DefinitionShot 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.
Explained
What Shot Size Means 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 to 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.
Key takeaways
- Shot size is the total plastic one cycle needs, including every cavity plus the sprue and runners.
- It is measured by weight or volume and must be matched to the machine's barrel capacity.
- Shot capacity is the maximum a machine can deliver, which is different from the actual shot size used.
- Shot weight is calculated as (part weight × number of cavities) + runner and sprue weight.
- Very small shots can overheat and degrade plastic, while shots near the machine limit leave no process margin.
Example
Related terms
References
- US Patent 3,889,849 — patents.google.com
- US Patent Application 2015/0329720 — patents.google.com
- US Patent 8,273,283 — patents.google.com
- US Patent 9,610,721 — patents.google.com
- MIT OpenCourseWare 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II — ocw.mit.edu