OXYNONÒ Sintering Furnace

The OXYNONÒ furnace features distinctive characteristics such as an ultra-high temperature, inert atmosphere to achieve an extremely low oxygen partial pressure, and continuous belt operation. The resulting furnace is ideally suited for high temperature sintering of pressed or molded metals and ceramics.

1) Standard Model

The standard model OXYNONÒ furnace has a long tunnel shaped chamber with the entrance and exit at opposite ends. A part or all of the furnace walls are made of a carbon insulating material so as to keep the inside temperature high. The conveyor belt transferring the workpiece through the furnace is made of a C/C composite, which enables the belt to operate in an ultra high temperature with a high load. During the operation, the furnace uses an inert atmosphere containing no oxygen or water vapor. This atmosphere will protect the carbon materials and maintain a thermochemical equilibrium with the work. Although oxygen has been removed previously from the inert atmosphere, a small amount of oxygen contained in the work may affect the atmosphere and react with the carbon material of furnace wall to produce carbon monoxide. The amount of carbon material consumed by this reaction is small and does not affect the long-term stable operation of furnace.

The furnace is fully sealed from the outside air and openings are provided only at the entrance and exit. Atmosphere gas is intended primarily to prevent air from entering into furnace. It is best to consume the smallest volume of gas as practical so as to flow gas uniformly from the entrance and exit to the outside while keeping the inside at a positive pressure. Secondly, it helps to exhaust the contaminated gas that is generated by the work to the outside. If the volume of contaminated gas becomes too high to exhaust, which will effect the furnace operation, some pre-sintering of the work is recommended. The cause of the contamination is generally additives needed for powder molding. Light metal oxides, such as zinc should be particularly avoided, as they will deposit as solid oxides near the entrance of the furnace. Temperatures there may be sufficiently high to cause the oxide to dissociate and release liquid metallic zinc. The liquid metal is then transferred into the furnace where it evaporates into a gas at the higher temperature and returns to the entrance as a gas to be oxidized again. Zinc will forever repeat this process and eventually destroy the atmosphere and furnace structure. Generally a high-temperature-resistant C/C composite belt is used to convey the work and a non-reacting ceramic plate must be used as a buffer between the work and the belt.

In the case of a stationary or batch type furnace, the workload is usually heated up by a program matched to the heat source after having been placed into the furnace at room temperature. Alternately, the work is loaded into a pre-heated furnace and kept there for a predetermined period of time. In the case of the OXYNONÒ furnace, however, the method of heating is different from that of a stationary furnace. For instance, various temperature zones are programmed according to the belt speed and processing time through the furnace to maintain a continuous flow of work. Since this method of heating the work can closely control the heat load, the continuous furnace is ideally suited to produce work with very consistent quality in large quantities. In the case of the OXYNONÒ furnace, three or more independent temperature control zones are provided according to the production requirements of the user.

The temperature of the control zones can be easily set by the process engineer according to the material, weight, configuration, and production volume respectively.

 

2) Burn-Off Model (with pre-sintering chamber)

In the OXYNONÒ furnace, the inert gas is employed both as an atmosphere and as a means of purging the furnace. A metallic belt is used when operating below 1120OC, and a C/C belt is used at 1120OC or above. When a pre-sintering chamber with a metallic belt is provided, as a preliminary zone, air would be used as the furnace atmosphere. When a C/C belt is used, inert gas must be used in both the pre-sinter and high temperature furnaces. The composition of the gas exhausted from the pre-sintering process, and the correlation between exhausted volume and temperature, significantly affect the furnace structure and atmosphere. If the pre-sintering zone is located close to the sintering zone the contaminated gas generated in the pre-sintering chamber may readily flow into the sintering chamber. However, by feeding pure gas into the sintering chamber to raise the furnace pressure the contaminants are returned into the pre-sintering chamber and exhausted outside. Since this contaminated gas contains much polymerized oily mist, parts of the chamber and exhaust duct can be filled by the resulting high viscosity material, sufficient to cause unsafe operation. The organic binder that is to be decomposed and exhausted is evaporated at comparatively low temperature with sufficient time so that carbonization of binder will be minimized. Thus, the furnace as a whole must be kept at an adequate temperature and filled with appropriate atmosphere so as to prevent the volatile material from reaching the sintering zone.

3) Burn-Off & Gas Decomposition Model

(With pre-sintering and effluent gas destruction chamber)

In the process of sintering of molded powder parts, the polymerized oily material, which results from the pre-sinter operation, must be carried to the pre-sinter exhaust and it is desirable to decompose this material into carbon monoxide and water. Therefore, the decomposition model is provided with an oxidizing and combustion zone in the bottom part of pre-sintering chamber, which is controlled to contain free oxygen at 5% and, from which the polymerized material is exhausted after being held for several seconds at 850OC or above. The circuit connecting the pre-sintering zone to the gas-decomposing zone is kept at a negative pressure in order to prevent the gas from escaping. The heat source aiding the combustion may be either fuel gas or electric.

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