CPT International 02/2019

COATINGS
Figure 6: Schematic illustration of the Leidenfrost effect.
Figure 7: The formation of different structures is controlled, among other things, by the
coating.
Figure 8: When drawing the tubes, the coating should adhere to the cast part and be completely
removed.
with defined grain sizes and shapes,
water, binder, dispersion agent and surfactants
to control surface tension.
Figure 6 shows the various processes
that occur when an aqueous coating is
sprayed onto the hot centrifugal casting
mold. If drops of the coating hit the
mold, the coating is accelerated by the
rotary motion of the mold and pressed
against the hot mold as a result of the
centrifugal forces. The water in the coating
immediately begins to boil. Leidenfrost
effects cause gas layers or gas bubbles
to form. As the coating dries,
channels form in the coating. Figure 7
illustrates the concept of forming different
structures from the gas bubbles.
Depending on the coating composition,
the centrifugal forces and the mold
temperature, different channels are formed,
which in turn lead to different
surface structures during casting.
Additional coating requirements
In addition to the functions of coating
for centrifugal casting described above,
there are a number of additional, application-specific
requirements: A key characteristic
with regard to the process
steps is, for example, extraction behaviour:
When the tube is extracted from
the mold after solidification, the coating
should adhere to the casting and,
ideally, be completely removed from
the mold with the casting (Figure 8).
In terms of cleanliness in the workplace,
it is also desirable that a contiguous
layer of the coating remains on
the casting, and that it is as dust-free as
possible (Figure 9). If the next process
step involves blasting, the coating
should be easy to remove – in case of a
textured surface it may also need to be
removed from the undercuts. For health
and safety reasons, the coating must
not contain crystalline quartz.
Consistent quality is crucial
In order to gain a competitive advantage,
many foundries strive to increase
the productivity of their processes and/
or improve the quality of their castings.
This leads to ever narrower process
windows, with the risk that even the
smallest defects can cause major disruption.
Coatings must also meet these
more demanding requirements. It is
therefore becoming increasingly
important to use stable coating products
of uniform quality. Foundries
need products with consistent properties
in terms of composition, viscosity
and application behaviour. However,
natural raw materials are typically used
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