Guide to Controlling Pressure Die Casting Porosity
At Creatingway, we use pressure die casting to make high volume parts. It is a cost-effective method for making zinc, magnesium, and aluminum parts. The advantage of using die casting is getting a quality surface finish while still maintaining consistency. Also, little material goes to waste along with achieving tight tolerances. However, die casting has a common defect known as porosity. It thus means the part made has a problem.
How Porosity Occurs and Some Practices To Manage This Issue
What’s The Porosity?
Porosity is a void or hole found within a solid cast metal part. The sizes of the pores vary from micropores to large ones. They often take the shape of a circle in cross-section but can get the form of an irregular linear crack. How pores form in pressure die casting is mainly through gas porosity and solidification shrinkage.
Pores form through gas entrapment in the metal during die casting. While the cavity fills, gas pockets form when there is no full evacuation of the gas. We call this trapped gas entrained. Also, gas bubbles form through the mix of molten metal and other liquids. Such liquids include hydraulic fluid and release agents. They rapidly vaporize to form these gas bubbles. Liquids that do not vaporize such as oils later form impurities in the final part.
Porosity is likely to form during the cooling stage of the pressure die casting process. Molten metal starts solidifying after coming into contact with the cooler walls of the mold cavity. However, the parts further away from these cooler parts solidify at a much slower rate. This portion is known as slush. It is the area most likely to form pores. Also, during the solidification process, the solid parts block sections of the pressure die-cast. This prevents full liquefaction of these places, therefore, creating pores.
What’re The Types of Pressure Die Casting Porosity?
The type of pore begins on the part surface and makes a path through the part to the opposite surface wall. It results in leaks in the metal part and requires sealing on both sides to fix.
This type of pore begins on the part surface but end within the body of the metal part. It, however, does not upset mechanical strength but may end up causing corrosion. We fix it by sealing the pores especially after the casting process is complete. Parts that require to maintain pressure e.g. hydraulic cylinders require this solution.
Such pores only exist inside the body of the metal part. Unless revealed post-machining, they are not seen on the outside. We could otherwise use a CT scan to detect the presence of any fully enclosed porosity within a metal part.
Acceptable Tolerances for Pressure Die Casting Porosity
The average porosity occurrence for most die casting processes is about 5%. The complete elimination of porosity is not realistic. Therefore, the focus should be on achieving proper metal part function and appearance. A designer should clearly indicate the set of specifications allowable for such a defect in the drawing or CAD file. Such as the number of pores per given volume and max size of pores allowed.
Porosity Prevention Design Tips
Simple Part Design
Gas entrapment is a common cause of porosity. Often, designs with complex shapes and internal features only increase the chances of gas being trapped. The designer should first reduce the number of sharp corners or pockets to allow air to escape. We could also add more vents to optimize the air escape routes.
Even Thickness of Walls
A varying wall thickness causes unequal cooling of the molten metal within the cavity. It is a common cause of porosity. To fix this design flaw, the designer has to make consistent wall thickness of the part where feasible. This will result in even cooling of the molten metal, thereby reducing the chances of porosity occurring.
Use of Inert Gases
We can reduce porosity by not having gas used in the process of reacting with the molten part. A gas such as argon is an important inert gas for this task.
Controlling Shrinking Rate
We have seen how to shrink rate causes porosity in die-cast parts. Factors affecting it include the cooling time, the alloy used, and cooling temperature. We alter these factors to get the right conditions to lessen the chances of forming pores. For instance, we add silicon to aluminum to lower the shrink rate significantly by changing its metal structure. However, too much silicon leads to poor mechanical performance.
At Creatingway, we have a good understanding of pressure die casting works. We are thus able to lessen the possibility of defect occurrence on every part made. Contact Creatingway now for pressure die casting needs.
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