Shot Weight: The Real Amount of Metal Behind Every Casting

What counts in the shot

Each cycle meters more metal than the part weighs. In a cold chamber die casting machine, the shot consists of four components: the metal that fills the cavity and becomes the part; the overflows (also called overflow wells) that trap cold, oxidized front-end metal and local gas; the runner and gate system that carries metal from the shot sleeve to the cavity; and the biscuit — the disc of metal left in the shot sleeve after the plunger finishes its stroke. Add them together and you have the shot weight.

This total matters for two very different reasons. First, it sets the required machine shot capacity: a die can only run on a machine whose shot sleeve and plunger system can deliver the full shot every cycle. Second, it defines metal consumption per shot, and therefore the raw-material share of the part cost. Overflows and biscuit are recycled in the foundry loop, but they still cost energy and cycle time, so an oversized shot wastes real money on every single part.

A worked example

Numbers make this concrete. Take a compact aluminum ADC12 housing with a finished part weight of 250 g. Its die might be designed with roughly 40 g of overflows, a 60 g runner and gate system, and a 90 g biscuit for the chosen shot sleeve:

In this example the shot is 1.76× the part weight — the buyer pays for 440 g of metal per shot to receive a 250 g casting, with the difference returned to the loop as recycled in-house scrap. The ratio is fixed once the die is cut, which is exactly why shot layout belongs in the DFM review, not after tooling kickoff.

The biscuit: a small disc with big consequences

The biscuit looks like waste, but it is functional. It is the metal reserve that the intensification pressure pushes against during solidification — without it, final pressure cannot transfer into the cavity and porosity control falls apart. Biscuit thickness is therefore held inside a process window, commonly in the range of roughly 15–30 mm depending on sleeve diameter and part requirements.

Both extremes cause trouble. A biscuit that is too thin can solidify before intensification completes, which shows up as shrinkage porosity and unstable quality. A biscuit that is too thick wastes metal and energy every cycle and can mask metering drift. The right answer is a window set during process qualification and then monitored — not a habit inherited from a previous job.

Alloy density changes the machine, not just the mass

Shot weight interacts with alloy choice. The same cavity volume demands very different machine capacity depending on the alloy's density:

This is one reason alloy selection and machine selection are coupled decisions — see our guides on zinc vs aluminum and AZ91D vs ADC12 for the trade-offs.

Metal usage: what you actually pay for

When comparing quotes per part, metal usage per shot and cycle time together explain much of the price difference between similar castings. Metal cost per part is best understood as: shot weight × metal price per gram, minus the value of the recycled biscuit and runner returns at internal recovery rates, divided by cavities per die. Energy per part also tracks shot weight — more metal per cycle means more heat to maintain and more shot sleeve area exposed per cycle.

Our own loop closes most of this material: gate runners, overflows and biscuit are re-melted in-house, and the alloy's characteristics are monitored across the recycling loop — see our article on die casting scrap and the alloy recycling loop. What does not come back is the energy spent melting metal that never reaches the cavity, which is why lean shot design remains a genuine cost lever, not just a tidy number.

Keeping it lean — and watching it drift

Good tooling design sizes overflows to function — enough volume to trap cold metal where defects are expected, not more. Shot sleeve fill percentage is set within the process window rather than by habit; a sleeve filled too little risks air entrapment in the sleeve, while overfilling wastes metal and can destabilize the plunger stroke. Biscuit thickness stays inside its qualified window. Monitoring shot weight drift is also a quality signal: consistent weight means consistent metering, while a slow trend in one direction points at dosing, valve or plunger issues before they show up as porosity — a theme we cover in shot sleeve and plunger tip wear.

For buyers, three practical checkpoints cover most of the risk:

If you are planning a new part, our prototyping and DFM guide shows how we get from drawing to sample in 7 days with these questions settled up front — and our wall thickness guidelines explain how part geometry decisions feed directly into the shot. Production planning follows our standard 30–45 day production window once tooling is approved.