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SPECIAL
ALUMINIUM CASTING
country and several are in the process of modernisation
under the national government’s
industrial infrastructure upgrade scheme.
More of such clusters are likely to follow. Significantly,
inter alia, Disa has played a leading
role in parallel support technologies, such as
by introducing semi-automatic and automatic
high pressure moulding lines to the Indian
foundry sector.
Commenting on this situation for his own
company, Viraj Naidu, Disa India’s managing
director says: “The 5,000-strong domestic
foundry industry in India has immense potential.
Out of this total, the number of manually-operated
foundries is currently more
than 4,000. The industry, which in the past
has relied on manual and labour-intensive
techniques, is increasingly opting for automated
equipment due to acute labour shortage
and high energy costs. Disa India is expanding
capacity to meet the needs of customers
and we aim to make high-end technology
affordable by localising equipment supply.”
With infusion of technology, the industry
hopes to tackle the twin problems of labour
shortage and high energy costs.
Ken Stanford, contributing editor
Porosity – towards the perfect casting
Porosity is a phenomenon found in all castings
made from many materials, as well as
in aluminium or magnesium die castings that
are commonly used in volume automotive applications,
as they change state from liquid to
solid during the manufacturing process. Casting
porosity takes the form of surface and core
imperfections which either affect the surface
finish or act as a leak path for permeation of
gases and liquids. Resulting holes, voids in the
structure or defects can incur significant and
costly quality problems, even leading to the
failure of the component in service.
A casting defect is a deleterious irregularity
in the metal casting process: defects due to
casting porosity impact upon every stage in
the casting manufacturing supply chain since
it occurs to varying degrees in all castings.
Some defects can be tolerated, but otherwise
they must be eliminated. Defects can be categorised
into certain key types, based on their
causes:
• Flow porosity, resulting in surface or
internal defects caused by poor pressure
or metal flow conditions in the process
• Gas porosity, usually internal, caused by
trapped gases of various kinds in the die.
Gas porosity comes from three main
sources in die-casting, namely trapped air,
steam and burned lubricant
• Shrink porosity, the most common and
most difficult to control, caused by the
change in volume as the metal changes
state from liquid to solid.
These main types of die casting porosity can
also be due to mould material and metallurgical
and process parameters. This includes
variables such as temperatures; material
cleanliness; die casting mould and part design;
machine pressures and shot speed, and
the die casting spray or the mould release media
employed.
All types of casting porosity can be identified
in three basic forms:
• Fully enclosed porosity – this is not such
a problem unless it is uncovered by
subsequent machining
• Blind porosity – often the cause of
internal corrosion giving rise to ‘spotting
out‘ of paintwork
• Through porosity – an in-service problem
where gases or liquids can permeate
through the casting.
Consequently, it is vitally important to employ
an optimum combination of equipment, process
parameters and chemical sealant.
A cost-effective and permanent solution
to porosity problems in castings is offered by
vacuum impregnation, whereby any voids are
filled with a stable
yet flexible
material that is
resistant to attack
from heat,
oils, chemicals
or vibration. As
a leader in this
field, Ultraseal
International
explains that
the process is
sub-surface
and can be
performed on
raw materials
or the finished
machined part,
causing no
dimensional
change or contamination
to
the component.
Impregnation
as a means
of treating casting
porosity has
been employed
since the 1950s,
although the
sealant media traditionally used, such as sodium
silicate or polyesters, have since been
replaced by more effective and environmentally-friendly
methacrylate-based products.
Ultraseal International has invested substantially
in R&D and continuous improvement
to ensure that its latest equipment is designed
to accommodate and effectively work
with the latest products like MX2 and Rexeal
recyclable sealants which makes major contributions
towards the environmental issues and
providing considerable cost saving benefits
through less sealant usage. Its effective process
solutions over the years have convinced
Front-loading vacuum impregnation equipment from Ultraseal
Images: Ultraseal
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