Calculate injection molding shot weight by multiplying the molded resin mass per part by the number of active cavities, then adding the complete cold-runner and sprue mass discharged in one cycle:
[
m_{\text{shot}} = n,m_{\text{part}} + m_{\text{runner}}
]
Use this result as the estimated resin mass delivered into the mold per steady-state cycle—not as a machine shot-capacity rating or a complete resin purchasing quantity. For a fully hot-runner mold with no cold runner or sprue discharged each cycle, the runner term is zero. Material remaining molten inside the hot-runner system is not an additional runner mass consumed every shot.
Define the part and runner inputs without double-counting
Keep all mass inputs in the same unit. Grams are convenient for individual shots; the result is then expressed in g/shot. Although shop-floor terminology uses “weight,” the calculation adds masses.
| Input | Meaning | What to check |
|---|---|---|
| (m_{\text{part}}), g/part | Molded resin mass of one part, excluding the separately counted runner and sprue | Use an actual molded-part measurement when available. Exclude inserts and other components not supplied by the injection unit. |
| (n), parts/shot | Number of cavities actually producing parts in that cycle | Use active cavities, not automatically the mold’s nominal cavity count. Include molded parts that are subsequently rejected. |
| (m_{\text{runner}}), g/shot | Complete cold feed-system mass discharged per cycle | Include the sprue, runner branches, cold slug wells, and any detached gate material not already included in part mass. |
The sprue carries melt from the machine nozzle into the mold, while runners distribute it toward the cavity gates. They are separate features, but both belong in the cold feed-system mass when they solidify and leave the mold each cycle.
Choose a consistent boundary at the gate. If a gate vestige remains on the weighed part, it is already included in part mass. If gate material is trimmed off before the part is weighed, include that removed resin elsewhere in the shot total. Do not count it twice.
For insert molding, subtract the insert mass from the complete molded assembly mass before using it as (m_{\text{part}}). The calculation must represent resin injected during the cycle, not the combined mass of resin and preloaded hardware.
Use a sum when cavities produce different parts
The simple multiplication assumes every active cavity produces the same nominal part. For a family mold, or when using individual cavity measurements, calculate:
[
m_{\text{shot}} = \sum_{i=1}^{n} m_i + m_{\text{runner}}
]
Here, (m_i) is the molded resin mass from active cavity (i), in grams. Add every part produced in the cycle, whether or not it later passes inspection.
If a cavity is blocked, use the remaining active cavities and the runner mass for that configuration. Do not assume runner mass decreases in direct proportion to cavity count: the material discharged depends on which feed passages still fill.
Worked calculation: four parts and one cold-runner system
The following values are illustrative inputs, not recommended part or runner weights.
Assume a four-cavity mold produces identical parts with a measured average mass of 18.6 g each. The complete runner and sprue assembly weighs 12.8 g per shot.
| Quantity | Calculation | Result |
|---|---|---|
| Total molded-part mass | (4 \times 18.6) | 74.4 g/shot |
| Cold-runner and sprue mass | Measured separately | 12.8 g/shot |
| Total shot weight | (74.4 + 12.8) | 87.2 g/shot |
Therefore:
[
m_{\text{shot}} = (4 \times 18.6) + 12.8
= 87.2\ \text{g/shot}
]
The 87.2 g result includes all four parts plus one complete feed system. Do not multiply the 12.8 g runner input by four: it already represents the whole shot.
For the same arithmetic, use the Shot Weight calculator with part mass, active cavity count, and runner/sprue mass per shot. Keep the measured inputs alongside the calculated result so the estimate can be checked against production.
Measure a complete shot to check the estimate
A direct mass check compares the calculated total with everything molded and discharged during the same cycle. Use stable production conditions rather than startup cycles or deliberately incomplete shots.
- Collect all molded parts and the complete cold feed system from a known number of consecutive cycles. Keep purges and unrelated scrap out of the collection.
- Confirm that no parts, detached runners, sprues, or trimmed gate pieces are missing. An automated runner-separation system may send them to a different container.
- Weigh the material using a suitable balance, subtracting the container tare. Use a consistent cooling and conditioning interval.
- Divide the net collected mass by the number of cycles.
[
\overline{m}{\text{shot, measured}}
=
\frac{M{\text{collected}}}{N_{\text{cycles}}}
]
Both (M_{\text{collected}}) and the resulting average use the same mass unit; (N_{\text{cycles}}) is the number of collected molding cycles.
For example, a hypothetical collection of 872.0 g from 10 complete cycles gives:
[
\overline{m}_{\text{shot, measured}}
=
\frac{872.0}{10}
=
87.2\ \text{g/shot}
]
That agrees with the worked calculation. It demonstrates the arithmetic, not a production validation.
For identical parts, calculate average part mass by dividing the combined part mass by the number of parts weighed—not the number of cycles. If cavity-to-cavity differences matter, weigh and record each cavity separately instead of relying only on the overall average.
When calculated and collected totals differ, first check the cavity count, container tare, missing feed-system pieces, and whether the part measurement includes inserts or trimmed material. Confirm that both measurements represent the same process condition and configuration before attributing the difference to molding variation.
If the complete shot and all its parts have been weighed separately, runner mass can also be obtained by subtraction:
[
m_{\text{runner}}
=
m_{\text{complete shot}}
–
\sum_{i=1}^{n}m_i
]
The measurements must cover the same cycle or equivalent averaged samples. The related procedure, Estimate Runner and Sprue Weight from a Measured Molding Shot, addresses this input.
Count only the runner material discharged each cycle
A hot-runner system keeps resin molten in its heated distribution passages rather than ejecting those passages as a solid runner after every cycle.
Apply the runner term according to the material actually discharged:
- Fully hot-runner mold with no discharged cold feed system: set (m_{\text{runner}}=0).
- Hot-runner mold feeding cold subrunners: include the cold subrunners and any other cold feed material discharged each cycle.
- Cold-runner mold: include the complete ejected sprue and runner system once per shot.
Do not add the entire hot-runner manifold inventory to every cycle. Initial filling, color changes, and purging affect job-level material accounting, but they are not recurring cold-runner mass.
Keep shot weight separate from cushion, losses, and machine capacity
The part-plus-runner calculation gives a mass balance for the molded output. It does not need an arbitrary percentage added to make that mass balance correct.
A screw cushion is material remaining ahead of the screw at the end of injection and holding; it is not another part or runner discharged with that shot. Keep cushion and machine-setting allowances separate from the calculated molded-output mass.
Likewise, do not add packing material twice. A weighed, fully molded part already includes the resin retained during filling and packing. A short-shot sample does not represent the final production part mass.
For material planning, multiply average shot mass by the planned number of cycles, then account separately for startup, purging, rejected production, and the actual treatment of runner material. A recoverable runner is still part of each shot’s mass even when it is not ultimately discarded. The parent guide, Injection Molding Shot Weight and Resin Planning, connects the per-shot quantity to job-level planning.
For machine comparison, check the specification basis before comparing numbers. ARBURG’s ALLROUNDER 570 A technical data, for example, lists calculated stroke volume in cm³ separately from maximum shot weight in g PS and provides material-dependent theoretical shot weights. A rating in g PS is not a universal gram capacity for every resin.
No density input is needed when both part mass and runner mass are already known. Converting the result into an injection-volume requirement is a separate step requiring appropriate material data and the machine manufacturer’s calculation basis; room-temperature solid density should not be substituted uncritically for melt density.
Record the result as, for example, “87.2 g/shot, four active cavities, including 12.8 g cold runner and sprue,” together with the resin grade, part revision, and whether the inputs were estimated or weighed. That makes the number usable for comparison without implying that shot mass alone establishes machine suitability.