When making parts with pressure die casting, 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.
What Is Porosity in Pressure Die Casting?
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.
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 microporosity 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.
These are caused by impurities in the casting metal, contamination from the environment or oil and moisture inside the mold tool. These can only be controlled by using correct material and quality control processes.
How to Control Porosity In Your Product Design
The first thing to understand is that molten parts solidify at the skin first and the cooling extends inward to the thicker sections. The thinner outer layer, up to about .5mm, is fine-grained with little porosity and can be stronger than thicker core sections.
From a designer’s point of view, this means that many areas that require greater strength can achieve this without necessarily adding more material.
Allow porosity in areas with lesser mechanical stress
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.
Consistent Wall Thickness
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 example on the left shows the location where two walls meet. The area will be too thick and will cause excessive porosity unless the design is modified to eliminate this thick section, as shown on the right.
Sharp, 90° angles should be avoided whenever possible.
Rounded corners will fill faster in the mold and will avoid hot and cold spots, without compromising strength or functionality.
Rounded transitions, such as from this base to the wall, will also help to prevent hot spots which can cause pores and sink marks.
Gussets are angular plates that join different structural members together. Gussets add strength while helping to reduce overall thermal mass.
The example on the right is just as strong as the one on the left, but the curved profile makes a smoother transition to the wall surface which eliminates sharp corners and hot spots.
A boss is a raised column or protrusion that is used as the anchor point for a screw or other similar fitting. They can be hard to fill in a die casting without causing thermal stress.
Gussets can be used to strengthen a boss, and help it to fill more easily. Bosses are often set apart from a wall surface to minimize wall thickness.
Many of these design tips are similar to those for plastic injection molding.
How Does Machining Affect Porosity?
Because most pores occur in the deeper sections of castings, machining processes like tapped and threaded holes will expose pores. Since some castings must be able to hold air or liquid pressure, such as for hydraulic cylinders or manifolds, it’s critical to seal these pores after machining.
How Can Pores Be Sealed?
It’s common in industry to apply vacuum impregnation of the surface of die castings to effectively seal them. This is generally a three-step process:
- The part is placed in a chamber and vacuum is used to remove any trapped air in micropores;
- A sealant such as a liquid polymer resin is then put into the chamber and forced into the micropores with positive air pressure;
- Once impregnated, the part is removed from the chamber and the sealant fully cured. This is considered a one-time, permanent surface treatment.
We have a standard we apply to pressure die castings. In this standard we determine the minimum and maximum allowable number and size of pores, depending on wall thickness. Again, thicker walls will tend to have more and larger pores.
It needs to be repeated that porosity of some kind is almost impossible to prevent without expensive and specialty processing. This includes cyclic cooling or the use of conformal cooling channels. It may also be possible to die cast within an atmospherically controlled environment, which prevents gas from being trapped in the mold.
But these are expensive remedies for a problem that may not exist. The product engineer is well advised to consider where porosity can safely be ignored and plan their design accordingly. We can offer good advice and help to point out areas where a design can be improved to help you get the best results. To find out more and get a free quotation contact us today!