Introduction
Die casting is an important manufacturing process in many industries. It provides consistent quality and repeatable results in the production of components used in the automotive, aerospace, electronics, and medical industries. Die casting is a complex manufacturing process, and the end results are dependent on a variety of factors, including tool design, metal alloy, die casting machine configuration, casting process parameters, and more.
One of the greatest challenges faced by die casting process experts is reducing porosity, or voids, in the components. Porosity, or improperly filled areas inside the diecasting, can lead to faulty parts and expensive scrap. This article discusses in detail the cause and prevention of porosity, or collapsed pockets in die casting parts.
Causes of Porosity
Porosity, or voids in the die casting, can be caused by shrinkage that occurs during the cooling process after the die is cast. Shrinkage occurs as the die casting cools and at the same time, the molten alloy inside the die contracts. As the molten alloy contracts at different rates than the solidified die casting shell, it can create voids, the technical term for “pockets” or “pits” inside the casting. This phenomenon is known as shrinkage porosity or shrinkage cavities.
Shrinkage porosity can occur due to several factors, including a number of different process parameters. Examples of process parameters that could lead to shrinkage porosity include:
• Casting mold design
• Casting machine temperature and speed
• Casting temperature
• Casting pressure
• Prolonged holding time
• Feed material properties, such as composition and size of the alloy particles
• Casting process parameters, such as tool design, injection time and speed, etc.
• The number of cycles the diecasting is subjected to
Shrinkage porosity can also be caused by improper pouring of molten metal into the mold. If the molten alloy is poured too quickly or if the correct flow rate and pressure is not maintained, it can result in pockets or voids in the diecasting. Additionally, if the mold is filled too slowly, the molten alloy can cool faster in certain areas, resulting in pockets or voids as well.
Prevention and Control
Fortunately, porosity due to shrinkage can be controlled and prevented through careful management of the die-casting process parameters. By closely monitoring the machine parameters and carefully controlling the following variables, die casters can ensure that their parts are free of voids or pockets.
• Tool Design: The tool design should be carefully considered to ensure that adequate feed material can reach all areas of the die casting and reduce the risk of shrinkage porosity. For example, elongated tool design can enable a greater flow of molten alloy, and also ensure better filling of the mold.
• Injection Pressure: The injection pressure should be closely monitored and adjusted to ensure that the parts are filled uniformly, and that the molten alloy reaches all sections of the die casting.
• Metallurgical Properties: The metallurgical properties of the alloy should be closely monitored to ensure that they are conducive to the injection process and help minimize shrinkage porosity.
• Cooling Rates: The cooling rate of the casted part should be closely monitored to ensure an optimum temperature during the cooling process.
• Feed System: The feeding system should be checked and readjusted as necessary to ensure that an uninterrupted and consistent flow of feed material is maintained.
• Holding Time: The holding time should be carefully monitored and adjusted to ensure that the molten alloy does not cool too quickly and lead to shrinkage porosity.
Conclusion
Shrinkage porosity is a common issue encountered in die casting, and can lead to faulty components and high scrap rates. To ensure that parts are of good quality, it is important for die-casters to closely monitor and adjust the process parameters to ensure that shrinkage porosity is avoided. By carefully considering the tool design, injection pressure, metallurgical properties, cooling rates, feed system, and holding time, die casters can ensure the production of parts free of pockets or voids.
