[Trade Journal]
Publication: Journal of the American Institute of Electrical Engineers
New York, NY, United States
vol. 42, no. 8, p. 858-863, col. 1-2
The Production of Porcelain for Electrical Insulation-V
BY FRANK H. RIDDLE
Champion Porcelain Company, Jeffery-Dewitt Insulator Company
Review of the Subject.—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 dens' ity 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.
CONTENTS
Review of the Subject. (350 w.)
Forming. (6.5 w.)
1. Dry or Semi-Dry Pressed Ware. (175 w.)
Turning from Pugged Blanks, Throwing and Jiggering. (20 w.)
Casting. (475 w.)
Turning from Pugged Blanks. (300 w.)
Throwing. (90 w.)
Jiggering. (16.5 w.)
Hot Press Dies. (155 w.)
Casting. (450 w.)
Drying. (350 w.)
FORMING
HERE are several methods of forming the body. The best one to use depends not only upon the working qualities of the body but also upon the character of the finished product. Each method requires a different way of preparing the body, but the various methods need only be mentioned briefly as the previous descriptions of the different operations cover the details.
1. Dry or Semi- Dry Pressed Ware. Filter press cakes are dried out, then broken up into lumps by passing them through coarse rolls. The dry lumps are sprinkled with a definite amount of water (10 to 15 per cent), allowed to stand for 24 hours in a damp room to equalize the moisture content, and finally run through a disintegrator. This machine, which has a rapidly revolving set of hammer-shaped arms, disintegrates the slightly moist body by impact and pulverizes it sufficiently to pass through a 20- mesh screen. It is then in condition that when pressed tightly in the hand it will retain its shape. This "dust" when pressed in metal molds can be made into many complicated shapes, largely used in low tension work such as switches, fuse plug boards, lamp sockets and the like. In spite of the fact that a high density may be obtained by this process, the pressed mass is lacking in the ultimate cohesion of the particles, so that large amounts of plastic clay and of fluxes must be used.
Turning From Pugged Blanks, Throwing and Jiggering are all executed on the body as prepared by the pug mill.
Casting. There are two general methods for preparing body for casting. In the first method the filter press cakes, while still wet, are placed in a blunger with just sufficient water to yield the proper specific gravity of the final slip, together with small amounts of electrolytes, usually sodium silicate and sodium carbonate. The latter are added to produce a slip having a degree of fluidity which will make it the most workable under the particular conditions desired.
The above method necessitates several extra operations which are eliminated in a second alternate procedure. Here a grinding charge is prepared and ground in the ball mill as usual, and with the clays there are added the electrolytes, so that thorough mixing is obtained. This produces a rather thick, stringy slip, but it is surprising how readily it will pass through a lawn. This slip is then stored in tanks, checked, corrected for specific gravity and fluidity if necessary, and held for use. The first method is preferable for casting exceedingly fine ware with thin walls or where a very high specific gravity is required. The second method should not be employed with blungers, not only on account of the inferior mixing, but also because of the coarseness of the slip, which would necessitate greater care in lawning.
Casting slip usually has a specific gravity ranging from 1.7 to 1.9 and a fluidity which will permit of easy pouring, but is not great enough to permit the heavier particles to settle out.
The following values apply to some casting slips:
The reason for using electrolytes or salts in casting slips is that they make it possible to produce a mixture in which the water content is exceedingly low, in fact so low that if the salts were not present the mass would not even pour. It is well known that any wet clay body will shrink, the shrinkage varying practically in proportion to the water content. It is obvious, hence, that the lower the water content the lower will be the shrinkage. The importance of this is readily seen when it is realized that severe strains may result from high shrinkage. Where all variables are properly controlled and the shrinkage reduced to the minimum, casting can be done very successfully. Different ways, on account of their different colloidal contents, are affected differently, and it is necessary that great care be taken in the selection of the raw materials which are used.
Turning From Pugged Blanks. There are two methods of doing this, i. e., when the clay is either in the leather hard or dry condition (a water content of 0.7 per cent). For leather hard turning the blanks are partially dried. This used to be done rather crudely in warm rooms, the conditions depending entirely upon the skill of the potter. Modern processing dryers are now used which yield a much more uniform quality of the product. As the blanks at this stage are only partially shrunken, and the degree of shrinking depends upon the degree of dryness, it is evident that lack of uniformity of moisture content will effect the size of the finished product. A piece of clay ware is said to be in the leather hard condition when it is possible to whittle it easily with a knife and still have it stiff enough to retain its shape. When in this condition the piece to be formed can be placed in a lathe and turned in the same manner that wood is turned. If turned by hand considerable skill and time are required. Dry process turning is done by revolving the piece to be turned and facing it with a rapidly revolving grinding wheel. This process is not generally used, as it is controlled by patents. Its advantages are that all the drying shrinkage has been eliminated, all pieces thus being a definite size and only the burning shrinkage left to contend with. The profile of the shape to be made is the reverse of the profile of the grinding wheel and is retained within working limits for a considerable time, which results in great uniformity. The disadvantage of this method is that the excess material ground off is dry dust, the re-use of which presents some unusual and difficult problems, while the shavings from the leather hard turning can be reblunged and worked over if not used in too great a percentage.
Throwing is the most ancient of the forming methods, and is done on a potter's wheel. A lump of thoroughly wedged or uniformly mixed clay of a plastic consistency is placed in the center of the rapidly revolving horizontal wheel and pressed down or centered by hand into a hemisphere very much like forming the hub of a wheel. When thus placed in the center, the piece can be pulled up by hand (drawn) and shaped into the desired form. This method requires considerable skill and is slow as compared with modern methods.
Jiggering is a development from throwing. Here the wheel consists of a head containing a plaster of paris mold, and is revolved in a horizontal plane.
The inside of the mold has the hollowed-out shape the reverse of the outside of the piece to be jiggered. A ball of properly pugged clay is then thrown into this mold and pressed down by hand. The shaping is done by pulling down a profile, corresponding to the shape of the inside of the piece, to a position fixed by a guide. The profile cuts off the excess amount of clay in the mold and shapes the piece. This method is very good for thin walled pieces, but in forming thick walled articles there is a tendency to drag. The clay in the mold is revolved with the mold, and as the tool or profile which is stationary is .pressed down into this revolving mass it will tend to draw and retard the clay on top while that in contact with the mold is turning. It is necessary to condition the clay so that the chances of a strained structure are reduced to a minimum.
Hot Press Dies are machines which have been developed for electrical porcelain manufacture and are particularly good where it is desired to form thin webs which would tear if made with an ordinary jigger tool. These dies are given the reverse shape of the part to be formed. In the case of an ordinary type of strain insulator made with thin walls, the mold which revolves horizontally would be the shape of the outside or top of the insulator, and the hot press die would correspond to the shape of the bottom, including not only the thin, high petticoats, but also the threaded center hole in which the pin is fastened. The hot die moves up and down and is heated with a gas flame and kept at such a temperature that when the die is brought down against the clay and mold steam is generated and acts as a lubricant between the die and the clay. It is surprising how thin-walled, high petticoats can be spun by this method.
Casting has been known of and done in a small way for many years; however, it has only been used as a process for forming heavy insulators for the last five or six years. The process of casting consists in pouring the body slip into properly dried plaster of paris molds and allowing the plaster to absorb the water from the slip, leaving the partially dried body hard enough to be handled. When a thin-walled piece is being cast the process is continued until a layer of proper thickness is formed in the mold, the remaining liquid slip being poured out. The cast piece is then allowed to dry and shrink free of the mold. When heavy solid pieces are cast, as the water is absorbed into the mold, and the slip settles down in the latter on account of this displacement, additional slip is added as required until finally sufficient water is removed so that no more settling will take place. The piece will then harden in a solid mass. The conditions of the mold, quality of slip, time of settling, and the temperature of the room are all factors that govern the success of the operation.
In casting a piece like a heavy strain type disk insulator, the molds are filled with slip in the morning, then refilled about three times during the day to replace the water absorbed. The mold is arranged with a collar or reservoir on top to allow an excess of slip to be held each time. The pieces are then allowed to stand over night, a slight amount of steam being turned on to gradually warm the molds and their contents. In the morning the top halves of the molds are removed and the spare or collars cut off the top of the insulators with a thin, fine wire. The pieces are allowed to stand in this condition for another 24 hours to avoid any possible chance of the insulators, while they might still be soft at the cores, from being strained. Slip which has been treated with alkaline electrolytes has the peculiar characteristic that while it may appear to have dried sufficiently to maintain its form, vibration or continued handling will quickly revert it so that it will not only deform but may even become liquid again.
At the expiration of 48 hours, the insulators are removed from the molds and placed on portable racks which are located as near the casting benches as possible, so as to avoid any unnecessary handling. The ware is again held at this point for another 24 hours to be air dried before it is removed to the dryers. In order to obviate any chances of trouble during the air-drying, the entire casting and work room is kept within certain temperature and humidity ranges.
Drying. The drying of clay wares, particularly large, irregularly shaped pieces, is a very important operation, and involves not only a thorough understanding of the piece, but exact control of the drying conditions as well.
Before modern automatic drying equipment had been developed large rooms were used, which were maintained at normal temperatures and as free from air currents and drafts as possible. Many weeks were required to dry large, irregularly shaped pieces. This of course meant either a small production or else an enormous amount of drying space.
The reason for the difficulty in drying clay wares is, as previously stated, the shrinkage which occurs as the clay body gives up its water. It is obvious that if the evaporation is not uniform, strains are produced which may eventually cause cracks. It is evident that the warmer the place is while drying the more readily it will give up its moisture. But while the piece is being warmed up it tends to dry and shrink on the outside, while the inside is wet and cold. This has been overcome by first subjecting the ware to a treatment in saturated, or at least very humid air, and warming it up under such conditions. The result of this is that the ware will be warmed through uniformly without drying on the surface, since the surrounding air already contains all the moisture it can hold. Additional heat is then applied, not only to heat the clay ware, but also to make the surrounding air capable of carrying more moisture. This results in evaporation of water from the wet clay ware into the air of the dryer. But since the ware has been warmed uniformly the flow of moisture from all parts is uniform. The result is that the drying is done not only at higher temperatures, but more quickly and safely. Where in the past considerable care was used to prevent drafts of all sorts, it has been found that the more completely the air is circulated and the better this circulation bathes all parts of the ware, the more effective and uniform is the drying process. On the other hand, a sluggish draft will expose one side or the top to one condition and the remainder of the piece to another, with the result that the drying proceeds irregularly.
Glazing. Practically all types of insulators are glazed by what is known as the one fire process; i. e., the glaze is applied to the dried, unfired piece. It is then placed in the kiln and fired to maturity. When properly done this is an excellent process, as it not only insures a very close chemical union between the body and the glaze, but it is less expensive than firing twice, as is done with some types of ware such as table ware. In the two fire process there are two methods. The one commonly used in Europe is to first fire to a low temperature, i. e. about 950 deg. cent. (1742 deg. fahr.) to 1000 deg. cent. (1832 deg. fahr.), for the purpose of dehydrating the clay or driving off the chemically combined water and hardening the ware so that it may be handled. The articles are then dipped in the glaze and re-fired to the maturing or vitrifying temperature of the porcelain, the glaze being so adjusted that it will mature or brighten at the same temperature at which the body vitrifies. The second method, more commonly used in America, is to mature the bisque in the first fire, then apply a low fusing glaze which will mature at a lower temperature than was used in the first burn. This is the easier of the two processes, and yields a brighter glaze and a higher percentage of grade 1 ware, but the intrinsic quality is not as high as that obtained by the former process, all things considered. The only difference between the one fire and the low bisque, high-glossed process is that the former cannot be used on thin ware such as table ware and the like without causing increased losses. Practically speaking, for heavy ware such as insulators, there is nothing to be gained by using the two-fire method.
There are many different formulas from which glazes are compounded. A brown or "Albany" glaze, suitable for high-tensioned insulators, would have a formula as follows:
Add manganese dioxide in a small amount depending on color desired.
The albany slip is used, as it not only fuses to a glass at a lower temperature than that to which the body and glaze are fired, but also because it burns to a beautiful mahogany color. The manganese is added to strengthen this reddish brown color. Other materials not shown here but sometimes used in the glaze with the exception of the whiting or calcium carbonate, which is an active flux in the presence of acids such as silica, are the same as are used in compounding the body, the proportions of flux to refractory, however, being greater in the case of the glaze.
A clear white glaze would have a formula about as follows:
In compounding the glaze there are several important factors to take into consideration. First, the glaze must mature or become a clear glass at the same temperature as that at which the porcelain body matures. Second, and equally important, is that the glaze must fit the body. This means that the coefficient of thermal expansion and contraction of the body and glaze must be such that when the two cool down after the firing, after the body has matured, they will contract at substantially the same rate, so that no strains are produced. After the finished insulator is cooled and later put into use, it is still subject to thermal changes. They are not so great as those produced in cooling the ware in the kiln, but they are not uniform, hence may become more severe. When an insulator is in use, a sudden change in the weather may cause a sudden rise or drop in temperature, which will result in the outside of the insulator, chiefly the glaze, being subjected to an expansion or contraction, thus causing a severe strain. The lower the thermal expansions of the glaze and body are, and the greater the thermal conductivity of the body, the more quickly will the latter be able to absorb and distribute the thermal changes. Low thermal expansion and contraction cause the volume changes to be smaller per degree of temperature change than when the coefficient is greater. High thermal expansion and poor heat conductivity cause the conditions to be very bad. During the past two or three years an understanding of how to control these various conditions better has made possible improvements in the qualities of the various products which are of considerable value. In the cooling of body and glaze after firing, if the glaze contracts faster than the body the glaze will be in tension when cold. This tension may be great enough to pull the glaze apart into sections or "craze" it. This can crudely be illustrated by comparing it to a mud puddle which is formed in clayey soil and just about to dry up. The skin of mud on the surface, as it dries and shrinks, cracks into many small cakes, leaving spaces between the cakes. This surface might be said to be "crazed".
On the other hand, if the glaze does not shrink or contract as much as the body, the glaze will be under compression when the ware is cold. If, in the final fusion of the glaze on the ware in burning, the glaze has not thoroughly fused, and does not show good cohesion with the body, it will simply shell off the porcelain as the cooling takes place. This is called "shivering". Usually this does not occur excepting in extreme cases, and when the coefficient of thermal expansion of the glaze is slightly less than that of the body.
Quite often expansion or contraction strains exist, but are not sufficient to cause immediate rupture by "crazing" or "shivering". Fracture is sometimes delayed and occurs when the ware is in service. This is often seen on bathroom wall tile and in some cases on table ware, and is much more likely to happen on ware of this sort, which is not vitrified, than in such ware as insulator porcelain. It has been shown that where the body and glaze in insulators are under strain, their strength and resistance to hot and cold tests are not as great as they otherwise would be. This is easily demonstrated by making up tensile test bars of porcelains glazed with various glazes, determining the breaking points of the various specimens and also subjecting similar specimens to heat tests.(1)
It is quite likely that these unbalanced conditions have considerable to do with the failure of insulators, due to time, whether they be in service or in storage.
(1) "The Control of Glaze-Fit by Means of Tensile Test specimens." Journal American Ceramic Society, Vol. 5, No. 8, August 1922. By F. H. Riddle and J. S. Laird.
