When making parts with pressure die casting, zinc die casting manufacturers know that there is a risk that holes or voids may form within the part, much to the frustration of every product developer. While pressure die casting can and does produce excellent surface quality and close tolerances, every product designer and engineer needs to know about the formation of holes, or pores, and what can be done about them.
Porosity is a concern in all pressure die casting projects. Porosity cannot always be eliminated but it can be controlled through careful design, process control and finishing. Let’s take a look at what causes porosity and how to control it. As mentioned, porosity refers to any holes or voids in a pressure die cast part. The exact cause of the defect can be a little different depending on the material being cast, whether aluminum, zinc or magnesium. Porosity is usually caused by gas or inclusions.
Gas porosity incorporated by zinc die casting manufacturers
Within this subset there are two types. Ambient air can be trapped inside the mold and must be evacuated via vents in the mold. As the air escapes, molten metal is pressurized to fill the cavity. It is possible, however, that some air molecules have been captured inside the metal in suspension – this is called entraining. Good part design will seek to avoid places where air can be trapped, such as in sharp corners and deep pockets.
There is also gas micro-porosity that takes place in the deeper sections of a die casting. At the surface, close to the tool walls, the metal cools quickly and solidifies with fine grain texture. However, further away from the cooler tool walls, molten metal takes longer to solidify. During this longer cooling cycle, the metal slowly contracts. This process of internal contraction creates minute voids. Into these voids hydrogen molecules, especially in aluminum, migrate into the void and change to a gaseous state, making gas filled pockets in the metal.
How to control porosity?
Porosity is not always detrimental to part function. It’s possible to design parts and mold tools such that greater areas of porosity are concentrated in zones with lesser mechanical stress or which do not otherwise impair functionality. In many cases it’s better to leave such areas alone rather than engage in costly, time-consuming configurations of complex mold tools.
By far the most common cause of porosity is unequal cooling of the part inside the cavity, which is in turn a function of different wall thicknesses. Therefore, the easiest and most expedient way to prevent this is to maintain consistent wall thicknesses whenever possible. The area will be too thick and will cause excessive porosity unless the design is modified to eliminate this thick section as controlled by zinc die casting manufacturers.