 Lustrous carbon defects generally occur in iron castings made
using urethane bonded sands, shell molds, expanded polystyrene
(EPS) molds or green sand molds. Also known as resin, kish or
soot, they often appear as adherent, shiny "wrinkled" deposits
of carbon resembling alligator skin on or just under the surface
formed by the cope mold or top of the core.
Carbon-Rich Gases
While the exact nature of the mechanism that forms lustrous
carbon is not completely apparent, defect formation and severity
are caused by three primary factors: pouring time, temperature
and turbulence; organic volatile gas content; and mold/core
composition and permeability.
The most important cause of lustrous carbon is the presence of
high levels of volatile gases trapped at the mold or core
surface. These gases are the hydrocarbon breakdown product of
organic binders - especially urethane-based cold-set and shell
mold systems - released during pouring as the metal comes in
contact with the mold or core surface.
The carbon-rich gases recondense as a very thin film or sheet of
graphite flushed ahead of the leading edge of the incoming
stream of metal in the mold. There is a tendency for the defects
to form along the edges of the first metal streams in the
ingates.
Defect Formation
In lesser amounts, the carbonaceous materials in the gases are
actually beneficial. They provide a reducing atmosphere blanket
against the casting surface in the mold, which minimizes surface
oxidation and improves surface quality, or "peel." Often, they
can be easily removed from the casting surface by regular
cleaning operations.
As the level of volatile gases increases, however, the severity
of the defect also increases, and the lustrous carbon folds into
solidifying metal, causing unacceptable cold shuts and laps.
These occurrences also affect micro-structure, creating
differences in graphite formation in the layers created by the
defect.
Defect Control
Foundrymen can take several steps to control the frequency and
severity of lustrous carbon defects. These include:
* lowering the binder content in urethane-bonded systems,
especially the isocyanate component of the binder;
* increasing the mechanical venting and permeability of the core
and mold;
* increasing pouring temperature - hotter metal tends to
dissolve the carbonaceous material;
* reducing the fill time and pouring turbulence;
* applying a low-carbon coating to the core and/or mold surface;
* adding 0.5%-1% oxidizing materials (iron oxide, for example)
to the core sand.
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