Furnaces for normal sintering applications are continuous-belt furnaces (Fig. 1). This type of furnace automatically conveys the parts through the furnace on an alloy belt. Mesh-belt furnaces are usually not used above 2100°F/1100°C due to the limitations of the metallic alloy belt and muffle.
Typically, higher-temperature continuous furnaces are known as “pusher furnaces” or “walking-beam furnaces.” A pusher furnace moves the work through on a series of boats or plates (Fig. 2). One boat is pushed against another in a continuous train. A pusher furnace only pauses long enough to remove a boat at the exit end and add one at the entrance end. This is considered a constant push.
A walking-beam furnace utilizes a pusher mechanism to bring the boat into the furnace and place it on the beams. These beams are analogous to a series of rails. The rails are on cams, which lift up, forward and down, essentially walking the boat or carrier through the furnace. At the exit end, the boats are then commonly transferred onto a belt for the cooling section (Fig. 3).
High-Temperature Sintering FurnacesHigh-temperature sintering is utilized in powder metallurgy for sintering stainless steel and, in some cases, iron-based materials. It is exclusively used in refractory-metal fabrication of molybdenum, tungsten and rhenium. High-temperature sintering is also utilized in the nuclear-fuel industry for sintering uranium oxide. The ceramic industry has always used high-temperature processing for sintering, co-firing and metallizing.
To properly select and size a continuous high-temperature furnace, a number of qualifying questions must be answered.
- What is the operating temperature?
- Is there an existing profile?
- What is the process atmosphere?
- What size furnace opening is required?
- What is the boat/carrier size?
- What is the mass of the component?
- What is the material being processed (if not proprietary)?
- What is the required output?
Many furnace manufacturers have standard-size furnaces that they have built in the past. Most, however, customize the furnace to the client’s needs. Because the units are produced one at a time, it is not difficult to have the furnace built to the customer’s exact specifications.
AutomationIn both the pusher and walking-beam furnace there is a high degree of automation. This automation allows the operator to run multiple furnaces. A return conveyor runs the length of the furnace, connecting the entrance and exit ends. This return conveyor is where the operator will load and unload product. The boats/carriers are automatically conveyed around the furnace. There is usually no reason to remove these boats. The return conveyor delivers the boats to a furnace entrance crossfeed. This crossfeed then moves the boats in front of the main pushing device. The boats are then pushed through the furnace at a constant speed. Upon exiting the furnace the boats move in front of an exit crossfeed, which delivers them back to the return conveyor.
The advantage of this type of furnace, if you have the volume, is guaranteed throughput and repeatability. Every boat goes through a given profile at a constant speed. This ensures that every part will see the exact same conditions. The parts enter the furnace at room temperature and exit at room temperature. The interior of the furnace is in a protective atmosphere. Therefore, oxidation is not a problem.
OutputIn several industries, output is discussed in terms of pounds/hour. This is an interesting number. However, it is misused most of the time. The origins of this output rating came from lower-temperature furnaces, specifically traditional mesh belts. If you were to speak with the belt manufacturers themselves, they would say you cannot exceed more than 10 pounds/foot2on the belt. Most people routinely run at 20 pounds/foot2and experience a shorter belt life. Pounds/hour is not an accurate number because you do not know if the part is solid or shaped like a doughnut. Depending on this, you would not get as many parts or as many pounds/foot2, so the output is actually incorrect.
Higher-temperature pusher or walking-beam furnaces are not load limited. Pusher furnaces can push in excess of 500 pounds/foot2and walking beams approximately 400 pounds/foot2. The problem still remains that the shape of the part dictates the load you will get in a boat or on a plate. A more meaningful output number for high-temperature sintering is boats/hour. The furnace does not care if you put lots of small parts or a very large, heavy part into it. By design, higher-temperature furnaces have sufficient power for almost any application. Many of these units are designed for refractory metals with tremendous densities. If you have enough power for these very dense materials, you need not be concerned about more traditional materials.
LayoutThe typical layout of a continuous high-temperature furnace is a preheat/debinding zone, a sintering section and a cooling section (Fig. 4). To provide maximum versatility, each of these sections has multiple-zone control. All of these zones are independently controlled. The user has the ability to vary the push rate, thereby changing the throughput of the furnace. Typically, these furnaces have complete gas systems based on the processing atmosphere that has been selected.
High-temperature furnaces have gone through many changes in terms of improvements in automation. These furnaces have extensive self-diagnostics and data-acquisition capabilities. These improvements enable higher output with less operator intervention, thereby lowering material processing costs.
Operating CostsMore than ever, the focus is on operation costs. A continuous furnace is used when warranted by volume. Which is better, continuous or batch? This question is constantly asked, and the answer is both. Either furnace will make an excellent part. It is strictly a question of volume. If your volume is low or uncertain, batch would be the proper choice. With a batch furnace you are only paying for operation of the unit while you are processing parts. If your volume does not warrant constant production, batch is a better solution.
If you have high volume, continuous is the proper choice. In addition to the throughput and repeatability previously mentioned, there is operational size. These are massive systems that are extremely well insulated. The insulation packages are analogous to a sponge. Once the furnace is initially heated, the insulation package absorbs and holds the heat and the power levels drop off considerably. These furnaces are also efficiently designed in terms of atmosphere flow. High-temperature furnaces are not open at both ends – they have door assemblies, which minimize gas flow. With a continuous furnace, the processing costs per boat or component are at the lowest possible level.
When selecting a high-temperature furnace, it is important to work very closely with the supplier and provide as much information as possible. This will ensure proper selection and result in a furnace that is optimized to best fit your production requirements. Continuous high-temperature furnaces offer a growing array of sizes and features to help meet today’s challenging production goals.IH
For more information:Contact Jim Neill, vice president, CM Furnaces Inc., 103 Dewey St., Bloomfield, NJ 07003; tel: 973-338-6500; fax: 973-338-1625; e-mail: firstname.lastname@example.org
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