Vacuum sintering is one of the unsung heroes of industrial metallurgy. In this crucial process, materials are heated in a vacuum environment until they are almost at the point of melting. As heat is applied, the materials become compacted, creating new materials with completely different properties.
What is the right way to carry out the sintering process? This quick run-through of vacuum sintering will provide a solid grounding for companies that want to introduce it into
their operations.
At its root, vacuum sintering involves two things: a vacuum furnace and a combination of metallic powders. By using one properly and mixing the other in the right proportions, several useful properties can be promoted.
For instance, sintering can create metallic compounds that reduce the incidence of component failure in machinery or vehicles. And it is also very useful in creating components that demand high porosity – such as in plumbing or systems involving ball bearings. So if you need to strengthen a product or alter its porosity, sintering is often the best option available.
However, it’s important to remember that sintering generally cannot happen before another important process has taken place: debinding. And the two processes go together when planning purchasing decisions.
Debinding prepares materials or components for vacuum sintering, and it needs to be done thoroughly. All components will have impurities, often as a result of injection-molding processes. When these impurities are left on the surface of the component, they can easily contaminate the binding process.
Debinding refers to the removal of “binders,” which are deposited during production processes. Done poorly, it can result in blistering of component surfaces or the creation of pores that cannot be removed during the sintering phase.
That’s why it really matters how debinding is carried out. The exact process used depends completely on what type of binder is present. It could involve the use of specialist solvents but almost always involves decomposition of organic binders through heat treatment, generally at temperatures of 150-600°C (300-1110°F). Multiple passes through the furnace are often needed to ensure that all binder has been removed, and it pays to be cautious because even trace amounts can contaminate the sintering phase.
At this stage, an important consideration enters the equation: Should you use the same vacuum furnace for sintering and debinding? This is a vital question because it influences the cost of the process and the likelihood of success. So it’s definitely worth considering in more detail.
When choosing your sintering furnace, insulation is one of the most critical factors. There are two major types of insulation to pick from – metallic and graphite – and both have their own special properties.
The shielding on metallic chambers is usually made from molybdenum, tungsten or stainless steel. They can cater for a range of temperatures (with tungsten useful for high-temperature processes and molybdenum better for lower-heat levels). But the real strength of metal chambers is purity. Unlike graphite alternatives, they have a much lower risk of contamination caused by carburization or wafer degassing. That’s why metal chambers are often favored by aerospace firms or medical organizations – where exact purity is essential.
Another strength is cost. Metal chambers often come with a higher initial cost, but this disappears when you factor in shorter pump-down times, quick heating and cooling times, and uniform heat distribution. But molybdenum has its problems. Most notably, it can become brittle at high temperatures and is vulnerable to oxidation, which can compromise the vacuum. So that needs to be considered as well.
Graphite is the other option. Again, it has its own strengths and weaknesses. On the plus side, graphite can operate at very high temperatures (up to 3000°C), has a low density and weight, has excellent emissivity and produces a high degree of uniformity. You can easily replace graphite hot zones and repair shields when needed, while laminate CFC can be added for extra protection. Graphite is useful for most conventional sintering tasks, but it has its negatives. Most importantly, graphite tends to absorb vapors and release micro-particles, particularly if bonded layers are used, so contamination can be a factor.
Several factors come into play when making the decision about whether to combine your sintering and debinding furnace.
First, it’s important to bring the fragility of your components into the picture. When components undergo debinding, the stripping of binders and the heat involved can leave them much more frail and prone to breakage than before. Moving many components between furnaces can result in losses as these parts fail, making a single furnace more advantageous. However, it’s important to note that this can often be resolved by applying a presintering stage in the debinding furnace.
Ensuring a clean process is also absolutely essential. At no stage do you want contaminants interacting with the sintering chamber. So, on the face of it, sintering and debinding appear to be in conflict. Debinding removes impurities, making it inherently “dirty,” but that isn’t the end of the story. In many cases, the binders being removed can be kept separate from sintering powders when proper processes are followed (see the section on boxes).
Cost concerns also matter. Not all factory owners can afford to run both sintering and debinding furnaces, particularly when the volume of material being processed is relatively low.
Time comes into play as well. In general, sintering is far quicker than debinding, but the gap varies depending on the materials being used. If the gap is large, then a far-larger debinding furnace will be needed so that the production line can maintain a steady pace. In those cases, having separate furnaces makes sense.
Manpower has to be considered too. In smaller production facilities, combining sintering and debinding furnaces can allow companies to make best use of their human resources. When staff doesn’t need to move components around or manage two furnaces, they can be much more productive.
Finally, energy costs matter. In many cases, combining the two vacuum furnaces results in energy-efficiency gains, driving down costs. If done properly, there will be far less need to cool and heat the furnace – the main consumer of energy in the process.
So, there are arguments for or against using separate debinding and sintering furnaces. In general, if you have challenging debinding requirements or you are particularly concerned about the fragility of components, a separate furnace will be advisable. However, if these don’t apply, you may be able to realize cost and energy savings by combining furnaces without compromising the quality of the product.
Of course, there’s another side to the equation as well: the requirements of the sintering stage. Let’s consider that process in more depth.
Aside from insulation, the hot zone also needs to be brought into the picture. Generally, it’s important to think about keeping the mass of the hot zone low and finding a system that operates at the right temperature range, with low operating costs and efficient operation at peak power levels.
In terms of dimensions, a square cross section ensures optimal gas flow through the hot zone and also tends to reduce costs when compared to useful volume. You can be flexible here, depending on what needs to be processed. If very high temperatures are needed, however, you will have fewer options. In these cases (over 2000°C), a suspended cylindrical resistor supported by current feed-throughs is often the only option.
Gas-flow distribution is also something to consider, and here you have three broad options: gas-flow distribution with a box, without a box or no distribution at all.
Distribution with a box allows operators greater control over gas flows. Keeping a slightly higher pressure outside the box helps to prevent contamination of the heat zone by debinding products. By pumping gas directly into the box, users can keep the gas flowing over sintered components as pure as possible, resulting in fewer failures and a purer result. This tends to be the best solution for setups where debinding and sintering take place in the same furnace.
However, vacuum sintering can also take place without a box. In this case, gas distribution can be achieved via a series of points within the chamber, ensuring uniformity during the sintering process. This system tends to enlarge the usable volume of the chamber and heats and cools faster than boxed versions. It also means that the furnace can be adapted for other tasks such as tempering – a major plus for smaller workshops.
Finally, systems without gas distribution exist as well. These basic systems can be created by upgrading conventional furnaces but are not suitable for high-end specialist sintering tasks.
If you’ve chosen a boxed gas distribution system, loading is the final thing to think about (if not, you’re ready to make a furnace purchase or arrange an upgrade). The main issue here is whether to use a fixed or removable box to hold the load during sintering.
As usual, the right solution depends on your needs. Removable boxes suit operations with a single-flow mode and loads that can be cooled in static gas. In these cases, boxes can be mounted on trolleys, making removal simple and safe.
In more complex furnaces where multiple flows are employed, removal of the box may well be impractical. In these cases, boxes need to be fitted with removable shelves, allowing you to remove the load without disturbing the sintering box. Hybrid solutions are also available that feature removable head frames that slot into fixed casings, providing a middle ground. Be aware that these hybrid systems can occupy more chamber space with effects on its thermal efficiency and usable volume, however, so they aren’t common.
Hopefully, you now have a clearer idea of whether you want to combine debinding and sintering processes, the size of the furnace you require, the kind of insulation you need for your thermal chamber and whether you need a boxed or non-boxed configuration.
All of these factors feed into the decision about choosing a sintering furnace, and they all matter. Choose wisely, and you can easily create a cost-effective, efficient sintering process for almost any industrial application.
For more information: Contact Andrea Alborghetti, Vice General Manager, TAV VACUUM FURNACES SPA, Via dell’industria 11- 24043 Caravaggio (BG) - ITALY; tel: +39 0363 355711; e-mail: info@tav-vacuumfurnaces.com; web: www.tav-vacuumfurnaces.com
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