Improved quality and process control with EXACTPORE 3D-printed filters

The limitations of sintered ceramic foam filters do indeed pose a challenge in the foundry industry. Here are some specific aspects that highlight the problem of filter particle formation and the advantages of 3D-printed filters:

1. Filtering liquid metals

The quest for purity in the melt has been a constant feature of casting since its inception. It makes no difference whether the metals in question are ferrous or non-ferrous and their alloys – non-metallic impurities in the melts are fundamentally undesirable. Their presence almost always leads to a deterioration in the properties of the casting, and may even result in scrap castings, which represents a financial loss. It may be possible to rework castings, but even these measures should only be considered as a last resort, as they involve additional effort. In some cases, repair is not even possible, as certain applications of these parts do not allow for it.

Preventing impurities during the casting process is therefore crucial. Two main aspects must be taken into account here: the removal of existing impurities from the melt using suitable methods, and the reduction of re-oxidation during casting by ensuring that the mould cavity is filled as uniformly and with as little turbulence as possible. In particular, minimising turbulence plays a key role in the production of clean castings. This aspect should be taken into account right from the conception and design of the casting and gating system. The use of existing simulation programmes is extremely helpful in this regard.

Why is a smooth filling process so important? To understand this, it is worth taking a brief look at physics – more specifically, the fluid mechanics of liquids – and examining the interactions between flow velocity and the geometry of the areas through which the fluid flows.

1. Is the Reynolds number a troublemaker?

In short, the Reynolds number Rn is the ratio of the inertial forces acting on the flow particles (in this case, a liquid) to their viscous forces (friction forces). It is dimensionless and is used, amongst other things, to distinguish between laminar and turbulent flow. Laminar flow is attributed a Reynolds number of < 2000, whilst turbulent flow has a Reynolds number of > 4000. The range in between is referred to as laminar-unstable.

What does this have to do with casting? A great deal. To determine their casting system, foundries use an equation derived from the basic formulas of fluid mechanics to calculate the gating of the castings:

This equation uses a foundry-specific coefficient ξ, which is intended to account for flow-induced velocity losses. In addition to the usual flow deflections in the casting system, the Reynolds number is also taken into account. The casting method (e.g. gravity, riser, etc.), the cross-sectional ratios (riser/runner/sum of all gates) and the effective casting heights are taken into account. Further data are also incorporated.

Even a simple runner can lead to contamination. In a very interesting paper by Majidi, S.H. and Beckermann, C. on air ingress during mould filling, it was shown that this is one of the main sources of oxygen, which leads to the formation of re-oxidation inclusions and thus to contamination. At an impact velocity of approximately 5 m/s of the poured material stream into a stationary liquid bath, a ratio of 1:1 between liquid volume and entrained air was observed.

This illustrates that reducing turbulence plays an extremely important role in preventing non-metallic inclusions in melts. Many measures are taken both during the melting of the metals and outside and inside the mould cavity. Regardless of whether the aim is slag retention or specialised gating, one tool is particularly well-suited to this purpose: the ceramic foam filter.

1. Ceramic foam filters

The use of refractory ceramic foam filters for filtration and, more importantly, for smooth mould filling is highly valued. Ceramic foam filters are available in various materials, all of which must meet certain fundamental requirements: good chemical, thermal and mechanical properties are just a few of these. Materials used include silicon carbide, zirconia and alumina, with possible stabilisers and binders. Partially stabilised zirconia is very often used for the most demanding applications. Depending on the manufacturing process used for the base material, it is available in two colours (white and yellow/orange). The disadvantage of manufacturing such filters from this material is the shrinkage that occurs during the firing process, which can be as much as 20 per cent; this makes it quite challenging to ensure that the filters meet the relevant dimensional tolerances. If one examines the normal filter structure more closely, one can see repeating patterns. This explains the terms ‘rib’, ‘cell’ and ‘pore’.

Although the distribution of pores and cells exhibits a certain regularity, it nevertheless varies in its exact arrangement, size and interconnection, meaning that foams within the same PPI rating exhibit variations in their distribution. Filters are classified by PPI (Pores per Inch), which makes it possible to estimate their capacities and potential flow rates depending on the metal flowing through the filter. The division into PPI classes and other classifications is usually carried out visually by specially trained staff using reference standards (retainers). This procedure is entirely manual and is the subject of some controversy, but has been a common and sufficiently accurate system for many years. For specialised applications, other solutions may also be used, such as agreed weight checks or similar methods. Specialised test procedures, such as an impingement test, can also be employed to obtain more precise data and utilise it in a targeted manner.

Despite all the efforts of filter manufacturers, the quality of ceramic foam filters depends on the quality of the phenolic polyurethane foams. Despite precise and rigorous checks, structures may be present in the foams that negatively affect filtration efficiency and the behaviour of molten metal as it flows through the filter. The possible presence of thin, discontinuous webs, uneven structures and pore sizes can have a negative effect.

The smallest particles, known as filter bits, which may not be sintered firmly enough to the base material, could become dislodged when the molten metal is poured.

1. 3D-printed EXACTPORE filters – the latest innovation in filtration

The solution to effectively prevent filter bits whilst ensuring 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 filter’s structure is available as a CAD file on the computer, clean geometries are created that can always be reproduced exactly. The 3D-printed filter (Fig. 5) represents a new level of reproducibility. A printed filter is identical to the one printed before it, and the next filter will be exactly the same. This significantly improves process reliability.

All the benefits of EXACTPORE:

• Consistent flow characteristics for every filter
• Significant reduction in potentially detaching particles
• Improved melt flow behaviour
• Increased filter capacities
• Precise and uniform pore sizes, even tailored to individual requirements
• Freely scalable pore sizes
• Customised solutions with maximum design freedom
• More precise simulation options
• EXACTPORE 3D filters prevent filter bits and enable a significant increase in flow capacity compared to standard filters.

The freedom of design and scalability allow for entirely new approaches to filter manufacture. Previously, the casting and casting technology were adapted to the filter – now the filter can be adapted to the specific requirements of the casting.

1. 3D-printed filters: possibilities and product variations

These new possibilities are just one feature of using printed filters. Another feature is the prevention of filters tipping over. When pouring the melt into a riser, a standard round filter can tip over.

The solution is a 3D-printed EXACTPORE filter with a special geometry. The integration of an additional edge into the filter’s geometry successfully prevents tipping.

Furthermore, the 3D-printed EXACTPORE filter prevents fluctuating, insufficient casting rates. The required casting rate (kg/s) fluctuates considerably when using the standard filter and occasionally leads to scrap. The problem can be solved by switching to a printed EXACTPORE filter with a pore size precisely adjusted for the application. Thanks to the manufacturing process, this unique setting is now precisely replicated in every filter. This results in a constant casting rate.

A medium-sized steel foundry presented a challenge for the EXACTPORE filters. The requirement was to improve filter efficiency by increasing capacities and flow rates whilst maintaining at least the same filtration performance (compared to standard foam ceramic filters) through the use of printed filters.

To maximise the validity of the planned test, various filter sizes and melt volumes were combined. Above all, however, castings were selected that are produced in large quantities and cast using the comparable standard foam ceramic filter, in order to provide a good basis for comparison.

The filter sizes used were round filters with a diameter of 75 mm and a thickness of 25 mm at 10 PPI, as well as square filters measuring 150 x 150 x 30 mm at 10 PPI, both types made from partially stabilised zirconia.

The use of 3D-printed EXCATPORE filters enables the following: The reduction in post-processing effort by 33% and 38%, achieved by using these filters compared to conventional foam ceramic filters in the example above, is immense and clearly demonstrates the economic benefit.

1. And even more benefits thanks to 3D-printed EXACTPORE filters

In many cases, the use of 3D-printed EXACTPORE filters represents a technical and economic optimisation of the process. The advantages of these filters, together with the product variations and potential applications, make them a forward-looking innovation for foundry applications. EXACTPORE 3D filters offer unbeatable advantages, particularly for large castings (> 1,200 kg).