Permanent mould die casting is a process that uses iron or steel moulds (dies), enabling large numbers of the same part or castings to be produced. These permanent mould dies erode and quickly dissolve with molten metal contact. This erosion is even quicker in areas with turbulent metal flow or high drag. A die coating protects the surface of the die from molten aluminium and prevents this erosion from occurring. Once molten aluminium begins to erode the die, castings will stick or drag on the die and eventually wear it down. As the aluminium and metal die solder or stick together, sticking and dragging of the casting to the die will occur. These areas will appear as indentations and other defects on the casting.
Die coatings provide insulating properties to the die and reduce thermal shock during the cast. Thermal shock can lead to a roughness defect on the die called heat checking (many small cracks on the die face), which may appear as small protrusions on the casting. Primarily, die coatings are designed to achieve controlled directional solidification by promoting solidification of the cast metal in one direction. If heat transfer is properly controlled in all the various parts of the die, then directional solidification can be achieved.
Heat Transfer Control
- Heat transfer through the various areas of a die is the most important characteristic of a die coating, because it can control filling of the die and directional solidification.
- Insulation properties of the coating(s), methods used to apply the coating(s), and thickness of the coating(s) determine the coating's ability to control heat transfer and directional solidification.
Metal Flow Control
- A coating assists in filling the die because it insulates the molten aluminium from the die. The coarseness of the coating, the size of the refractory charge within the coating and the insulation (or thermal conductivity) of the coating play a major role in how molten metal flows within the die.
- When a casting solidifies, it begins to contract, which creates compressive forces which can attach the casting to the die that can be strong enough to create cracks in the casting and drag on the die. Typically, very thin layers of release coatings are applied as a sacrificial topcoat to the die coating(s) used on the die.
Good Surface Finish
- A good surface finish is directly related to the choice in die coating. Without heat transfer control, metal flow control, and easy release, metallostatic pressure will dictate the quality of the casting.
Protect the Die
- Die coatings cannot provide permanent protection. Although factors such as binding agent, surface preparation, and chemistry influence coating life, all die coatings must be maintained. Typically, the longer the performance life of the coating, the more consistently the dies will operate and last.
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