For a cold-runner mold, shot weight includes the molded parts plus the sprue, runners, and any other polymer ejected each cycle. For a direct-gated hot-runner mold, it normally includes the molded parts without adding the melt held inside the manifold and hot-runner nozzles. If a hot runner feeds a cold subrunner, include that ejected subrunner too.
The distinction is discharged material versus retained melt. Cold-runner channels solidify and leave the mold; a hot-runner system maintains its internal melt for continued operation. That difference—not simply whether the mold is labeled “hot runner”—determines what belongs in the recurring shot-mass estimate.
Count the Material That Leaves the Mold Each Cycle
For steady-state thermoplastic injection molding, calculate the discharged shot mass as:
[
m_{\text{shot}}=\sum_{i=1}^{n}m_{\text{part},i}+m_{\text{ejected feed}}
]
where:
- (m_{\text{shot}}) = total polymer discharged per cycle, in g/cycle.
- (m_{\text{part},i}) = polymer mass of the part molded in active cavity (i), in g.
- (n) = number of active cavities.
- (m_{\text{ejected feed}}) = combined mass of the sprue, runners, and other feed-system polymer ejected each cycle, in g/cycle.
For identical parts, the equation becomes:
[
m_{\text{shot}}=n,m_{\text{part}}+m_{\text{ejected feed}}
]
Use the actual operating cavity count. For a family mold, add the individual part masses rather than multiplying one part weight by the total number of cavities.
| Mold configuration | Include in recurring shot mass | Do not add as a recurring runner mass |
|---|---|---|
| Cold runner | All molded parts, the cold sprue, runners, and other ejected feed-system polymer | Melt remaining in the machine barrel or nozzle |
| Direct-gated hot runner | All molded parts and any separately discharged polymer produced during normal cycling | Melt retained in the hot-runner manifold and nozzles |
| Hot runner feeding cold subrunners | All molded parts plus every ejected cold subrunner, sprue segment, or feed-system remnant | Melt retained in the heated portion of the delivery system |
A small cold runner still counts even when most of the melt-delivery system is heated. Conversely, a large hot-runner manifold does not become a large recurring runner allowance merely because it contains a substantial volume of resin.
Count each piece of polymer once. If the measured part weight includes an attached gate vestige, do not add that same material again under runner weight.
Why Hot-Runner Melt Inventory Is Not Added to Every Shot
A hot runner contains a standing melt inventory. During normal cycling, incoming melt flows through the system and replaces material delivered to the cavities; the entire manifold is not emptied and refilled as a separate addition to every shot. Husky’s description of hot-runner operation distinguishes this heated delivery network from cold runners that solidify and are ejected.
For a stable process, with no significant leakage or recurring drool and no net change in retained melt inventory, the mass supplied by the injection unit per cycle approximately equals the mass discharged from the mold.
Retained melt still matters, but it belongs in different calculations:
- Initial charging of an empty system.
- Material remaining in the equipment at the beginning and end of a production run.
- Purging and color or material changes.
- Residence-time assessment.
Do not multiply the hot-runner inventory by the number of production cycles. Also, do not automatically classify all retained melt as a loss: whether it becomes usable production material or discarded material depends on the startup, shutdown, and changeover procedure.
Establish Part and Runner Mass Without a Generic Allowance
For an existing mold, use complete shots collected after the process has stabilized.
- Collect the output from several consecutive cycles.
- Keep all molded parts and all ejected feed-system material.
- Weigh the parts and feed-system material separately.
- Divide each collected mass by the number of cycles.
- Record the active cavities, resin grade, and whether the parts were weighed before or after trimming.
Use a scale with sufficient resolution for the parts being measured. Keep the weighing condition consistent when moisture uptake or other conditioning could affect the comparison.
For insert-molded parts, subtract the mass of metal inserts or other nonpolymer components. Those components contribute to finished-part weight but not to the resin shot mass.
If tooling does not yet exist, estimate solid part and cold-runner masses from their modeled volumes:
[
m=\rho_{\text{solid}}V_{\text{solid}}
]
where:
- (m) = estimated solid polymer mass, in g.
- (\rho_{\text{solid}}) = solid-material density for the selected resin grade, in g/cm³.
- (V_{\text{solid}}) = modeled polymer volume, in cm³.
Use the same method for the actual cold sprue and runner geometry. Label these values as design estimates until measured production weights are available.
A universal runner percentage is not a substitute for runner geometry or measured mass. Runner length, cross-section, sprue dimensions, cavity spacing, and the amount of cold distribution remaining downstream of the hot runner all affect the result.
Worked Comparison: Cold Runner, Hybrid, and Direct Hot Runner
Assume three proposed layouts mold the same four parts using the same resin. Each finished polymer part weighs 18 g. The following feed-system masses are hypothetical design inputs, not typical values:
- Cold-runner layout: 24 g of ejected feed-system material per cycle.
- Hybrid layout: 6 g of ejected cold subrunners per cycle.
- Direct-gated hot-runner layout: no separately ejected feed-system material.
Assume stable operation, no flash, and no recurring drool.
The combined part mass is:
[
4\times18=72\text{ g/cycle}
]
| Quantity | Cold runner | Hot runner with cold subrunners | Direct-gated hot runner |
|---|---|---|---|
| Combined part mass | 72 g | 72 g | 72 g |
| Ejected feed-system mass | 24 g | 6 g | 0 g |
| Total discharged shot mass | 96 g | 78 g | 72 g |
| Ejected feed-system share of total shot | 25.0% | 7.7% | 0% |
For the cold-runner layout:
[
\text{Feed-system share}
=\frac{24}{96}\times100
=25.0%
]
The runner-to-part ratio uses a different denominator:
[
\text{Runner-to-part ratio}
=\frac{24}{72}\times100
=33.3%
]
Label the denominator whenever reporting a “runner percentage.” A runner equal to 33.3% of part mass is not 33.3% of total shot mass.
In this example, the direct-gated layout reduces discharged mass by 24 g per cycle, or 25% relative to the cold-runner shot. That is a mass comparison—not proof of a particular cycle-time improvement, machine fit, or total production-cost saving.
Keep Shot Weight Separate From Machine Shot Settings
The calculated shot mass is a starting input for an injection-unit capacity check. It is not, by itself, a screw-position setting or a complete machine-selection result.
Check the machine’s capacity basis
A published shot capacity in grams may refer to polystyrene rather than the resin being processed. For example, ARBURG’s technical data list calculated stroke volume in cm³, maximum shot weight in g PS, and separate theoretical shot weights for different materials. Comparing an actual-resin shot mass directly with a g PS rating can therefore give a misleading capacity assessment.
Use the machine manufacturer’s material-specific capacity data or its recommended conversion method. If converting required mass into melt volume, use a suitable melt-density basis for the resin and processing condition—not the room-temperature solid density used to estimate a molded part’s mass.
Do not add packing mass twice
A complete, normally packed molded part already contains the resin delivered during both filling and packing. When shot mass comes from finished parts and their runners, do not add another arbitrary packing percentage.
An intentionally short shot or a fill-only sample is different: its measured mass does not represent a complete production shot.
Treat cushion as retained material
The melt cushion remaining ahead of the screw at the end of holding is not ejected with the parts, so it does not belong in discharged shot mass. It still matters when establishing metering position, usable screw travel, and a stable process.
Do not turn cushion into a fixed percentage added to every material-consumption calculation. Establish it through the machine’s setup procedure and the validated process.
Check more than maximum shot capacity
A smaller shot does not automatically make a smaller injection unit suitable. Required injection pressure, flow rate at that pressure, plasticizing output, metering behavior, and residence time still need evaluation.
ARBURG’s equipment data specifically caution that maximum injection pressure and maximum injection flow may not be available simultaneously, depending on the drive configuration. A nominal capacity comparison cannot establish that the unit will fill the mold under the required conditions.
Separate Gross Resin Throughput From Virgin-Material Purchasing
For a planned number of complete production cycles:
[
M_{\text{gross}}=\frac{N,m_{\text{shot}}}{1000}
]
where:
- (M_{\text{gross}}) = cumulative resin mass discharged over those cycles, in kg.
- (N) = number of production cycles.
- (m_{\text{shot}}) = discharged shot mass, in g/cycle.
At 10,000 cycles, the example layouts discharge:
- Cold runner: 960 kg.
- Hot runner with cold subrunners: 780 kg.
- Direct-gated hot runner: 720 kg.
These figures exclude startup, purging, and other events outside the assumed production cycles. If the job is specified as an accepted-part quantity, determine the required cycles using the active cavity count and a stated production-yield assumption before calculating throughput.
Gross throughput is not necessarily the amount of virgin resin to purchase. A cold runner that is reground and fed back into production still passes through the injection unit again, so its full mass remains part of every applicable shot.
For purchasing and job costing, account separately for:
- Approved regrind actually available and reused during the job.
- Runner and reject material that is discarded, sold, or retained for another job.
- Startup shots, purging, sampling, and changeover losses.
- Changes in resin and regrind inventory.
Do not subtract all theoretically recyclable runner material from a purchase estimate. Reuse depends on material requirements, customer approval, contamination control, and the permitted blend; Husky identifies these restrictions as practical considerations when evaluating cold-runner regrind.
Keep the resulting records distinct: shot mass describes what the injection unit must deliver each cycle; gross throughput describes cumulative processing; and the purchasing estimate describes fresh material required after actual reuse, losses, and inventory changes are accounted for.