While manufacturing parts with pressure die casting, there are chances of voids and holes forming within the part, affecting the quality of the part that is being manufactured. Pressure die casting can offer close tolerances and excellent surface quality. However, porosity is a big concern in most of the casting projects that make use of pressure die casting.
Every die caster should know about the formation of pores or holes in the process and the steps to control or avoid them. It is not always possible to eliminate porosity bit can be controlled with process control, careful design, and efficient finishing. Let us look into the detail of the causes of porosity in die casting.
Porosity in Pressure Die Casting
Porosity is the term used to refer to holes and voids formed in pressure die casting. The exact reason of porosity can differ depending on the material that is cast. Below is a rundown of gas porosity and its effects on die cast parts.
In pressure die casting, there are chances of ambient air being trapped inside the mold. This air should be evacuated via vents and when the air escapes, molten metal will fill the cavity as the metal is under high pressure. There are chances for some of the air bubbles being captured inside the molten metal. This is usually referred to as “entraining”. While designing parts, die casters will try to avoid places like deep pockets and sharp corners where air can be trapped.
In deeper sections of the part, gas micro porosity can take place. At the surface, near to the walls of the tool, the molten metal can cool down fast and can solidify with a grain texture. However, molten metal in the inside, which is away from the cool walls of the tool, will take more time to solidify. The metal can contract slowly in the long cooling cycle and this internal contraction can lead to small voids. Air can migrate into these voids (hydrogen molecules in the case of aluminum casting), resulting in the formation of gas filled pockets in the part.
Porosity can also happen due to inclusions, which is caused by the impurities that are present in the casting metal. Presence of moisture and oil in the tool or contamination from the environment can lead to this type of porosity. The only way to control porosity due to inclusions is to use proper material and to use efficient quality control processes.