When talking about a casting plant’s capability, the familiar question is “how many machines, and how many tons each?” Clamping force is the easiest spec to put in a capability brochure, but it’s only part of the story. Two machines with the same clamping force can still deliver very different quality, because the quality of a casting depends on how the machine controls the delivery of metal into the mold, not just on how large the machine is.
This article starts from the classification of die-casting machines into hot-chamber and cold-chamber, explains why aluminum almost always uses cold-chamber machines, then walks through the machine’s main units and the practical criteria for matching a machine to a product, while suggesting how to read a plant’s capability from its machine setup.
Two machine families: hot chamber and cold chamber
Die-casting machines are divided into two families according to how molten metal is delivered into the injection system.
Hot-chamber machines. The injection components, a plunger and a metal-feed tube, are submerged directly in a bath of molten metal attached to the machine. When the plunger moves down, metal is pushed through the feed tube into the mold; when the plunger moves up, metal from the bath refills the injection chamber. Because the metal is always available inside the system, cycle times can be very fast.
Cold-chamber machines. The bath of molten metal sits separately from the machine. In each cycle, a quantity of metal is ladled or poured from the furnace into a shot sleeve, and a plunger then pushes the metal from the shot sleeve into the mold. The injection components contact molten metal only for the short portion of each cycle, rather than being submerged continuously.
The table below summarizes the differences at a general level, following common industry practice.
| Criteria | Hot-chamber machine | Cold-chamber machine |
|---|---|---|
| Metals typically used | Low-melting-point alloys that don’t attack the injection components much (for example zinc) | Aluminum and alloys with higher melting points or that tend to attack injection components |
| Where the molten metal sits | Metal bath attached to the machine, injection components submerged in the bath | Separate holding furnace; metal is delivered into the shot sleeve each cycle |
| Cycle | Typically faster because metal is already in the system | Typically longer because of the step of delivering metal into the shot sleeve |
| Injection components’ contact with molten metal | Continuous | Short within each cycle |
| Suitable for aluminum | Typically not | The common choice |
Why aluminum almost always uses cold chamber
Molten aluminum is strongly corrosive to iron and steel. If a machine’s steel injection components were submerged continuously in molten aluminum, as in a hot-chamber setup, they would gradually corrode and dissolve into the aluminum, damaging the injection components and contaminating the alloy with iron. The cold-chamber machine solves this by letting molten metal contact the shot sleeve and plunger only for the short portion of each cycle, and by keeping the holding furnace separate from the machine.
This is why, when an aluminum die-casting plant talks about “the casting machine,” it means a cold-chamber machine by default. The cost of this design is an extra step in each cycle of delivering metal into the shot sleeve, and the metal begins to cool slightly as soon as it sits in the sleeve, so how this step is controlled significantly affects quality, as later sections discuss.
Anatomy of a cold-chamber machine: three main units
The clamping unit
The clamping unit opens, closes, and holds the mold tightly while metal is injected. The very high injection pressure tends to push the two mold halves apart, so the clamping unit must generate enough holding force to keep the parting line sealed. This clamping force is the “tons” figure usually quoted. The clamping unit also determines the mold space: platen size, the distance between platens when open, and the opening stroke — that is, which molds fit on which machine.
An important but rarely mentioned parameter is the uniformity and rigidity of the clamping force. If the clamping unit doesn’t hold evenly across the entire parting line, metal can escape and form flash even when the nominal clamping force is sufficient.
The injection unit
The injection unit consists of the shot sleeve, the plunger, and the hydraulic drive system that moves the plunger. This is the unit that determines fill quality. The injection process is typically divided into stages: in the first stage the plunger moves slowly to push metal along the shot sleeve while limiting air entrainment; in the next stage it accelerates to fill the cavity in a very short time; and in the final stage pressure is increased to compensate for shrinkage as the metal solidifies.
What’s worth evaluating in the injection unit isn’t just “strong or weak” but how precisely and repeatably those stages can be controlled from one cycle to the next. An injection unit with a stable response produces more consistent parts than one that is powerful but fluctuates.
The control system and auxiliary equipment
The control system records and adjusts the injection parameters. Around the machine are auxiliary devices that directly affect results: the holding furnace and the device that ladles or pours metal into the shot sleeve (manual or automatic), mold temperature controllers, release-agent spray equipment, part-extraction equipment, and for some products, a vacuum system in the mold cavity to reduce gas.
The level of automation in these steps affects consistency, especially for delivering metal into the shot sleeve: manual ladling easily varies the amount of metal and the waiting time, whereas an automatic device makes each cycle more alike.
Criteria for matching a machine to a product
For a specific product, the question is “which machine fits best,” not “which machine is the largest.” Some main criteria:
- Clamping force versus product area and injection pressure. Clamping force must be enough to keep the mold sealed under the pressure acting on the entire projected area of the part and gating system. A machine too small causes flash, but a machine far too large for the mold isn’t beneficial either (wasted energy, less stable in the machine’s low working range).
- Shot capacity versus the amount of metal needed. The shot sleeve should be filled to an appropriate level; too little metal in the sleeve increases the surface exposed to air and makes the metal cool early.
- Mold space. The mold must fit and open far enough to remove the part.
- Injection control capability. The number of programmable stages, precision and repeatability, and the ability to log data for every cycle.
- Accompanying equipment the product needs. For example vacuum assist for parts needing tightness, temperature controllers for complex molds, automated ladling for products needing high consistency.
It’s also worth remembering that a pressure die-casting machine isn’t the only choice for every product. For small volumes or other requirements, technologies such as gravity casting may be more reasonable.
Reading a plant’s capability through its casting machines
When evaluating a plant, the machine list is only the starting point. The following questions usually reveal more than counting machines and tons:
- Does the plant have a wide enough range of machines to match the right machine to each product? A plant with only a few machine sizes is forced to fit products onto non-optimal machines. What to look at is how they choose a machine for a product (is there logic around clamping force, shot capacity, and mold space, or do they just use whichever machine is free).
- How is the injection unit controlled and logged? Are parameters recorded for every cycle for traceability and comparison between batches, or set once and left to run?
- How is the delivery of metal into the shot sleeve controlled? Manual or automatic, and how is a stable amount and temperature of metal ensured?
- How are the machines maintained and calibrated? An older machine that is well calibrated and maintained can deliver more stable results than a new one that is neglected.
- Does the plant have a backup plan when a machine goes down? A machine that can substitute for your product, or a mold that can run on two different machines while still holding quality.
Discuss matching your product with the right casting machine
If you’re preparing an aluminum pressure die-cast product and want to discuss the right range of machines, have your drawing, expected volume, and any special requirements for tightness or consistency ready. Daiwa’s technical team is ready to discuss based on this information.