The limitations of sintered ceramic foam filters can indeed lead to challenges in the foundry industry. Below are some specific aspects that further illustrate the issue of filter particle formation and the benefits of 3D printed filters:
- Filtering liquid metals
The pursuit of purity in the molten metal has accompanied casting since its beginnings. It does not matter whether it is ferrous or non-ferrous metals and their alloys - non-metallic impurities in the melts are fundamentally undesirable. Their presence almost always leads to an impairment of the properties of the cast part, up to and including casting scrap, leading to an economic loss. It may be possible to rework cast parts, but even these measures should be seen as a last resort. They mean additional expenses. In some cases, repair is not even feasible because certain subsequent applications of these parts do not allow it.
Avoiding contamination during the casting process is therefore crucial. This involves two main aspects: the removal of existing impurities in the melt using suitable subsidies and the reduction of reoxidation during casting by filling the mold cavity as smoothly and with as little turbulence as possible. The avoidance of turbulence, in particular, plays a central role in the production of clean castings. This aspect should already be considered during the conception and design of the casting and gating system. The use of existing simulation programs is extremely helpful here.
But why is a smooth filling process so important? To understand this, it is worth taking a brief look at physics, or more precisely, the fluid mechanics of liquids, and considering the interactions between flow velocity and the geometry of the areas through which the flow passes.
- Is the Reynolds number the troublemaker?
In short, the Reynolds number Rn is the ratio of the inertial forces acting on the flow particles (in this case of a fluid) to the viscous forces (frictional forces) of the same. It is dimensionless and is used, among other things, to differentiate between laminar and turbulent flow. Laminar flow is specified with a Reynolds number of < 2000, and turbulent flow with a Reynolds number of > 4000. The area in between is referred to as laminar instability.
What does this have to do with casting? A lot. To determine their casting system, foundries use an equation formed from the basic formulas of fluid mechanics to calculate the gate of the castings:
In this equation, a casting-related coefficient ξ is used, which is intended to consider flow-related velocity losses. In addition to the usual flow deflections in the casting system, this includes the Reynolds number. The casting method (e.g., falling, rising, etc.), the cross-section ratios (sprue/runner/sum of all gates), and the effective pouring heights are considered. Other data is also included.
Even a simple inlet can lead to contamination. In a very interesting paper by Majidi, S.H. and Beckermann, C. about air entrainment during mold filling, it was shown that this is one of the main sources of oxygen, which leads to the formation of reoxidation inclusions and thus impurities. At an impact velocity of approx. 5 m/s of the poured stream of material into a standing liquid bath, a ratio of 1:1 liquid volume to entrained air was determined.
This makes it clear that the reduction of turbulence plays an extremely important role when it comes to avoiding non-metallic inclusions in melts. Many measures are taken not only during the melting of the metals, but also outside and inside the mold cavity. Regardless of whether slag retention or special sprues are involved, one aid is particularly suitable here: The ceramic foam filter.
- Ceramic foam filter
The use of refractory ceramic foam filters for filtration and, more importantly, for calm mold filling is highly appreciated. Foam ceramic filters are available in different materials, that must fulfill some basic common requirements: good chemical, thermal and mechanical properties are just some of them. Materials used include silicon carbide, zirconium oxide, aluminum oxide, with possible stabilizers and binders. Partially stabilized zirconium oxide is very often used for the highest demands. Depending on the manufacturing process used for the base material, it is available in two colors (white and tan/orange). The disadvantage of manufacturing such filters from this material is the shrinkage process that takes place during the firing process of the filters, which can amount to up to 20% and makes compliance with the corresponding dimensional tolerances of the respective filters quite challenging. If you take a closer look at the normal filter structure, you will notice repeating structures. This explains the terms web finger, cell and pore.
Although the distribution of pores and cells has a certain regularity, it nevertheless varies in its exact arrangement, size and connections, so that foams have a distribution within the same PPI number. Filters are classified in PPI (pores per inch), which enables the capacities and possible flow rates to be estimated depending on the metal flowing through the filter. The classification into PPI classes and other classifications is usually carried out visually by specially trained personnel using comparative standards (retainers). This procedure is purely manual and is the subject of controversial debate but has been a common and sufficiently accurate system for many years. For special applications, other solutions tailored to the respective application can be used, such as agreed weight checks or similar. Special test methods, such as an impingement test, can also be used to obtain more precise data and use it in a targeted manner.
Despite all the efforts of the filter manufacturers, the quality of the ceramic foam filters depends on the quality of the phenolic urethane foams. Despite all precise and strict controls, structures may be present in the foams that have a negative influence on the filtration efficiency and the behavior during the flow of liquid metal through the filter. The possible presence of thin, interrupted webs, non-uniform structures and pore sizes can have a negative influence.
The smallest particles, so-called filter bits, which may not be sintered firmly enough to the base material, could be detached during the pouring of the molten liquid.
- 3D-printed EXACTPORE filters – the latest filtration innovation
The solution to effectively prevent filter bits and at the same time ensure the repeatability of the filter structures is to manufacture the filters using suitable 3D printing processes. The advantage of this process is obvious: exact reproducibility. Once the structure of the filter is available as a CAD file in the computer, clean geometries that can always be reproduced exactly are created. The 3D printed filter (Fig. 5) represents a new quality of reproducibility. A printed filter is identical to the previously printed one, and the next filter will be the same. This significantly improves process reliability.
The advantages of EXACTPORE:
- Consistent flow properties with every filter
- Significant reduction of potentially dislodging particles
- Improved flow behavior of the melt
- Increased filter capacities
- Precise and consistent pore sizes, even according to individual requirements
- Freely scalable pore sizes
- Customized solutions with the greatest possible design freedom
- More precise simulation options
EXACTPORE 3D filters avoid filter bits and enable a significant increase in flow capacity compared to standard filters.
The free design options and scaling possibilities allow completely new approaches in the manufacture of filters. Previously, the casting and its casting technology were adapted to the filter - now the filter can be adapted to the special requirements of the casting.
- Product variations and possibilities with 3D printed filters
The new possibilities are just one feature of the use of printed filters. Another feature is the avoidance of filter tipping. During the pouring of the melt into a sprue hopper, the normal, round standard filter can tilt.
The solution is a 3D printed EXACTPORE filter with a special geometry. By incorporating an additional edge into the filter geometry, tilting is successfully eliminated.
In addition, the 3D printed EXACTPORE filter avoids changing, insufficient casting capacity. The required casting capacity (kg/sec) fluctuates greatly when using the standard filter and sometimes leads to rejections. The problem can be solved by switching to a printed EXACTPORE filter with a pore size that is precisely set for the application. Thanks to the manufacturing process, this one-off setting is now repeated the same for every filter. This leads to a constant casting performance.
A medium-sized steel foundry presented the EXACTPORE filters with a challenge. The requirement was to improve the filter efficiency by increasing the capacities and flow rates with at least the same filtration effect (compared to the standard foam ceramic filters commonly used) by using printed filters (Fig. 7).
To maximize the significance of the planned test, various filter sizes and melt quantities were combined. Above all, however, castings were selected that ware produced in large quantities and with a comparable standard foam ceramic filter in order to have a good basis for comparison.
The filter sizes used were round filters with a diameter of 75 mm and a thickness of 25 mm in 10PPI, and square filters with dimensions of 150 x 150 x 30 mm in 10PPI, both types in partially stabilized zirconium oxide.
The following can be achieved with the use of 3D printed EXCATPORE filters: The reduction in post-processing effort of 33% and 38% achieved by using these filters compared to the conventional foam ceramic filters in the above example is immense and clearly shows the economic advantage.
- Further advantages of 3D printed EXACTPORE filters
The use of 3D printed EXACTPORE filters is, in many cases, a technical and economic process optimization. The advantages of these filters, as well as the product variations and application options, make them a pioneering innovation for foundry applications. EXACTPORE 3D filters offer unbeatable advantages, especially for large castings (> 1,200 kg).
