The proper treatment of binders is crucial for the manufacture of ceramic and metallic products via powder technological process routes. Binders are used to adjust the flowability of bulk materials for the shaping process (e.g., extrusion, compression molding, injection molding or tape casting). They are also required for the perpetuation of form stability until the products get their final strength through thermal heat treatment.
During the production process, different methods can be used to remove the binder from the solid part. Several binders are dissolved with acid or water, while others are removed catalytically. Binders are most frequently driven out thermally, which means the binder is converted from a solid to a gas under thermal influence, thereby releasing hydrocarbons and water. This typical debinding step takes place mainly at temperatures of up to 500°C.
A binder’s vaporization dynamics are dependent on numerous factors. The properties of the sintered parts, the amount and vaporization behavior of binder, and the temperature curve tailored to the individual debinding process all play a role for the validation of phase transformation. For example, heat needs more time to pass through large-scaled goods with low porosity than for smaller goods with high surface areas or high porosity. At the phase conversion from the solid to gas, the binder material expands. Poor fume conduction can lead to product damage, which degrades the optical and mechanical performance of the finished part.
In addition, the binder’s vaporization behavior is determined by its chemical composition. Volatile organics can disappear abruptly in a small temperature range, whereas binders with long-chained molecules need higher debinding temperatures and can be driven out over a longer period. The heating rate and holding time determine the vaporization velocity and period. Hence, the respective temperature curve has a decisive role on vaporization.
The vaporization of organic binder may cause potential hazards. The released hydrocarbons in the furnace chamber can accumulate and ignite by contact with an ignition source (or self-ignite at certain temperatures). The abrupt release of energy and the enormous pressure increase in the chamber may trigger uncontrolled explosions with unforeseeable risks for personnel and equipment.
Over the last few decades, this risk has been increasingly acknowledged by producers of metallic and ceramic goods. As a result, a generation of mandatory safety measures for the respective industrial furnaces has been developed. Today, state-of-the-art safety equipment and controls are integrated in all thermoprocessing equipment to help ensure safe operation.
Safety requirements are met by keeping the maximum accumulation of hydrocarbons out of range of a potentially explosive mixture at release. The critical range is limited by the lower explosion limit (LEL) and the upper explosion limit (UEL). Developing a process in the safe range beyond the UEL is very difficult because the critical phase would have to be passed through before reaching the UEL. Therefore, the safety concept is designed to make sure that the LEL is not exceeded at vaporization. This concept requires a continuous dilution of the air/gas mixture with fresh air.
To reach this requirement in electrically heated furnaces, there are two alternatives: active gas monitoring or a passive safety concept. The active safety concept provides for the continuous monitoring of the exhaust gas line and control of the fresh air supply. The disadvantage of such a concept is the high purchase price for the exhaust gas analysis equipment and the inertia of the system. At a sudden increase of the hydrocarbon amount in the furnace, the system may react too slowly.
The passive safety concept with a focus on constant air dilution of the furnace atmosphere represents a more cost-efficient solution. In cooperation with well-known ceramic manufacturers, such a concept has been developed* and successfully established in the market.
For the design of a passive safety system, the dimensions and complexity of technical components (e.g., inlet and exhaust air vents, air circulation fans and afterburner devices) are based on application-specific charge data, binder, and process. Thermogravimetric analysis (TGA) data of defined temperature curves can be consulted to determine the expected vaporization behavior. If the explosion limit and temperature dependencies for the vaporization of the applied binder are unknown, the safety limit values to be maintained can be consulted in available standards.
Typical vaporization curves have no linear progression (see Figure 1). Over the debinding period, the vaporization peaks have to be taken into account for calculation by means of safety factors. The maximum debinding rate results from the comparison of the maximum valid binder concentration for compliance with LEL and the maximum fresh air rate that can be supplied continuously by the vent during the debinding phase.
In reverse conclusion, the performance of the fresh air vent defines the maximum debinding rate. In order to adjust the debinding system to the process demands, either the performance of the vent has to be changed or the vaporization behavior has to be adapted by changing the temperature curve. For example, the debinding velocity can be reduced by decreasing the heating rate (e.g., by providing for a longer dwell time). When designing the debinding concept, an optimal compromise between process duration and performance of the debinding process has to be worked out.
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.