PERMANENT MOLD CASTING
Permanent mold casting, often referred to as gravity die casting, is a high precision method of metal casting that is normally used to cast complex components such as cylinder heads requiring sand cores for the internal areas which need to be produced in high volumes, with dimensional accuracy and with a finer grain microstructure.
Permanent mold casting uses a mold made of steel, cast iron or graphite and the filling process simply uses the force of gravity with no other external pressures being employed.
The molten metal is gravity poured into the mold cavity reducing the risk of turbulent flux and any resulting low gas porosity.
The Permanent Mold Casting Process
The process of permanent mold die casting is fairly straight forward and can be summarized as follows:
- the mold is preheated and the cavity surface is coated with a permanent mold refractory coating to facilitate the removal of the finished part and enhance the life of the mold
- sand cores (cold box or shell sand), used for the more complex features of the part that could not be shaped as part of the mold, are placed in the mold halves and the mold is then clamped closed
- the molten metal, poured in through the sprue at the top of the mold, flows through the runner and enters the mold cavity. The filling process takes place without any additional pressure being applied. As cooling and metal contraction takes place molten metal is fed in from risers that have been put in place for this purpose
- the cast is usually cooled down by water jets on the outer surfaces. After a preset dwell time allowing the metal to solidify, the cores are removed, the die is opened and the component is ejected
- the part may then undergo any necessary trimming, machining or finishing necessary, and the mold is then prepared for the next component to be produced
Use of steel molds result in a faster cooling of alloys with respect to sand molds. As a consequence there is a significant increase in the mechanical properties and tensile strength of alloys such as Aluminum 319 and 356, magnesium and bronze when compared to the micro structural properties of those same alloys cast with the sand casting method, based on the Secondary Dendrite Arm Spacing (SDAS) value.
Today, industries such as the automotive industry make heavy use of parts that are created through the permanent mold casting method. It is used to produce parts such as the power train, supports, suspensions, casings, gears and many others.
Notwithstanding the obvious benefits of this type of metal casting, permanent mold casting is not defect-free and typical part defects can include cold shuts, oxide films, inclusion, core erosion and shrinkage porosity.
Permanent Mold Gravity Casting from the Numerical Simulation Perspective
Numerical simulation can be used to support and improve the process layout and optimization of permanent mold casting with the aim of reducing defects and materials used. A simulation model involves a complete fluid-dynamics simulation, including the free surface, and the change of a laminar flow of molten metal to an undesired turbulent flow. The model also needs to account for all thermo-physical properties because these vary significantly over the temperature range of the process. For example, the metal front of a thin wall could freeze when the filling time is too long. Some typical process parameters and their corresponding outcome that the model should include are:
- the risk of inclusion in the case of a turbulent vortex and molten metal velocity above the suggested 0.5 m/s range
- the temperature changes during the filling process in order to predict the emergence of cold shuts
- the thermal behavior of the mold and its role during the solidification process. The thermal study should include the energy balance of the different thermal parameters involved such as that of the die mass, thermoregulation channels, core overheating etc. This is needed to predict the feeding path from the risers to hot spots and the corresponding risk of shrinkage porosity
Mastering the Permanent Mold Casting Process through Simulation
Using Engineering Simulation to optimize the permanent mold casting process we hope to lead foundries to “zero defect” manufacturing and lengthen the life cycle of a permanent die. In so doing we will create a finer microstructure and improved mechanical properties of each finished component. This can be achieved by:
- optimizing the gating system design to avoid filling defects
- avoiding the presence of shrinkage porosities by acting on the process variables and feeder geometry right at the design stage
- reducing casting trials
- prolonging the mold or die life and minimizing any extraordinary maintenance
- predicting the local microstructure and mechanical properties
And all of this can be done while shortening the time-to-market of the finished product.