When casting metals such as non-ferrous metals like zinc and aluminum, it is not uncommon for defects to occur, especially if the methods used by manufacturers are incorrect. One of the most common defects that can occur is shrinkage defects.
Here at SEI Castings, we will be sharing a blog post discussing how to prevent shrinkage defects on die cast products. If you want to know more information, we encourage you to keep reading below to find out.
What is solidification shrinkage?
Metal can shrink when it transforms from liquid to solid state and it may experience additional thermal contraction as its temperature cools down to room temperature.
Companies that manufacture casting parts often design their products with shrinkage allowances so that they can achieve the desired dimensions accordingly. For example, die cast aluminum shrinks by over 6% during solidification and may produce rough-looking castings.
It is important to have shrinkage allowances and an experienced mold designer must take the shrinkage into account when creating a mold for the die cast aluminum.
When shrinkage occurs, it can also cause defects in cast products that can result in leakage, failure, and even cause stress on the product.
Most of the time, shrinkage defects may appear on the cast products’ surface which can be easily detected by visual inspection or via dye penetrant.
Other shrinkage defects are found inside the casting and may require X-ray inspection or destructive test to locate them. These types of defects are known as open and closed shrinkage defects.
What are open shrinkage defects?
Open shrinkage defects can occur on the surface of a cast product either as a dip or also known as a caved surface or a hole or also known as a pipe. When the molten metal cools and shrinks unevenly, it can draw air inside the mold, therefore, causing open shrinkage defects on the cast product.
An open shrinkage defect can occur when the cast product cools and shrinks with not enough molten metal available to fill up any void, pipes can form on the surface of the cast product and it can even extend into the other areas of the material.
When a cast product has shrinkage defects that occur on the surface and spreads across the face, they are also known as cave defects or sinks. The shrinkage defects that are open to air can allow air to take the place of the molten metal.
Cracks and hot tears are common to occur during the final stage of the cast products’ solidification and are usually found in areas where stress concentration occurs like the thin web connecting two heavy parts.
They can also occur when there is not enough draft in the part and in areas where heat pools especially in the heavy sections of a cast product.
What are closed shrinkage defects?
Closed shrinkage defects occur in the form of holes in the inner part of a cast product. It normally happens in the area where the molten metal has a hotter temperature than the rest of the material. It can either appear in macro or micro form.
Micro shrinkage, also known as shrinkage porosity may appear as jagged lines or marks. While the holes on the cast product look angular and it is only visible using a microscope.
Closed shrinkage defects can occur when porosity becomes a problem in castings. It occurs when gases are trapped inside the molten metal and the cast product shrinks during its cooling phase.
Shrinkage porosity is one of the most common types of shrinkage defects that can occur on the surface of a cast part which may appear like small cracks or holes. The holes may appear circular but they are actually angular in shape and may even form branching internal fractures.
Cast products that are thick and with multi-angles are the most prone to have closed shrinkage defects which can occur during the cooling phase and may result in a solidified non-uniform pattern.
Porosity can also occur in the inner part of the casting and may not be obvious on the surface or exterior part of the cast product. This can occur when the molten metal is surrounded by solid metal and molten metal cannot fill in behind the liquid when it cools down and shrinks.
One of the most common causes of shrinkage defect is related to the casting sprue. The casting sprue is the passage where the liquefied metal is poured into the mold or die.
In some cases, the heavy parts of the mold are where the molten metal may take a long time to cool down and solidify so this can reduce feed material availability and increase the possibility of the shrinkage defect occurring.
For example, the casting sprue used for the casting process is too small and narrow for the molten metal to flow properly.
A casting sprue with an appropriate size must be attached properly to the heavy section so it can fill the shrinkage cavity immediately and prevent any shrinkage defect to occur during the cooling phase.
Furthermore, you can use a rounded gate on the casting sprue so it can prevent the risk of forming shrinkage defects in the cast product.
When a tapered or narrow sprue is used for the casting process it can result in the liquefied metal being sprayed all over instead of being poured slowly into the mold or die.
When this occurs, certain areas of the cast product will solidify earlier while the molten metal is filling the entire mold. The flow of the molten metal must be uniform as much as possible and using a bigger central sprue can help achieve an even flow of the molten metal into the mold or die.
Shrinkage defects can be prevented by applying local heat dissipation like chills or using a metal that will be inserted in the mold or die that will melt simultaneously during the pour of the molten metal. This is done for areas where the temperature is hotter than the rest of the material such as in heavy sections of a cast product.
This defect can occur when the cast product solidifies or after it solidifies thus causing it to change its dimensions and shape. This can cause stress on the product and even cause it to curve, which normally occurs in castings with large and flat sections.
Other shrinkage defects that can occur are the following:
Sponge shrinkage – This defect can occur in a casting product that has a thick mid-section causing a thin lattice texture that causes a filament to develop.
Filamentary shrinkage – This defect can result in continuous cracks of different densities and dimensions and it is often seen below a thick section of the casting product. It is often hard to detect and the fracture lines are usually interconnected with each other.
Dendritic shrinkage fracture – This defect is narrow and often produces randomly distributed cavities or lines that are connected. They appear thinner and less dense than the filamentary shrinkage defect.
What causes shrinkage defects?
Shrinkage defects can occur when the molten metal in the mold or die does not have the same temperature. Two possible causes for this are the molten metal poured into the die has a very high temperature or when the molten metal hardens unevenly.
Uneven solidification occurs when the manufacturer uses an incorrect mold design. They should follow the rule of directional solidification.
This rule means the thinnest part of the cast product must solidify first while the thickest part will solidify last to make sure that it will not have a shrinkage defect, preventing the alteration or damage to the resulting product.
How to prevent shrinkage defects on die cast products
When a problem occurs with the continuous flow of molten metal at the correct temperature it can result in the uneven solidification of the product. This can be corrected by making sure that a continuous supply of molten metal will be filled into the mold.
The manufacturer must use a runner and gate system with risers to provide the molten metal which involves channels for the molten metal to flow continuously through the mold while the reservoirs of molten metal on the top of the mold will fill in where the metal can shrink.
Risers are used during the casting process to make sure enough molten metal will fill in the mold, especially the part where it is solidifying and shrinking. It should be designed to have a size that allows them to solidify last. Insulation can also be added to make sure it happens accordingly.
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If you are searching for a company that offers standard and custom aluminum and zinc die cast products, SEI Castings is worth checking out. We are offering die cast products using nonferrous metals such as Zinc and Aluminum.
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