CPT International 02/2019

act the casting and extends the life of
the mold.
Avoiding casting defects
Casting defects such as inclusions, gas or
reaction defects, which cannot be completely
eliminated simply by changing
the process parameters, are a significant
problem in the centrifugal casting process.
As the metallic permanent mold is
impermeable to gas during centrifugal
casting, the mold coating plays an
important “gas management” role. In
order to avoid gas defects such as pinholes,
the coating must have a certain
porosity and be able to absorb the
decomposition gases produced.
By targeted optimisation of the
composition and structure of the coating,
casting defects can be effectively
avoided. If, for example, organic components
are replaced with inorganic
ones, or calcined raw materials are
used, the loss on ignition and thus the
risk of gas defects is reduced. Selecting
raw materials with low reactivity also
helps reduce the risk of reaction errors
(Figure 3).
In centrifugal casting castings can be
manufactured without voids, where you
can dispense with the use of feeders.
Photo: ??
Figure 1: The mold is coated using a spray
lance.
coatings for centrifugal casting make it
possible to achieve clearly defined properties
and advantages. The various
functions that a centrifugal casting coating
can perform are described below
and illustrated using practical application
examples (Figure 2).
Coatings as separating agents
The coating can be applied by
sprinkling a dry powder into the rotating
mold. An alternative frequently
used process involves applying the coating
by spraying a coating dispersion
into the mold.. The coating’s primary
task is to achieve a separating effect
between mold and melt. Powder coatings
can, for example, consist of a mixture
of ferrosilicon, graphite, quartz
dust, bentonite or diatomaceous earth .
The addition of water and a dispersion
agent makes it possible to paint or
spray the coating. The uniform coating
of the mold wall makes it easier to ext-
Insulating effect
The insulating effect of the coating is
an important factor in the composition
formation and the hardness profile. It
regulates the heat transfer from the
molten metal to the mold, thus contributing
to the control of solidification
and the casting structure.
The insulating properties depend on
the one hand on the composition of the
coating, and on the other hand, on the
layer thickness on the mold. Both para-
the quantity of metal supplied determines
the wall thickness.
In horizontal casting, the melt is
poured into a lying mold. In the case of
particularly long castings or during continuous
centrifugal casting, the runner
can be moved during the casting process
(Figure 1). In vertical casting, the
casting takes place in a mold with a vertically
rotating axis. Vertical casting can
be used to produce conical or spherical
outer contours. The rotational speed is
often significantly slower than with
horizontal casting.
In centrifugal casting, the mold is
usually lined with a ceramic protective
layer, the so-called coating. Since the
development of the centrifugal casting
process, the demands placed on the casting
parts and, at the same time, the
functions to be met by the coating have
continuously increased. Today, modern
Figure 2: Several molds can be arranged in a casting carousel to produce large numbers of
castings.
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