To calculate gross resin feed for an injection molding order, divide the required good-part quantity by the expected good parts per shot, round up to a whole number of shots, and multiply by the total molded shot weight. Add startup shots, purge, and other separately identified losses afterward.
For a mold producing identical parts at a constant rejection rate:
[
M_{\text{production}}
=
\frac{
\left\lceil \dfrac{Q}{C(1-r)} \right\rceil
(Cm_p+m_r)
}{1000}
]
Here, (Q) is the required good-part quantity, (C) is the number of active cavities, (r) is the part rejection rate expressed as a decimal, (m_p) is the weight of one molded part in grams, and (m_r) is the total sprue and runner weight discharged per shot in grams. The result is in kilograms.
This gives the cumulative material processed through the planned production shots, including material in rejected parts and runners. It is not automatically the quantity of virgin resin to purchase: regrind reuse, usable inventory, and packaging increments require separate accounting.
Establish the part count, weight, and rejection basis
Use inputs that describe the intended mold configuration and the same production conditions.
| Input | What to enter | Check before calculating |
|---|---|---|
| Required good parts, (Q) | Accepted parts needed to complete the order | Exclude parts already accepted and allocated to the order. Include additional accepted samples only if they must also be produced. |
| Active cavities, (C) | Cavities producing the ordered part during the planned run | Do not use the mold’s nameplate cavity count if cavities are blocked off. |
| Part weight, (m_p) | Molded weight of one part, excluding the separately counted runner | Use a representative measured weight or an identified preliminary estimate. |
| Runner weight, (m_r) | Total sprue, runner, and other separately ejected feed-system material per shot | Enter the total for the mold, not a per-cavity value. Match the active-cavity configuration. |
| Part rejection rate, (r) | Rejected parts divided by all parts molded during the production interval | Use a part-count percentage, not a scrap-weight percentage. Keep separately counted startup losses out of this rate. |
Use a consistent mass basis. If a molded tab is removed after molding, either include it in the molded part weight or count it separately as per-shot scrap—not both. A finished, trimmed part weight alone will understate material consumption if the removed material is omitted.
For runner input, count material actually discharged with each shot. Do not count the entire retained melt volume of a hot-runner manifold as recurring runner scrap. Include any cold sprue or runner that the particular system still ejects, and account for manifold filling or cleanout separately when it consumes material attributable to the order.
The equations below assume identical parts from all active cavities. A family mold producing different components requires separate demand and yield calculations for each component; a single combined good-part count can hide a shortage of one component.
Convert accepted demand into whole production shots
First calculate the expected good parts per shot:
[
G_{\text{shot}}=C(1-r)
]
Then calculate the planned production shots:
[
N_{\text{production}}
=
\left\lceil
\frac{Q}{C(1-r)}
\right\rceil
]
The ceiling brackets mean round up to the next whole shot. Keep intermediate values unrounded.
For example, enter a 3% rejection rate as (r=0.03), so the expected yield is (1-r=0.97). Dividing by 0.97 accounts for the additional parts that must be molded to replace rejects. Multiplying demand by 1.03 is not equivalent.
The rejection rate must satisfy:
[
0\leq r<1
]
A 100% rejection rate cannot produce an accepted order. The active-cavity count must be a positive integer; if no parts remain to be produced, production-shot demand is zero.
The rounded shot count is a planning quantity based on the entered average yield. It does not guarantee the required accepted quantity because actual rejects vary. During production, update the remaining requirement from accepted counts and revise the material balance if yield changes.
Multiply by the complete molded shot weight
Calculate the material discharged in one production shot:
[
m_{\text{shot}}=Cm_p+m_r
]
Then calculate production feed:
[
M_{\text{production}}
=
\frac{N_{\text{production}}m_{\text{shot}}}{1000}
]
With weights in grams, division by 1,000 converts the result to kilograms.
If a representative complete shot has been weighed directly—all molded parts plus the sprue and runner—use that measured shot weight instead of rebuilding it from individual weights. Do not add runner weight again.
This shot weight is the mass of molded output per cycle, not the machine’s rated injection capacity or the weight of the melt cushion retained in the barrel. The material calculation therefore does not establish machine suitability.
Keep the runner in the shot weight for every production shot, including shots that produce rejected parts. Removing runner weight from rejected shots would understate the material processed.
Worked example: 10,000 accepted parts from a four-cavity mold
The following are demonstration inputs, not recommended production allowances:
| Parameter | Assumed value |
|---|---|
| Required accepted quantity | 10,000 parts |
| Active cavities | 4 |
| Molded part weight, excluding runner | 18.0 g |
| Total sprue and runner weight per shot | 12.0 g |
| Production part rejection rate | 3.0% |
| Startup shots, excluded from the production rejection rate | 20 shots |
| Additional same-material purge | 2.0 kg |
Expected good parts per production shot:
[
G_{\text{shot}}=4(1-0.03)=3.88
]
Production shots:
[
N_{\text{production}}
=
\left\lceil\frac{10{,}000}{3.88}\right\rceil
=
\left\lceil2577.32\right\rceil
=
2578
]
Complete molded shot weight:
[
m_{\text{shot}}=(4\times18.0)+12.0=84.0\text{ g}
]
Production feed:
[
M_{\text{production}}
=
\frac{2578\times84.0}{1000}
=
216.552\text{ kg}
]
The production plan therefore requires approximately 216.6 kg before separately counted startup and purge losses.
As a count check, 2,578 shots produce 10,312 molded parts. Applying the assumed 97% yield gives an expected accepted quantity of 10,002.64 parts. That fractional result is an average-yield calculation, not a physical final count.
If the 20 startup shots each consume the same 84.0 g as a normal shot:
[
M_{\text{startup}}
=
\frac{20\times84.0}{1000}
=
1.680\text{ kg}
]
Adding the separate purge allowance:
[
M_{\text{gross}}
=
216.552+1.680+2.0
=
220.232\text{ kg}
]
Report the estimated gross feed as approximately 220.2 kg. The extra decimal places above show the arithmetic; they do not imply that actual consumption can be predicted to the gram.
Add startup and other losses without counting them twice
A production rejection rate and a startup allowance describe different losses only when their accounting boundaries are separate.
For example, if the rejection rate comes from stable production after first-piece approval, add material consumed before approval separately. If the historical rate already includes all startup rejects, adding the same startup shots again overstates the requirement.
For separately recorded losses:
[
M_{\text{gross}}
=
M_{\text{production}}
+
M_{\text{startup}}
+
M_{\text{purge}}
+
M_{\text{other}}
]
All terms in this equation are in kilograms.
Use the following accounting rules:
- Startup shots: Multiply by normal shot weight only when startup shots actually consume approximately that amount. For short shots, interrupted fills, or varying setup conditions, use recorded startup mass when available.
- Purge: Count purge discharged outside the mold separately from complete setup shots. If a different purge compound is used, list that compound separately rather than treating it as production resin.
- Interrupted production: Add identifiable restart losses only if the production rejection rate does not already include them.
- Material left in the system: Account for changes in material retained in the dryer, hopper, barrel, and manifold when planning material issued to the job. Retained material is not automatically scrap or consumption.
- Handling losses: Use a separately identified quantity supported by shop records. Do not hide an unexplained allowance inside the part weight or rejection rate.
When a material-issue balance is needed, the relationship is:
[
M_{\text{issued}}+M_{\text{opening system}}
=
M_{\text{discharged}}+M_{\text{closing system}}
]
This assumes no material recovery back into the process within that balance. Opening and closing system material must be measured on the same boundary.
For the related task of separating setup losses from production rejection, see How Setup Shots and Part Rejection Change a Resin Order.
Keep gross feed separate from virgin-resin purchasing
Gross feed counts every pass through the molding process. If runners or rejected parts are ground and fed back into the machine, their mass remains part of cumulative gross feed, even though the same material is being processed again. Subtracting all generated scrap from gross feed would therefore confuse process throughput with new-material demand.
Only credit regrind toward new-material demand when it is approved for the application, usable in the required blend, and available when needed. BASF’s Elastollan processing recommendations illustrate why this is material- and application-dependent: they exclude contaminated or degraded material from reprocessing and state that some part specifications prohibit regrind. Those recommendations are specific to Elastollan, not a universal regrind allowance.
For a purchase calculation, reconcile:
- The gross process requirement and separately identified losses.
- Approved regrind actually available for use, including its timing and blend restrictions.
- Usable virgin resin already allocated to the order.
- Any required ending inventory.
- Supplier package size or minimum order quantity.
When no regrind is reused and opening and closing system inventories are equal, gross feed provides the starting material requirement before subtracting usable stock and rounding the purchase quantity. If the feed contains separately purchased masterbatch or additives, split the blend into its individual material requirements before ordering.
Use the Resin Purchase Quantity page for the related purchasing task. Keep the unrounded engineering estimate in the job record and record packaging rounding separately, so a purchasing increment is not mistaken for predicted molding consumption.