CPT International 04/2015

Figure 2: Dosing (right) and motion simulation
of the shot curve reveals possible
oxide entrainments and air inclusions
(Figures on the right show differences in
air entrainment for changed shot curves)
gas inclusions and misruns are hardest
to control and often even modern
simulation strategies struggle to provide
adequate solutions. Problems occur
because the mold filling is not well
enough understood, making it hard to
design the mold efficiently, by minimizing
excess material, while at the
same time respecting the die-casting
machine’s clamping forces. In addition,
entrapped gas can lead to blistering
during heat treatments, rendering
the part useless for structural components
or requiring too much fettling
in areas where appearance of the part
is critical.
Why STAR-Cast?
STAR-Cast is a powerful casting simulation
module jointly developed by
Access e.V., Aachen, Germany, and
CD-adapco, Melville, USA. Drawing
on CD-adapco’s 35 years of expertise
in thermal-fluid simulation and Access’
29 years of experience in casting
and metallurgy, STAR-Cast integrates
industry-leading computational fluid
dynamics (CFD) technology with the
specific models required by the casting
engineer, and brings a new level of precision
into casting process simulation
for the manufacturing industry.
The simulation software can be used
to address flow related issues more accurately,
allowing for a better understanding
of the complete casting process.
The key to better predict the flow
starts with a faithful representation of
the plunger motion and identify possible
shortcomings in the shot curve design
early on. Achieving a higher fidelity
flow description not only requires
more detailed physics, but more attention
needs to be paid to the discretization
of the model as well (Figure 2).
Getting the physics right
Many cast parts need to be heat-treated
after the fact and gas inclusions in
the metal can lead to undesired blistering.
Therefore the simulation model
needs to accurately predict the entrapment
of air and gas during the
filling process to aid understanding
leakage problems and gas porosities.
The software allows the interaction
between the molten metal and the air
to be correctly described by modeling
the air as a separate phase which can
be displaced or entrapped within the
melt, and either find its way to a appropriately
placed bean or vent, or get
compressed and remain inside critical
areas of the cast part (Figure 3).
The volume of fluid method is commonly
accepted as a valid approach
to capture multiphase problems with
a sharp front between phases and neglected
mixing. The turbulent flow, including
phase changes in the melt, is
usually computed using Navier-Stokes
equations, but often the air phase
is not accurately described. This is
caused by applying a bulk pressure
boundary condition on the free surface
with no consideration for the air
whatsoever. The more complex the
geo metry gets and the faster the filling
process occurs, the more difficult
it becomes to get accurate answers.
STAR-Cast solves these difficult problems
as it follows the continuum mechanics
approach and allows to accurately
calculate both air and melt flows
inside the mold. This comes at a slightly
higher computational expense but is
an important step towards predictive
mold filling analysis.
Ensuring accurate geometry
and mesh representations
Accurately capturing geometric details
and discretizing the geometry in
a way that accommodates the steep
gradients in temperature and velocity
inside the casted part are other
Casting Plant & Technology 4/2015 39