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In Figure 2, the vaporization rate, fresh air flow and vaporization period are taken into context for binder amounts in the range of 200-15,000 g. The process parameters can be adjusted depending on the maximum valid vaporization rate (red curve).
Process safety needs to be guaranteed during debinding. A corresponding flow rate monitoring system ensures the conduction of exhaust gases from the heating chamber, while furnace pressure control maintains a slight under-pressure in the chamber. Thus, no organic gases can enter and contaminate the environment in case of leakage.
The continuously supplied fresh air can be pre-heated by an air heater to avoid broad temperature distributions and disruptions of the target temperature profile by large temperature gradients between fresh air and furnace atmosphere. Perforated injection tubes provide for uniform fresh air input. The outgoing exhaust gases can optionally be cracked and cleaned by a thermal or catalytic afterburner. Thus, unpleasant odors or contamination of the environment are prevented. In case of operational disturbances (e.g., sudden power loss), elaborate emergency programs ensure the process transfer to a safe state.
An alternative debinding concept involves debinding under inert conditions, which is suited for processes where charging materials with oxidable surfaces get debinded. Without a protective atmosphere, the released binder starts with oxidation at the charge surface. Especially for goods produced through powder metallurgy, this result is undesirable. Therefore, binder is driven out under inert atmosphere and burned in either a torch or thermal afterburner for these processes.
Basically, two alternatives exist for the debinding and sintering process. The full process can either be done in two steps (in two furnaces) or in one combination furnace. Furnaces for just the debinding step can be used up to approximately 850°C and feature an air circulation fan installed in the furnace chamber. The constant air circulation leads to an optimal temperature distribution to provide for a homogeneous thermal soaking of the load. The subsequent sintering step is carried out in a separate furnace.
This two-step process alternative is the right choice when high production rates have to be achieved and the cycle time of both the debinding and sintering are close together. Disadvantages of this two-step process include higher capital spending for two separate furnaces, the risk of charge damages during transfer from the debinding to the sintering furnace, and the higher energy consumption due to the additional cooling and reheating step.
Combination furnaces need less capital spending and shorten the cycle time, as both process steps run consecutively in the same furnace. Especially for sensitive goods, this solution is recommended as no transfer damages can occur between the process steps.
For more information, contact Nabertherm GmbH at 28865 Lilienthal, Bahnhofstr. 20, Germany; or visit the website at www.nabertherm.com.