1. Field of the Invention
The present invention relates to a ceramic injection molded body and a method of molding the ceramic body by controlling distribution of fine pores in the molded body after removal of organic binders to improve yield in removing of organic binders and characteristics of a sintered body.
2. Related Art Statement
Recently, there has been actually made research performed on utilizing silicon ceramics to make engine parts or the like because silicon ceramics, such as silicon nitride, silicon carbide, sialon or the like, are more stable and less susceptible to oxidation or creep deformation at high temperatures, when compared to metals. For example, radial turbine rotors made of these ceramic materials are lighter and superior in thermal efficiency, allowing the operating temperature of engines to be raised, as compared with metallic rotors, so that these materials are drawing attention as turbocharger rotors, gas turbine rotors, etc. for automobiles.
However, since such a turbine rotor has complicated three-dimensional shaped blades, naturally it is almost impossible to finish by grinding a sintered body having a simple shape such as a round rod, square rod or the like into a desired shape and also it is difficult to obtain such a ceramic body having a complicated configuration by only one molding operation. Other than the above, injection molding methods have been extensively employed for molding the complicated ceramic bodies.
The method of injection molding of ceramics is an application of the conventional injection molding method which has been used for molding plastics. Generally, in injection molding of a ceramic body, ceramic powder is mixed in a kneader with organic binder consisting of a binding agent such as polyethylene, polystyrene or the like, wax, and lubricant. The mixture is subsequently heated to plasticize the binder and then injection molded in a mold. The thus molded body is heated to remove the binder, hydrostatically pressed and finally sintered to obtain a ceramic sintered body. According to the injection molding method, complicated ceramic articles can be rapidly and accurately obtained by only one molding operation with less finishing allowance, otherwise many manufacturing processes are required.
In the aforementioned conventional injection molding method for producing a ceramic article, the raw material powder is granulated. However, when the strength of the granulated particles is low, the granulated particles are ground into fine particles during kneading of the mixture of ceramic powder and binder. As a result, the fluidability of the mixture is deteriorated, and cracks are often observed in the molded body after removing the binder.
On the other hand, when the strength of the granulated particles is high or the pressure level of the hydrostatic pressing is low, wrecks of coarse granulated particles remain in the pressed body after pressing. Consequently the pressed body is not so uniform and accordingly, a uniform sintered body cannot be obtained.