Porosity is one of the more serious and most common die casting defects. In die-cast components, porosity refers to small voids, holes or pockets or air spaces that exist within the metal part. As porosity is a structural defect, the effects can be very dangerous. Porosity can result to structural failure, leakage, cosmetic concerns and even affect part functionality.
So, how then can porosity be controlled? Well, like every other problem, porosity is better prevented than controlled. However, the risk can never be fully mitigated. To reduce porosity in your die-cast parts, you can take certain steps from process control, and others from material preparation. Here’s how to control porosity in die casting.
Causes of Porosity in Die Casting
There are two main sources of porosity in die casting. Porosity usually arises as a result of injecting molten metal into the die. The two most common culprits are shrinkage and impurities. Here’s how they come into play – The first type of porosity, called by solidification shrinkage, is occasioned by the volumetric shrink that happens when the material cools and solidifies from its initial molten state. This shrinkage can cause some pores/voids to open up in the material, breeding opportunities for porosity.
The second type of shrinkage is caused by gas impurities. This can arise due to numerous sources of impurity in the material to be casted. It can also occur when gas becomes trapped within the material on forming, leading to increased material porosity.
Controlling Porosity Through Design
From a design standpoint, product developers and design experts can prevent or mitigate porosity by adhering to all the design for manufacturing (DFM) guidelines for die casting. Some simple tips include:
• Ensure that walls have uniform thickness
• Manage transitions across cross sections by making them gradual rather than steep
• Thinner walls tend to have less porosity but thicker walls are likelier to shrink
• Design die casting operation to house overflows, cooling channels, waterlines, vents and runner gates in crucial locations
Controlling Porosity Through Process Parameters
Process parameters refer to a measurable set of variables that is used to initiate, modify or monitor a process. In the case of die casting, the process commences with certain set of parameters. These parameters are then continuously modified to ensure that the outcome of the casting are products with acceptable levels of quality. The following are some process parameter that can be optimized to reduce the chances of encountering porosity:
The shot speed refers to the velocity at which the molten metal is poured into the mold by the machine. A variation in the shot speed and machine speeds during pouring can affect the pressure of the operation, paving avenue for porosity. To mitigate this, ensure that you put optimal parameters in place to ensure consistent shot speed during your casting operation.
A key part of your process cycle time is the solidification time. This refers to the time taken for your molten metal to solidify and form the physical die-cast shape. Through process control, a number of variables can be controlled to help ensure the best solidification rate for reduced porosity.
Mold designs with sharp corners and edges, poor surface texture, insufficient hardness and steep changes in cross-section areas will likely have an adverse effect of porosity on your final product.
Heat and temperature thermodynamics play a huge role in die casting forming and quality. The temperature at which the molten metal is poured can affect the porosity of your parts. Like pressure, temperature is a die casting parameter that has to be kept optimal, as low temperatures can result in parts with greater porosity.
Keep your alloy clean to reduce the entry of contaminants into the process. By controlling process parameters and environmental conditions, you can help your alloy stay as clean as possible.
Firstpart Die Casting Manufacturing Services
Firstpart can help you optimize your production, design, budget and supply chain to choose what is best for you. Our experience in metal die casting makes us one of the most competent metal casters, using a wide array of methods to ensure high-quality, defect-free parts every time.
Please get in touch with one of our experts and determine the best process for you.
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