1. Field of the Invention
This invention relates to a measurement of a dimension of non-metallic material molded bodies having a shape that is impossible to measure by a usual means, such as vernier calipers, micrometer calipers and the like.
2. Description of the Prior Art
As means of measurement of a dimension, particularly a thickness, of a body, vernier calipers and micrometer calipers have been most usually and extensively used. However, as is easily understood from the general shape of the vernier calipers or micrometer calipers, when a body to be measured has a form such that the body cannot be interposed between the calipers, the measurement of the thickness is, as a matter of course, impossible. Namely, what cannot be measured with calipers are, for example, a wall thickness of a middle portion of a slender hollow tube, a wall thickness of a closed tip portion of a cul-de-sac i.e. a hollow tube with a closed end, etc., as into these tubes, measuring parts of the vernier calipers or micrometer calipers cannot be inserted.
Incidentally, amongst non-metallic material molded bodies, ceramic products are generally manufactured by granulating a starting powder added with a bonding agent such as a binder or the like, forming the thus granulated powder into a ceramic green molded body, by a molding means such as mold pressing, isostatic pressing or the like, and thereafter firing the ceramic green molded body at a predetermined temperature in an electric furnace, etc. The dimension of the products is influenced by conditions of firing which is accompanied by contraction, and the conditions of firing are usually constant, so that the dimension of the products depends upon that of the ceramic green molded bodies in the molding process. Accordingly, precise measurement and control of the dimension of the ceramic green molded bodies are very important, in the ceramics manufacture, for lessening dispersion of dimension of the products after firing.
Thus, the ceramic molded body before firing is particularly pertinent to the present invention and herein called a ceramic green molded body, though the ceramic product after firing also can be understood to be a kind of a non-metallic material molded body to which the invention is applied.
As means of thickness measurement of the ceramic green molded bodies, a direct method as mentioned above has been generally employed, wherein a measuring implement, such as vernier calipers, micrometer calipers and the like, is used. However, as was mentioned hereinabove, when a ceramic green molded body to be measured is in a form of a hollow tube or a cul-de-sac, i.e., a hollow tube with a closed end, it has been almost impossible to make a measure of the wall thickness of the middle portion or near the closed end portion, due to the inadequate shape of the vernier calipers or micrometer calipers. Further, since ceramic green molded bodies are generally brittle and fragile wherein particles of the material powder are merely bound together by the action of a binder, hollow tubular such bodies with a thin wall have frequently suffered impairment, such as cracks, etc., during the thickness measurement.
Furthermore, like a transparent alumina ceramic blow-molded part which is used as a light-emitting tube in a high pressure sodium-vapor lamp, etc., when the hollow tube is in a bulbed form, with a middle portion having a diameter (about 5 mm) larger than that of its end portion (about 3 mm), the vernier calipers cannot be inserted thereinto, so that the measurement of the wall thickness of the middle portion (about 0.2 mm) has been absolutely impossible. Besides, it is very important for ceramic products having such a form, in view of their use, to have a small dispersion of wall thickness, particularly, at the middle portion thereof.
On the other hand, although there has been an indirect method for measuring the thickness by making use of ultrasonic, yet attenuation of the ultrasonic wave is generally so rapid due to the fragility and softness, as mentioned above, of the ceramic green molded bodies immediately after molding, that the thickness measurement with a high precision has been difficult.