Shot size is the amount of molten plastic an injection molding machine injects in one cycle: enough to fill every cavity plus the sprue and runners. It is measured by volume or weight and must suit the machine's barrel capacity; a shot that uses too little of the barrel leaves plastic in the heat long enough to degrade.
In short: shot size is how much plastic one cycle needs, and matching it to the right machine is part of getting consistent parts.
What Does Shot Size Mean in Injection Molding?
Between shots, the machine's screw rotates and melts plastic pellets, conveying the melt to the front of the barrel. As the melt builds up, the screw is pushed backward until enough has accumulated to fill the mold. That accumulated charge is the shot. The screw then moves forward like a plunger and injects it into the mold under pressure.
The shot includes everything that fills on each cycle, not just the part. In a cold-runner mold the sprue and runners are part of every shot and come out as scrap, so a large runner can add a surprising amount to the shot. A hot-runner mold keeps its runner molten between cycles, so the shot is mostly the parts themselves.
Machine specifications often use "shot size" for something different: the maximum the injection unit can deliver in one stroke. This page calls that shot capacity, to keep the two apart.
Shot Size vs Shot Capacity vs Short Shot
| Term | What it means | Why it matters |
|---|---|---|
| Shot size (shot weight) | The plastic one cycle actually uses: parts + runners | Sets how much melt the machine must prepare each cycle |
| Shot capacity (barrel capacity) | The most the machine's injection unit can deliver | The shot has to sit comfortably inside it |
| Cushion | A small amount of melt left in front of the screw at the end of injection | Lets the machine keep packing pressure on the part |
| Short shot | A part that did not fill completely | Can be caused by too little shot volume or pressure |
How Do You Calculate Shot Size?
Shot weight = (part weight × number of cavities) + runner and sprue weight
Worked example: a four-cavity mold making 12 g (0.42 oz) parts, with a 10 g (0.35 oz) cold runner, needs a shot of 4 × 12 + 10 = 58 g (2.05 oz). Divide weight by the plastic's melt density to get shot volume, which is what the machine actually meters.
Once you know the shot, compare it with the barrel capacity of the press:
- Avoid very small shots on a large barrel. The melt then spends several cycles in the heated barrel, and heat-sensitive plastics can degrade. One molding guide for a rigid vinyl compound recommends using 50–80% of barrel capacity, with 30–90% workable.
- Avoid shots near the machine's limit. There is no margin left for cushion and process adjustments.
- Cut the runner before you upsize the press. A smaller runner, or a hot runner at high volumes, reduces the shot and the scrap together.
Shot size is worked out alongside cavity count and clamping force when a mold is planned. For production volumes, NICE Rapid's high-volume manufacturing runs multi-cavity hardened steel tooling at cycle times of 10–60 seconds per shot.
Frequently Asked Questions
What is shot size in injection molding?
It is the amount of plastic injected into the mold in one cycle, including every cavity and the sprue and runners. It is usually stated in grams or ounces, or as a volume in cubic centimeters or cubic inches.
What is the difference between shot size and shot capacity?
Shot size is what one cycle of your mold actually uses. Shot capacity is the most a machine's injection unit can deliver in one stroke. The shot should use a comfortable share of the capacity, not a tiny fraction and not all of it.
How do you calculate shot size?
Multiply the weight of one part by the number of cavities, then add the weight of the sprue and runners. For example, four 12 g parts plus a 10 g runner make a 58 g shot.
Related Services at NICE Rapid
- Plastic Injection Molding — single- and multi-cavity tooling in ABS, PP, PE, PC, nylon, POM, TPU, TPE and engineering plastics.
- High-Volume Manufacturing — multi-cavity hardened steel tooling with cycle times of 10–60 seconds per shot.
- Request a Quote — share your design files and annual volume; our team confirms tooling lead time and unit cost.
Sources
- US Patent 3,889,849, Background — the screw plasticizes material and "is necessarily pushed backward until enough plasticized material is accumulated ahead of the ram to fill the mold"; the accumulated "shot" is then injected — patents.google.com
- US Patent Application 2015/0329720, Description — "Shot size refers to the amount of polymer it takes to fill the mold for producing a part"; "Higher barrel capacity leads to longer residence times" — patents.google.com
- US Patent 8,273,283, Description — for a rigid vinyl compound, "Shot weight which uses 50%-80% of the barrel capacity is recommended, but a shot weight using 30%-90% of the barrel capacity may be possible"; a low shot-weight percentage "may lead to slight degradation of the material in the barrel" — patents.google.com
- US Patent 9,610,721, Background — cold-runner volume is part of each "shot" and is "discarded as scrap or reground" — patents.google.com
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — short shot cause: "The shot volume or injection pressure is not sufficient" — ocw.mit.edu
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