Bonded Particle Filter - Pyrotek

Filtration is especially beneficial in removing ‘hardspots’ (oxide and
sludge inclusions) from net and near-net-shape foundry castings,
resulting in significantly increased machinibility, less tool wear
and less tool breakage! Clearly, gains in product quality, productivity
and profitability make filtration a very necessary and desirable
process for all aluminium casting applications.
2.2 COMMERCIALLY AVAILABLE FILTER MEDIA
Four major types of filters for aluminium are found in the
marketplace. Along with their predominant characteristics, they
are the following:
A. Alumina bed filters
Bed filters are comprised of ‘layers’ of tabular and ball alumina to
create ‘depth’ as the metal flows between the ‘balls’ and flakes.
The porosity of the bed is not fixed, however, so channelling of the
aluminium flow can occur, either spontaneously through surge
flow, or inadvertently through mechanical disturbances. This can
lead to dumping of already-trapped inclusions and discharge of
unfiltered metal. While deep-bed filters are usually large and fairly
costly, they do provide excellent filtration if the life is not over-
extended.
B. Ceramic foam filters
Ceramic foam filters are bonded or sintered alumina with
porosity controlled during manufacture to provide a gradation
of pore sizes 20 or 30 ppi, or grades 40, 50, 60, 65 and 80. With
high average porosity (80-85%), fairly rapid metal flow rates can be
achieved, but at the expense of filtration efficiency. With such high
porosity, mechanical strength is relatively low, and handling
damage or break-up may occur during melt flow. Providing the
proper filter grade/flow relationship is maintained for an initial
metal cleanliness, ceramic foam filters are low-cost and find their
best application as a single-use (single-cast) application in the
casthouse or foundry.
C. Bonded ceramic particles
This is the type of filter supplied by Metaullics - a bonded granular
ceramic media, fabricated either from aluminium oxide or silicon
carbide. The proprietary bonding agent employed is both strong
and resistant to chemical attack by molten aluminium.
Substantially lower porosity (nominally 40%) and the inherent
properties of the hard alumina or silicon carbide granules
provides a much stonger filter structure which aids in handling
and durability. Especially with silicon carbide construction, thermal
conductivity is substantially higher which aids in preheat and in
thermal transfer. The complex, fully-interconnected internal pore
structure provides a ‘tortuous path’ for metal flow. The bonding
agent also has a particular affinity for inclusions. This combination
of lower porosity, tortuous flow path and binder chemistry allows
the Metaullics bonded particle filter to exhibit the best balance
between cake-mode and depth-mode filtration.