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The die lubrication phase has a
determinant role on both production
rate and production quality.
Water-based release agents are
sprayed to remove most of the
heat deriving from aluminium
cooling and solidification, which
amounts to about 250 Kcal per
kilogramme of alloy. These create
a film on the die cavity which
facilitates aluminium flow and
reduces the physical-chemical
adhesion effect. The “sticking” effect
of aluminium on steel is well
known and becomes more evident
as the metal temperature, speed
and final pressure increase. This
is also known as “metallization”.
The present paper describes the
results achieved “on site” with
the “Smart Lubrication System”
(SLS) technology: a die lubrication
control system according to the
surface temperature.
During die filling, excess amounts of
release agents develop gas that causes
casting porosity and can cool the die
excessively during heating, so increasing
the reject rate.
At production start up when the
die is in the warm-up phase, the optimum
solution is to use release agents
with specific low heat and high lubrication
capacity, and to be able to
vary the spray time during production
as a function of the die temperature
variations when the machine is in full
operation. During production the die
temperature should remain uniform
all over the surface area, and with the
same cycle time, but in reality this is
determined by the down-times due
to various reasons, and by variations
caused by other factors such as metal
temperature, cooling water temperature,
and in some cases also environmental
conditions.
The use of die thermo-regulators
has increased, but the excess quantity
of 250 Kcal to be extracted from the die
ALUMINIUM · 6/2007
per kg of cast alloy is removed by the
release agents sprayed on the die surface.
The cooling channels in the die
have geometrical restrictions due to
the shape of the cavity, the presence of
ejectors and the need not to pass near
the cavity surface, in order to avoid
premature fracture of the inserts.
To obtain fast production cycles
there must be high cooling capacities
and therefore high fluid flows at low
temperature, but this can cause fracture
due to the temperature gradient.
The hot steels with 5% Cr used
for inserts are poor heat conductors.
Thus, the temperature difference
between the surface in contact with
the cast metal and where the cooling
channels extend generates stretching
due to thermal expansion.
The most critical phase for die
life occurs during cooling, when the
water based-release agents nebulize
on the hot surface of the cavities,
because of high tensile stress due to
the contraction of superficial layers
relative to the underlying part, which
cools much more slowly because of
the reduced thermal conductivity. It is
known that the tensile breaking load
is lower compared to the compression
and fatigue strength. This shows
that die pre-heating is important for
RESEARCH
Smart Lubrication System
Die lubrication control system according to
the surface temperature
L. Baraldi, R. Boni, Italy
Schematic diagram of the Smart Lubrication System
speeding up production, but has a limited
effect on surface fracture due to
thermal fatigue.
Another important factor is the
effect of die temperature on casting
quality, and it is evident that die thermo-regulation
units are not very efficient
because the fluid temperature
is adjustable, but not the die surface
temperature. Thermocouples can be
fitted directly into the die but unless
they are very close to the surface,
causing mechanical resistance, they
have a large hysteresis and the correction
effect given by the fluid temperature
variation is limited and very
variable in terms of time.
Using water limits the fluid temperature
to 120°C (pressurized circuits),
whereas thermal fluids that can
reach a temperature of 350°C have a
specific heat which is half the value of
water and are therefore not suitable
for removing large heat quantities in
short periods.
The simulation models used allow
precise simulations of the die thermal
dynamics to be carried out, with different
geometries of the cooling channels.
As a result, it can be said that
a uniform die temperature is necessary
for product quality and for die
life, and:
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