A quotation can show a healthy margin while the completed order tells a different story. Injection molding costing needs to account for the material actually consumed, the time the job occupied the press, and the number of parts that passed inspection. Finished part weight and an ideal cycle time give you a starting estimate, not the full result.
To calculate the cost per accepted part, add the costs attributable to the completed batch and divide by its accepted output. Apply your approved treatment of recovered material, overhead, and tooling. Keep the scope consistent when comparing that result with the original quotation.
For an ERP implementation, the practical challenge is making those inputs traceable. A useful cost report should let the production head and finance team open a figure and see where it came from.
The basic formula for injection molding costing
Manufacturing cost per accepted part = attributable batch manufacturing cost ÷ accepted finished quantity
The batch total normally includes net material cost, machine usage, production labor, secondary operations, and the applicable production overhead. Add a tooling-recovery allowance separately when the commercial arrangement requires it. Include each cost once.
Define what the report means by “actual.” Resin quantities and work durations can come from recorded transactions, while machine and overhead costs may use approved rates. Until finance reconciles those rates and late costs, the result is a job cost based on actual activity and applied rates.
Also separate cost from selling price. Profit margin, commercial risk, outbound delivery, and sales expenses may belong in the pricing decision, depending on the contract. They should not silently become manufacturing expenses.
Start with the completed job and its good output
Use one manufacturing order, batch, or other clearly defined costing scope. Identify the product revision, resin grade, mold, press, and active cavities. This makes comparisons useful when the same part runs under different conditions.
Collect the following records before finalising the report:
- Material issues, unused returns, and opening or closing material held for the job.
- Accepted output, rejected parts, and quantities awaiting inspection or rework.
- Cycling time, setup time, and relevant stops outside the cycling period.
- Production labor assigned to the job.
- Secondary processing, packaging, and any external service charges.
- Applicable overhead rates and the tooling-recovery agreement.
Use accepted output at the costing stage you have defined. If parts still need an outsourced operation before they become finished goods, keep that status visible. Do not treat unresolved quality holds as confirmed good output.
Calculate material consumption beyond finished part weight
For a closed job with no opening or closing material held at the machine, net consumption equals material issued minus unused material returned. If material remains in a dryer, hopper, or work in progress, reconcile it before calling the cost final. Measure it rather than assigning the entire issue to the order automatically.
Consider a batch receiving 470 kg of resin and returning 20 kg unused. The net consumption is 450 kg. At an approved valuation rate of ₹150 per kg, the job carries ₹67,500 of material cost before any applicable recovery credit.
Finished-part weight provides a useful reconciliation. Suppose 20,000 accepted parts each weigh 20 g, representing 400 kg. Another 1,000 rejected parts account for 20 kg, runners account for 21 kg, and purging consumes 9 kg. Together, those quantities explain the 450 kg used.
The material cost is therefore ₹3.375 per accepted part, rather than the ₹3 obtained from good-part weight alone. The additional consumption has a source that production can investigate.
Treat scrap and recovered material consistently
Once actual consumption includes rejected shots and purging, do not add another blanket scrap percentage to that same cost. A scrap allowance can help estimate a future order. It should not duplicate a loss already captured in the completed job.
Regrind needs a recorded quantity and an approved value. If recoverable material transfers to stock, recognise its value according to your costing policy and link it to the originating job. Charge subsequent consumption consistently, including the relevant processing cost.
Material circulating within the same job should not become a new virgin-resin purchase every time it passes through the grinder. Likewise, do not assume that every runner has full virgin-material value. Its permitted use and condition affect the recovery decision.
Calculate machine cost from the time actually used
For a quotation, a useful steady-production estimate is:
Expected cycling hours = required good parts × cycle time in seconds ÷ (3,600 × active cavities × expected yield)
Express yield as a decimal, such as 0.96 for 96%. Add setup and other justified allowances separately. Avoid including the same allowance in both the formula and an extra efficiency factor.
For a completed job, use recorded time instead. Distinguish cycling from setup and stops, then apply the job-cost policy to those categories. A timer left running after an operator changes jobs can distort the result even when the ERP calculation is correct.
In the example batch, four active cavities produce 21,000 total parts in 5,250 shots. At an average 30-second cycle, those shots occupy 43.75 cycling hours. Two hours of setup and 1.25 hours of stops outside cycling bring the occupied time to 47 hours.
At ₹900 per hour, charging those 47 hours gives a management machine cost of ₹42,300, or ₹2.115 per accepted part. Keep the time categories visible so finance can apply the appropriate inventory-valuation treatment separately.
Do not divide recorded time by an OEE or yield factor again. Its actual delays are already in the measured duration, while rejects are reflected in the accepted-output denominator.
Define the machine rate before adding labor and overhead
Injection molding costing can go wrong when different departments use the same hourly rate label for different things. Document what each press rate contains: equipment ownership, energy, routine maintenance, operator labor, or other costs.
For the worked example, the ₹900 machine rate covers press ownership, energy, and routine machine maintenance. It excludes production labor, mold recovery, and the separate factory overhead shown later. These are illustrative rates, not market benchmarks.
If an operator attends several presses, allocate that person’s time using a documented method. One eight-hour shift cannot become eight labor hours on every machine attended. Record setup and specialist assistance consistently as well.
For overhead, state the allocation basis and the costs it covers. Check for overlap with the press rate and direct labor. Review applied rates against period costs rather than treating them as permanently accurate.
Job economics and financial inventory valuation can differ. For example, IAS 2 Inventories bases fixed production-overhead allocation on normal capacity. Finance should apply the relevant accounting framework and keep exceptional losses separately visible. Dividing all factory costs by an unusually small month’s output can mislead pricing decisions.
Add tooling recovery according to the contract
Separate the mold’s capital cost from routine production costs. If the molder funds a ₹600,000 mold and plans to recover it over 300,000 accepted parts, the commercial allowance is ₹2 per part.
That allowance depends on the agreed recovery quantity, ownership, and payment arrangement. If the customer has already paid for the tool, charging its purchase cost again through the part price may duplicate recovery. Customer-owned tooling can still generate agreed storage, maintenance, or repair costs.
Keep the commercial recovery schedule separate from statutory depreciation. Track how much has been recovered and review volume assumptions when demand changes. Otherwise, a pricing model can continue charging a completed recovery or fail to recover a tool used for fewer parts than expected.
Protolabs’ molding-cost guide explains that multi-cavity tooling can lower piece price while affecting tooling cost. Evaluate that trade-off using realistic accepted output and process time, rather than cavity count alone.
Worked example of actual injection molding costing
The example uses 20,000 accepted parts and 1,000 rejects from one four-cavity job. All parts weigh 20 g; net resin consumption is 450 kg and occupied press time is 47 hours. No material recovery credit applies in this example.
Production labor totals 26 allocated hours at ₹180 per hour, including the job’s setup and production attendance. Secondary operations and packaging cost ₹12,000; this excludes the production labor already recorded. Separate factory overhead is ₹8,000 under the approved allocation policy.
| Cost category | Batch cost | Cost per accepted part |
| Net material | ₹67,500 | ₹3.375 |
| Machine usage | ₹42,300 | ₹2.115 |
| Production labor | ₹4,680 | ₹0.234 |
| Secondary work and packaging | ₹12,000 | ₹0.600 |
| Separate factory overhead | ₹8,000 | ₹0.400 |
| Manufacturing subtotal | ₹134,480 | ₹6.724 |
| Commercial tooling recovery | ₹40,000 | ₹2.000 |
| Total including tooling recovery | ₹174,480 | ₹8.724 |
The manufacturing job cost is ₹6.72 per accepted part after rounding. Including the separate tooling-recovery allowance gives ₹8.72 per part. Label both figures so the sales team knows which one its pricing model needs.
This is a management costing example with actual quantities and applied rates. It is not a claim that every amount belongs in financial inventory value. Final accounting reconciliation may change the treatment or amounts.
Compare the quotation with actual cost on the same basis
Suppose the quote assumed 20 g of resin per good part and a 25-second cycle across four cavities, with no losses or setup allowance. At the same rates, that gives material of ₹3 and machine usage of ₹1.5625 per part.
If quoted labor was ₹0.20, and secondary work, overhead, and tooling allowances matched the example, the quoted cost was ₹7.7625. The completed job’s comparable cost is ₹8.724. The gap is ₹0.9615 per accepted part, or ₹19,230 across 20,000 parts.
The material gap contributes ₹7,500, machine usage ₹11,050, and labor ₹680. Investigate those drivers separately. They point to different actions: revise consumption assumptions, understand cycle and setup losses, or check labor allocation.
Keep the original quote assumptions unchanged for this comparison. If the ERP replaces them with the latest actual values, the team loses the evidence needed to improve the next quotation.
Build the ERP around traceable costing inputs
A Manufacturing ERP should connect the cost to its source records. Material values need links to stock movements and valuation; time costs need links to work records. Accepted output must reconcile with quality results and finished quantities.
During implementation, test units of measure first. A BOM using grams, stock tracked in kilograms, and timers recording minutes can produce very expensive conversion errors. Check active cavities and changes to the mold configuration too.
Odoo for manufacturing provides expected and real manufacturing-order cost views. Its manufacturing-order cost documentation describes differences arising from component quantities, work-order duration, and employee costs. Accurate records and rate configuration therefore matter as much as the report itself.
However, validate the complete injection molding costing workflow in your installed version. Cavity-specific output, recovered-material valuation, commercial mold recovery, and preserving quotation baselines may need configuration or extensions. A field labelled “real cost” does not confirm that every factory-specific expense has been captured.
Use Apagen’s guide to ERP software for plastic injection molding to frame the evaluation. Ask the implementation team to reconcile one closed order from stock transactions through time records to accepted output.
Mark provisional and final costs clearly
A completed production run can still have an open material return, pending rework, or an unposted subcontracting charge. Show what remains unresolved and who owns the next action. Update the cost through controlled adjustments rather than silently changing a report already used for a pricing decision.
Use the result to improve the next order
Review repeated runs by product revision and machine-and-mold combination. Separate changes in resin price from changes in consumption. Likewise, distinguish a longer cycle from a short batch carrying the same setup time over fewer parts.
Use those findings to update the next quote’s assumptions. Do not hide an unusually poor run inside an unexplained average, or treat one exceptional trial as the permanent production standard. Keep the underlying conditions visible.
For management, the useful outcome of injection molding costing is a decision that can be explained: adjust batch size, fix a recurring loss, revisit the machine assignment, or renegotiate an outdated price. A precise-looking number is helpful only when its inputs stand up to review.
Frequently asked questions
How do we calculate cost per part for a multi-cavity mold?
Estimate cycling time using active cavities, cycle time, and expected yield. For actual job cost, divide attributable batch costs by accepted output. If one cavity is disabled, update the forecast and record the change rather than continuing to assume the full tool capacity.
Should runners and rejected parts be included in material cost?
Yes, account for material consumed beyond accepted-part weight. Record unused returns and any legitimate recovery credit separately. If actual consumption already includes the losses, adding a further scrap percentage will charge for them twice.
What hourly machine rate should we use for injection molding costing?
Build an approved rate from your equipment costs and allocation policy. Document whether it includes energy, maintenance, depreciation, labor, and overhead. Use it with the corresponding time basis, and avoid treating another factory’s selling rate as your production cost.
How do we cost a job when the customer supplies the mold and resin?
Calculate the conversion and other costs your company actually bears under the agreement. Track customer-owned quantities separately without presenting them as your purchased material expense. Include agreed handling, inspection, consumables, and tooling-service costs where applicable.
How should we divide costs between parts from a family mold?
Separate directly identifiable costs, such as material weight and part-specific finishing. Allocate shared cycle and setup costs using a documented method that suits the products and contract. Reconcile all allocated amounts to the job total; equal quantities do not necessarily mean equal cost.
Can Odoo calculate the actual cost of an injection molded part?
Odoo supports manufacturing-order costing from configured components, operations, and labor. Validate your material valuations, time records, output quantities, and additional cost rules. Specialised tooling recovery and regrind treatment may need extensions, while finance still needs to reconcile the final result.
Why does a small repeat order cost more per part?
Setup, purging, and some inspection work can occur again even when the part and mold are unchanged. Fewer accepted parts absorb those batch costs. Compare orders using both batch size and production conditions before concluding that the press has become less efficient.
Book a demo using one completed molding order
Bring a quotation, a closed production order, resin issue-and-return records, and actual output to an Apagen ERP demo. Ask the team to show how the proposed injection molding costing workflow would explain the cost per accepted part and its difference from the quotation.
Email sales@apagen.com or call +91-9971800665 to discuss your requirements. Start with a job your production and finance teams both know, then see how ERP can make the calculation easier to verify and use.