Excerpt from the report by Ohio State Industrial, Welding and Systems Engineering professor R. Allen Miller.
2.1 Characterization of loads on die and machine
The die and the machine together function as a system. Analysis of both is important in understanding the distortions of the die. The die and the machine are subject to thermal and mechanical loads during the course of a casting cycle. These forces cause the die and machine behavior to change due to deflection and distortion of the components in their assembly. For studying the loads on the die casting die, the process has to be studied over an entire casting cycle. The forces can broadly be categorized as mechanical loads and thermal loads.
The different stages of a casting cycle are listed below:
1. The die locks up and a clamping force is applied.
2. Hot molten metal is injected at high temperature. The loads involved are due to momentum of incoming metal, heat released during the filling process, and the sudden pressure spike as the cavity becomes completely full.
3. The molten metal is held inside the die and an intensification pressure is applied during solidification.
4. The part is held in the die for a short time to remove additional heat.
5. The die is opened and the part ejected.
6. The die remains open for lubricant and cooling spray and the cycle repeats.
Ahuett-Garza  made some recommendations for modeling of deflections taking into account the loads during the cycle. The conclusion was that the momentum from molten metal and hydrodynamic loads can only be important for poorly designed structures or uncharacteristic filling condition. Injection pressures may produce large deflections based on the dynamic response of die and machine. However, in majority of cases, these maximum deflections are achieved at intensification pressure which was considered important in the study of deflections. Also heat released from solidification is important to study thermal growth and its effect on die deflections.
Thermal and mechanical loads over many cycles may or may not effect deflection of dies during normal course of running. Over many cycles, thermal fatigue may crack the die surface, causing checking failure. Coining of dies and catastrophic failure could occur due to constant mechanical fatigue.
The clamping force has to hold the dies shut and withstand the metal pressure. However, simple free body relations balancing forces may not indicate how that clamp force is being transferred across the die face. Experience has also shown that balancing of forces on the tie bars may not guarantee flush parting surface. In final analysis, the clamp should not only hold the dies together, but should also be able to seal the parting surface to prevent shooting of metal across its face.
Another point that has to be accounted for is that the load path across the parting surface will change as the die grows by thermal expansion due to heat which is accumulated over multiple cycles. This is the main mechanism that explains why a cold die behaves differently than one which has been ‘run’ for many cycles. Typically, die casting process reaches a steady state after tens of warming cycles. Most notably the temperature in the die stabilizes assuming that the temperature of cooling water is stable and repeatable spray cooling can be achieved. Thus a quasi steady state is maintained over a large number of production cycles and this is the stage which can be simulated. The number of cycles to reach of quasi steady state can vary over different dies with varying cooling and spray conditions and even ambient temperatures.