[Trade Journal]
Publication: Bulletin of the American Ceramic Society
Columbus, OH, United States
vol. 12, no. 5, p. 138-140, col. 1-2
MANUFACTURING CONTROL OF DRY-PRESSED PORCELAIN*
BY FRANK CERMAK AND L. E. THIESS
ABSTRACT
The methods used for manufacturing control of raw materials and manufacturing processes are described. The continuous filtering machine gives greater uniformity and other advantages to a dry-pressed porcelain body
I. Introduction
The ever-increasing demands for better quality, greater uniformity, and more accurate dimensions, together with lower prices, command the manufacturer of electrical porcelain to make use of improved and controlled methods of manufacture.
Dry-process electrical porcelain has been manufactured in our plant for nearly forty years. The variety of such porcelain parts is very great. Some smaller pieces weigh less than one gram while others weigh as much as thirty pounds. Some of the larger types are shown in Fig. 1.
II. Control of Raw Materials
The manufacture of ceramic ware begins with the raw materials, which are supplied in most instances as unrefined, natural products, such as clays, feldspar, and flint. After one raw material has been accepted for production it is important that the supply be as uniform as may be expected of such natural materials.
For controlling purposes, a small amount of each approved material is put aside in containers, so that at any time a comparison between the original sample and the last shipment may be made.
For dry-process porcelain body an orthoclase feldspar is used.
The specifications for flint call for a silica content of not less than 99.5%.
Every carload of feldspar and flint is tested for fineness of grain before being released for unloading in the bins. The specified total allowance on 325-mesh screen for feldspar is 10.00% and for flint, 12.00%. A fusion test of feldspar is made on each carload, and the method is the same as has been described by Dubois.¹
There are no formal purchasing specifications for china clays or ball clays, but these materials are tested periodically, and tests such as modulus of rupture, shrinkage, and porosity determinations at different firing temperatures are made in conformity with the standard specifications of the American Ceramic Society.² Such detailed laboratory tests are always required for new materials which are to substitute old materials in the regular body mix.
Unless clays arrive in dry condition in the factory, moisture determinations are made twice a week and the laboratory informs the sliphouse foreman how much compensation to the batch weights must be made for water.
III. Body Preparation
The present method of dry-process body preparation constitutes a great change over that of a few years ago. It would require too much space to describe here the details of mixing, etc., and therefore the old and new methods are hereby given graphically on the flow chart (Fig. 2).
The advantages of the new method may be summarized as follows: (a) a more uniform porcelain body, as no segregation of the coarser particles, such as frequently occur in filter-press cakes, is possible; (b) instead of a wet, heavy filter cake the clay is received in small broken-up layers, about 1/16 inch thick, with an almost uniform moisture content; the drying of the filter cakes, the crushing, re-wetting, and aging before the process of pulverization, are entirely eliminated; (c) a more uniform drying and firing shrinkage is obtained and fewer losses in oversized or undersized porcelain parts occur; and (d) a saving of labor.
IV. Control of Manufacturing Processes
Such control can successfully be carried out only by the cooperation of laboratory and shop foremen. The methods employed are herewith given as follows:
(1) Moisture Control The amount of moisture in the pulverized clay has a great influence upon the shrinkage, density, and appearance of the pressed porcelain part.
Spurrier³ has shown the effect of moisture content upon the shrinkage of dry-process porcelain. This laboratory makes regular checks upon the water content of the pulverized body as it is delivered to the press room. The press foreman is notified at once if the moisture content is below or above the specified limits.
(2) Shrinkage Control Special shrinkage disks (as shown in Fig. 3), approximately 3 1/2 inches in diameter by 3/4 inch thick, are pressed daily and dated. These disks are dried, fired, and gaged for fired size. Pyrometric cones are always placed with these trial pieces. Sometimes some of these disks are also used for porosity tests.
(3) Die Wear In order to check the die wear and to eliminate oversized ware, the inspector is supplied with a few pieces of every article which has been pressed during the day. These parts are then put into the next available kiln and the fired dimensions are compared with the drawings. In this way irregularities in the dimensions are early detected and great losses avoided.
(4) Electrical and Mechanical Control of Finished Ware Dielectrical and mechanical tests on Standard test specimens, as shown in Fig. 3, are made from time to time in the laboratory. For transverse tests dry-pressed cylinders 7 inches long by 3/4 inches in diameter are used. The electrical test is carried out on 1/2-inch thick disks which are pressed on Standard presses. There are also some other tests provided, such as workability tests (pressing behavior of clay body) and tests for glaze fit and appearance, but the latter are usually made only for a comparative tryout of new raw materials in the body mix.
V. Summary and Conclusions
In outlining the above control methods for ceramic materials and processes for the manufacture of dry-process porcelain, attempts have been made to make use of methods best adapted for individual needs of our plant. These tests are simple and easy to carry out and have given satisfactory esults. The new process of dry-pressed porcelain preparation has great merits over the old system.
GENERAL ELECTRIC COMPANY
SCHENECTADY, NEW YORK
* Presented at the Annual Meeting, AMERICAN CERAMIC SOCIETY, Pittsburgh, Pa. February, 1933 (White Wares Division). Received December 12, 1932.
1 H. B. DuBois, Jour. Amer. Ceram Soc., 15 [2], 144-48 (1932).
2 Standard Specifications, ibid., June (1928).
3 H. Spurrier, "Investigation of Dust-Pressed Practice as Influencing Finished Sizes and Cracking," Jour. Amer. Ceram. Soc., 5 [11], 798 (1922).
