Die casting is the fastest and most cost-effective way to manufacture large volumes of aluminum, zinc or magnesium parts. Die castings have excellent surface finishes, maintain consistent characteristics and tolerances, and waste little raw material.
If you are developing a product and considering die casting for a project, you should be aware that all castings must deal with the problem of porosity. It is unavoidable, but it can be controlled. At Gestión de Compras we have more than 20 years of experience in this manufacturing process, dealing with porosity in our projects and mitigating the effects of porosity in parts. Let’s take a look at what causes it and how to use best design practices to manage porosity and achieve superior results.
What is porosity?
Porosity refers to a hole or void in an otherwise solid metal casting. Pore sizes can range from microscopic (micropores) to large voids measurable in cubic millimeters or more. Pores are not always circular in cross-section, but can also take the form of irregular linear cracks.
Porosity has two main causes: solidification shrinkage and gaseous porosity. Let us take a closer look at the root causes of these two types and then propose strategies for dealing with them.
Solidification shrinkage
After molten metal is injected into a mold cavity, it begins to solidify when it comes into contact with the relatively cold walls of the tool. This solidification causes the metal to shrink, but the rate at which this occurs varies depending on the geometry of the molding tool and the type of alloy used.
The part of the semi-liquid metal furthest away from the tool wall is called slush, and it is in this area that pores are most likely to form. In addition, as the molten metal becomes a solid, it can block the passage of liquid to other parts of the mold. This can create pores by preventing complete liquefaction of all design features.
Gas porosity
Gas pockets form in several ways. In the case of aluminum, hydrogen can leak out of suspension and fill the voids with hydrogen gas.
There may also be air trapped inside the molding tool that was not completely evacuated or vented as the cavity was filled. This trapped air is called entrained air.
Finally, other liquids may mix with the molten metal during injection. These may be mold release agents, spilled hydraulic fluid or even atmospheric moisture. Any of these can vaporize quickly and form gas bubbles. Liquids or oils that do not vaporize become contaminants that can form inclusions in the final part.
Three types of porosity
We have just seen the two main causes of pores: solidification shrinkage and gas porosity. In both cases, pores can occur that form one of three main subcategories.
- Blind porosity: The pore begins at the surface of an element and ends somewhere within the body of the metal. These types do not usually affect mechanical strength, but may invite corrosion. It is possible to seal these pores after casting, especially if the part has to maintain pressure, as in a hydraulic cylinder.
- Through porosity: The pore starts at the surface and creates a channel through the part and out the opposite wall. This causes a leak and would have to be sealed on both sides.
- Totally closed porosity: These pores exist within the body of the metal and are not exposed to the outside unless they are subsequently penetrated during machining. The existence of these pores is usually not evident unless the part is subjected to a computed tomography (CT) scan after casting or if the part is opened for diagnostic reasons.
Allowable tolerances for porosity
Porosity usually averages 5% of the total volume of the part. It is not realistic to eliminate porosity, but rather to ensure that it is contained in those areas where it is not detrimental to the function or appearance of the part.
Therefore, when a design drawing or CAD file is produced, these areas should be clearly designated with a set of specifications and tolerances for allowable defects, just as would be the case with dimensional tolerances. Porosity specifications typically take the form of: number of pores in a given volume; maximum allowable size per pore; and percentage of total volume per part.
Manufacturers use this information to adjust molding parameters accordingly. This will mean allowing porosity in some less critical areas and avoiding it in others. These trade-offs will always need to be taken into account, so it is best to consider them at the beginning of a project.
Tool design tips to prevent porosity in die casting
There are some recommended tool design practices that should be employed to help prevent the most common causes of porosity.
Wall thickness: By far the most common cause of porosity is uneven cooling of the part within the cavity, which is itself a function of varying wall thicknesses.
The easiest and quickest way to avoid this is to maintain uniform wall thicknesses wherever possible. That is the task of the mold designer. Many other important considerations, such as the design of bosses, ribs, gussets and other features, are similar to those of plastic injection molding.
Shrinkage rate: is affected by the melting temperature of the alloy, the cooling time and the cooling temperature. In the case of aluminum, which is by far the most common die casting material, the addition of silicon can significantly reduce the shrinkage rate, but only within a certain percentage of alloy. Too much silicon can adversely affect the mechanical performance of the alloy.
It is best for the product developer to work closely with the die caster to discuss raw material options depending on the application and design.
Entrainment: It is difficult to completely eliminate entrained air from a mold, especially for complex shapes that have many internal features where air can be trapped.
There are several strategies to mitigate air entrainment. One is to improve the mold tool design so that there are no sharp corners or pockets through which air cannot escape. One can also add more vents or optimize the design of the gating/channel system to allow air escape paths.
Changing the injection rate and pressure can help with venting, but can adversely affect the part in other ways, so this should be done with care.
Is die casting the solution for you?
At Purchasing Management, our team works with customers to ensure that designs are optimized to avoid the risk of porosity. If you would like to work with us on your next project, we can offer a free project review and work with you to source die castings that exactly meet your specifications.
Contact us today to get started on your next die casting project.
