Growing demand for higher and higher quality die castings with
increasingly complex geometrical shapes requires the components to be
designed together with their production processes in order to minimize
time, cost and waste on the one hand, and maximize mechanical
performance on the other.
Product design is, in fact, evolving towards increasingly intricate and
lightweight components. In addition, the evolution of electronics in the
automotive field is drastically boosting the performance of larger frame
components characterized by reduced wall thicknesses, complex shapes,
and exceedingly small tolerances.
However, these factors also affect the development of residual stresses
and deformations during the production process, which can influence
proper assembly of the parts and component performance during use.
Simulation serves as a useful tool to predict all the component’s qualitative
information, including the stresses and deformations occurring at the end
of the process.
Stresses and deformation can form in different phases of the melting
process, specifically: during the solidification of the component inside
the mold; during the ejection of the casting from the mold; during the
cooling phase after extraction; during blanking; or following any heat
treatment.
In die casting, one of the main causes of residual stresses and
deformations typically arises from the fact that the component does not
cool simultaneously at every point, with
the outer surface cooling first followed
by the core of the material and the thinwalled
areas, before the more solid parts.
There are also the bonds with the mold,
for instance to prevent part ejection,
during the solidification and cooling
phase that may induce particularly high
contact pressures in some areas of the
component.
Given this tight contact between the part
and the mold, ejection by the extractors, if badly positioned, may cause
deformations in the component because the alloy has a low mechanical
resistance while it is still at a fairly high temperature (about 300 °C- 350
°C), see Fig.1.
Residual stresses and deformations can be corrected with manual
techniques later in the production process, for instance, by straightening
with jigs, or with dimensioning by machining specific allowances;
alternatively, virtual simulation allows you to predict these problems in
advance and identify the appropriate corrective actions to be taken.
Starting with the simulation of the thermal treatment, which mimics the
production reality, you can study the component’s thermal evolution
through the various phases of the production cycle from injection and
solidification, to the opening of the mold with the lubrication and blowing
steps, while taking into account the effect of the
thermoregulation circuits up to extraction and
subsequent cooling in water, as shown in the
case study below (Fig.2).
The thermal analysis can be combined with an
analysis of the tensions and deformations in
the part while designing the mold to identify
which crucial steps of the production process
contribute most to the deformations and
tensioning of the casting, as shown in the
following images (Fig.3 to Fig 6).
Fig. 3 - Stress distribution and deformations before extraction
Fig. 4 – Stress distribution and deformations after extraction
Fig.5 – Stress distribution and deformations after cooling in water
Fig.6 – Stress distribution and deformations after blanking
Consequently, it is possible to virtually
evaluate the effects of modifications to the
geometry of the component rather than to the
equipment used to produce it. In particular, the
casting and venting system can be modified,
or the heat of the mold can be adjusted by
varying the efficiency of the thermoregulation
system and the cycle times that govern the
opening, lubrication and blowing phases.
The extraction process can also be evaluated
by examining the contact pressure between
the part and the mold and the pressure
exerted on the component by the extractors
(Fig.7), to evaluate the efficiency of the
various extractors during the thrust phase and to identify which are key
to eject the casting from the molds. In addition, a virtual optimization of
the number and position of the extractors can be added to reduce some
deformation problems caused by extraction (Fig. 8). For example, if we
observe the positions of the extractors in the two extreme configurations
A and B, we can see the resulting deformation of the component, which
is much more homogeneous in case A, as shown in Fig. 10.
In conclusion, we can affirm that simulation
is already becoming an essential tool in the
product and process design phases, not only to
evaluate the classic defects of the die-casting
process (such as air entrapment, shrinkage
porosity and cold joints), but also to reduce
distortions and residual stresses.
These latter are becoming more and more
widespread due to the growing geometrical
complexity of the components to be produced
for an increasingly competitive market requiring
high quality standards, low costs, and reduced
time to market.
Newsletter EnginSoft Year 17 n°3
By Giampietro Scarpa, Francesca Lago | EnginSoft
