In order to understand how to control porosity in die casting, one must first understand the basics of the process. To create a cast part, first the tool closes, molten metal is injected into the die at a high pressure and then tool and die cast machine holds the pressure until the metal solidifies—dwell time is dependent on material, shape, and size of the part. Water lines are put in place to cool the metal and casting at a controlled rate. By injecting the molten metal into the die you create an economical, high-speed process, however, it creates porosity, which is generally the inevitable outcome of one or more of the following:
- Gasification of impurities at high temperatures
- Shrinkage that takes place as the metal solidifies
- Unexpected or uncontrolled changes in temperature
- Variations in wall thickness
- Die cast machine shot speed
- Part and/or tool design
Porosity can directly affect the structural integrity of a part. For example, too much porosity in a door hinge can impact the functionality of the part, creating a weak point and potentially part failure. If you are concerned with pressure tightness, porosity can lead to leakage. In some cases, porosity can also have an indirect impact on customers’ finishing requirements.
The key to understanding porosity and launching a successful project is knowing the industry porosity standards (ASTM E505) and making sure your porosity requirements are clear to your supplier. While it is not possible to achieve zero porosity in the die casting process, with well-planned tool design and a carefully controlled process you can minimize it.
The first step to minimizing porosity is finding an engineering team that can provide design for manufacturing (DFM) insight on the front end of the project. Suppliers can use software like MAGMA to simulate the casting process to optimize the die beforehand. To reduce porosity, our engineers strategically place runner gates, water lines, vents, overflows, and cooling channels to protect areas of concern in the component. In some cases, we add vacuum assists to the injection process to further improve the casting. When the molten alloy is injected into the die at high speeds, the air in the die cavity creates pressure and resistance, which in turn creates porosity. Adding vacuum vents to pull the air as the metal is being injected allows the air to escape the cavity faster, therefore reducing the amount of porosity in the part and potentially eliminating the need for secondary impregnation. Vacuum casting also has the benefit of removing a portion of the residue materials (i.e. die lubrication droplets) prior to the metal entering the cavity. This will minimize the chance of gas porosity forming during solidification.
Additionally, suppliers should use sophisticated process control and advanced product quality planning (APQP) to evaluate the overall tool design and die casting process. Once the best design is agreed upon, the machines are set up to produce the first shots. It is critical to take precautions to control porosity from a process standpoint. Despite great efforts toward the tool design, there is still a chance of creating porosity in a component by incorrectly melting and injecting the metal. Always start with a clean mold and control the melting of the material—melting too fast or too slow will affect the process. The amount of mold release used can affect porosity as well. If your die casting company is using recycled metals, it is important to mix the pot often to create a homogeneous mixture and temperature before casting, while continuously monitoring the integrity and composition of the alloy.
Testing for Porosity
There are a number of different ways to test for porosity. The customer usually determines which testing method they would like to use. The easiest way to detect flaws in the part without taking it apart it is a simple weight or density measurement. In more complex situations porosity can be measured using a 3D x-ray machine or deconstruction testing (e.g. cross-sectional evaluation). Several of our locations offer x-ray testing and can very accurately measure porosity. Additionally, leak testing with purpose built fixtures is a frequently used method to verify pressure tightness of casting.
Porosity Concerns in Secondary Operations
If your part requires machining or other secondary operations after casting, porosity can be a major concern. One of our consumer electronic customers experienced issues with porosity when their part was cosmetically plated during the trial run. Our engineers quickly adjusted the gating method, overflow locations, and applied vacuum casting to solve the problem.
The most important thing to remember when starting a new project is the importance of working with an expert that knows what casting factors and part features will most likely contribute to porosity. If porosity is a concern for your next project, sign up for a free design seminar. Our engineering team will schedule an on-site meeting with your staff and provide expertise and insight on part design and the die casting process—they can tailor their presentation to meet any of your needs.