What Is Vacuum Die Casting?
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 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 Process
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:
#1. Melt the Metal Alloy.
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:
#2. Transfer the Molten Metal to the Shot Chamber Using a Plunger.
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.
#3. Vacuum Creation.
After the mold is ready, a vacuum is created within the mold cavity using one of the three methods mentioned earlier. This ensures the proper removal of entrapped air
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.
#4. Allow the Metal To Solidify and Cool Down.
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.
#5. Retrieve the Finished Casting by Ejecting the Die.
The die is then opened so the solidified cast metal object can be extracted:
Ejector pins move the ejector die away from the stationary cover die. The finished component can then be removed.
Metals Used in Vacuum Die Casting
It is a special way to make high-quality metal parts with better precision. The charge is sucked up by a tool placed above the molten reservoir, using a vacuum delivered to the cavity. They remove the air and gases from the mold before pouring in the hot metal. This makes the parts better and more precise.
Vacuum die casting is commonly done with aluminum and magnesium. It helps create strong and detailed parts that are used in cars, airplanes, and electronics.
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.
Which Defects can Vacuum Die Casting Reduce?
In die casting, the common defects are:
#1. Porosity.
When small bubbles of air/gas are trapped inside the cavity, it results in porosity. If there is too much porosity within the part, it loses its strength. That is why porosity is one of the worst defects in die casting.
#2. Misrun.
Misrun is the defect that occurs because of the failure of the molten metal to fill up the cavity properly. Therefore, it results in defective casting, which you have to discard and cast again.
Consequently, it results in a waste of production time and effort, which in turn increases the production cost.
#3. Blisters.
Blisters mostly occur during or after heat treating the parts. It occurs due to the entrapped gas or air inside the metal parts after solidification. Blisters can cause uneven surface finish in your parts.
#4. Cold Shuts.
When the molten metal enters the cavity from two different holes, it fails to merge properly sometimes. In this case, a void space occurs. This is cold shuts. Cold shuts make the parts weak and fragile.
Vacuum die casting can greatly reduce or eliminate most of the common die casting defects. Therefore, it can substantially increase the casting quality and reduces unnecessary post-processing operations.
Applications of Vacuum Die Casting
Here’s a glimpse of a wide range of industrial applications, where the polyurethane casting technique is used:
| Industry | Applications |
| Automotive Industry | Prototype parts: dashboards, door handles, Functional components: light covers, air vents |
| Consumer Electronics | Enclosures for devices: smartphones, laptops, Prototypes for wearable tech, Buttons, and switches |
| Medical and Healthcare | Medical device prototypes: enclosures, components, Custom orthopedic devices, Equipment casings |
| Aerospace Industry | Prototype components: interior cabin parts, Air ducts, and vents, Non-structural parts: covers, enclosures |
| Food and Beverage | Packaging prototypes |
Advantages and Disadvantages of Using Vacuum Die Casting
Advantages of Vacuum Die Casting
Vacuum die-casting offers several advantages over traditional methods:
- Reduced porosity in castings due to minimal air entrapment
- Elimination of surface defects like blisters and blowholes caused by air entrapment
- Improved mechanical properties due to reduced porosity
- Ability to cast thin walls without miss-run issues
- Increased production capacity by reducing post-processing operations
- Minimized waste due to fewer defective parts
- Enables heat treatment and welding for certain metal alloys
Disadvantages of Vacuum Die Casting
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.
Cost of Vacuum Die Casting
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.