Cold Chamber Die Casting -Process Guide

Cold chamber die casting is the standard process for aluminum, magnesium, and copper alloys. "Cold" refers to the injection cylinder being external to and separate from the molten metal holding furnace -not to any cooling of the metal.

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Why Aluminum Requires Cold Chamber

Aluminum's casting temperature (620-700°C) is aggressive enough to dissolve iron from steel components in sustained contact with the melt. A hot-chamber injection system submerged in aluminum melt would dissolve within hours. Cold-chamber design solves this: the steel plunger contacts aluminum only during the brief injection stroke, limiting dissolution to an acceptable rate.

Zinc, by contrast, casts at 385-430°C -low enough that the hot-chamber gooseneck (permanently submerged) has a practical service life of years. This fundamental temperature difference is why zinc is hot-chamber and aluminum is cold-chamber.

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Process Sequence (8 Steps)

1. Die close and clamp: Moving platen advances; die halves close under full clamping force
2. Metal ladle: Automated robot ladle (or manual) pours a controlled shot of molten alloy into the shot sleeve
3. Slow shot: Plunger advances slowly (0.1-0.5 m/s) to fill the shot sleeve and push metal to the gate entrance without creating turbulence or splashing
4. Fast shot: Plunger accelerates to final velocity (achieving 20-30 m/s at the gate), filling the die cavity in 50-100 milliseconds
5. Intensification: Secondary hydraulic pressure (up to 175 MPa) is applied as the metal enters final solidification -pushing semi-solid metal into shrinking regions
6. Dwell under pressure: Part solidifies under full intensification pressure (2-10 seconds depending on wall thickness and alloy)
7. Die open and ejection: Moving platen retracts; ejector pins push the solidified casting from the cavity
8. Die spray and reset: Die sprayed with release agent (robot-applied); shot sleeve cleaned; ready for next cycle

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Key Process Parameters

Parameter	Typical Range	Effect
Metal temperature	620-700°C	Fluidity, die thermal load, die life
Slow shot velocity	0.1-0.5 m/s	Shot sleeve fill without air entrainment
Fast shot velocity	2- m/s plunger (20-30 m/s at gate)	Fill speed, turbulence, porosity
Peak injection pressure	35-75 MPa	Cavity fill completeness
Intensification pressure	70-75 MPa	Shrinkage porosity reduction
Die temp (cover half)	180-240°C	Solidification rate, surface quality
Die temp (ejector half)	180-240°C	Distortion prevention at ejection
Cycle time	30-120 seconds	Wall thickness and weight dependent

At KastMfg, all parameters are logged every shot by the machine control system. SPC charts monitor key parameters throughout production runs.

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Machine Tonnage Selection

Required clamping force: F (tonnes) = Projected area (cm²) x Mean cavity pressure (MPa) / 100

Example: 500 cm² projected area, 60 MPa mean cavity pressure →300T minimum. Select next available machine: 400T.

Machine	Max Projected Area (60 MPa)	Max Shot Weight
160T	~270 cm²	2.5 kg
250T	~415 cm²	4 kg
400T	~665 cm²	6 kg
630T	~1,050 cm²	10 kg
800T	~1,330 cm²	14 kg
1,000T	~1,650 cm²	20 kg
1,600T	~2,650 cm²	30 kg

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Cold Chamber vs Hot Chamber

	Cold Chamber	Hot Chamber
Alloys	Aluminum, Magnesium, Copper	Zinc, Tin, Low-melting-point alloys
Why	Metal temp too high for submerged system	Low metal temp, fast cycling practical
Injection pressure	Up to 175 MPa	Up to 35 MPa
Cycle time	30-120 s	15-25 s
Metal oxidation	Moderate (ladle exposure)	Low (enclosed gooseneck)

Alloy choice determines process choice. There is no overlap in commercial production.

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Slow Shot -The Underappreciated Parameter

The slow shot phase is often overlooked in process optimization -operators and engineers focus on fast shot velocity and intensification. But incorrect slow shot is a primary cause of gas porosity.

Too fast slow shot: Metal splashes in the shot sleeve, creating turbulence and entraining air before the fast shot phase even begins. The fast shot then injects this pre-aerated metal into the cavity.

Too slow slow shot: Metal has time to start solidifying in the shot sleeve, creating a slug that the fast shot must push -cold metal in the flow front creates cold shuts.

Correct slow shot: Plunger advances at the rate that keeps the metal flowing steadily without splashing. The optimal velocity varies with shot sleeve diameter and fill ratio and is determined empirically on each program.

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Cold chamber HPDC inquiry: yaoqingpu1983@gmail.com | +86 138 1403 4409