Estimate runner and sprue weight by subtracting the combined weight of all molded parts from the measured weight of one complete molding shot. Both measurements must represent the same production conditions and use the same trimming boundary. If the shot also contains separately removed flash or other non-runner material, subtract that material too; otherwise, the result includes it.
For identical parts from a normally filled mold:
[
m_{rs}=m_s-nm_p
]
where:
- (m_{rs}) = combined runner and sprue mass, in g/shot.
- (m_s) = measured mass of all material ejected from one molding cycle, in g/shot.
- (n) = number of active, filled cavities.
- (m_p) = average molded-part mass after the defined gate trim, in g/part.
Although shop-floor records commonly call these measurements “weights,” the calculation uses mass. Grams are convenient, but any consistent mass unit works. No resin-density value is needed when both shot and part masses are measured directly.
Collect a Complete Molding Shot, Not a Purge
For this calculation, a measured shot means everything ejected from a completed molding cycle: all parts, the cold runner, the sprue, and any detached gates or other molded material included in the collection boundary.
Do not substitute a free-air purge, screw-stroke setting, or the machine’s rated shot capacity. None is a measurement of the complete ejected molding shot used in this subtraction.
The boundary between “part” and “runner” must remain consistent. The sprue feeds the runner network, and the runners distribute material toward the cavity gates, as described in Protolabs’ injection molding terminology guide. Gate material may remain attached to a part after ejection and require trimming, so weighing before or after that trim changes which category receives its mass.
Use the following assignments when collecting and separating a sample:
| Collected item | Treatment in the calculation |
|---|---|
| Molded parts after the specified gate trim | Include in total part mass |
| Ejected cold sprue, runner branches, and cold-slug wells | Include in runner and sprue mass |
| Detached gates or gate tabs | Assign to runner mass or record separately; use the same rule throughout |
| Flash or unrelated molded trim | Weigh separately if the result must represent runner and sprue only |
| Purge, nozzle drool, and startup cleanout | Keep outside the normal molding-shot sample and record separately |
| Inserts or pre-existing substrates | Keep their mass on both sides of the subtraction, or remove it from both sides |
If you subtract the weight of fully finished parts after machining, drilling, or additional trimming, the residual will include material removed by those operations. Use the molded-part weight at the chosen gate-trim boundary instead, unless you separately account for those removals.
Weigh Matched Shots and Parts
-
Tare the collection container and confirm that the scale can resolve the expected runner mass. A scale adequate for a heavy shot may still be inadequate for a small difference between shot and part weights.
-
Collect complete shots after the process has stabilized. Count the actual active cavities, not merely the mold’s nominal cavity count. Keep startup shots, incomplete samples, and short shots separate from the normal production sample.
-
Cool the collected material to a consistent weighing condition. Weigh the complete shot or batch before discarding any detached runner pieces, gates, or trim.
-
Separate the parts using the defined trimming boundary. Weigh all parts from the same collected shots. For a multicavity mold, weighing every part avoids relying on one cavity’s weight as though it represents all cavities.
-
Weigh any separately classified flash or other non-runner material. Subtract it only if it was included in the complete-shot measurement and is not already included in the part measurement.
-
Calculate the residual, then divide by the number of collected shots if you weighed a batch.
For (k) complete shots:
[
\overline{m}_{rs}
=
\frac{M_s-M_p-M_o}{k}
]
where:
- (\overline{m}_{rs}) = average runner and sprue mass, in g/shot.
- (M_s) = total collected shot mass, in g.
- (M_p) = total part mass from those same shots, in g.
- (M_o) = separately measured non-runner material included in (M_s), in g.
- (k) = number of complete shots collected.
Set (M_o=0) only when no separately classified material is present, or when you deliberately want the result to include all non-part molded material.
Batch weighing can make a small residual easier to resolve, but it does not correct missing pieces, inconsistent trimming, or a biased scale. If cycle-to-cycle variation matters, retain individual-shot measurements rather than recording only the batch average.
For a family mold, or cavities with materially different part weights, use the actual total part mass:
[
m_{rs}=m_s-\sum_{i=1}^{n}m_{p,i}-m_o
]
Do not multiply one part’s weight by the cavity count when the cavities produce different parts. Likewise, an active cavity producing a rejected part still contributes part material to the shot; its mass must not disappear into the runner calculation.
Worked Example: A Four-Cavity Cold-Runner Mold
Assume the following demonstration inputs, not recommended production values:
- Ten complete, stable shots are collected.
- Four identical cavities are active on every shot.
- Total collected mass is 1,240.0 g.
- The 40 parts weigh 980.0 g after a consistent gate trim.
- All detached gate trim is assigned to the runner system.
- No separate flash, inserts, or other molded trim is present.
The average complete-shot mass is:
[
\overline{m}_s=\frac{1240.0}{10}=124.0\ \text{g/shot}
]
The average part mass is:
[
\overline{m}_p=\frac{980.0}{40}=24.5\ \text{g/part}
]
The combined part mass per shot is:
[
m_{parts}=4\times24.5=98.0\ \text{g/shot}
]
Therefore:
[
\overline{m}_{rs}=124.0-98.0=26.0\ \text{g/shot}
]
The batch contains 260.0 g of runner and sprue material. Its share of the total shot mass is:
[
f_{rs}
=
\frac{m_{rs}}{m_s}\times100
=
\frac{26.0}{124.0}\times100
\approx21.0%
]
That percentage uses total shot mass as its denominator. A runner-to-part ratio uses a different denominator:
[
r_{rs/p}
=
\frac{m_{rs}}{m_{parts}}\times100
=
\frac{26.0}{98.0}\times100
\approx26.5%
]
Both figures are valid, but they answer different questions. Record the denominator whenever comparing jobs or reporting runner percentages.
The runner allocation per molded part is:
[
\frac{26.0}{4}=6.5\ \text{g/part}
]
This allocation assumes four parts are produced per shot. It is not automatically the runner allocation per accepted part if some parts are rejected.
For a related calculation, use Runner Weight and Shot Waste with the same shot, part, cavity, and trimming basis used in your weighing record.
Check What the Residual Actually Represents
A positive residual is not automatically a pure runner measurement. It represents everything included in the complete-shot weight but excluded from the subtracted part weight.
| Result or observation | Check before using the number |
|---|---|
| Negative runner mass | Confirm units, cavity count, container tare, and whether shot and part weights came from the same samples |
| Unexpectedly high runner mass | Look for uncounted parts, flash, gate tabs, downstream material removal, or a part-weight value that is too low |
| Runner estimate changes when cavity count changes | Reweigh the current mold configuration; do not assume the previous runner network or part weights remain applicable |
| Small residual compared with scale resolution | Weigh a larger matched batch or use a more suitable scale |
| Large shot-to-shot differences | Check sample completeness and actual process variation before reporting a single representative value |
The subtraction is particularly sensitive to the part-weight input in a multicavity mold. With four cavities, a part-weight value that is 0.1 g too high reduces the calculated runner mass by 0.4 g/shot. Weighing all parts from the same sample avoids that particular reliance on a separate nominal part weight.
When the runner and sprue can be collected intact, weigh them directly as a cross-check:
[
M_s \approx M_p+M_{rs,\text{direct}}+M_o
]
A difference larger than the weighing method can reasonably explain calls for checking lost fragments, sample mix-ups, trim assignments, or scale performance. This is a check on the collected sample’s mass balance, not proof of process capability.
Apply the Method Correctly to Hot-Runner and Hybrid Molds
A hot-runner system keeps material molten within its heated distribution system rather than ejecting that entire inventory with each shot. Do not add the mass of resin retained in the manifold to the measured ejected mass for every cycle.
- For a direct-gated hot-runner mold with no ejected cold runner or sprue, the runner-and-sprue residual should be close to zero after separately accounting for gate trim, flash, and measurement uncertainty.
- For a hot runner feeding cold sub-runners, include the ejected cold sub-runners in the residual.
- For a hot sprue feeding a cold runner network, measure the ejected cold network; do not assume the heated sprue is ejected.
- Record startup filling, purging, and color-change losses separately rather than treating them as recurring runner mass.
The related guide, Cold Runner vs. Hot Runner: What Belongs in the Shot-Mass Estimate, addresses this material-accounting distinction.
Use Runner Mass Without Calling It Total Waste
Runner mass measures material leaving the mold outside the defined parts. It does not, by itself, measure material discarded from the production operation.
For a planned number of molding cycles (N_s):
[
M_{rs,\text{run}}
=
\frac{N_s\overline{m}_{rs}}{1000}
]
where (M_{rs,\text{run}}) is runner and sprue material generated in kilograms when (\overline{m}_{rs}) is entered in g/shot.
Keep the following quantities separate in the production record:
- Runner and sprue material generated.
- Runner material recovered for permitted reuse.
- Runner material discarded.
- Rejected-part material.
- Purge, startup, and other handling losses.
Do not add runner mass again if a resin estimate already uses the measured complete-shot mass; that shot measurement already includes it. Equally, do not treat all collected runner material as recoverable resin without checking the applicable material, product, and customer requirements.
For broader material accounting, carry the measured values into Injection Molding Shot Weight and Resin Planning. Retain the sample count, active cavity count, weighing units, and trim definition alongside the result so the estimate can be reused without changing its meaning.