CPT International 02/2019

COATINGS
Figure 3: A coating with good
gas permeability helps to
avoid gas-related casting
defects such as “dents” (left)
or “pinholes” (right)
Figure 4: High-quality
centrifugal casting products
after finishing.
Figure 5: Castings
before further processing.
meters have a significant impact on the
cooling rate of the melt and consequently
the mechanical properties of
the casting. These, in turn, are a crucial
component of customer requirements.
Particularly with thin-walled pipes, insulating
properties need to be specifically
adjusted in order to achieve maximum
workability.
The selection of suitable raw materials
has an impact on thermal conductivity,
but also on factors such as chemical
reactivity, fire resistance, hardness and
application properties of the coating.
The grain size distribution is also
important: a fine grain size has many
grain boundaries and thus slows down
heat transfer. A large grain size, on the
other hand, creates more cavities between
the individual grains, which also
leads to an insulating effect.
Solids content, viscosity and rheological
properties must be balanced in
such a way that, on the one hand, the
desired layer thickness is achieved
while, at the same time, the coating is
easy to spray on and allows an even
application. Since the coating is a dispersion
that may sediment out during
storage, appropriate preparation is
required before application. Before use,
the coating is adjusted to the desired
processing viscosity by determining the
density or efflux time.
Controlling surface textures
Since most castings are machined after
production, there are no special requirements
regarding the roughness of the
casting surface for many applications.
For certain applications, the shape and
height of any roughness are clearly
defined. This applies, for example, to
grey cast iron wastewater pipes: during
their manufacture, the aim is to achieve
smooth cast surfaces. Although this
requirement has no direct influence on
the functionality of the casting, some
foundries regard it as an externally
visible quality criterion with which they
want to score points with their customers.
There are, however, also customer
specifications that require the production
of rough surface textures: For cylinder
liners, a rough surface with a precisely
defined surface structure is
expressly desired (Figures 4 and 5).
Cylinder liners can be pressed into
engine blocks. It is also possible to cast
the liners into the engine block after
machining. With the further development
of casting processes and coatings,
it is now possible to produce defined
surface structures that enable the liners
to be cast directly into the engine block.
The molten metal flows around the
structured surface, filling in recesses
and undercuts so that the engine block
and liner materials form an extremely
strong and resilient bond.
The aim of this process is to obtain a
rough surface structure during casting
so that the outside of the cylinder liner
does not require any further processing.
The depth and shape of the structure
can be determined according to customer
specifications. Hedgehog or mushroom-shaped
elevations are possible,
as are structures with undercuts. The
structure depths vary between 0.3 and
1.1 mm, depending on customer requirements.
The structures with undercuts
can only be processed in pressure die
casting, while the liners with hedgehog
or mushroom structure can be processed
in conventional gravity die casting.
In addition to the special configuration
of certain machine parameters, the
coating plays a decisive role in achieving
these surface structures. It can therefore
be said that the coating acts as a
forming factor (molding material).
A modern coating for the production
of structured cylinder liners is composed
of special refractory materials
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