Ask three die casting suppliers to quote the same aluminum part and you may get three different price-per-part numbers that vary by 40%. The difference is rarely the alloy. It is usually how well the shot parameters are tuned on the machine, because the shot profile decides how much air and porosity end up inside your casting — and porosity decides your scrap rate, your leak-test results, and whether the part can even be machined where you need it.
The cold chamber process has three distinct phases, and each one has a job. When a supplier gets one of them wrong, the defect signature is predictable. Knowing the phases means you can describe your problem to a caster in their own language — and ask better questions before you place a fast-track order.
The Three Phases of a Cold Chamber Shot
Phase one is the slow shot. The plunger moves the molten metal through the shot sleeve toward the gate at a deliberately low speed, typically well under 1 m/s. This phase has one purpose: fill the shot sleeve and let the trapped air ahead of the plunger escape through the pour hole. If the slow shot is too fast, the metal ahead of the plunger curls and folds over itself, trapping air in waves inside the sleeve. That air has nowhere to go except into the cavity, where it becomes distributed porosity that no overflow or venting will fix.
Engineers who run well-tuned cells keep the shot sleeve fill between roughly 30% and 60% by volume. A sleeve that is too full leaves no room for the air to be pushed out; a sleeve that is nearly empty lets the metal splash and freeze prematurely. If your supplier complains that parts leak "sometimes," ask what their sleeve fill percentage is and whether the slow-shot velocity is tuned per shot weight. It is a fair question, and a good caster will have an answer.
Phase two is the fast shot, triggered once the metal reaches the gate area. Now the plunger accelerates hard, driving the metal through the gate at velocities that typically fall between 30 and 50 m/s. At that speed the metal atomizes and fills the cavity as a spray rather than a coherent front, which is why the fill takes only tens of milliseconds on most parts. The gate velocity is what the famous pQ² diagram governs: there is a working window of machine pressure and gate area where the metal fills fast enough to avoid cold shuts but not so violently that it erodes the die and drags lubricant into the casting.
Phase three is the intensification shot. After the cavity fills, the hydraulic system slams a much higher pressure — commonly in the 400 to 800 bar range on casting surface, depending on part size — onto the still-molten core of the casting. This squeeze compensates shrinkage as the thick sections solidify. Without adequate intensification, your heavy boss areas and thick junctions shrink from the inside out: that is classic shrinkage porosity, the kind that shows up as sink marks on the surface or as leak paths in pressure-tight housings.
Matching Defects to the Phase That Caused Them
- Random, distributed porosity throughout the section — usually phase one. Air folded into the metal in the shot sleeve. Check slow-shot speed and sleeve fill; venting will only partially mask it.
- Cold shuts, flow lines, or unfilled thin features — usually phase two. Gate velocity too low for the flow length, or the metal lost too much temperature in the sleeve and runners. Longer flow paths need higher gate speed or hotter metal.
- Shrinkage porosity under thick bosses, sink marks, or leak-test failures at machined bosses — usually phase three. Intensification pressure arrived late, was too low, or the gate froze before pressure could transfer.
- Flash around parting lines and into slides — the opposite of phase three problems. Excess intensification pressure, or a die and tie-bar system that is not clamped rigidly enough to hold it. More flash also means more trimming cost on every part.
This mapping is a simplification — real parts can have overlapping causes — but it is the first diagnostic pass any experienced casting engineer runs. For a broader look at how metal actually fills the cavity, our guide on die casting fill defects and mold flow basics covers the gate-side view of the same physics.
Why You Cannot Just "Crank Up" the Pressure
A common request from buyers facing leak failures is: raise the intensification pressure. Sometimes that works. Often it makes things worse. Higher cavity pressure multiplies the force trying to open the die, which increases flash, wears the parting line faster, and can actually drive more metal into the vent channels so they stop venting at all. The clamping force requirement grows with projected area, so a part that was comfortable on a 350-ton machine may need 500 tons — and the piece price reflects that.
The productive path is usually geometry, not brute force. Keep wall sections as uniform as practical — 2 to 4 mm is a healthy band for aluminum alloys like A380 and ADC12. Avoid isolated hot spots by coring or repositioning bosses. Provide clean escape paths: generous overflow wells and vents at the last places to fill. If you need a leak-tight zone around a machined port, design it so the caster can put feed metal and an overflow right there. Our die casting service page lists the DFM review we run on every new casting, and the two die casting design rules articles cover draft, wall, and fillet geometry in detail.
What This Means for Prototype and Low-Volume Buyers
Here is the practical catch: shot tuning is a per-part effort. Every new casting starts with a trial sequence — the caster guesses a profile, shoots parts, sections or X-rays them, and adjusts. That tuning effort is a real cost component in your first article, and it is one reason a die casting program rarely makes sense below a few hundred parts.
If your leak-test or machining problems only appear at specific locations, ask the supplier to section or X-ray those exact regions on the next lot and tell you whether the porosity is gas-type (round, evenly scattered) or shrinkage-type (ragged, clustered near hot spots). Gas porosity points to phase one and two problems; shrinkage points to phase three and thermal design. This single distinction will save you a week of back-and-forth.
For truly pressure-tight requirements on low volumes, it is also worth comparing against an alternative route entirely. Machined CNC metal parts from wrought stock carry no casting porosity at all, and where quantities are under a few hundred pieces per year, the absence of leak-test fallout often closes the cost gap on its own. Cast when the volume and the geometry reward it; machine when leak-tightness and speed matter more than unit price.
Understand the three phases and you will never again accept "that is just die casting" as an explanation for porosity. The physics is knowable, the defects map to causes, and a supplier who can walk you through their shot profile is a supplier worth keeping.
