The production of Porcelain for electrical insulation — V

Abstract

Forming the plastic clay body into the finished insulator shape prior to the firing to vitrification is of great importance. Among the methods used to accomplish this are: 1. Dust or Semi-dry Pressing 2. Plastic Forming 3. Casting. Dust pressed insulators are used chiefly in low-tension work. The prepared plastic body is dried out, dampened with a definite amount of water, pulverized and pressed in steel molds. The density of this product is not so great as that made in other ways, but accurately sized, difficult shapes can be made. Turning from pugged blanks, throwing and jiggering are processes in which the body is roughly shaped while in the plastic condition, and might be termed “plastic forming.” Pieces formed this way are trued up or fettled while still damp but stiff enough to hold their shape. The most common method of shaping thin-walled insulators is with a hot press die. Here the plaster of paris mold, having the shape of the outside or top of the insulator, is placed on a horizontally revolving wheel, the plastic clay body being placed in the mold and carefully pressed down so as to conform with the mold. A heated metal part is forced down onto the clay in the revolving mold, and shapes the spinning clay into proper form for the bottom of the insulator. The hot metal causes steam to be formed from the water in the plastic clay body, and this acts as a lubricant, preventing the clay from tearing, and also making it possible to form extremely thin petticoats or walls. Casting is done by preparing the clay body with sufficient water and electrolytes to form a liquid (slip). This is poured into dry plaster of paris molds of proper shape. The plaster absorbs water from the clay, and allows it to stiffen and become firm enough to be handled when the mold is removed after a proper period of time. Properly prepared slip has no more water (hence no more shrinkage) than plastic body. Pouring in the liquid condition makes it possible to form heavy pieces comparatively free from strains.