Aluminum die casting is a type of method used to produce a wide variety of parts from a variety of materials. It involves pouring molten metal into a die under pressure, allowing it to cool, and then the die is opened, releasing the part.
One of the most common problems encountered in aluminum die castings is the “black spot” issue. Black spots are the result of oxidation or heat stress of the aluminum during the molding process. The oxidation occurs as gases trapped between the aluminum and the walls of the die rust, which reacts with the aluminum to form dark spots. Heat build-up inside the die can also cause black spots. The stiffer the structure of the part being cast, the worse the problem will be since a stiff structure tends to trap heat more quickly.
There are a few solutions to rid an aluminum casting of its black spots, including sandblasting, chemical dipping, and mechanical polishing. For sandblasting, the casting is subjected to a pressurized stream of sand. The sand penetrates beneath the oxide layer and dislodges the particles. This method is usually preferred for removing smaller areas of oxidation.
Chemical dipping involves submerging the part in a series of chemical baths. This method is more suitable for parts that don’t have complicated features, as the chemicals can damage intricate features.
For those parts that need a finer finish, mechanical polishing is generally the best option. In this process the part is put onto a polishing wheel and is placed beneath several brushes that remove the oxide layer. This method is the most time consuming, but it often produces the best results. It is preferable for large or intricate castings, as the brushes can easily reach every small corner.
In addition to this process, it is important to take preventative steps to minimize the likelihood of oxidation affecting the final product. Proper use of release agents and proper cooling of the die accounts for much of preventing oxidation. Additionally, it may be beneficial to lower the molten metal pouring temperature since hotter temperatures increase the rate of oxidation.
In conclusion, the “black spots” issue is a common occurrence in aluminum die castings. With the proper processes and preventative measures in place, parts can be successfully freed of their spots. Sandblasting, chemical dipping, and mechanical polishing are all effective ways to remove black spots from a casting. Understanding and controlling the temperature, pressure, and alloy type are also important for creating castings free of oxidation.
