Die casting is a process whereby liquid metal is injected into a steel die to form a die cast part. The filling and solidification occur rapidly under high pressure, and the part “freezes” quickly. The very nature of this fast-cycling process creates castings that contain some degree of porosity.
What is porosity?
Porosity in a die casting occurs when an air pocket forms because air or gas is entrapped in the metal as it solidifies (gas porosity), or it is caused by the metal shrinking away from thick sections and toward thin sections, leaving voids in the center (shrink porosity).
Gas Porosity
The formation of “bubbles” is a sign that gas is trapped within the die cast part, which indicates gas porosity. When close to the surface of the casting, these pores can cause surface blisters to form. A die casting engineer will examine venting, gating, lubrication, an increase in time held within the die, and other factors that may contribute to the defect.
Shrinkage Porosity
These pores are larger and irregular in size, occurring in heavy sections of the die casting. Shrinkage porosity decreases the integrity of the die cast part, especially when machining operations are located in close proximity to the pores. A die cast engineer will consider eliminating thick walls and redesigning the section where shrinkage porosity occurs.
Porosity can be influenced by:
- Material cleanliness
- Part Design
- Die Design
- Metal & Die Temperatures
- Die Casting Machine Pressure
- Spray or Mold release
- Shot Speed
- Wall Thickness
- Shrinkage
A basic knowledge of the die casting process is helpful for better understanding the formation of porosity.
Die casting is a process that involves molten metal being placed inside a shot cylinder. A piston uses great force and speed to inject the liquid metal into the die mounted to a machine. The machine provides the power to inject the metal and hold the die shut until solidification occurs, aided by using water or oil cooling in the die. The machine then opens the die, and a series of ejector pins built into the die move forward to push the part out.
This process is repeated with every die cast part.
CWM casts aluminum, magnesium, and zinc, the three most common alloy families. We employ state-of-the-art engineering systems to design the dies and develop the process parameters to minimize porosity.
For more information or if you would like to discuss your project further, feel free to contact one of our CWM engineers at sales@cwmtl.com or call us at 630-595-4424.
