Understanding and Dealing with Porosity in Die-Casting Operations
Rapid die casting is a metal manufacturing process used to create high-quality metal casted parts with very good dimensional stability and surface finish. The process, which may be used in place of injection molding and machining remains very economical for high-volume manufacturing. And while die-casted parts are often close to perfection, porosity in castings remains a big challenge as it is almost inherent, sometimes even, inevitable.
Die-casting porosity may be defined as the presence of voids or holes within a casted metal part. These pores are often a consequence of the high-speed that is employed during the process of injecting the molten metal into the die. Porosity in die-casting may be shrink or gas-based.
Gas and Shrink Porosity
Gas porosity occurs when air is trapped within the die-casted product. It may be caused:
1：By the action of the machine pushing air into the metal which leaves some space atop the die.
2：When the air used in forcing the molten material into the mold is not properly forced out or unable to escape through the provided overflows and vents.
3：Due to bad temperature control where the molten material solidifies quicker than expected or unregulated processes when a mold is being filled too slowly.
4：By the inability of the solidified metal casting to hold as much gas as the liquid could during the casting process.
Shrink porosity is the more common type of porosity that will occur on metal die-casted parts. They are visible on the casting, appearing as cracks or small holes forming as a result of internal fractures. Shrink porosity occurs as a result in the difference in the densities of molten and solid metal. Consequently, when the alloy changes from liquid to solid casted product, this difference in density results in a non-uniform cooling, causing shrinkage.
Shrink porosity is an internal fracture in the die-casted parts. It occurs when the metal casting begins to solidify inside the die-casting die, forming small voids in the casted part. Unlike gas porosity, these pores hold no air or gas, they are simply a vacuum in the casted part.
Causes of Porosity in Die-casted parts
There are many reasons why porosity is the most common defect in die-casted parts. Some of the causes of porosity include:
1：Flawed design and structure
2：Impurities in the metal or alloy material
3：Speed and pressure of the die-casting machines
4：The presence of excess lubricant in the die
5：Air bubbles trapped in the molten metal
6：Low temperature control
7：Shrinkages due to differences in material density at different states
Although the risk of having a porous die-casted part is inherent because of some factors, the presence of this defect may result in component failure or loss of functionality, especially in applications where pressure integrity is critical to the structure of the part.
Dealing with Porosity
The first step to dealing with porosity is trying to minimize and prevent it. To address this, we must begin from the basics – the design of the die-casting. With a design-for-manufacturing (DFM) approach, you should expect and prepare for some shrinkage during the solidification of the molten metal. For this, factor in some allowance into the design to accommodate any shrinkage that may occur.
Next, ensure that you are working with a high-quality material that is devoid of impurities to a reasonable extent. Also, ensure that you begin your process in a clean mold. If you are using recycled metals for your process, ensure that you use a stirring action to ensure an homogeneous mix of the recycled molten metals. This is important because impure metals and alloys significantly increase the chances of porosity defects.
With the right software solutions, it is possible to simulate the casting process, detect critical issues and proffer solutions to mitigate the risk of porosity. At FirstPart, our experts will use specialized software to position runner gates, outflows, overflows, vents and water lines to efficiently manage the flow and cooling process. Emphasis should be put on the pressure and temperature during the process as fluctuations and inadequate or excess parameters. Defects such as hot tears can occur if the cooling process is inefficient. If you notice any hot tears or spots, you will need to take care of the cooling operations or increase the efficiency of the cooling channels to solve this issues.
Vacuum vents should be used to enable faster air release from the cavity. This in turn helps to reduce or eliminate the need for secondary impregnation as the process is great at removing a large portion of left-over materials that are used in the pre-casting process.
Low-solubility gases such as Argon can be used to curtail the formation of air bubbles by melting the metal in the presence of these gases inside a controlled environment or vacuum. De-gassing subsequent to melting or filtering can prevent porosity by inhibiting the formation of oxides in the molten material.
FirstPart Die-casting Solutions in China
Looking for a renowned brand to handle your metal die-casting services in china? Firstpart is your one-stop shop for all range of manufacturing service. Our expertise in die-casting enables us to deliver defect-free parts with high quality with excellent surface finish and dimensional stability. With our specialized software and techniques, we are able to prevent gas and shrink porosities by optimizing process parameters to the perfect output. To contact our experts and have a talk about your project, please click here now. After completing the form, one of our engineers will be in touch to discuss the project design and give you a free quote!