Before ordering resin, reconcile three quantities: the total material expected to pass through the molding process, the virgin resin needed after an approved regrind allocation, and the purchase quantity remaining after usable inventory is deducted. Then round the purchase requirement to the supplier’s actual package size.
The handoff should preserve the assumptions behind each quantity. A resin order based on an obsolete part weight, unavailable regrind, or inventory already reserved for another job can be mathematically correct and still leave production short.
Match the Estimate to the Released Job
Use one job revision for the material estimate, production order, and purchasing request. Resolve differences before carrying quantities forward.
| Input | What to confirm before ordering |
|---|---|
| Required good quantity | The number of acceptable parts needed, including any separately authorized samples or retained parts |
| Resin specification | Supplier, exact grade, color, and any customer restrictions on substitutions, additives, or regrind |
| Part weight | Resin-only weight per molded part, excluding metal inserts or other components not supplied through the barrel |
| Active cavity count | Cavities scheduled to produce parts, not simply the mold’s nominal cavity count |
| Sprue and runner weight | Total material discharged outside the parts on each cycle |
| Production yield | Expected acceptable parts divided by total parts molded during production |
| Setup allowance | Shots or measured material needed before the production yield period begins |
| Additional losses | Job-resin purge, shutdown discharge, handling losses, and other quantities not already included |
| Regrind allocation | Approved blend limit, eligible source, usable quantity, and availability during the run |
| Inventory and packaging | Uncommitted usable stock, confirmed receipts, supplier net package weight, and order increments |
Prefer measured part and runner weights from an accepted process for the same configuration. If weights come from CAD or an earlier job, identify that basis and any unresolved differences.
For a hot-runner mold, count only the sprue or runner material actually discharged each cycle. Do not add the manifold’s entire retained melt volume to every shot; account for job-attributable filling or clearing losses separately.
Calculate Gross Feed Without Counting Losses Twice
For a mold producing identical parts, calculate the discharged shot mass:
[
m_s = c,m_p + m_r
]
where:
- (m_s) = total discharged shot mass, g/shot
- (c) = active cavities, parts/shot
- (m_p) = molded resin weight per part, g/part
- (m_r) = total sprue and runner discharge, g/shot
This is the material mass leaving the mold each cycle. It is not the machine’s rated injection capacity.
Calculate the planned production shot count:
[
N_p = \left\lceil \frac{Q}{c,y} \right\rceil
]
where (Q) is the required good-part quantity and (y) is production yield expressed as a decimal. The ceiling symbol means round up to a whole shot. This calculation assumes a reasonably stable average yield across the active cavities; it does not guarantee the final acceptable count.
Then calculate gross feed:
[
M_g = \frac{N_p,m_s}{1000} + M_{\text{setup}} + M_{\text{other}}
]
where:
- (M_g) = planned gross feed, kg
- (M_{\text{setup}}) = setup material outside the production shot count, kg
- (M_{\text{other}}) = additional job-material consumption not already counted, kg
- 1,000 converts grams to kilograms
If setup consists of full shots at the normal shot mass, (M_{\text{setup}} = N_s m_s/1000), where (N_s) is the setup shot count. For short-shot trials, air shots, or extended purging, a separately estimated or measured setup mass is more appropriate.
Keep Yield, Runner Discharge, and Setup Separate
Production yield increases the number of shots needed to obtain the required good quantity. Runner discharge increases the material used on every shot. Setup material occurs outside the production period when setup is modeled separately.
Avoid these overlapping allowances:
- Do not add a reject percentage after dividing by a yield that already includes those rejects.
- Do not add runner scrap again when the full runner mass is already included in (m_s).
- Do not add separate setup shots if the historical consumption factor already includes startup.
- Keep a separate purge compound on its own material line rather than treating it as consumption of the production resin.
A loss allowance and a yield adjustment are not interchangeable. Adding 2% to net material gives a multiplier of 1.02; allowing for a 2% reject rate gives (1/0.98), or approximately 1.0204. Record which method the estimate uses.
If downstream trimming or machining removes material, retain the as-molded part weight in the molding calculation. If downstream rejects must be replaced, include their effect in the required molded quantity or a clearly defined downstream yield—not both.
Credit Regrind Only When It Is Approved and Available
Regrind reduces virgin demand only to the extent that it can actually be fed during the job. An approved maximum blend percentage is not a promise that enough eligible material exists.
Confirm the permitted source, resin grade, color, processing history, condition, and blend range against the released material requirements. Supplier guidance is material-specific: Avient’s GLS TPE processing guidance, for example, distinguishes SEBS and SBS formulations and identifies color and contamination considerations. Those recommendations are not blanket approval for another resin or application.
For each production phase (i):
[
M_{v,i} = M_i – R_i
]
[
0 \leq R_i \leq r_{\max,i}M_i
]
where:
- (M_i) = gross feed for that phase, kg
- (R_i) = approved regrind scheduled to be fed in that phase, kg
- (r_{\max,i}) = approved maximum regrind mass fraction
- (M_{v,i}) = virgin requirement for that phase, kg
The scheduled (R_i) must also be supported by eligible material available when needed. If the process requires a fixed blend rather than a permitted range, a regrind shortage requires an approved blend change or a different material plan—not an automatic reduction in the percentage.
Do not credit all expected runner and reject output as immediate regrind supply. Recovery requires collection, granulation, any necessary conditioning, and release for reuse. Startup also occurs before the job has generated that material. For same-run recycling, check the opening regrind stock and the generation-versus-use balance over time.
Count recovered material once: either as existing eligible inventory or as a planned recovery during the run. Do not subtract it again as a separate scrap credit. Gross feed still includes each pass through the process, even when some material passes through more than once.
For separately metered color concentrate or additives, split the blend into individual material requirements before purchasing. The simple virgin-minus-regrind calculation assumes those are the only two feed components.
Deduct Usable Inventory Before Rounding to Packages
After summing the virgin requirement across the job, calculate the net purchase requirement:
[
M_{\text{buy}} =
\max\left(0,\ M_v + B – I_u – D_u\right)
]
where:
- (M_v) = total planned virgin requirement, kg
- (B) = separately authorized contingency allowance, kg
- (I_u) = usable virgin inventory allocated to this job, kg
- (D_u) = confirmed, unallocated receipts usable by the required date, kg
- (M_{\text{buy}}) = net virgin quantity to purchase, kg
Exclude quarantined material, incompatible grades or colors, stock committed to other jobs, and quantities that cannot be released or prepared in time. Weigh partial packages rather than treating them as full units.
Keep any contingency allowance separate from calculated consumption. State what it covers and whether existing loss allowances already address the same risk.
For equal-size packages:
[
n_{\text{pkg}} =
\left\lceil\frac{M_{\text{buy}}}{P}\right\rceil
]
[
M_{\text{order}} = n_{\text{pkg}}P
]
Here, (P) is the supplier’s net material weight per package in kilograms, not the shipping weight including packaging. Use the quoted weight for bags, gaylords, or other containers; the container name alone does not establish its resin content.
Apply minimum order quantities or pallet-only increments after calculating the net requirement. Record package-rounding surplus separately from production losses.
Worked Example: Reconcile a Cold-Runner Job
The following values are hypothetical planning inputs, not recommended operating allowances.
| Input | Assumed value |
|---|---|
| Required acceptable parts | 50,000 |
| Active cavities | 4 |
| Resin weight per part | 18 g |
| Sprue and runner discharge | 12 g/shot |
| Production yield, excluding setup | 98% |
| Setup allowance | 40 full shots |
| Additional production-resin purge | 6 kg |
| Approved production regrind range | 0–15% by mass |
| Eligible regrind allocated before production | 120 kg |
| Regrind during setup and purge | None |
| Usable virgin inventory | 180 kg |
| Confirmed incoming virgin inventory | None |
| Additional contingency | None |
| Supplier package size | 25 kg |
No same-run recovery is credited. The material has only virgin and regrind components, with no separately purchased additive.
Shot mass:
[
m_s = 4(18) + 12 = 84\text{ g/shot}
]
Production shots:
[
N_p =
\left\lceil\frac{50{,}000}{4(0.98)}\right\rceil
= 12{,}756
]
The resulting material and purchasing quantities are:
| Calculation | Result |
|---|---|
| Production feed: (12{,}756 \times 84/1000) | 1,071.5 kg |
| Setup feed: (40 \times 84/1000) | 3.4 kg |
| Additional purge | 6.0 kg |
| Total gross feed | 1,080.9 kg |
| Maximum production regrind at 15% | 160.7 kg |
| Regrind actually allocated | 120.0 kg |
| Virgin requirement: gross feed minus allocated regrind | 960.9 kg |
| Net purchase requirement after 180 kg inventory | 780.9 kg |
| Whole 25 kg packages required | 32 |
| Virgin resin ordered | 800.0 kg |
| Package-rounding surplus | 19.1 kg |
Calculations retain unrounded values until the package count is determined.
The available regrind supports approximately 11.2% of production feed, within the hypothetical approved range. Applying the full 15% credit without confirming supply would understate virgin demand by about 40.7 kg.
The handoff therefore calls for 120 kg of eligible regrind, 180 kg of allocated virgin inventory, and 800 kg of purchased virgin resin. It does not call for purchasing the full 1,080.9 kg gross feed.
Release the Order With a Reconciliation Record
Keep the following with the material request or linked job record:
- Job number, part and mold revisions, estimate date, and estimate owner.
- Resin grade, color, approved substitutions, and applicable material restrictions.
- Part and runner weights, their measurement or estimation basis, and active cavity count.
- Required good quantity, production yield, setup quantity, and separately listed losses.
- Gross feed, regrind allocation by phase, and resulting virgin requirement.
- Inventory allocations, incoming receipt dates, package size, order increments, and final order mass.
- Any contingency and package-rounding surplus, identified separately.
- The change trigger: who must review the order if quantity, weight, cavity availability, material approval, or regrind supply changes.
At job close, compare actual production shots, part and runner weights, rejects, setup consumption, and purge against their corresponding estimate lines. Compare virgin and regrind requirements with actual withdrawals less usable returns, adjusting for material left in dryers, hoppers, the machine, or work in process.
Keep newly recovered regrind distinct from unrecoverable waste. Material collected for future use is closing inventory, not material that disappeared from the job. Likewise, a package-rounding surplus is not evidence of poor process yield.
Use the variance to update the specific assumption that changed. Higher startup purge should revise the startup allowance; heavier parts should revise shot mass; unavailable regrind should revise the material allocation. Increasing a single blanket scrap factor would hide the cause and make the next purchasing handoff less reliable.