“What’s the piece price?” sounds like a simple question. The answer depends on volume assumptions, tooling amortization, secondary operations, and a dozen other factors that vary by supplier. Two suppliers quoting the same part may arrive at different piece prices through different calculation paths, making direct comparison misleading unless you understand what’s included.
Per-part cost isn’t a single number but a formula with variables. Knowing the formula enables meaningful quote analysis, informed negotiation, and accurate comparison between suppliers or between in-house and outsourced production.
The Basic Formula
Per-part cost breaks into components that can be calculated independently then summed:
Per-part cost = Material + Production + Labor + Tooling amortization + Secondary operations
Material is the cost of plastic consumed to make one part, including waste.
Production is machine time cost for one part, based on cycle time and machine rate.
Labor is direct operator cost allocated to one part.
Tooling amortization is mold cost divided by expected production volume.
Secondary operations include any post-molding processes.
Each component has its own calculation method and its own sources of variation between quotes.
Material Cost Calculation
Material cost per part accounts for actual consumption, not just finished part weight.
Shot weight is part weight plus runner weight plus any sprues. A 50-gram part in a two-cavity mold with cold runners might have 20 grams of runner per shot, meaning each cavity’s share is 50 + 10 = 60 grams consumed per part.
Scrap factor accounts for startup parts, rejected production, and end-of-run waste. If scrap rate is 3 percent, multiply consumption by 1.03 to account for material that doesn’t become saleable parts.
Regrind credit reduces consumption when runners and scrap can be reground and reused. If 25 percent regrind is acceptable and runners represent 15 percent of shot weight, effective virgin consumption decreases. The calculation depends on specific regrind policies and allowances.
Material cost formula:
Material cost per part = (Part weight + Runner share) × Scrap factor × Material price / (1 – Regrind rate × Runner fraction)
Example: A 30-gram part with 5-gram runner share (runner split between two cavities) using material at $2.00/lb:
Without regrind: (30 + 5) × 1.03 × ($2.00 / 454 grams/lb) = 36.05 × 0.00441 = $0.159
With 25% regrind recovering runner: Approximately $0.145
The difference seems small on one part but scales significantly over production volume.
Machine Cost Calculation
Machine cost converts hourly rate to per-part cost based on production rate.
Hourly rate varies by machine size, technology, and geographic location. Rates range from $30/hour for small machines to $150/hour or more for large specialized equipment.
Cycle time determines parts per hour. A 30-second cycle produces 120 cycles per hour; in a two-cavity mold, that’s 240 parts per hour.
Efficiency factor accounts for real versus theoretical production. Startup, changeover, minor stops, and unplanned downtime reduce actual output versus theoretical. Efficient operations achieve 80 to 85 percent of theoretical capacity; less disciplined operations may be 65 to 75 percent.
Machine cost formula:
Machine cost per part = Hourly rate / (Parts per hour × Efficiency factor)
Example: $60/hour rate, 30-second cycle, two-cavity mold, 80% efficiency:
Parts per hour theoretical = (3600 / 30) × 2 = 240 parts/hour
Parts per hour actual = 240 × 0.80 = 192 parts/hour
Machine cost per part = $60 / 192 = $0.313
| Cycle Time | Cavities | Parts/Hour (80% eff) | Cost at $60/hr |
|---|---|---|---|
| 20 sec | 1 | 144 | $0.417 |
| 20 sec | 4 | 576 | $0.104 |
| 30 sec | 1 | 96 | $0.625 |
| 30 sec | 4 | 384 | $0.156 |
| 45 sec | 1 | 64 | $0.938 |
| 45 sec | 4 | 256 | $0.234 |
The table illustrates why cavitation and cycle time dramatically affect production cost.
Labor Allocation
Labor cost depends on operation requirements and automation level.
Fully automated cells may require one operator monitoring multiple machines. If one operator tends four machines, labor cost per machine is one-quarter of operator cost. Per-part cost is that fraction divided by parts per hour.
Semi-automated operations require periodic operator interaction: loading inserts, trimming gates, or packing parts. Labor allocation increases with interaction frequency.
Labor-intensive operations with continuous operator attention dedicate full labor cost to that production.
Labor cost formula:
Labor cost per part = (Operator hourly cost × Attention fraction) / Parts per hour
Example: $25/hour operator giving 50% attention to a machine producing 200 parts/hour:
Labor cost per part = ($25 × 0.50) / 200 = $0.0625
Tooling Amortization Methods
Tooling cost allocation significantly affects piece price, and different approaches suit different situations.
Include in piece price: Divide mold cost by expected volume, add to piece price. Buyer pays for tooling gradually through production. Simple to administer, but buyer must meet volume to avoid underpaying for tooling.
Example: $50,000 mold over 500,000 parts = $0.10 per part tooling amortization.
Separate tooling payment: Buyer pays for mold upfront or in installments. Piece price excludes tooling amortization. Buyer owns the mold regardless of volume achieved.
Hybrid approaches: Buyer pays portion upfront, remainder amortized into price. Or supplier owns tool until volume threshold, then transfers ownership.
What happens when volumes don’t materialize: If tooling is amortized into piece price at 500,000 parts but only 100,000 parts are ordered, the supplier hasn’t recovered tooling cost. Contracts should address this scenario.
For quote comparison, ensure all quotes handle tooling consistently. A quote with tooling included at different volume assumptions can’t be directly compared to a quote with separate tooling payment.
Secondary Operations
Post-molding operations add cost that may or may not appear in piece price quotes.
Per-piece operations like printing, assembly, or inspection add a cost per part that compounds with production volume.
Batch operations like setup for printing or machining spread setup cost across batch size. Smaller batches mean higher per-part setup cost.
Secondary operation formula:
Secondary cost per part = Operation cost per part + (Setup cost / Batch size)
Example: Pad printing at $0.05/part plus $75 setup per batch of 5,000:
Secondary cost = $0.05 + ($75 / 5,000) = $0.05 + $0.015 = $0.065
Quotes should itemize secondary operations to enable comparison. A quote offering “complete parts” at one price may or may not include the same operations as a quote with itemized line items.
Getting Comparable Quotes
Meaningful quote comparison requires structured requests that specify what’s included.
Specify volume scenarios. Request pricing at multiple volumes (e.g., 10,000, 50,000, 250,000 parts) to understand volume effects on cost structure.
Separate tooling from production. Request both tooling and piece price separately, even if you intend to amortize tooling into piece price. This enables understanding of true cost drivers.
Define material handling. Specify whether regrind is acceptable and at what percentage. Different regrind assumptions create different material costs.
Specify secondary operations. List all required post-molding operations and request either inclusion in piece price or separate line items.
Request itemization. Ask suppliers to break down piece price into material, production, labor, and secondary operations. This reveals whether differences come from efficiency, overhead, or margin.
Ask about assumptions. What cycle time is quoted? What efficiency is assumed? What machine rate underlies the calculation? Understanding assumptions enables meaningful comparison.
Common quote comparison pitfalls:
Supplier A quotes $0.75 including tooling amortization over 500,000 parts.
Supplier B quotes $0.65 plus $45,000 tooling.
At 500,000 parts: A = $0.75; B = $0.65 + $0.09 = $0.74. Nearly equal.
At 250,000 parts: A = $0.75; B = $0.65 + $0.18 = $0.83. A is cheaper.
At 1,000,000 parts: A = $0.75; B = $0.65 + $0.045 = $0.695. B is cheaper.
The “cheaper” quote depends on volume that hasn’t happened yet.
Putting It Together
A complete per-part cost calculation:
| Component | Calculation | Example |
|---|---|---|
| Material | 35g × $2.00/lb × 1.03 scrap / 454g/lb | $0.159 |
| Production | $60/hr / 192 parts/hr | $0.313 |
| Labor | $25/hr × 0.50 / 192 parts/hr | $0.065 |
| Tooling | $50,000 / 500,000 parts | $0.100 |
| Secondary | $0.05 print + $0.015 setup | $0.065 |
| <strong>Total</strong> | <strong>$0.702</strong> |
At this level of detail, cost drivers become visible. Reducing cycle time affects production cost. Increasing cavitation reduces both production and labor cost per part. Volume affects tooling amortization dramatically.
Per-part cost depends on assumptions as much as calculations. Understanding what’s included in quotes enables meaningful comparison and negotiation. A quote isn’t just a number; it’s a set of assumptions about volume, efficiency, and scope that must be understood before comparison or commitment.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- CustomPartNet. “Injection Molding Cost Estimator.” https://www.custompartnet.com/
- Plastics Technology. “Cost Analysis Tools.” https://www.ptonline.com/
- Society of Plastics Engineers. “Injection Molding Economics.”
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.