Vacuum Die Casting: Definition, Importance, How It Works, Applications, and Advantages
Vacuum die casting is a method for manufacturing high volumes of identical precision parts, usually with complex geometries. It is very similar to traditional die casting, but with an added vacuum system that removes almost all of the air and other gasses from the die before the molten metal is introduced. This improves product quality overall and, in particular, reduces porosity. Lower porosity leads to more consistent mechanical behavior.
This article will discuss vacuum die casting, its importance, how it works, its applications, and its advantages.
Vacuum die casting is a variation of traditional die casting. Like most such methods, molten metal is injected into a metal die cavity under pressure. However, the key difference with vacuum die casting is that a vacuum system is used to remove all the air and other gasses from the die cavity before the molten metal is injected. For this reason, vacuum die casting is also referred to as gas-free die casting.
Variations of this technique, such as high vacuum die casting (HVDC) or even ultra-high vacuum die casting (UHVDC), are named for the internal vacuum’s intensity. Higher vacuum means less gas in the die, and therefore better quality casting from a material porosity perspective.
Vacuum die casting was developed in Japan in the 1980s as an improvement on traditional die casting. As the benefits became clear, the technique quickly spread around the world. Trapped air in the die commonly causes defects in die cast products, and so gas-free die casting was meant to combat these issues. By removing the air from the die before injecting the molten metal, you can avert the most common die-casting failures.
The gas-free process developed in Japan was expanded upon in Europe. Over the years, the process has been refined and developed further to include variants like ultra-high vacuum die casting (UHVDC). These further limit the porosity of the final product and reduce the risk of defects or uneven strength.
No, vacuum die casting and centrifugal casting are not the same. In vacuum die casting, a vacuum is created within the die to remove all air before the molten metal is injected into the die under pressure. In centrifugal casting, the mold rotates while the molten metal is poured into it, allowing the centrifugal force to distribute metal evenly throughout the mold. Centrifugal casting is used mainly for cylindrical components such as pipes, where it creates a consistent wall thickness and surface finish over the whole area of the pipe. Vacuum casting is used for more complex shapes, and relies on the vacuum in the mold and the pressure of the metal injection to ensure good distribution of metal within the mold.
The main importance of vacuum die casting in manufacturing is that it can produce parts with very low porosity, making them suitable for heat treatment or welding. Die casting is a valuable process for mass-producing thousands of identical parts, even with complex geometries. However, some of the air in the mold dissolves into the molten metal, causing porosity and manufacturing defects. When heated, the dissolved gas escapes and alters the surface quality of the heated area. Vacuum die casting preempts these problems.
Vacuum die casting works by evacuating most of the air and other gasses from the die prior to injection. This means that there is very little gas present in the die when the molten metal is injected. The resulting parts are far less likely to have the porosity defects that plague die-cast items. The vacuum in the mold also helps draw the molten metal into every channel. An evacuated die therefore provides numerous benefits over traditional die casting.
Vacuum die casting stands out because of the exceptionally low porosity of its completed components. This is achieved because the air is removed from the die before molten metal is introduced. There are thus very few air molecules available to create defects. Parts with minimal porosity also exhibit: more consistent mechanical properties, higher dimensional accuracy, and better surface finishes. To learn more, see our guide on Casting Types.
The process of vacuum die casting is similar to traditional die casting and is quite straightforward. Naturally, though, the creation of a vacuum inside the die is a critical step. Let’s look at the process:
The first step in vacuum die casting is to melt the metal alloy. This is the source material that will later be cast. The metal needs to be molten enough to accurately fill the shape and crevices of the die. Figure 1 depicts a melting crucible:
Melting the metal alloy.
Image Credit: Shutterstock.com/Serdar Tibet
2. Transfer the Molten Metal to the Shot Chamber Using a Plunger (Hot Chamber) or a Ladle (Cold Chamber)
The second step in the vacuum die casting process is to transfer the molten metal to the shot chamber. The transfer method depends on the type of die casting machine. For a hot chamber machine, the pot of molten metal forms part of the molding equipment. In that case, a plunger is integrated within the chamber and descends to force molten metal into the shot chamber. A cold chamber die-casting machine, by contrast, does not contain its own inventory of molten metal. The shot chamber is filled manually by transferring molten metal from a pot by use of a ladle, as shown in Figure 2:
Pouring molten metal with a ladle.
Image Credit: Shutterstock.com/mehmetcan
High-capacity vacuum pump.
Image Credit: Shutterstock.com/engineer story
This sucks nearly all the gas molecules from the die cavity to create near-vacuum conditions. The molten metal is then injected into the die from the shot chamber by a ram.
The vacuum created within the die cavity helps draw the molten metal into all of its fine channels. Removing the air allows the liquid metal to be pulled in and distribute itself evenly throughout the die. This is critical for complex geometries such as that shown in Figure 4:
Die casting complex geometries.
Image Credit: Shutterstock.com/Surasak_Photo
A vacuum is particularly important for thin-walled parts.
The molten metal will immediately begin to cool within the die. Dies are manufactured with cooling channels so that coolant can be circulated to control the temperature and remove heat once the metal has been injected. An integrated cooling system in the die allows the metal to cool and solidify in a quick but controlled way. Figure 5 is an example of a die-casting cooling system:
Die casting cooling system.
Image Credit: https://www.thermalcare.com/die-casting/
Ejecting the metal part.
Image Credit: Shutterstock.com/Surasak_Photo
Ejector pins move the ejector die away from the stationary cover die. The finished component can then be removed.
The metals used in vacuum die casting usually have moderate melting temperatures and moderate densities. Most common are:
- Aluminum Alloys: The common aluminum alloys for die casting contain constituents like: silicon, magnesium, zinc, and copper. A380 is the most common aluminum alloy used in die casting.
- Magnesium Alloys: Magnesium alloys such as AZ91D are also popular for use in vacuum die casting. Typically, magnesium alloys are used to cast structural components.
- Zinc alloys: Zinc alloys include the elements of aluminum, magnesium, and copper, termed the Zamak series. Zinc alloys with high aluminum content are part of the ZA series. These alloys have a broad range of properties and fit well into the vacuum die casting process.
No, copper-nickel alloys (also generally referred to as cupronickels) are not used in vacuum die casting. The melting points of most copper alloys, including copper-nickel alloys, are too high for the process to be done economically. Copper-nickel alloys are typically cast with other processes such as: sand casting, investment casting, or centrifugal casting.
Yes. Many alloys are well suited to vacuum die casting. The most common are aluminum, magnesium, and zinc alloys, though lead alloys are not unusual either. However, this doesn’t mean that all alloys can be used in vacuum die casting. Alloys are incredibly varied — far more so than the elemental metals themselves — and so there are numerous alloys that are not suited to vacuum die casting, primarily due to high melting points or high material densities. To learn more, see our guide on Regular Alloys.
Yes, magnesium can be cast using vacuum die casting equipment. In fact, it is one of the more common targets for the process. It is usually used in alloy form with constituents like: aluminum, zinc, manganese, or silica.
The biggest application of vacuum die casting is the mass production of automotive parts. The process is frequently applied to manufacture engine, transmission, and suspension components. It’s most important for these components that need to be either heat-treated or welded after fabrication. Metal cast with the aid of a vacuum is far less porous than other cast metals, so welds and heat treatments are far less likely to cause blistering or other defects. Another application of vacuum die casting is in the aerospace industry, where highly accurate parts with predictable mechanical properties are required.
The automotive industry uses vacuum die casting more than any other in the world. This is for a number of reasons, such as: the high volume of identical parts produced, the high strength-to-weight ratio it lends thin-walled components, and the parts’ dimensional accuracy. The alloys used for automotive manufacture — especially aluminum and magnesium alloys — are well suited to vacuum die casting.
The aerospace industry also uses vacuum die casting products, as even complex shapes can be made this way. Plus, parts can be designed with thin walls that give them good strength-to-weight ratios. Vacuum die casting machines are also used in many industries with large assembly lines that produce thousands of identical components. These include the manufacture of electrical machinery, appliances, and power tools.
Products made via vacuum die casting are typically of very high quality. Die casting in general provides components with very good surface finishes that rarely require post-processing. Vacuum die casting further improves the quality by precluding gas porosity and other defects from the start. This gives them more consistent mechanical properties and also allows thinner walls to be cast reliably. Therefore, vacuum die cast products are highly durable, according to the properties of the materials used.
Yes, vacuum die casting products are as heat resistant as the metal alloys they’re made of. Compatible metals tend to have relatively low melting points compared to some others like steel, but they can still usually withstand temperatures above 600 °F! Furthermore, vacuum die casting products are far more suitable for heat treatment and welding than normal die cast products. This is because the vacuum in the die casting process results in significantly lower porosity and less trapped gas within the finished component. This means that the components can be heat treated or welded without trapped gas forming blisters and other defects.
Vacuum die casting is a very accurate production method. Nearly all gas gets evacuated prior to metal injection, so the molten metal can flow unrestricted into every channel. That lack of gas also staves off potential defects or imperfections caused by dissolved gasses in the solidified metal part. Therefore, the dimensional accuracy of vacuum die cast parts is exceptional.
The lifespan of vacuum die casting products will depend on the source material (alloy) and the demands of its actual application. You can typically expect product lifespans of around 15 years, particularly for key automotive components. Products manufactured by vacuum die casting typically have longer functional lives than those made via traditional die casting.
Vacuum die casting is a valuable process, particularly because of the following advantages:
- Very little gas can get trapped within the solidified component because the air in the die is evacuated before the molten metal is injected. This means that parts that are manufactured with vacuum die casting are far less porous than traditional die cast parts.
- Without the interference of air, the metal exhibits consistent mechanical properties throughout the part rather than unpredictably porous or weak patches.
- Consistent properties and low porosity give designers the opportunity to create thinner walls that could reliably be cast with traditional die casting.
- The much lower porosity of vacuum die cast components means that they can be welded or heat-treated without the risk of interference from dissolved gasses.
No manufacturing process is perfect, and there are some disadvantages to vacuum die casting as listed below:
- Vacuum die casting like ordinary die casting, is not compatible with all metals. Ferrous metals (any alloy with significant iron content) and those with particularly high melting points are not well suited to vacuum die casting.
- The additional equipment in the vacuum system makes it more expensive than a traditional die casting arrangement.
- The vacuum system introduces many new pumps and valves that need to be calibrated and controlled properly. This adds to the complexity of the operation.
The cost of vacuum die casting a component depends on size and material, but it usually falls in the range of $10 for a small part (<1 kg) to $100 for a larger part (~20 kg). Vacuum die casting is one of the more expensive manufacturing methods, but it is well suited to producing very high volumes of identical high-quality parts.
Yes, vacuum die casting is more expensive than sand casting. Sand casting is a simple and accessible casting method but is far less precise. Sand molds can be created fairly easily and cheaply for undemanding geometries. The metal dies required for vacuum die casting, on the other hand, are more expensive to make, both due to the material cost and the expertise required to make them. The overall machinery necessary for vacuum die casting is also significantly more expensive than for sand casting. However, for very large volumes, the cost of vacuum die casting becomes more competitive per unit.
The difference between vacuum die casting and traditional die casting is that the latter simply allows the air to exit the die through vents when the molten metal is injected. The molten metal itself pushes air out of the die. However, if the air doesn’t vent correctly, it can cause problems in the resulting manufactured component such as porosity or incomplete casting. When a vacuum pump is instead used to evacuate air from the die prior to injection, those trapped-air defects cannot occur.
This article presented vacuum die casting, explained it, and discussed its importance and how it works. To learn more about vacuum die casting, contact a Xometry representative.
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