Determine Injection Molding Cost per Unit to Forecast Accurate Profit Margins

If you can’t trust your cost-per-unit (CPU), you can’t trust your profit forecast. This guide shows a practical, engineering-first method to calculate CPU for injection-molded parts and turn that number into reliable margins—from prototype to mass production. Use it as a checklist, or send your CAD to TaiwanMoldMaker.com for a 48-hour DFM & Cost Pack.

The Cost-Per-Unit Formula (FOB, ex-works)

CPU = Material + Machine Time + Labor + Overhead + Tool Amortization + Secondaries + Packaging + Freight/Duty (if applicable)

What each term means (and how to estimate fast)

• Material = (Part wt. + Runner wt.) × (1 + scrap %) × (Resin price × blend)

• Machine Time = Press rate ($/h) ÷ (3600 / cycle time) ÷ cavities × (1/OEE)

• Labor = (Operators × labor $/h) ÷ parts/hour

• Overhead = % × (Machine + Labor)

• Tool Amortization = Tool price ÷ program quantity (or expected life)

• Secondaries = In-cell print/laser, weld, leak-test, assembly

• Packaging = Bags, trays, cartons, labels, desiccant

• Freight/Duty = Per-piece share of logistics + any import charges

Tip: Put OEE (Overall Equipment Effectiveness) in the denominator. For stable programs, OEE ≈ 0.85–0.92 is realistic.

Worked Example (4-cavity, PC/ABS housing)

Assumptions
Part 80 g + runner 10 g; scrap 1%; resin $3.20/kg + 3% MB at $6.00/kg; cycle 22 s; 4 cavities; OEE 0.90; press rate $14/h; 0.4 operators at $6/h; overhead 15%; tool $40 000; program qty 400 k; pad print $0.06; packaging $0.04; freight/duty $0.05.

Results

• Material: $0.299 /pc

• Machine: $0.0238 /pc

• Labor: $0.0041 /pc

• Overhead: $0.0042 /pc

• Tool amort.: $0.1000 /pc

• Secondaries + pack + freight: $0.15 /pc
  CPU (total): $0.581 /pc

Margin check: At a sell price of $0.78, gross margin = 25.6%.

Price Needed for Target Margin (from the example CPU = $0.581)

Formula: Price = CPU ÷ (1 − target margin)

Sensitivity: What Moves Your CPU the Most?

Starting from the example above:

Takeaway: Resin, tool amortization, and cavity count typically drive CPU more than tiny tweaks to press rate.

How to Forecast Accurately (and Defend Your Quote)

1. Lock part weight early. Every extra gram at $3–6/kg compounds across millions of parts.

2. Model real cycle time. Use Moldflow® cooling and conformal inserts; don’t guess.

3. Right-size cavities. Match demand curve; avoid paying for unused capacity.

4. Amortize truthfully. Tie tooling amortization to realistic program volume, not marketing hopes.

5. Include secondaries & pack. In-cell finishing (print/laser/weld) reduces freight legs and surprise costs.

6. Track OEE and scrap. Quote with evidence—shot weight curves, V/P transfer, and MES trends.

7. Stress-test resin price. Quote a ±10–15% resin escalator to protect margins.

What You Get from TaiwanMoldMaker.com

• 48-Hour DFM & Cost Pack (gate, draft, cooling, cycle model, CPU breakdown)

• Bridge-to-Steel Tool Credits (reduce amortization risk)

• Live MES Dashboard (CpK, OEE, shot weight, energy per kg)

• Service-weighted billing options to reduce dutiable value on the tooling portion

• Inline finishing (pad print, laser mark, weld, leak-test) to keep “all-in” CPU predictable

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