JPH0565321B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a method for molding a ceramic molded body in the production of a ceramic sintered body, and more specifically, to a method for molding a ceramic molded body in the production of a ceramic sintered body. The present invention relates to a method for forming a ceramic molded body by plasticizing and then pressure molding to prevent the occurrence of internal shrinkage cavities and cracks.

[Background of the invention]

Generally, ceramic molded bodies having complicated shapes are manufactured by a known injection molding method. That is, a non-plastic ceramic powder and an organic material that becomes plasticized by heating are heated and mixed, and then cooled and crushed to produce a pellet-shaped injection molding material. Next, this is plasticized using an injection molding machine, and pressure is applied to cause it to flow into the mold cavity, where it is cooled and solidified within the mold.
This is taken out to obtain a molded body.

Generally, when the molding material is introduced into the cavity of a mold, the injection temperature is 100 to 200°C, and the injection pressure is 500 to 3000 kg/cm ² . On the other hand, the molding material
It contains 30-70 vol% organic fluidizing agent and exhibits volumetric shrinkage when cooled from a plastic state to a solid state. Moreover, the cooling and solidification of the molded body in the mold progresses inward from the portion that is in contact with the inner surface of the mold, and the maximum thickness portion becomes the final solidification portion. In this maximum thickness portion, shrinkage cavities caused by volumetric contraction and tensile stress due to volumetric contraction are generated, and internal cracks occur due to this tensile stress.

Conventionally, methods to solve the above problems include lowering the injection temperature during molding, reducing the proportion of organic material in the molding material, and using a two-stage injection pressure method during molding. effect was used.

The idea behind this is to minimize the difference between the injection temperature and the solidification temperature of the molding material to reduce volumetric shrinkage. However, when the injection temperature is lowered slightly, the viscosity of the molding material increases rapidly, and as a result, the deformation resistance increases, making molding impossible. Therefore, since the amount of decrease in the injection temperature is small compared to the normal injection temperature, the effect of reducing the amount of shrinkage is extremely small.

The idea behind this is to plasticize the non-plastic ceramic powder using the minimum amount of organic fluidizing agent necessary to reduce the amount of volumetric shrinkage of the molding material. As an example, JP-A-55
In No.-23097, stearic acid wax, oil, etc. are added as a solvent, and in JP-A-55-113510, stearic acid is added. However, these molding materials also contain 18 to 25 wt% of an organic fluidizing agent containing a thermoplastic resin, and have little effect on reducing the amount of shrinkage.

In addition, the plasticized molding material is injected into the cavity of the mold, and it is continued to be added even after filling is completed, and during the cooling and solidification process, it fills the voids caused by volumetric contraction. This is based on the idea of supplying a molding material in a plastic state to reduce volumetric shrinkage, which is a common practice in the field of plastic injection molding. However, if the shape of the molded product is complex and there is a thin wall between the filling port of the mold cavity and the maximum thickness part of the molded product, solidification of this part will proceed quickly, and this part will become a link to the maximum wall thickness. The supply to the thick portion becomes insufficient, and as a result, volume shrinkage nests occur in the thickest portion.

[Purpose of the invention]

An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to prevent the generation of volumetric shrinkage cavities and internal cracks inside the molded product when the ceramic injection molding material is filled into the cavity of a mold to produce a molded product. The objective is to provide a new molding method that does not cause

[Summary of the invention]

In order to achieve this object, the present invention has been made based on the following findings.

That is, when the organic fluidizing agent contained in the ceramic injection molding material is heated, it expands, particularly when it changes from a solid state to a plastic state, it expands significantly, and conversely, when it is cooled from a heated state, it contracts. On the other hand, the proportion of the organic fluidizing agent in the injection molding material is 30 to 70 vol%. Therefore,
When the plasticized molding material is filled into the cavity of the mold and cooled in the mold, the molded product exhibits volumetric shrinkage. Normally, cooling and solidification of a molded article in a mold starts from a point in contact with the inner surface of the mold and progresses toward the inside of the molded article. When cooling and solidification progresses from the surface toward the inside in this way, the maximum thickness part of the molded body becomes the final solidification part, and large shrinkage cavities are generated in this final solidification part, that is, the maximum thickness part. In addition, tensile stress is generated internally due to the volumetric contraction, and this force causes internal cracks to occur around the location where the volumetric shrinkage foci occur.
These internal defects are a phenomenon that occurs due to the large volumetric shrinkage of the injection molded material when it is cooled from a high temperature in a plastic state to room temperature.

Therefore, the present inventors believe that if the cavity of the mold is filled with ceramic injection molding material in a solid state and molded, there will be no volumetric shrinkage, thereby preventing the occurrence of volumetric shrinkage cavities and internal cracks. Based on this knowledge, we have come up with the present invention.

That is, in the present invention, when molding a ceramic molded body, a mixture of ceramic raw material powder and an organic fluidizing agent is filled in a solid state into a cavity of a mold, and then the filling mixture is heated by heating the mold. The method is characterized in that the vicinity of the inner surface of the mold is plasticized, and then pressure molding is performed through the mixture filling port.

The present invention will be explained in detail below.

First, it is preferable to fill the ceramic molding material into the mold cavity while reducing the pressure inside the cavity to 1 to 100 Torr.There is no problem with filling even if the degree of vacuum inside the cavity is 1 Torr or less.
In order to achieve a value of 1 Torr or less, the mold alignment accuracy must be reduced to 1/1000 mm or less, which makes the mold expensive and the cost of the molded product significantly increased, making it uneconomical. On the other hand, if it is 500 Torr or more, it becomes difficult to fill the molding material densely and uniformly.

Further, the heating temperature of the mold is preferably set to be higher than the softening temperature (plasticization temperature) of the organic fluidizing agent and lower than the thermal decomposition temperature. Below the softening temperature, it is difficult to integrate the molding material filled with particulates even if pressure is applied, and it is difficult to obtain a sound molded body. Moreover, if the temperature is above the thermal decomposition temperature, the organic fluidizing agent that binds the ceramic powder will be thermally decomposed and removed, making it difficult to obtain a sound molded body.

Furthermore, plasticization means that an organic fluidizing agent mixed with ceramic powder is heated to a high temperature higher than its softening temperature to bind the particulate ceramic powder together.
This refers to the state in which the ceramic molding material becomes fluid.

In addition, in pressure molding, pressurization is started when the plasticized thickness of the molding material reaches a predetermined thickness, preferably 0.2 to 3 mm, from the surface in contact with the inner surface of the mold, and the pressure during pressurization is reduced. It is desirable to carry out at 100-5000Kg/ ^cm2 . If the plasticized thickness is less than 0.2 mm, the strength of the molded product will be weak and it will be difficult to take it out from the mold, and it will be difficult to smooth the surface of the molded product. Furthermore, if the plasticized thickness exceeds 3 mm, the absolute amount of shrinkage upon cooling and solidification will increase, causing internal shrinkage cavities at the center of the wall thickness, making it difficult to obtain a sound molded product.
If the molding pressure during pressurization is less than 100Kg/ ^cm2 , it will be difficult for the pressure to be transmitted to the plasticized part that is in contact with the inner surface of the mold, and if it exceeds 5000Kg/ ^cm2 , the mold structure will be quite strong. As a result, the mold manufacturing cost increases significantly, and the cost of the molded product increases.

According to the molding method of the present invention described above, molding is performed by only plasticizing a part of the molding material that is in contact with the inner surface of the mold, so a healthy molded product without internal shrinkage cavities or internal cracks can be produced. I can do it. What is particularly noteworthy is that it is possible to produce a sound molded body with no internal defects even if the wall thickness is 30 mm or more. This is because effective molding can be achieved by plasticizing only a part of the molded body, that is, the surface thereof, so that the amount of volumetric shrinkage of the entire molded body can be reduced.

Next, an embodiment of the present invention will be described.

[Embodiments of the invention]

In this example, SiC powder 72 with an average particle size of 0.7 μm was used.
%, Sic sintering aid 8%, polyethylene resin 8.5
%, paraffin wax 8.5%, stearic acid 3
% of the powder was premixed in a dry state, the kneading temperature was set at 130°C, and the mixture was kneaded for 30 minutes using a pressure kneader. This kneaded product was formed into a plate with a thickness of 1 to 3 mm using twin rolls heated to 130°C, and cooled to room temperature. This was further immersed in liquid nitrogen to be cooled to below 0°C, and then finely pulverized using a crusher.

Next, inspect the inside of the mold cavity shown in Figure 1 by 2,
While reducing the pressure to around 100 Torr using a vacuum pump from 2' and 2'', finely pulverized molding material was filled in a solid state.After filling, a heating medium was sent to the flow path provided inside the mold to cool the mold. After heating the inside of the mold to 160℃ and keeping it at that temperature for 5 minutes, 3000Kg/
A pressure of cm ² is applied for 60 seconds, and then a refrigerant is sent to the flow path of the mold to lower the mold inner surface temperature.
When the temperature reached 80°C or lower, it was removed from the mold to obtain a molded body. Note that the pressurization method was performed using a normal injection molding machine. That is, the filling port of the mold and the tip of the molding machine nozzle are brought into contact, and the same molding material filled in the mold is put into the molding machine, and after plasticizing, it is injected at an injection temperature of 160°C, and the mold Pressure was applied to the inner filling material.

In addition, as a method for evaluating the internal properties of molded objects,
The molded body was cut lengthwise, and the cut surface was polished with emery paper, and then visually inspected for shrinkage cavities and internal cracks. The results are shown in FIG. In the figure, A is a molded article according to a comparative example, and B is a molded article according to this example. In addition, the comparative example was molded using the same molding material as the present example at a mold temperature of 80° C., an injection temperature of 160° C., and an injection pressure of 3000 Kg/cm ² .

As is clear from the figure, in comparative example A, the axis 1'
While shrinkage cavity 1'' and internal crack 1 were generated in the central part of Example B, no shrinkage cavity or internal crack was observed in Example B.

〔Effect of the invention〕

As detailed above, according to the present invention, it is possible to effectively prevent the occurrence of shrinkage cavities and internal cracks, thereby making it possible to obtain a healthy ceramic molded body. It is possible to produce a healthy molded body without defects such as.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of the mold, and FIG. 2 is a longitudinal sectional view of the ceramic molded body. 1...Molding material filling port, 1'...Molded body shaft,
1″……Shrinkage nest, 1

Claims (1)

[Scope of Claims] 1. After filling the cavity of a mold with a mixture of ceramic raw material powder and an organic fluidizing agent in a solid state, heating the mold causes the filling mixture to be heated in the vicinity of the inner surface of the mold. 1. A method for forming a ceramic molded article, which comprises plasticizing it and then press-molding it through a mixture filling port. 2. The method for molding a ceramic molded article according to claim 1, wherein the mixture is filled while reducing the pressure inside the mold cavity to 1 to 500 Torr. 3. The method for molding a ceramic molded body according to claim 1, wherein the heating temperature of the mold is set to a temperature above the softening temperature of the organic fluidizing agent in the mixture and below the thermal decomposition starting temperature. 4 In the pressure molding, the plasticized thickness of the mixture is 0.2 to 3 mm from the surface in contact with the inner surface of the mold.
2. The method for molding a ceramic molded article according to claim 1, wherein pressure is started when the thickness reaches 100 to 5000 Kg/cm ² . 5. The method for molding a ceramic molded body according to claim 1, wherein the organic fluidizing agent is plasticized when heated.