JPH0315521B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a carbon fiber reinforcement for ceramic molding that improves strength, thermal properties, etc. without reducing water absorption by mixing carbon fibers with excellent strength characteristics and extremely small single fiber diameters into plaster. This invention relates to a method for manufacturing a plaster mold. Conventionally, various methods have been adopted to increase the strength of a plaster mold for molding ceramics (hereinafter simply referred to as a plaster mold). For example, there are methods such as mainly using β-type hemihydrate gypsum and mixing α-type hemihydrate gypsum into it to reduce the amount of mixed water, or mixing cement or resin into the plaster. There is also a method of mixing natural fibers such as, or glass fibers. However, when α-type hemihydrate gypsum is mixed into β-type hydrogypsum to reduce the amount of mixed water, the strength itself is improved, but the water absorption rate is reduced. One of the most important conditions required for a plaster mold is that the water absorption rate of the plaster mold, especially the water absorption rate when pouring slurry, be good. In other words, if the water absorption rate of the plaster mold is poor, the molding time for each product will be longer, resulting in a decrease in molding efficiency.If the water absorption rate is particularly poor, the green strength will be weak when removed, resulting in deformation. The finished shape of the product deteriorates. Therefore, a decrease in water absorption is a fatal drawback in plaster molds. Also, when cement or resin is mixed into plaster, the strength itself can be increased, but it also has the disadvantage of decreasing water absorption. Furthermore, when mixing natural fibers such as hemp into plaster, the tensile strength of natural fibers is low, so the strength cannot be increased unless the amount of natural fibers mixed into the plaster is increased. This increase inevitably leads to a decrease in the water absorption rate of the plaster mold, and since the single fibers of natural fibers themselves are thick, the ends of the fibers that have entered the surface layer of the plaster mold are likely to be exposed on the molding surface. It has the disadvantage that the fiber ends damage the surface of the molded product and are likely to cause molding defects. In addition, when glass fiber is mixed into the plaster, the strength of the plaster mold is slightly improved, but since glass fiber has rigidity, the ends of the glass fiber that have entered the surface layer are exposed on the molding surface. This has the drawback of damaging the surface of the molded product. Therefore, the present inventor focused on the characteristics of carbon fiber, such as its excellent strength characteristics, flexibility, lightness, small diameter, and extremely low coefficient of thermal expansion, and cut countless carbon fibers into predetermined lengths. Various experiments and research were carried out in order to increase the strength of the plaster mold without reducing the water absorption rate by dispersing it into the single fibers of books and uniformly dispersing the countless single fibers within the plaster mold. As a result, the present invention was completed. The present invention, based on the above-mentioned conventional technology, uniformly diffuses a small amount of carbon fiber inside a plaster mold, thereby reducing the mechanical and thermal properties of the plaster mold while maintaining its original function without reducing the water absorption rate. This was done with the aim of increasing the strength of the carbon fibers, and the gist of the process is to heat and remove the sizing agent applied after cutting the carbon fibers into lengths of 5 to 70 mm, and to cut the carbon fibers into lengths of 5 to 70 mm. A process of stirring carbon fibers in water while applying ultrasonic vibration to disperse the carbon fibers into countless single fibers, gypsum powder, and the dispersed carbon in an amount of 0.03 to 1% by weight based on the gypsum powder. A process of mixing and stirring fibers, water, and other additives as necessary to create a carbon fiber-containing gypsum slurry in which single carbon fibers are uniformly dispersed, and placing the carbon fibers in a case mold. A process of pouring gypsum slurry, removing it from the mold after hardening, and drying it, or pouring pure gypsum slurry into a case mold in advance to form a thin film, then pouring the carbon fiber-containing gypsum slurry, and removing the mold after hardening. The drying process allows the carbon fibers to be uniformly dispersed within the plaster mold. Hereinafter, the present invention will be explained in detail by citing preferred embodiments. The carbon fibers used in the present invention may be polyacrylonitrile-based, pitch-based, or rayon-based, but carbon fibers with high strength or high elasticity are desirable from the standpoint of increasing the strength of the plaster mold. is tensile strength 200Kg/mm ² (Kg/mm ² )
As mentioned above, a tensile modulus of elasticity of 20,000 Kgf/mm ² (Kg/mm ² ) or more is desirable. Furthermore, in the present invention, it is extremely important to uniformly diffuse carbon fibers, which tend to form nodules, into the base material plaster without forming nodules, and from this point of view, the carbon fibers are mixed into the plaster as a reinforcing material. The length of the carbon fibers and the weight percentage relative to the plaster is determined. The carbon fibers are dispersed into countless single fibers and mixed into the base material, gypsum.For reasons explained later, the length of the single fibers needs to be 5 to 70 mm, preferably 20 to 30 mm. . Figure 1 shows the relationship between the length of carbon fibers and the bending strength (flexural strength) in a 15 x 25 x 250 mm plaster specimen made of 100 parts by weight of plaster, 60 parts by weight of water, and 0.5 parts by weight of carbon fiber. A graph of the test results is shown, and it is clear from this that the bending strength decreases rapidly when the length of the carbon fiber is 15 mm or less. Here, the length of the carbon fibers to be diffused into the base material plaster was limited to 5 to 70 mm because if the length was less than 3 mm, the total adhesion area between the base material plaster particles and the carbon fiber single fibers would be insufficient. If it is not possible to improve the strength of the plaster mold sufficiently, or if the thickness exceeds 10 mm,
This is because handling becomes troublesome when dispersing into single fibers, mixing and stirring with gypsum powder and water, or pouring into a case mold, and it becomes difficult to uniformly disperse into a gypsum mold. First, carbon fiber manufactured by a conventional method was
After cutting the carbon fiber to a length of 70 mm, the carbon fiber is heated in an oxidizing atmosphere to oxidize and remove the handling stabilizing sizing agent applied to the surface. The heating temperature is relatively determined depending on the relationship with the sizing agent applied to the surface, but it is desirable to perform the heating at around 300°C, which is the generally safe maximum temperature for use of carbon fibers. The carbon fibers from which the sizing agent has been removed by heating are easily dispersed into countless (usually 1000 to 2400) very small diameter (usually 5 to 10 μm) single fibers that constitute the carbon fibers. Next, remove the sizing agent by heating and
Put the carbon fiber cut into length into the aquarium,
When the carbon fibers are rotated slowly using a stirring blade while applying ultrasonic vibrations, the sizing agent is removed and the carbon fibers become easily dispersed. Due to the synergistic effect of the ultrasonic vibrations and gentle stirring, the carbon fibers do not form lumps. It is dispersed into countless single fibers with extremely small diameters. To avoid damaging the carbon fibers by the stirring blades during stirring, the rotation speed is set at
It is necessary to set the speed to 40 to 60 rpm in an aquarium with a diameter of about 60 cm. After the dispersion treatment, the countless dispersed single fibers are taken out of the water tank, dehydrated, and dried. Next, a gypsum slurry containing the carbon fibers dispersed in single fibers is prepared, and the ratio of carbon fibers to the gypsum powder is 0.03 to 1% by weight, preferably 0.1 to 0.3% by weight for the reasons described below. It is necessary to. Figure 2 shows 100 parts by weight of gypsum,
Graph of test results showing the relationship between the weight percent of mixed carbon fiber to plaster and the bending strength in a 15 x 25 x 250 mm plaster test piece made of 60 parts by weight of water and the required weight part of carbon fiber with a length of 20 mm. In addition, FIG. 3 shows a graph of the test results showing the relationship between the proportion of carbon fiber mixed in gypsum and the water absorption rate. As is clear from FIGS. 2 and 3, it can be seen that the higher the proportion of carbon fibers mixed in the plaster, the higher the bending strength and the higher the water absorption rate. The reason why the water absorption rate is high is that the gypsum particles are acicular, the cross-sectional shape of the carbon fibers is circular or a shape similar to this, and the size of the gypsum particles and the diameter of the carbon fibers are not much different. This is thought to be due to the formation of new voids between the carbon fibers and the gypsum particles. Here, the mixing ratio of carbon fiber to gypsum powder is 0.03 to 1% by weight, which means 0.01% by weight.
At the end of the weight percentage, the ratio to plaster is too small to improve the strength of the plaster mold sufficiently, or
If it exceeds 1.2% by weight, the proportion of carbon fibers will be too high when making the gypsum slurry, making it impossible to spread the carbon fibers uniformly within the gypsum slurry, making it easy to form lumps, and making it difficult to fit the gypsum slurry into the case mold. The fluidity during pouring deteriorates, making it difficult to work, and the physical properties of the plaster mold to be molded, such as water absorption, change, and it does not meet the requirements for a mold material for ceramics, and the inside of the plaster mold contains carbon fiber. This is because baby booms are more likely to occur. In order to make a gypsum slurry in which carbon fibers are uniformly dispersed, the container 1 shown in Fig. 4 must be filled with water suitable for the amount of water mixed at one time, a curing retardant, a water reducing agent, etc. Add additives in advance,
Next, a predetermined amount of dispersed carbon fibers weighed in advance is put into this container 1, and finally a predetermined amount of gypsum powder is added, and this container 1 is attached to a vacuum stirrer, and the diameter is about 60 cm. When mixing and stirring the stirring blade in the container 1 at a low speed of 100 to 200 rpm for several minutes, the carbon fiber single fibers are uniformly dispersed in the gypsum slurry, and the carbon fibers are degassed. A gypsum slurry is obtained. here,
Carbon fibers are uniformly dispersed within the gypsum slurry because
This is understood to be because the ratio of carbon fiber to gypsum is extremely small, and the reason why the stirring blades are rotated at low speed during mixing and stirring is to prevent the carbon fibers from being damaged by the stirring blades. be. Next, the above-mentioned gypsum slurry in which carbon fibers are uniformly dispersed is gently poured into the case mold 2 as shown in Fig. 5, and left to stand for a predetermined period of time. When the molds 2 are separated from each other and demolded, and then sufficiently dried at a predetermined temperature, a plaster mold (used mold) A for external troweling for potter's wheel molding as shown in FIG. 6 is obtained.
Because carbon fiber has a lot of flexibility, it is understood that even after casting, it can be freely deformed and fit easily between the plaster particles, and the molding surface of the cast plaster mold. The ends of the carbon fibers that have penetrated into the surface layer are almost never exposed, but even if they are exposed, as mentioned above, the diameter of the single carbon fiber is extremely small, and it is highly flexible. Therefore, the surface of the molded product is hardly damaged by the carbon fibers exposed on the molding surface of the plaster mold. In this way, even if the ends of the carbon fibers are exposed on the molding surface, the surface of the molded product will hardly be damaged, but especially if the molded product is a high-quality product, an extremely smooth molding surface can be obtained. If necessary, use case type 2 as shown in Figure 7.
A thin film 3 with a thickness of about 1 to 3 mm is formed in advance by pouring a small amount of pure gypsum slurry while rotating at a low speed, and then the rotation of the case mold 2 is stopped and the carbon fibers are immediately poured into the gypsum slurry in which carbon fibers are uniformly dispersed. When the gypsum is poured gently and the same operations as described above are performed, as shown in FIG. A plaster mold for use (used mold) A′ is obtained,
Carbon fibers can be reliably prevented from being exposed on the molding surface. The above-mentioned plaster mold for external trowel (used mold) A' is a plaster mold for molding the inner peripheral surface of a molded product, and the molding surface is coated with a thin film 3 made of pure gypsum, and the molding surface is coated with carbon fiber. As shown in Fig. 9, in the case of preventing the exposure of
A thin film 5 with a thickness of about 1 mm to 3 mm is formed in advance on the outer peripheral surface of the protruding molded part 4a of the case mold 4 by pouring pure gypsum slurry while rotating the case mold 4 at a low speed. Immediately after stopping the rotation, the gypsum slurry in which carbon fibers are uniformly dispersed is gently poured in, and the same operations as described above are performed. As shown in FIG. Plaster mold for inner troweling (used mold) for potter's wheel molding of a cup covered with a thin film 5 made of plaster
B is obtained. Incidentally, the plaster mold for molding ceramics that can be manufactured according to the present invention is not limited to the above-mentioned plaster mold for potter's wheel molding.
Plaster molds for cast molding, gypsum molds for press molding, gypsum molds for injection molding, gypsum molds for molding refractories, etc. Furthermore, clays are not included in the raw material formulations of alumina, silicon carbide, silicon nitride, partially stabilized zirconia, sialon, etc. Almost all plaster molds for ceramic molding, such as press molding plaster molds and injection molding plaster molds for so-called new ceramics, are included. Next, examples of the present invention and comparative examples will be given. Example 1 Polyacrylonitrile fibers were heat-treated at about 300°C, and then further heat-treated at about 1300°C in a nitrogen gas atmosphere to graphitize them to form single fibers with a diameter of about 7 μm.
A bundle of 6,000 carbon fibers was used. The physical properties of this carbon fiber include tensile strength of 300 Kgf/mm ² (Kg/mm ² ), tensile modulus of elasticity of 23000 Kgf/mm ² (Kg/mm ² ), and density of 1.75.
g/cm ³ , linear expansion coefficient 0.0×10 ^-6 /℃, thermal conductivity
It had a heat capacity of 15 kcal/m・hr・℃ and a heat capacity of 0.17 cal/g・℃. This carbon fiber is cut into lengths of approximately 20 mm and dispersed into countless single fibers in water using the synergistic effect of ultrasonic vibration and stirring, and then gypsum powder
Mix and stir 100 parts by weight of carbon fiber, 0.1 part by weight of carbon fiber, and 0.2 part by weight of borax (hardening retardant) to create a plaster slurry in which carbon fiber single fibers are evenly dispersed, and mold this plaster slurry into a case shape. A plaster mold for potter's wheel molding was obtained. In this plaster mold for potter's wheel molding, single fibers of carbon fiber were uniformly diffused over the entire surface including the cross section, and the diffusion state was such that it could be seen with the naked eye. Example 2 A gypsum slurry in which carbon fiber single fibers were uniformly dispersed was made under the same conditions as in Example 1, and pure gypsum slurry was poured in advance while rotating the case mold at low speed to form a thin film with a thickness of about 1 to 3 mm. After that, the rotation of the case mold was stopped and the plaster slurry mixed with carbon fibers was poured in to obtain a plaster mold for potter's wheel molding for molding a plate. The outer peripheral surface, which is the molding surface of this plaster mold for potter's wheel molding, was covered with a thin film made of pure gypsum, and no carbon fibers were exposed on the molding surface. Example 3 After heat treating polyacrylonitrile fiber at about 300°C, it was further heated at about 300°C in a nitrogen gas atmosphere.
Carbon fiber was used, which was graphitized through special heat treatment at 2500℃ and made into a bundle of about 6000 single fibers with a diameter of about 7μm.
The physical properties of this carbon fiber include tensile strength of 250Kgf/mm ²
(Kg/mm ² ), tensile modulus 35000Kgf/mm ² (Kg/
mm ² ), density 1.77g/cm ³ , linear expansion coefficient 0.0×10 ^-6 /℃, thermal conductivity 100kcal/m・hr・℃, heat capacity 0.17cal/
It was warm at g・℃. After cutting this carbon fiber to about 25 mm, it was dispersed in the same manner as in Example 1, and then 100 parts by weight of gypsum powder, 60 parts by weight of water, and the carbon fiber
A mixture of 0.3 parts by weight and 0.2 parts by weight of Pufftard (hardening retardant) manufactured by Ajinomoto Co., Inc. was mixed and stirred to create a gypsum slurry in which carbon fiber single fibers were evenly dispersed, and this gypsum slurry was molded into a case shape. A plaster mold for casting was obtained to form an elliptical plate by pouring into the mold. The diffusion state of the carbon fibers in this plaster mold for cast molding was almost uniform as in Example 1. Comparative example: 100 parts by weight of gypsum powder, 60 parts by weight of water, and 0.2 parts by weight of borax
A pure gypsum slurry containing no carbon fibers was prepared by mixing and stirring in proportions of parts by weight, and this pure gypsum slurry was poured into a case mold to obtain a potter's wheel molding plaster mold for molding a plate. The flexural strength, water absorption, difference in breaking temperature in the atmosphere, bulk specific gravity, and expansion coefficient upon curing of each of the plaster molds of Examples 1, 2, and 3 and Comparative Example were as shown in the table below.

[Table] As is clear from the table above, plaster molds mixed with carbon fiber have mechanical and
It was found that the thermal strength was greatly improved, and the water absorption rate was also slightly improved, indicating that all of the above-mentioned physical properties were excellent. Due to the large improvement in bending strength, the mechanical strength of the plaster mold increases, and in particular, the mechanical strength of the parts that were conventionally damaged due to external pressure during molding is increased, thereby preventing damage to the plaster mold. In addition to preventing this, high-cycle molding becomes possible, and as a result, molding efficiency improves. Furthermore, it is possible to eliminate the trouble of interrupting production due to damage to the molding machine due to breakage of the plaster mold during molding, or of replacing damaged parts and readjusting them. Furthermore, the significant improvement in mechanical strength makes it possible to reduce the thickness of the plaster mold itself, which in turn reduces the amount of plaster used. Also, the reason why the difference in fracture temperature in the atmosphere improves due to the inclusion of carbon fibers is that while the plaster itself expands by a certain amount due to temperature rise, the carbon fibers themselves hardly expand. This is because a tensile force is applied in the length direction of the carbon fibers and a compressive force is applied to the plaster itself, which creates internal stress in the length direction of the carbon fibers and introduces prestress. It is understood that there is. The large increase in the breakdown temperature difference in the atmosphere is
This means that the plaster mold can withstand large temperature changes (temperature differences), making it possible to increase the drying temperature of the plaster mold. Therefore, the drying time of the plaster mold for each molding can be shortened, the molding cycle can be improved, and the molding efficiency can be improved. Therefore, it is possible to reduce the number of operating plaster molds relative to the number of molded products produced, making it suitable for molding a wide variety of products in small quantities, and reducing product costs. For the same reason, the molding efficiency of the plaster mold itself is also improved during plaster mold molding. In addition, improvement in water absorption rate improves molding efficiency by shortening the time from completion of molding to demolding in plaster molds for potter's wheel molding, and improves molding efficiency in plaster molds for casting molding. is shortened, and the molding efficiency is also improved. The reduction in bulk specific gravity reduces the weight of the plaster mold itself, which in turn improves the transportability and handling of the plaster mold. Furthermore, a slight decrease in the expansion coefficient upon curing reduces the pressure applied to the case mold during plaster molding, making demolding easier and at the same time preventing damage to the case mold. In summary, the present invention has the following effects. (1) Carbon fibers are cut into 5-70mm pieces, the sizing agent is removed by heating, and the sizing agent is stirred in water while applying ultrasonic vibrations to create bundle-shaped carbon fibers with an extremely small diameter and high flexibility. Since carbon fibers can be easily dispersed into countless single fibers, countless single fibers of carbon fiber can be uniformly dispersed within the plaster slurry, and as a result, the carbon fibers can be uniformly mixed into the plaster mold. (2) Carbon fiber, which has an extremely small diameter, high strength, and high flexibility, is used as the reinforcing material for the plaster mold, so even if the proportion of reinforcing material mixed into the plaster is small, the strength of the plaster mold can be increased. In addition, since the proportion of reinforcing material mixed in is small, the water absorption rate, which is the basic function of plaster molds, does not decrease. Furthermore, after molding, the carbon fibers deform freely within the plaster mold, and the It is understood that the carbon fibers are intercalated between the particles, so that the gypsum particles and carbon fibers blend well within the gypsum mold. (3) Even if the ends of the carbon fibers mixed into the surface layer of the plaster mold are exposed on the molding surface, the diameter of the single carbon fibers is extremely small and is highly flexible. Molded products are rarely damaged during molding. In particular, when covering the molding surface of a plaster mold with a thin film made of pure gypsum,
It is possible to reliably prevent the ends of the carbon fibers from being exposed to the molding surface. (4) Since combustible carbon fiber is mixed as a reinforcing material, it is possible to easily burn off and remove only the carbon fiber from the plaster mold that is damaged during use or becomes unusable. , plaster molds can be reused. In this regard, when noncombustible materials such as glass fibers are mixed in as a reinforcing material, it is extremely difficult or impossible to remove only the mixed materials and regenerate or reuse the plaster mold. (5) The mechanical strength of the plaster mold is greatly increased, preventing damage to the plaster mold due to external pressure during molding, and at the same time, the thermal strength is also greatly increased, reducing the drying temperature of the plaster mold for each molding. As a result, the drying time can be shortened, and the molding efficiency can be significantly improved.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the length of carbon fibers mixed in plaster and the bending strength of plaster.
The figure is a graph showing the relationship between the weight percent of carbon fiber mixed in plaster with respect to plaster and the bending strength of plaster.
Figure 3 shows the weight percent of carbon fiber mixed in plaster,
Graph showing the relationship between the water absorption rate of gypsum, Figure 4 A,
B is a plan view and a front sectional view of a container for making a gypsum slurry in which carbon fibers are diffused, respectively, and FIG. Figure 6 is a partially cutaway perspective view of the molded plaster mold A for an external trowel, and Figure 7 is a
FIG. 8 is a sectional view of the molded plaster mold for an outer trowel for molding the inner peripheral surface of a dish, the outer periphery of which is covered with a thin film 3 made of pure gypsum. FIG. 9 is a partially cutaway perspective view, and FIG. 10 is a cross-sectional view of a plaster mold for an inner trowel for molding the outer circumferential surface of a cup whose inner circumferential surface is covered with a thin film 5 made of pure gypsum. 1 is a longitudinal cross-sectional view of a molded plaster mold B for an inner trowel. Explanation of the symbols of the main parts, 1: container, 2, 4: case type, 3, 5: thin film, A, A': plaster mold for external trowel,
B: Plaster mold for inner trowel.

Claims (1)

[Claims] 1. A step of heating and removing the sizing agent applied after cutting the carbon fibers into lengths of 5 to 70 mm, and applying ultrasonic vibration to the carbon fibers cut into the lengths in water. A step of stirring to disperse the carbon fibers into countless single fibers; gypsum powder; the carbon fibers dispersed in an amount of 0.03 to 1% by weight based on the gypsum powder; water; and other materials as necessary. A process of mixing and stirring the carbon fiber-containing gypsum slurry with additives to create a carbon fiber-containing gypsum slurry in which carbon fiber single fibers are uniformly dispersed, and pouring the carbon fiber-containing gypsum slurry into a case mold, and removing the mold after hardening. A method for producing a carbon fiber-reinforced plaster mold for molding ceramics, comprising a drying step. 2 A process of heating and removing the sizing agent applied after cutting the carbon fibers into lengths of 5 to 70 mm, and stirring the carbon fibers cut to the above length in water while applying ultrasonic vibrations to form carbon fibers. A step of dispersing the fibers into countless single fibers, gypsum powder, 0.03 to 1% by weight of the dispersed carbon fibers based on the gypsum powder, water, and other additives as necessary. A process of mixing and stirring to create a carbon fiber-containing gypsum slurry in which carbon fiber single fibers are uniformly dispersed, and pouring pure gypsum slurry into a case mold in advance to form a thin film, and then pouring the carbon fiber-containing gypsum slurry into a thin film. A method for manufacturing a carbon fiber-reinforced plaster mold for molding ceramics, comprising the steps of pouring slurry, removing the mold after hardening, and drying.