Batch Kilns for Binder Removal
Once the fuel cell, honeycomb, threaded component or other precision part has been formed and placed into the kiln, the useful life of the binder is over. The binder must be removed as quickly and evenly as possible to avoid cracking, warping, bloating, tearing, swelling or otherwise distorting the parts. In the worst possible case, the binder stays in the middle of the piece as a carbon core, undetectable from the outside, except when the product fails under a low load.
To achieve the required temperature uniformity and volume exchange in an electric batch operation, ceramic manufacturers have typically used a separate burnout oven to remove the binders from their ware before transferring their product to a kiln for sintering. The transfer step requires handling the product at its weakest point. A new technology exists that allows manufacturers to burn out the binder and sinter their products in one piece of equipment.* The furnace uses a dual heating system that gives good uniformity at low temperatures for binder removal and at high temperatures for sintering in the same process (see Figure 1).
*The Combi-Furnace, developed and supplied by Nabertherm.
Development ChallengesRemoving binders from ceramics typically takes place below 500°C (932°F), while sintering typically occurs above 1000°C (1832°F). Both of these processes require uniform treatment of the load, but the ways to achieve these uniform conditions are completely different.
Binder removal requires large volumes of air to be transferred inside the furnace for two reasons. First, the heat transfer at these low temperatures is primarily by convection. Second, as the binders evolve out of the product, air is required to sweep the binders away to prevent an explosive atmosphere from forming. The typical solution is to use an oven with a circulation fan to force convection, heating elements to provide heat, and a second exhaust fan to force air exchanges in the oven.
In a high-temperature kiln, heat transfer is primarily by radiation. Elements are placed along the walls of the kiln to radiate heat to the products. The high temperatures needed to sinter the products usually require that the entire surface of the walls be covered with heating elements to ensure that heat is radiated to the ware evenly and to minimize the potential for hot or cold spots within the kiln (see Figure 2).
Developing a system that allows for air to be circulated within the chamber at the low-temperature binder burnout portions, but is also able to withstand the high-temperature sintering portion of the cycle, is challenging. The method used to heat the air and deliver it to the products must be able to deliver that heated air uniformly to the ware space during the binder burnout portion, while also being robust enough to survive repeated trips to the maximum furnace temperature.
Technology OverviewWhen designing the new furnace, engineers looked at both low-temperature oven and high-temperature kiln technology and considered ways to combine them. Using the high-temperature kiln as the basis made the most sense, since the resulting kiln would need to fire to the sintering temperature, and using a standard design proved to be the most robust solution.
Distributing the air inside the kiln was another challenge. Using a circulation fan in a separate heating chamber-the standard practice in heat-treating furnaces-was ruled out since the best fans will only survive to about 1100°C (2012°F). To reach the high sintering temperatures required, either a new fan technology would need to be developed, or a means to isolate the fan from the heat would need to be implemented. Both of these prospects pointed to a less reliable system.
A single fan system would not give sufficient energy to both circulate the air through the furnace and force the required number of air changes. More importantly, using a single fan is not permitted under the current version of the NFPA 86 industrial furnace safety standards. If pollution control needed to be installed, this would also require the addition of a separate exhaust fan. The cost of two high-temperature fans, one for circulation and one for exhaust, would make the system too expensive.
To eliminate this problem, the engineers decided that the air supply should be external to the furnace. A low-temperature pressure blower was selected as the solution. Since it draws room-temperature air, the blower does not have the reliability problems of the high-temperature circulation fan. It is slightly less efficient than a circulated air system, but it is also safer, since all the input into the kiln is exchanged. To provide uniform temperatures during the binder burnout portion, an external preheating system is used.
Several methods were tried to distribute the air into the chamber, but plenums made from vertical ceramic tubes were found to give the best result. The distribution of air into the kiln was uniform all through the ware space and could be tailored to meet the individual end user's needs. Using tubes allows the standard heating element design to be used, as these tubes permit the heating elements behind them to radiate heat freely to the ware during the sintering portion of the cycle. Since the heating elements and air distribution systems are separate, any type of heating element can be used (see Figure 3).
Increased ProductivitySince its development, the combination furnace has been used in myriad applications. Manufacturers have used the technology to sinter such diverse parts as precision structural components, fuel cell substrates and electronic assemblies. The ability to burn out binders and sinter the parts in the same furnace has helped these manufacturers increase their productivity.
For example, one leading fuel cell component manufacturer uses a combination furnace to produce tape-cast substrates. In earlier attempts at production, the manufacturer had tried a very slow burnout in its high-temperature sintering kiln but could not get the low-temperature uniformity needed to keep the pieces from breaking. The company then switched to a two-furnace design, using a convection oven for the low-temperature burnout and moving the parts to the high-temperature furnace to finish the cycle. This extra handling step added to the per-piece cost and often caused breakage when the fragile parts were moved between the oven and the furnace. With the new combination furnace, the company has been able to eliminate this extra step and improve its product quality and productivity.
A manufacturer of injection-molded parts still uses a two-furnace system but has found that the combination furnace improves its yield. The amount of time required to remove the binders from the company's parts would tie up the combination furnace for too long if it were used exclusively for this purpose. Instead, the manufacturer uses a convection oven to remove most, but not all, of the organics from the parts. The remaining binder gives the parts stability when they are transferred to the combination furnace, which removes this extra binder and sinters the parts.
For these manufacturers and numerous others, the combination furnace is providing a welcome alternative for binder burnout and sintering operations.
For more information about the combination furnace, contact Nabertherm Inc. at 54 Read's Way, New Castle, DE 19720; (302) 322-3665;