Published on 10/9/2017
Categories: Applications

Perfect vacuum sintering step by step [4/4]

Perfect vacuum sintering step by step [4/4]

Here we are! This is the final part of our article on vacuum sintering process. In the first part, I provided a satisfactory introduction of two processing steps, debinding and sintering. In the second part, I focused on debinding-sintering vacuum furnaces by analyzing the specific features such as hot zone, gas fluxing, and loading/unloading operations. In the third part, I went through binders and binder removal techniques, leaving you free to concentrate on the characteristics of the binder condenser. So now, let’s go on to the next and last step. After the condenser, what will happen next to the binder residues pumped up with the process gas?

The primary pump and the binder residues: what happens?

After the condenser, the pumped gas reaches the primary pump. This poses two types of problem: one is associated with any residues of binder disassociation, and the other is in any reactivity of the process gas itself.

As regards the first point, it is no surprise that some of the binder dissociation residues reach the pump, and it should not therefore be assumed that the condenser is not working. The point is that, for any substance, a vapor pressure is associated with each temperature, or a vapor pressure in equilibrium with the solid or liquid phase of the same substance that exists at that temperature. Therefore, even a perfect condenser would not have a partial pressure of binder residues at the outlet lower than the steam pressure these substances have at the water temperature used to cool the condenser.

In theory, these residues could be further reduced by using other types of filtration, similar to those used in collector towers for industrial emissions. In this case they are referred to as wet captured substances. In practice however, the cost for this type of solution would be too high, both for a tower which must operate under a vacuum, having to filter the gas before reaching the pump, and for a fluid capable of not causing problems of retro-diffusion towards the vacuum chamber, which would have to be changed periodically through being polluted by the binder residues.

For these reasons, usually the preference is to intervene on the primary pump, either by trying to avoid the binder condensing in the pump, or by making sure that the pump can be cleaned. In reality, we have seen that some use less expensive pumps, accepting the fact that they will have to change or repair it whenever it is damaged. Generally however, it is possible to design less extreme solutions.

How to avoid the binder condensing in the pump?

To avoid the binder condensing in the primary pumping unit, we first need to ask: why should it condense in the pump if it is not condensed in the condenser? Simple: because there is compression in the pump, and compression favors condensation of vapor. The two ways for avoiding the problem follow logically from this: limiting the compression or countering the condensation resulting from compression by changing another parameter at the same time, i.e. temperature.

As regards limiting compression, the gas ballast technique for the rotary pump is widely known: in practice, during the compression phase of the gas extracted from the pump , a valve is opened that allows intake of air or another gas from outside, enabling the pump discharge valve to open without additional compression of the extracted volume, and therefore without condensation of any vapor it contains.

As regards temperature, a quite natural solution would be to lower the temperature of the captured material. However, it is too costly to go below the values typically reached by industrial chillers. Instead it is easier to change to rotary pumps that work at higher temperatures, not least because these pumps are usually inexpensive. In fact when intending to use a vacuum system without being familiar with the problems of sintering, people tend to use high-performance, high final vacuum level pumps. To obtain this however, it is important to limit retro-diffusion of the oil vapors, and therefore the pump must operate at low temperatures. But doing this facilitates condensation of the vapors coming from the binder residues which contaminate the oil and damage the pump. This brings you to the apparent paradox that a less refined pump which works at a higher temperature at the cost of a poorer final vacuum proves to be more suitable for this process and lasts longer under these operating conditions.

The search for the possibility of periodically cleaning pumps to avoid problems however leads us from common rotary pumps to dry pumps. Although neither of these pumps is immune from condensation problems, at least they do not contain oil, which can get contaminated. As well as this, various manufacturers have models of pump specifically designed to be disassembled for cleaning the rotors or, even more simply, have washing kits duly designed for sintering applications, enabling the pump to be cleaned without removing it and clearly, without having to damage it.

So far, I hope the article has been helpful and satisfactory. And now, I should like to focus on two more points: high vacuum and pressure in the sintering furnaces.

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Sintering furnaces and high vacuum

Although there are clients who ask us for sintering furnaces with the capability to work in a high vacuum, i.e. between 1E-3 mbar and 1E-6 mbar, we find that the applications in this area are rather limited.

Some are concerned about certain materials that are particularly sensitive to oxygen residues, such as titanium. However, given that the major part of the process takes place under partial pressure, through the effect of cleaning that we mentioned above, using a diffusion pump is reduced in practice to a high vacuum purge before the process starts, which can be effectively replaced by a short inert gas (typically two or three) cleaning sequence, thereby simplifying the system.

Sintering furnaces and HIP process

Usually, sintering furnaces do not need to work under pressure, unless a slight overpressure is necessary for making the gas flow without passing from the vacuum pump or you do not want to temper the sintered pieces within the same cycle.

The most notable exception is where you expect to have problems related to the density of the sintered material, and therefore want the option of a HIP (Hot Isostatic Pressing) process immediately after the sintering. In this case, depending on the application, pressures varying between 30 and 150 bar can typically be used, which significantly increases the cost of the system.

As always, I hope that this article will be helpful to you in any case. Nevertheless, in order to gain a comprehensive view about the vacuum sintering, have a look at the first article on debinding and sintering. Then go on with the second article on vacuum sintering furnace, and the third one on binder removal.

If you have any questions about sintering in a vacuum furnace, I’ll be more than happy to help you with it. Just ask! Any of your questions are useful for deepening our knowledge of vacuum sintering and can be used to explore aspects of the overall issue which had not been described yet.

In this regard, I have a question for you. In previous comments, we have taken into account furnaces with an inert process atmosphere (typically vacuum, argon or nitrogen). What would happen, on the other hand, if an atmosphere with inflammable gases were required, such as hydrogen? What are your views on the benefits, added value or weaknesses of this choice? I would love to deepen this topic. If you have a field experience, I would be interested in knowing your opinion. Write to us in the comments section below.

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