The reasons for using a filter varies from method to method but the most common ones include ensuring cleanliness of the melt when it enters the mould cavity, and the catching of slag particles of varying kind and sizes, as well as sand grains. The filter is also used to reduce flow velocity in order for the melt to enter the mould in a laminar way. Sometimes it is hard to build a gating system that can effectively reduce melt velocity beneath the turbulence threshold and then the filter can work as a fix for that.
A number of variants of filters exist on the market, their construction and shape makes them suitable for different purposes. These include:
• Textile type which is mostly used for filtering particles. Speed reduction is small.
• Extruded filters having holes that go straight through the filter. The shape of the holes, as well as the shape of the actual filter dimensions, can be quadratic or circular. Extruded filters are suitable for both filtering and speed reduction.
• Foam filters consist of multi-complex holes in the shape of a sponge and are even more suitable for filtering and speed reduction than extruded filters.
• Combined filters consisting of both extruded and foam filters assembled together.
Most studies of the effect of filters have so far only been made with so-called water trials and a few studies using real-time x-ray where the whole mould has been x-rayed during filling. Simulating filters has mostly been done using ‘nudge factors’ with inbuilt so called filter functions.
Simulations that have been made with real filters have taken an extremely long time due to the fact that a huge number of elements were needed in the ’old’ simulation technologies in order to simulate the filling through the filter. The author has heard about a specific simulation made in a competitor system using an actual foam filter. The simulation time was more than two months, despite the fact that it was run on a super computer using a cluster solution connecting more than 10 computers.
One can say that it has not been practical to use simulations to verify the effect of different types of filters and, because of this limitation, facts have been very limited on how filters really work in different kinds of casting, indeed some of what is considered to be ‘facts’ might be just myths.
Now there is a program called
NovaFlow & Solid CV that can simulate all types of filters as long as you can provide a 3D model of it. The program uses a different type of meshing than other programs on the market, called
VOF (volume of fraction) or as
NovaCast calls it Control Volume
(CV).
VOF can handle parts of cells during filling and solidification and it describes the 3D model 100%. The finite difference method is used in earlier versions of NovaFlow &
Solid and in most other software on the market. Finite difference approximates the contours of the casting with the help of cubic mesh and therefore is less accurate and much more cells are needed.
The figure displays a test part simulated with an extruded filter that has holes evenly distributed on the complete filter area, hole diameter being 1.5 millimeter.
Here can be seen the velocity decrease, pressure drop and the temperature cool down through the filter. The ability to feed through the filter, which some suppliers claim they can do, can be tested. Now the efficiency can be seen and the mechanisms investigated.
So what is the simulation time for the part above? It is two hours.
If you decrease the accuracy by
3-4% by using less cells, you can easily simulate this part within 10 minutes. Provided you have a 3D model of foam filter the simulation time is the same.
The VOF method has no lower limit as to how small a fraction of a cell it can simulate.
Instead of relying only on experience, the world is now open for simulation of ‘real’ filters. This provides more knowledge on how to use filters in the future, as well as how they really work. The simulation time is so much shorter that there is no excuse not to simulate with a real filter in order to improve simulations.
NovaCast Foundry Solutions AB.
Tel: +46 (0) 457 386320; email: hakan.fransson@novacast.se web: www.novacastfoundry.se
Fig. 1 Fig. 2
Figs. 1-6 Simulation of an extruded filter
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Fig. 3
Fig. 6