What Is Shot Size in Injection Molding — and Why Does the Window Around It Matter So Much?

Last updated: 2026-09-01 · Reading time: about 4 min

Shot size is the amount of melt delivered each cycle — part plus runner — and the rule that governs it is a window, not a number: the shot should sit inside roughly 20–80% of the machine’s injection capacity, with the comfortable middle around half. Too small a shot leaves resin cooking in the barrel; too large a starves the melt of the plasticating it needs. Both failures are predictable, and both are decided when the mold and the press are matched.

Key facts

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  • Shot size counts everything the machine injects: the cavities’ parts plus the full runner weight — hot tools inject only the parts, which shrinks the shot and raises the residence question.
  • Residence time is the hidden cost of a small shot: resin sitting through ten, twenty, thirty cycles of heat history degrades — drying-sensitive and splay-prone materials feel it first.
  • Machine capacities are quoted in a reference resin, not yours: the comparison always runs through your material’s density and your shot’s real weight.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

    What does shot size actually describe?

    Shot size
    The volume or weight of melt injected per cycle — set by the screw’s recovery position — usually discussed as a percentage of the injection unit’s rated capacity.

    The confusion starts with naming: machine datasheets quote a shot capacity (in grams or ounces of a standard test resin), the process sheet sets a shot size (the actual metered stroke), and the part pays for a shot weight (part plus runner, every cycle). The three meet in one check: is the actual shot a healthy fraction of the barrel’s capacity?
    A shot far below the capacity does not hurt by being small — it hurts because the resin it displaces stays behind, cycling through heat, shear and time until the embrittlement shows up in the parts.
    A shot at or beyond the capacity fails differently: the screw cannot recover a full charge in the cycle time, and injection pressure runs out of melt to push — a machine matched to the wrong job, discovered at the first full shot.

    What happens across the window?

    Shot vs capacity What the machine does What the parts do
    Under ~20% Barrel mostly idle, resin residence stretches over many cycles. Degradation, splay, black specks on sensitive grades; process drifts with barrel history.
    20–40% Workable with attention: purging discipline and barrel temperatures matched to the short stay. Fine for stable resins; watch sensitive ones closely.
    30–60% The comfortable middle — full recovery, stable melt, honest residence. The window every quotation should aim for when the press can be chosen freely.
    60–80% Working hard: recovery time eats the cycle margin. Capable, with the cycle watched and the plasticating checked.
    Over ~80% Recovery cannot keep up; shot weight drifts shot to shot. Short shots, weight variation, pressure starvation at the end of fill.

    The window is why mold and press are matched as one decision, together with the clamp arithmetic: a tool that flashes on the right tonnage but rides a 10% shot on that press is still the wrong pairing.
    Hot-runner tools deserve the explicit note — removing the runner shrinks the shot, which is good for the system but pushes the residence question harder, and small shots on large barrels are exactly how sensitive resins get cooked quietly.

    What does the design review check?

  • Shot weight stated — parts plus runner, in the quotation line, so the press match can be audited per the itemized assumptions.
  • Percentage inside the window — the shot-to-capacity ratio written down, not left to the press-floor guess.
  • Residence risk priced for the resin — drying-sensitive and regrind-bearing materials get the purging and barrel discipline named in the process sheet.
  • Recovery time against the cycle — the screw has to recharge within the cycle the price assumes, checked at the trial, not discovered in production.

    • Frequently asked questions

    Why do our parts splay on a press that seems too big for the job?

    Because the shot rides far below the barrel’s capacity, and the resin sits through dozens of heat cycles before it is injected.
    The fix is a press matched to the shot — or purging discipline and barrel temperatures set for a long residence, not a brochure setpoint.

    Is shot capacity the same as shot size?

    No — capacity is the machine’s maximum in a reference resin; size is your actual metered shot in your resin. The comparison between them is the whole check, and it runs through your material’s density, not the datasheet’s test resin.

    Does a hot runner change the shot calculation?

    It removes the runner from the shot weight, so the same part count needs less melt per cycle — but the window still applies, and the smaller shot makes the residence check more important, not less.

    Key takeaways

    1. Shot size is the metered melt per cycle; the rule is the window — roughly 20–80% of capacity, best near the middle.
    2. Small shots cost residence time; large shots cost recovery and pressure — the failures sit at both ends of the window.
    3. Mold and press match as one decision: shot weight, window ratio and clamp are quoted and checked together.

    If a part is riding a barrel far larger than its shot, or a quote names a press nobody sized for the shot, send the part and the resin. You will get the shot weight computed, the window checked and the press match stated — from a named tooling engineer, before the machine is booked.

    Sources
    Plastics Industry Association — industry resources on machine sizing and molding practice
    Autodesk Moldflow — injection molding simulation reference
    MatWeb — material property database, density and processing data by grade

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet