Why Aren't You Firing Faster?
Sometimes it seems like someone has stomped on the gas pedal of the world's economy. Time is the commodity that no one has enough of anymore. With e-mail and the Internet, information can travel instantly around the world. With overnight couriers, almost anything can travel across the country overnight. So it's only natural that people expect everything to be available just as quickly. This rapid pace isn't going to get slower any time soon. As manufacturers, we must shave every minute possible from our production time to respond to our customers more quickly.
For most ceramic manufacturers, the easiest place to look is in the kiln. The firing cycle for most ceramic products consumes hours-if not days-of time. It is the longest single process in almost every ceramic plant. However, many people believe that the firing time is fixed and cannot be changed. This is not true. Over the past several years, all types of ceramic manufacturers have reduced their firing times to one sixth of what used to be normal, simply by changing their thinking about the kiln.
The Science of Firing
Before changing the firing process, it makes sense to take a look at the science of firing, and the physical principles that govern it. When firing a ceramic, the goal is to heat the ware so it develops a microstructure that gives the finished part the desired properties. This takes place by one of two mechanisms: sintering or vitrification.Sintering is the bonding of discrete particles into a single cohesive mass. It is a physical rearrangement of the boundaries of the particles. Mass is transferred within each particle and between particles. In a ceramic material, mass cannot move unless there is sufficient energy for atoms to break free of their bonds and move to a different location. The rate at which this happens is determined by three key variables:
- Time, which has a linear effect on the sintering. Doubling the sintering time doubles the amount of sintering that takes place.
- Particle size, which has an inverse power relationship on the amount of sintering. Reducing the average particle size will reduce the amount of sintering time required.
- Temperature, which affects sintering exponentially. A small change in the temperature will have a large effect on the sintering rate.
Sintering is the primary densification process in the production of technical ceramics. For traditional ceramics, such as most whitewares, however, the primary process is by vitrification. Vitrified ware is bonded with a glassy phase. This glass envelops the other ceramic particles in the body, creating a dense, cohesive part when cooled. The time required for a product to reach a given density is determined by the viscosity of the glassy phase. Empirical evidence shows that a 50°C (90°F) increase in the peak firing temperature reduces the time required to achieve a given density by a factor of 10.
What this means to you is that if you are firing dinnerware for three hours at 2100°F (1150°C), you could achieve similar results by firing for 18 minutes at 2200°F (1200°C). This has been validated in development tests for one manufacturer. Parts were taken off of their production lines, and ware that was fired in a 24-hour cycle in their existing kilns was successfully fired in a 3.4-hour cycle to 100% density. Limitations of the test kiln prevented faster cycles from being attempted.
The Path To Fast Firing
It is possible to fire ceramics on faster cycles. However, in the past it was not always practical. Firing cycles needed to be longer than necessary for the ware due to other constraints, such as kiln furniture, kiln insulation, ware loading and other factors. Advances in firing technology have eliminated these obstacles.Kiln Furniture. Most kiln furniture is made from an alumina silicate ceramic, a composition similar to the composition of the ware it is supporting. As a result, while the kiln is at peak temperature, the furniture is undergoing the same changes as the ware: The grains are growing, and any glassy phases are viscous. Of course, unlike the ware, the furniture needs to survive multiple trips through the kiln while supporting its own weight, plus the weight of the ware and the weight of other pieces of furniture. With this load at temperature, the furniture is subject to slumping under the load, or failing due to creep. To keep this from happening, the furniture is made with thick cross sections. These sections require time to reach thermal equilibrium. If the furniture is not at thermal equilibrium, stresses develop. When these stresses are too high, the furniture fails from thermal shock. A long firing cycle is needed to prevent such failures.
Kiln furniture based on other ceramic systems, such as silicon carbide, has eliminated many of the problems with alumina silicate furniture systems. To begin with, silicon carbide sinters at a much higher temperature, over 3800°F (2100°C). From the information above, at a typical peak temperature of 2200°F (1200°C) in a whiteware kiln, the sintering rate is many times slower than at the normal sintering temperature for silicon carbide. This and the absence of a glassy phase make silicon carbide stronger at temperature and less susceptible to creep, which allows thinner cross sections to be used. Another advantage is its higher thermal conductivity, which is almost as high as the thermal conductivity of steel. This allows the temperature of the furniture to come to equilibrium quickly and makes it less susceptible to thermal shock.
Changing to silicon carbide kiln furniture opens the path to faster firing since it is stronger at temperature, sinters more slowly and is more thermally conductive. Silicon carbide also has additional benefits: It stays flat for a long time, improving ware quality; it weighs less, improving the efficiency of the kiln; and it has a longer lifetime, reducing inventory of spare kiln furniture and the time it takes to change it out.
Kiln Insulation. Not so many years ago, the only option for lining a kiln was with firebrick. Firebrick is dense, and stores a tremendous amount of energy. In a tunnel kiln this is not a problem, since a zone of a tunnel kiln is nearly always at the same temperature. But in a periodic or tunnel kiln car, a firebrick lining affects the firing cycle both during heating and cooling. When heating, the insulation has a high heat flux into it, causing a heat demand on the kiln heating system. If a very aggressive heating cycle is followed, there will be cold spots near the lining. Of course, like kiln furniture, the kiln lining is susceptible to thermal stresses. As the lining is heated and cooled, cracks develop in the bricks. As cracks in the firebrick grow together, pieces of the surface spall away. As more material spalls, the lining loses its ability to insulate. In kilns firing glazed ware, these pieces may stick in the glaze, causing rejects from kiln dirt. To prolong the lifetime of the lining, the cycle needs to be long.
Changing the lining of a periodic kiln or tunnel kiln cars to ceramic fiber opens the path to faster firing, since fiber is not damaged by rapid heating and cooling like bricks are. Ceramic fiber is less susceptible to cracking because of its structure: It is a mass of fibers held together loosely rather than a solid brick. Since the heat storage is lower, it makes the kiln more efficient.
But there is a problem with this rationale: Heat transfers slowly. The objective in the kiln is to bring all the ware to the peak temperature evenly, so that all pieces exit the kiln with the same properties. In the kiln, radiation and convection transfer heat most efficiently. These both become more efficient as the surface area of the exposed ware is increased. However, in the loading patterns shown in Figure 3, the outside pieces have more surface area exposed than the inside pieces, so the pieces on the outside are heated more efficiently than the pieces in the center. To compensate for this, a long soak is required to get the pieces in the center to the same temperature as the pieces on the outside. To keep from over firing the pieces on the outsides of the stack, the firing temperature must be relatively low to keep the sintering rate slow.
These pieces could be fired more efficiently in a more open loading pattern. Changing the loading pattern to maximize the surface area of each piece exposed to the furnace will dramatically decrease the time it takes to bring all the pieces in the kiln to thermal equilibrium. Since heat does not need to soak into the center of a pack, a short soak is required. Of course with the short soak, the temperature can be raised slightly, which reduces the time required for sintering. The net result of opening the load and reducing the kiln cycle can amount to significantly higher production than would be possible with a dense load.
Other Factors. Many other factors could be forcing a slower firing rate than necessary. Some of these include:
* Insufficient drying before the kiln. To remedy insufficient drying, many people slow the kiln to prevent ware from exploding.
- Fast preheating that does not allow sufficient time for the decomposition reactions to come to completion before the surface of the ware seals. This causes black coring and blistering in the piece.
- Stress centers in the ware, causing cracks on heating or cooling.
The Value of Speed
Fast firing cycles are possible with nearly every type of ceramic body. However, the historical reasons for not firing on a fast cycle are due to the durability of the lining and furniture. Because these materials could not stand fast cycles, high-density loading patterns developed that were good enough for that firing cycle.Advances in materials have now removed many of the technical barriers to fast firing, and have made possible more open loading patterns. With these, firing times can be dramatically reduced. For instance, in roller hearth kilns in the tile industry, where all kiln furniture has been eliminated and all parts of the lining are stationary, full density tile can be produced in less than 30 minutes.
In the business world, speed is more valuable than price. Look at the overnight letter. It costs just $0.33 to send a letter through the U.S. mail, yet FedEx has become one of the world's largest companies charging nearly 50 times that to deliver the same letter the next morning. By reducing the amount of time it takes to fire your product, you can make your products more valuable than your competitor's-unless your competitor beats you to the punch.