3 The slurry for skin gypsum and the mother layer is prepared separately by the preparation conditions shown in the upper part of Table 6, and the two types of slurry for skin gypsum are molded using a spray gun. After applying two layers to the (rubber mold), the gypsum mold was inverted by casting the slurry for the mother layer. The results of the gypsum mold thus obtained and the castings cast using the same were as shown in the lower part of Table 6. Compared with Examples 1 and 2, it was possible to obtain a casting having the best overall quality.

  In these examples, in order to make the results easier to understand, the defects α and β that occurred in the mold were cast as they were, but when the actual product was handled, the defects α and β were “at the mold stage. It will be dealt with by "correcting". (Because it becomes a casting, it is almost impossible to correct.) For this reason, it can be said that the less difficult the number of defective parts in the previous embodiment, the more highly difficult casting can be produced with high productivity.

  As described above, by using the present invention, it is possible to accurately transfer a fine concavo-convex shape, which has been difficult in the past, using a gypsum casting method. From this point of view, it can be said that the present invention has a great significance in casting various decorative products, precision parts and precision molds.

It is explanatory drawing of the manufacturing process of a general gypsum mold. It is explanatory drawing which shows the application | coating method of skin gypsum. FIG. 2 is a process explanatory view showing the invention of claim 1. It is a contrast diagram which shows the generation | occurrence | production state of the defect in each process. It is process explanatory drawing of invention of Claim 2. It is explanatory drawing of the original shape in an Example. It is explanatory drawing of the casting shape in an Example.

Explanation of symbols

1 Mother layer slurry 2 Skin gypsum slurry 3 Mother layer gypsum powder 4 Skin gypsum powder 5 Mother mold 6 Skin gypsum layer 7 Mold inner layer 8 Gypsum mold 11 First layer gypsum slurry 12 Second Layer skin gypsum slurry

Claims (3)

When the gypsum mold used in the gypsum casting method is manufactured by the casting reversal method for the mother mold, the gypsum layer forming the surface in contact with the molten metal of the gypsum mold has a gypsum content ratio of 50 to 100% by weight . A method for producing a gypsum mold for casting, characterized in that the gypsum powder is used to form a mother layer that forms the interior of the gypsum mold using a gypsum powder having a gypsum content ratio of 30 to 40% by weight .   The casting gypsum mold manufacturing method according to claim 1, wherein the water mixing ratio of the gypsum mold material slurry used for the skin gypsum layer is higher than the water mixing ratio of the gypsum mold material slurry used for the mother layer. Method for producing gypsum molds.   3. The method for producing a gypsum mold for casting according to claim 1 or 2, wherein a gypsum powder ratio of a gypsum mold material slurry used for the skin gypsum layer is applied to the surface of the master mold as a first layer. Using the gypsum powder within the range of the gypsum content ratio value of the mother layer preparation powder from the same value as above, apply a mixture with a lower water mixing ratio than the slurry for the first layer as the second layer, A method for producing a gypsum mold for casting, characterized by casting.

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