Ceramic products can be divided into three broad groups according to the method of manufacture. They are as follows:
Group one comprises pulverized products in which the raw materials have been heated to impart latent cementitious properties which become active upon the addition of water. This group includes cements, limes, and plasters.
Group two consists of products which are heated until fluid and shaped while in this state prior to solidification by cooling. The most important material in this category is glass.
The third group to which ceramic tile belongs consists of products which are shaped by various methods of compacting finely divided powders, followed by a heat-treatment which promotes partial fusion or sintering of these particles thus imparting strength to the formed object. The great majority of ceramic products are manufactured by one or more of a number of forming methods such as dry pressing, plastic molding, extrusion, and casting.
The processes specifically used for ceramic tile manufacture include 1) dry (dust) pressing, 2) auger extrusion, and in some cases, 3) plastic molding.
The Dry Pressing method is used for forming many types of ceramic products, including tiles. However, the term "dry pressing" is not strictly accurate because in many cases, the materials being pressed contain from 3% to 15% water. Pressures used in pressing vary from a few hundred lbs/sq. in. to 100,000 lbs/sq. in. The pressure needed depends upon the material being compacted, the shape of the piece, and the quality necessary for the ultimate use for which the piece is intended. Pressing is usually accomplished in steel dies using either hydraulic or mechanical means of producing the desired pressures.
Plastic Molding is one of the oldest methods of ceramic forming, since it can be readily performed with little or no machinery. Tiles produced by ancient civilizations were made by plastic molding. Today, this method is not used much in the ceramic tile industry, except possibly in the production of ceramic-mosaic tiles.
In Auger Extrusion, the rotation of an auger-type screw is used to force a plastic body through a confining chamber with a die located at its end. Quarry tile is produced by this method. It is a continuous process with the material being fed into the chamber located at one end of the screw and forced out the die end. The continuous ribbon of extruded material is then cut to the desired lengths by a wire-type cutter.
As was mentioned earlier, ceramic tiles are processed at a high temperature at some time during their manufacture. In the ceramic industry, this heat treatment is usually referred to as "firing" and furnaces in which the heat treatment is performed are called "kilns." The temperatures used in firing ceramic products range from approximately 1100 degrees F. to 3200 degrees F. Temperatures as high as 4200 degrees F. can be obtained in some special type kilns.
A kiln, in the broadest sense, is merely an enclosure for containing heat and the material to be heated. The sources of heat are usually electric elements, gas, oil, or coal. The enclosure itself is built of ceramic refractories (materials which resist the effects of heat), while the inner lining of the enclosure is composed of high melting-point refractories. This inner lining is backed by ceramic materials which are good heat insulators. These serve the purpose of keeping as much heat as possible inside the enclosure.
The kilns used for the production of the great majority of ceramics can be divided into two major types, periodic and continuous. All of the earliest types of kilns were periodic. In this type, the tile to be heated is placed in the cool kiln. The kiln is then heated to the required temperature, allowed to cool, and the tile is then removed.
Continuous-type kilns are called "tunnel kilns" since they are essentially long tunnels through which the tiles pass on either cars, belts, or sliding slabs. Tunnel kilns are from 80 or 90 feet to over 300 feet in length. The entrance and the exits are only a few hundred degrees above room temperature, but the temperature at each succeeding point in the kiln increases as the midpoint is approached, and reaches a maximum at a point near the center. The temperature at any given point within the kiln remains constant and is monitored by sensing equipment in the form of thermocouples which feed data into a computer.
The tiles enter at one end, are gradually heated (Pre-heat zone) as they progress slowly towards the center (Firing zone) of the kiln, and then are cooled slowly as they approach the (Cooling zone) opposite end of the tunnel. This process is widely used in the ceramic floor and wall tile industry. The most efficient and economical use of these kilns requires constant flow of tiles through them twenty-four hours a day. Therefore, most large ceramic tile manufacturers have firing crews working around the clock.
Several of the tile-making processes as well as some of the specialized pieces of equipment were identified above. We will now describe the total production process for the manufacture of white-body, glazed ceramic tile. This type of ceramic tile is used widely throughout the US for a myriad of applications.
The production of white-body, glazed ceramic tile begins with what is known as "dust," a combination of raw materials which will ultimately be formed into the tile "body." As was discussed earlier, "dust" is composed mostly of talcs and clays which are formulated to give a desired strength and shape to the finished tile. In this case, the "dust" is composed of 62% talc and 38% clay (Ball clay + China clay/Kaolinite).
Quantities of these materials as determined by specific formulation are put into a ribbon blender which blends the clays and talcs together. From the blender, the mixture is transported to a mix-muller where it is mulled or kneaded by two steel wheels weighing 1800 lbs each. During the mulling procedure, water is added to the dusty mixture to bring its moisture content to 8% by volume. The moisturized mulling of the dust causes the separate particles of talc and clay to adhere to each other thus completeing the combination of the materials into a solid, dusty state.
From the mix-muller, the "dust" is transported to a pulverizer which breaks down the dust globules created by the muller into a fine, dusty form. From the pulverizer, the "dust" is put into a storage bin where it is allowed to set for a given period of time while the moisture content of the full load of "dust" becomes uniform due to capillary action of the moisture which has been added.
From the storage bins, the "dust" is transported to the press room where the second stage of the production process takes place. Electrically and/or hydraulically-driven presses are employed to transform the "dust" into a solid body of specific size, shape and tensile strength. In the pressing process, the "dust" is molded into shape with pressures of about 2,400 lbs/sq. in. on four cavity (4-1/4"x4-1/4", etc.) presses. The "greenware" or "body" thus formed by the press has a tensile strength of about 8 lbs/sq. in. The body is then stacked on metal racks and is now ready for the next stage of the production process.
From the press room, the racks of tile are moved to designated areas to await space in the drying rooms. The tiles are allowed to set at room temperature for a period of from 12 to 24 hours during which time they will lose by evaporation approximately 2% of the 8% moisture content. After this waiting period, the tiles are put into a drying room for the first stage of the drying process. The drying process consists of three 12-hour cycles. The cycle temperatures are approximately as follows: First Cycle - 100 deg. F .- 110 deg. F., Second Cycle: 160 deg. F. - 170 deg. F., and the Third Cycle: 210 deg. F. - 230 deg. F. After the 36-hour drying period, the "body" is void of moisture and has a tensile strength of 16 lbs/sq. in.
The "body" is then placed on the spray booth chain where the fourth stage of the production process takes place. This moving chain carries the tile body through the spray booth or area where the ceramic tile glaze is uniformly applied to the face of the tile to a thickness of between 17 and 20/1000ths inches, depending upon the glaze.
The glazed tile is then removed from the spraying area and placed in fire-resistant setters which are likewise placed on small cars which move on railroad-type tracks through the tunnel kiln. The cars are moved through the kiln by means of a chain which is driven by a motor and reduction gears so that the car with its tile travels at a speed of 1.4 inches per minute. During the approximately four hours the cars are passing through the firing zone of the kiln, the tile is exposed to radiant heat of about 2000 deg. F. At this temperature, the glaze on the face of the tile becomes fluid and gasses are released. The glaze actually attacks the body of the tile absorbing some of the chemical properties of the body and creating a bond between the glaze and the body. It is during this period that the inherent shade variation in ceramic tile actually occurs.
After the tile has completed the firing cycle, it is stacked on pallets and taken to the sorting department where it is separated by shade and grade. To insure uniformity in shading, one person does the shade separation for each complete run of a single color. The tile is shaded and graded from a continuously moving belt. The grades into which tile is separated are established in accordance with specifications written by the U.S. Bureau of Standards, and the U.S. Department of Commerce. The grading of the tile is the last step of the production process before the tile is packaged. The graders fill the cartons and seal them.
From the grading and packaging area, the cartons are moved by conveyor belt to the storage area where they are set aside by order to await transportation to the distributor.
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