The Benefits of Vacuum Sintering (part 2)
Many factors come into play during the vacuum sintering process, from the size and materials used in the furnace hot zone to whether debinding and sintering processes are combined in a single unit. But there’s one thing that all sintering operations have to consider: how to efficiently remove binding agents from materials.
This article will run through the fundamentals of removing binders during sintering operations without risking contamination. It is an important thing to get right, and there are numerous ways to approach it. Let’s start by thinking about one key aspect – whether to burn or capture binders as they are released by the heat of the furnace.
Capturing or burning? Which makes sense for your sintering furnace?
When binding agents are generated during sintering, they can be disposed of in two ways: capturing or burning. Both have their strengths and weaknesses, and one method could be particularly useful in certain situations.
The first thing to bring into consideration is pressure. Sintering can take place at less than atmospheric (partial) pressure, or it can be carried out in slight overpressure conditions. The pressure level in a furnace largely dictates the kind of binder removal method.
If a sintering furnace uses partial pressure, any gases created during the process need to be pumped away from the material being sintered. In these situations, pumped binder vapor should be prevented from entering the pump, and this can only be achieved using a condenser.
A burner might still be present after the pump, but its purpose is not to dispose of the binder but of flammable gases used in the process. Things are very different if sintering is carried out under overpressure conditions. If this is the case, users have a choice of binder removal methods.
On one hand, condensation and capture could be appropriate. Condensation can be carried out with varying gas flow rates and percentages of binder, so it’s a flexible option. But it comes with a catch. The condensates still need to be captured and disposed of, which adds costs on top of the sintering process.
Where labor and disposal costs are an issue, burning binding agents could be the right way to go. However, this also has its potential problems. On a mechanical level, it is vital to set up the burner efficiently so that binders don’t condense before they reach the burner.
Sometimes there can be an imbalance between the gas flow rate and the burner’s capacity to process gas, so this needs to be monitored carefully. And when materials with too high a binder percentage are processed, it can lead to residue buildup inside the burner’s pipes, raising the risk of failure. And on top of that, there’s the cost of running a burner, which must be added to the general furnace costs.
In some cases, furnace users opt to use the built-in burners on their vacuum furnaces to remove binder gases instead of investing in additional removal mechanisms. However, the flames found on vacuum sintering furnace outlets are generally intended for a specific purpose: preventing hydrogen accumulation in the environment to eliminate explosion hazard. They aren’t designed to filter emissions to remove toxic gases or to efficiently remove debinding agents from the kiln.
That’s why it makes sense to install a specialist combustion chamber to handle binder gases as they are released. These chambers can be precisely calibrated to different gas flow rates and binder percentages along with the right incineration period needed to oxygenate the binder.
How to Pick the Right Condenser for Your Vacuum Furnace
If you have made the decision to install a condenser to handle your binder removal needs, it’s essential to make the right choice. The most important issue here is whether you go for liquid or solid condensing units.
The choice you make will be determined by the chemical makeup of the binders that you use. These materials have varying liquefaction points and solidification temperatures, so they will create either liquid or solid residues when removed from the furnace. Each type of residue demands a specific type of condenser.
Solid residues are the most complex. Here, the challenge is to prevent condensation before the binders reach the capture chamber (hence avoiding pipe blockages or valve failures). Because of this, the piping and components in the condenser need to be properly heated, keeping binders at a constant temperature before they reach the condensing unit.
These condensers tend to have wide conduits to capture solid residues. This can limit the efficiency of their heat exchange. Because the binders they use condense easily, however, solid residue condensers don’t tend to need a large surface area or require low-temperature operation. But they do require cleaning, and this is a key element of a solid residue system.
Solid residues can be heated, liquefied and removed automatically, or they can be manually removed. Ideally, the first system would be used because it can be faster and demands less labor.
If the sintering process creates liquid residues, the condenser is likely to be far simpler. A large, cooled surface area is needed to convert as much binder as possible in the shortest possible time. They tend to be directed downward, leading to a removable collecting vessel, and might include filters to remove dust particles as well. Automation is much easier to implement with liquid condensates, which usually makes this kind of condenser much more efficient.
Choose the Optimum Binder Vapor-Removal System
When you have chosen the right type of condenser, one more task remains before your vacuum sintering furnace is ready to operate. We previously mentioned that the sintering process generates vapor from the binders, along with solid or liquid residues. We know what happens to those residues. But what can be done with the binder vapor?
The gases pumped from the chamber go through the condenser before reaching the vacuum pumps. Ideally, all the vapor should be condensed before reaching the pumps, but this is not necessarily the case.
Even if your condenser is working at 100% efficiency, there may still be some binder dissociation residues to deal with. It is possible to capture these residues before they enter the pump in a wet-capture system, which resembles the emissions scrubbing used in power plants. But this is an expensive solution and poses problems related to the retro-diffusion of residues back into the vacuum chamber.
This means that some binder vapor might actually reach the vacuum pump, and it is crucial that the pump is suitably chosen to cope with this issue in order to increase its lifetime. This can be achieved by either trying to prevent vapor condensation inside the pump or removing the condensation residuals from the pump.
How can you prevent condensation from occurring? There are basically two ways. The first method involves modulating pressure. By adding a gas ballast valve to the pump, users can ensure that the exhaust valve opens before the condensation starts, protecting the pump from contamination. Of course, if the gas being pumped can be ignited, the gas ballast should be fed with inert gas to prevent explosion hazard inside the pump.
The other method involves temperature. Pumps with higher working temperature are less likely to promote vapor condensation and are therefore more suitable for this kind of process. It is interesting that such pumps are usually cheaper and less sophisticated than higher-quality pumps, in which a lower temperature is attained in order to reduce oil backstreaming into the vacuum chamber.
A completely different scenario is when vapor is allowed to condense and is then removed. In this case, dry pumps should be used. Once again, not all the dry pumps are suitable. The best choice is a pump that includes a “cleaning kit,” which periodically isolates the pump from the process and literally washes it.
Final Considerations for Configuring a Sintering Operation
Choosing the right condenser and binder gas-removal system is an essential part of setting up a properly functioning sintering furnace, but they need to be put in a wider perspective in order to understand which type of furnace to install.
For example, you may have concerns about the reactivity of certain substances like titanium with oxygen and feel that a very high-vacuum furnace is the way to go. It’s possible to reach a vacuum level of 1x10-6 mbar, and it’s true that high vacuums can drastically reduce the risk of reactivity with binder gases. But this level of vacuum isn’t really crucial.
The cost and complexity of maintaining high vacuum levels is not necessary. Instead, the pump systems outlined here and the use of short cleaning sequences featuring inert gases can handle oxidization issues in the vast majority of cases.
The same applies to high pressure. Sintering generally doesn’t require very high pressure levels to be successful (aside from cases where overpressure is needed to get gases moving into or out of the chamber).
However, this doesn’t apply in all cases. Most importantly, some materials require the use of a hot isostatic pressing (HIP) process directly after sintering. HIP demands high pressures ranging from 30-150 bar and dramatically reduces the porosity of the materials being handled by increasing their density.
Ensure that you Make the Right Vacuum Sintering Investment
Both HIP and high-vacuum processing raise the costs of the sintering furnace and are not required in all cases. So it is important to be clear about your own precise requirements and to avoid overspending on the vacuum sintering furnace.
But it is equally important to invest wisely in key elements of the furnace. From the right hot-zone insulation to sourcing an easily cleaned and efficient rotary pump, many different factors need to be balanced. By making the right investment in these systems, users can reduce costs and workloads, raise efficiency and – most importantly –
ensure that their products are manufactured successfully.
Read "The Benefits of Vacuum Sintering (part 1)" here.
For more information: Contact Andrea Alborghetti, Deputy General Manager, TAV VACUUM FURNACES SPA, Via dell’industria 11- 24043 Caravaggio (BG) - ITALY; tel: +39 0363 355711; e-mail: firstname.lastname@example.org; web: www.tav-vacuumfurnaces.com. Our North America representative: FURNACARE Inc., a TAV GROUP company, 100 Corporate Dr Ste A, Spartanburg, SC 29303; e-mail: email@example.com; web: www.furna.care.
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