[Trade Journal]
Publication: Contract Record & Engineering Review
Toronto, Ontario, Canada
vol. 35, p. 729-731, col. 1-2
The Manufacture and Testing of High Tension Porcelain Insulators
A Practical Description of the Processes Involved in Making and the Care Essential in Inspecting. Some Interesting Illustrations.
By A. D. ALLEN, A. D. Allen Inspection Co., Hamilton, at C. E. A. Convention.
It was brought to our attention that most of those engaged in the electrical profession, have but a vague idea of the processes involved in the manufacture of porcelain insulators. Upon the suggestion of one of your executives we prepared, illustrated with lantern slides, a brief paper covering the manufacture, inspection and testing of porcelain insulators. We are dealing with this subject not as ceramic engineers, but from our knowledge and experience gained from the inspection of insulators, during the last ten years, at the various insulator plants in the States and Canada. These lantern slides, which we are using to-day, are those which we secured at the Canadian Porcelain Company's plant in Hamilton and probably represent the latest developments in the ceramic art of making and testing insulators.
The materials generally used in the manufacture of hard porcelain are ball clay, china clay, feldspar and flint. It is the function of ball clay to make the mass plastic or workable; the china clay strong and dense when burned; the feldspar of a definite fluxing point to render the mass perfectly homogeneous, and flint to prevent too great shrinkage. All materials must burn white and dense so that the final product may possess the requisite mechanical and electrical strength.
Flint occurs in nature as pure silica, which is washed free of impurities and pulverized to prepare for the potter's use. Feldspar occurs ais a rock which is carefully sorted at the quarry and pulverized. Ball clay and china clay are both aluminum silicates having their origin in feldspar deposits, which has weathered and from which the potash and soda have 'leached away. China clay usually occurs in nature mixed with mica and some free silica, which is removed by washing before shipment to the potter. It is weakly plastic. Hall clay is seldom treated in any way other than to weather in order to thoroughly break it up and permit maximum development of its prime characteristic of Plasticity
Flint and feldspar occur in nature as rock and require pulverizing before introduction into the mixture. For this purpose grinding cylinders as shown in photograph are provided and in order that no foreign matter may be introduced into the raw material these cylinders are lined with porcelain brick and half filled with flint pebbles. A charge of flint or spar is introduced through the manhole in the side of the cylinder, the proper amount of water added, cover put in place and the whole cylinder rotated till the flint or spar is reduced to the proper state of fineness, after which the cover is removed and the mixture of water and flint, or spar, run out into the blunger for final mixing with other ingredients.
The liquid flint and feldspar mixture from the grinding cylinders is received into a mixer or blunger to which is also added the requisite amount of ball clay and china clay. These latter materials being by nature very finely divided are merely dissolved or distributed in the flint-feldspar mixture. The resultant liquid of the four ingredients is at nearly the consistency of very thick cream and in patter's language is know as " slip."
"Slip" or liquid clay from the mixing machinery contains more or less foreign material which must be removed by careful sieving in order that the "Body" or porcelain structure may be free of voids which would be caused by the burning or fluxing of foreign materials. For this purpose a shaking sieve is provided having a bronze screen containing in excess of 15,000 holes to the square inch. Two such screens or lawns through which all liquid must pass give absolute assurance of the cleanliness of clay solution.
The mining and shipment of the potter's raw materials entails much handling with the consequent introduction of more or less foreign matter, much of which is entirely harmless, but a certain amount of metallic iron is certain to become mixed with the clay and as such iron is hurtful it is desirable that it all be removed before farming the articles. Such cleaning of metallic iron is most easily accomplished while the clay is still in a liquid state, when it is easily passed by the poles of a powerful electric magnet which effectively removes all magnetic materials.
Up to this stage the clay is in the state of thick " slip," of cream-like consistency and a considerable portion of the water must be removed to produce a workable plastic mass capable of being worked into the desired shapes. Fig. 1. shows a pressure pump and filter presses, by means of which excess water is removed leaving a so-called leaf of plastic clay ready for working. Liquid clay enters at one extreme of the press and is forced at considerable pressure into the canvas lined cavities where separation of the water and clay substance takes place. When filled the iron plates are separated one by one and the leaf of clay removed after which the plates are again put in position and filtering resumed.
Plastic clay from the presses is seldom sufficiently uniform to permit manufacture of ware. It is necessary therefore, to work out all air pockets and bring the mass to a state of perfect homogeneity in order that the finished product may be free of voids and planes of weakness. This necessary condition is performed by passing all clay through a mill. It is essentially a tube in which revolving cutters on a vertical shaft force the clay powerfully against a horizontal revolving worm or screw, which causes the clay mass to be extruded from the orifice at the bottom homogeneous in state, and of a form to permit ready working to the desired shape.
Art ware is formed by clay largely by hand and is essentially an expression of the individual workman's skill. Clay products of technical service must however, be made with great exactitude and it is necessary to substitute machine work for hand work. Nearly all electrical porcelain, especially high voltage insulators, are made in plaster of paris moulds Such moulds are cast from exact plaster models. The exterior of the mould is usually framed by a cast iron ring machined accurately so that the finished mould will produce a product surprisingly exact in dimension. It is in the mould the plastic clay is finally shaped and it is the office of the plaster to absorb water from clay, and stiffen sufficiently to be handled. Moulds are made oversize to compensate for the inevitable shrinkage which occurs in the subsequent drying and burning of the ware.
The Potter's wheel, familiar to all ages, is still an essential tool in many clay working plants, but it is to-day a motor driven affair arranged mechanically to produce rapidly and accurately,
When large production is essential or where a screw thread is required the insulator press is utilized. As in the potter's wheel a plaster mould is used to form the exterior of the article, but in this process the interior of the article is formed by a plunger, which is made to rotate in either direction by powerful reversing clutches. At the completion of the downward stroke prompt reversion takes place leaving a mechanically accurate screw thread in the ware where required. This machine is equally serviceable in producing ware without screw threads, since the powerful pressing tends to produce denser and more homogeneous structure in clay wares
The surface of clay ware made in plaster moulds is seldom perfectly smooth, and it is generally necessary to trim the surface of the clay product to an even surface. This may be done immediately after removal of ware from mould, when in a leathery state, or finishing may be deferred till the article is entirely dry.
Clay wares made in the plastic state contain considerable water, which it is necessary to remove before applying glaze. and in the case of heavy pieces required in modern insulators it is essential that this drying proceed properly else internal strains develop resulting in an electrically weak structure of short life. When dried in the open air heavy pieces are cooled by evaporation from the surface, with result that the drying is held up, and further, the interior is still moist when the outside is comparatively dry, resulting in minute hair cracks in the surface, as well as internal strains. A proper drying system requires the use of high temperature and high relative humidity, in order that the whole mass of clay may become thoroughly heated before any drying may take place. Once the clay mass is entirely heated the relative humidity may be reduced, while the temperature may be increased and drying progresses uniformly throughout the piece without producing strains or cracks Following complete drying of prepared ware all pieces are carefully inspected and blown clear of dust or other accumulations by a powerful stream of compressed air, in order that the glaze solutions in which they are dipped may adhere uniformly. The glaze solution is made up of clay, feldspar and cooling oxides, ground together to extreme fineness. The presence of excess feldspar in the glaze causes it to melt into a thin coating of glaze when subjected to the high heat of the kiln. No lead or other soluble oxides are used in coating electrical porcelain, since their presence would cause the insulators in the course of time to present a slightly conducting surface. Basically all glazes are alike, the only variation necessary being in the coloring oxide, for white—tin, for brown—iron, for green—chrome, etc.
At kiln temperatures the glaze substance becomes nearly liquid and so spreads itself uniformly over the surface leaving a perfect glass like finish. Parts in contact with the pot or saggers in which the material is placed for burning, must be left free of glaze to prevent adhesion. Where a complete envelope of glaze is required on the ware it may be supported, if light in weight, on small fire clay points, or if heavy, article may be placed in sand which of course adheres to the surface where in contact. Such sand surfaces are sometimes applied for the purpose of presenting a roughened surface to facilitate cementing.
The pots in which high grade ware are burned are termed "saggers" and are usually made of fire clay, formed on a potter's wheel to the required shape and burned sufficiently hard to develop the requisite strength for the support of heavy clay wares.
The final operation in the production of porcelain is the process of burning, which is accomplished by gradually heating the ware until vitrification takes place. producing a dense, homogeneous product.
The oven in which burning takes place is shown in section Fig. 2. It is essentially a bee hive affair 18 ft in diameter. by 50 ft. overall height. The interior space of such an oven is packed full of saggers containing the ware to be burned, and a slow fire started in each of the ten fire holes. Fuel may be coal, gas, or oil, all of which are in wide use. At the expiration of 24 hours the entire contents have reached a dull red heat, and as all water has been removed the fires are increased and the temperature raised to 2500° F. The tiring period usually extends 60 hours and indication of satisfactory finish is registered by electric pyrometers, pyrometric cones and sample discs of ware which are periodically withdrawn and measured to ascertain shrinkage. After firing has ceased, a period of 48 hours is allowed to elapse before opening the kiln in order that no cooling strains may be introduced in the burning ware.
After removal from the kilns the insulators are usually presented to the customer's representative for inspection in order to avoid further factory costs of handling defective ware. Much depends on the inspector, as no specifications have ever yet been devised which can be substituted for the good judgement of an experienced inspector in determining what porcelain should be put on the line and what should be rejected at the factory. Porcelain which is just under-fired or just over-fired is in general the most difficult to determine whether it should be accepted or rejected. Of course such defects as cracks, presence of an occasional foreign piece of matter in the body, warped, defective glazing, crooked assembly, etc.—are all readily detected and rejected.
The porcelain which has been accepted is then trucked to the pans for their first electrical test, where it is subjected to a voltage sufficiently high to break over the piece under such test. By proper proportioning of the piece the flashover voltage is usually 75 to 80% of its ultimate puncture strength, a safe value determined by many years of trial. The testing circuit is usually at 25 or 60 cycles, and so arranged that a snapping discharge around the porcelain is produced. Such a discharge oscillates at the natural period of the circuit and is found very effective in eliminating any defective porcelains.
The photograph, Fig. 3, shows a rack of insulators ready for test. The surface on which they are placed is metal and constitutes one electrode, the hanging chains dipping into the water with which the interior is practically filled from the other electrode, to which a voltage sufficient to flashover the articles is applied. The shells which have successfully passed over the tracks are then, if of the pin type, cemented, or if of the suspension, capped and pinned. After which operation they are inspected for cementing and assembly. Then those accepted are placed on the racks for their final flashover.
