Die casting machines split into two families according to how molten metal reaches the injection system: hot chamber and cold chamber. The difference fits in one sentence — in a hot chamber the injection system sits inside the molten metal, in a cold chamber it sits outside and metal is dosed in for every shot.
It looks like a technical detail, but it governs everything from alloy choice to cycle time, from part design to cost. For anyone sourcing aluminium parts the conclusion is this: in practice, aluminium is cast on a cold chamber machine. Here is why.
How a hot chamber machine works
In a hot chamber machine the melting pot is an integral part of the machine. The injection cylinder and the channel known as the gooseneck sit directly in the molten metal. When the plunger retracts, the channel fills with metal; when it drives forward, that metal goes into the die.
The advantage is obvious: metal is not transferred separately for every shot, there is no ladling step, and the system stays hot continuously. That means very short cycles. Zinc, magnesium and lead based alloys with low melting points are cast on these machines.
How a cold chamber machine works
In a cold chamber machine the melting furnace stands apart from the machine. For every shot, a measured amount of molten metal is transferred into the shot sleeve; the plunger drives it into the die and the shot ends. The sleeve, as the name says, is not a place where metal permanently sits.
That extra step adds a few seconds to the cycle. In return, the injection system is not under constant attack from molten aluminium.
Why does aluminium call for a cold chamber?
The answer lies in two properties of aluminium: its high melting point and its appetite for iron.
Molten aluminium interacts chemically with the steel it touches; it dissolves iron and solders to the surface. In a hot chamber the injection system sits permanently in molten metal, so that interaction never stops — the gooseneck and plunger wear out quickly. Beyond being a maintenance cost, it is also a material quality problem, because the dissolved iron ends up in the casting.
In a cold chamber the metal stays in the shot sleeve for seconds only, and the sleeve empties after every shot. With contact time short, wear stays at a manageable level.
On top of that, aluminium's melting point is above the range in which hot chamber components can run continuously. Both reasons point the same way: high pressure aluminium die casting is done on cold chamber machines.
Side by side
| Hot chamber | Cold chamber | |
|---|---|---|
| Injection system | Inside the molten metal | Separate, dosed every shot |
| Typical alloys | Zinc, magnesium, lead based | Aluminium, copper alloys |
| Cycle time | Shorter | Longer by the dosing step |
| System wear | Severe at high melting points | Manageable |
| Pressure level | Lower | Higher |
| Typical part | Small, thin, very high volume | Structural, thin walled, broad range |
How the choice lands on part design
Because the work runs on a cold chamber, three headings come to the front in design:
- The filling window is short. Metal starts cooling the moment it enters the sleeve; filling has to finish within milliseconds. That makes gate section and position the most critical design decision on thin walled parts. Filling simulation exists precisely to see that window.
- Venting is critical. High pressure and rapid filling compress the air in the cavity. If vents and overflows are not placed correctly, trapped air turns into gas porosity. See our porosity article for the detail.
- Wall thickness balance sets the cycle. The thickest section solidifies last and holds up the whole cycle. Even walls mean both faster production and fewer internal voids.
The shot sleeve: the critical component of a cold chamber
In a cold chamber the metal passes through a horizontal sleeve before it reaches the die, and that short journey decides much of the quality. Two headings stand out.
Fill ratio. The metal dosed into the sleeve does not fill its volume completely — an air space remains above it. As the plunger moves forward, the metal forms a wave in that space. If the wave breaks uncontrolled, it folds air into the metal, and that air is carried straight into the part as gas porosity. The first phase plunger speed is therefore set to advance the wave without breaking it: too slow and the metal cools, too fast and air is entrained.
Thermal balance. The sleeve fills and empties with molten metal on every shot, living through a constant heating and cooling cycle. A sleeve that runs too cold forms prematurely solidified particles at the metal front, which show up in the casting as cold shut marks. A sleeve that runs too hot accelerates wear.
These two headings explain why the dosing stage, which looks like a mere "extra step" of the cold chamber, is in fact a quality decision.
A frequent side question: low pressure and gravity casting
High pressure is not the only route. Low pressure die casting and gravity casting into a permanent mould fill the cavity far more slowly and calmly, which means less trapped air and a structure that can be heat treated. In return, cycle time lengthens noticeably and thin walls become harder to reach.
The decision follows what the part demands: high volume and thin walls point to high pressure, while a strength requirement that needs heat treatment points to the other routes.
A common mistake
Fixing the method by looking at a supplier's machine list. The correct order is the reverse: first establish what the part demands — volume, wall thickness, strength, tightness — then choose the method that matches. If you would like to discuss which is right for your part, share your model through the contact page.