This is the second part of our article on vacuum sintering. In the first part of the article, we went through the two processing steps - debinding and sintering. It is now time to go further in order to choose fittings, fixtures and materials with reference to your needs. We will then examine the vacuum sintering furnace, including the choice of the hot zone (materials and design), the distribution of gas-flow and the box for loading/unloading.
The right insulation for a vacuum sintering furnace
There are different kinds of thermal chambers that can be used in sintering furnaces, each with different characteristics, regarding both the kind of insulation as well as the overall geometry.
As far as insulation is concerned, the two most common types are graphite wafer and the reflecting metallic shields, in Molybdenum or Tungsten depending on the temperature.
As far as the insulation aspects regarding vacuum sintering furnaces are concerned, there are at least two considerations to make:
- Most of the materials commonly used, including stainless steels - AISI 304 L, AISI 316 L : austenitic stainless steels containing chromium; AISI 434 : ferritic stainless steel; AISI 630; Boron stainless steel; Duplex stainless steels class 300 and 400 - can be sintered in graphite thermal hot zones. The metallic chamber is principally required for certain biomedical applications and ceramic materials.
- Entirely metallic hot zones were originally designed for vacuum treatment, while for sintering, as we pointed out, partial, non-negligible pressure values are used. This translates into an increase in dispersed energy, which, depending on the kind of gas and pressure level, can also double compared to that in vacuum. This increase can be mitigated by making use of specific solutions for this kind of application, such as the addition of a ceramic layer to contain the convection effects.
The hot zone design for vacuum sintering furnaces
As far as the geometry of the chamber is concerned, due to the fact that the parts are generally very small, there is no requirement for a specific form, so a horizontal loading hot zone of square section is generally used as it is cheaper when compared with useful volume and ensures that uniformity of gas flow conditions, required to guarantee homogeneity of the end product, can be more readily guaranteed.
The main exception is very high temperature sintering furnaces (over 2000°C approximately), as the lack of ceramic material suitable to the application creates technical difficulties in supporting and insulating the resistor and limits the geometries that can be used. In this case, a suspended cylindrical resistor supported only by feedthroughs, is a simple, robust solution.
The gas flow distribution in vacuum sintering furnaces
Unlike more common heat treatment furnaces, in which the introduction of a gas flow during heating is designed solely to limit the level of vacuum and hence does not require a particular distribution, in the case of vacuum sintering (and this also applies to debinding), an appropriate distribution of flow can be essential for the quality of the product obtained. This is because, in the case of sintering and debinding, the gas introduced has two additional functions:
- A mechanical transport action aimed at binder heat separation residues, with a view to distancing it from the load and transporting it towards the pumping unit and condenser.
- A possible reducing action, if the process gas is not solely made up of inert gases.
Reviewing the technical solutions typically used, we find three main categories:
- Gas flow distribution with box. In this case, the load is not simply inserted in the thermal chamber, but is inside a closed box which is in turn inside the thermal chamber and, while not being fully watertight, allows for a certain degree of differentiation between the composition of the internal atmosphere compared to the external one. This box has its own direct connection with the pumping unit, it may or not have direct connections with the process gas supply and it is essential for carrying out debinding and sintering in the same furnace, as it ensures that the separation products of the binder are conveyed directly towards the condenser and that they cannot be deposited on cooler, internal parts of the furnace in the area used for working. If this were not the case, a further increase in temperature at the sintering stage could cause further evaporation of these substances, with resulting pollution of the process atmosphere and with a negative effect on the result. Therefore, at the debinding stage, the gas is generally introduced into the vacuum chamber, outside the box, and pumped from inside the box, in order to prevent emission of the binder. Entry into the box can only occur through draughts possibly found in the joints of the walls themselves, so without a specific direction, or can be guided by conveniently drilling a hole in the wall of the box opposite the suction position, in order to create a uniform flow along all the shelves for the reasons given above. As well as this method of emission, there could also be a gas input directly into the box, that can be distributed in a similar way to that above, mainly used at the actual sintering stage, designed to guarantee that the gas that flows over the sintered parts is as pure as possible, and does not run the risk of gathering contaminated substances along the way.
- Gas flow distribution without box. If the sintering stage is completed in a specific furnace, the box is no longer required, as the entire process can be carried out in optimum clean conditions. Distribution of the gas is only aimed at guaranteeing uniform, reproducible process conditions for all the parts, and can be achieved with a series of distribution points integrated with the same thermal chamber. This affords a number of advantages, such as the increase in usable volume when thermal chambers are compared, an increase in heating speed due to direct irradiation of the load by the resistor and an increase in cooling speed due to elimination of the obstacle to gas circulation created by the box, even if this is opened when necessary. Consequently the design of the furnace is simplified and the same furnace can be used for other vacuum treatments, such as quenching or tempering. This can be particularly useful if the sintered pieces need to be tempered.
- No gas flow distribution. There are some cases where satisfactory results can be achieved without any particular flow distribution. If the process to be done is one of these and you already have a common heat treatment furnace or you can buy one at a reasonable price, you will probably be able to achieve what you need with a simple upgrade of the control system.
Before concluding, let me just mention whether it is better to have the box fixed in the furnace or for it to be removable with the load each time the furnace is loaded.
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Loading in vacuum sintering furnaces: fixed or removable box
Assuming that, at the previous point, you decided that the best solution for your application was a box that encloses the load, the question then arises as to whether it is better to have the box fixed in the furnace (in which case the individual trays with the parts will be loaded) or for it to be removable with the load each time the furnace is loaded.
Again in this case, there is no best pre-set solution, it depends on other factors. On the positive front, this time the criterion of choice is extremely easy.
Generally, if you have a reduced number of flow modes (for example only one mode from the outside of the box from inside) and you envisage cooling in static gas or in any case, without opening the box, it is possible to have the entire box removable at each load with a suitable trolley, so simplifying and speeding up loading and unloading operations. In other cases, the interface operations of the box with further interfaces for gas purging or opening at the cooling stage would make insertion and extraction too difficult. In these cases, it is better to leave the box fixed and remove the shelves.
There is also a possibility of having a fixed casing and an internal head-frame that can be removed together with the shelves. This way there would be the advantages of both solutions. That said, this solution is not commonly used as the space wasted in the thermal chamber (referring to space not used for loading the parts to be sintered) becomes considerable.
So far, I hope to have given you some useful information on vacuum sintering with the first part of the article, mainly focused on debinding and sintering, and with this second part as well. I would like to remind you that this is the second part of a larger article on vacuum sintering that will be continued.
If you have any questions, comments or concerns regarding sintering in vacuum furnaces, I’ll be more than happy to help you with this topic. Just ask!
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