Material costs get the most attention. Tooling costs get the most complaints. But machine time, often ignored in cost discussions, determines whether a project is profitable or marginal. Understanding where money actually goes enables informed decisions about design, volume, and supplier selection that affect total cost more than negotiating price points.
Cost conversations typically focus on piece price, but piece price is an outcome, not a driver. The drivers are material consumption, cycle time, labor requirements, tooling investment, and secondary operations. Changing any of these changes cost; negotiating piece price without changing drivers just shifts margin between parties.
The Cost Categories
Injection molding cost comprises several categories, each with different drivers and different leverage points for optimization.
Material cost is the resin consumed per part, including material in runners and sprues that may be partially recovered as regrind. Material cost scales linearly with volume: twice the parts means twice the material. Material is typically the largest cost category for high-volume production.
Tooling cost is the mold investment divided by the number of parts expected over the mold’s life. Tooling cost per part drops as volume increases: a $50,000 mold producing 100,000 parts adds $0.50 per part; the same mold producing 1,000,000 parts adds $0.05 per part. Tooling dominates cost at low volumes and becomes negligible at high volumes.
Production cost is machine time and direct labor per part. Machine rates range from $30 to $150 per hour depending on machine size and capabilities. Cycle time determines how many parts that hourly rate produces. Production cost per part is machine rate divided by parts per hour, adjusted for labor and efficiency factors.
Secondary operations add cost for any processing after molding: assembly, printing, machining, inspection, packaging. These costs may be minor or may exceed molding cost depending on part requirements.
Quality and overhead include inspection, documentation, engineering support, and facility costs. These may be bundled into machine rates or broken out separately.
Material Costs
Material cost per part equals part weight plus waste, multiplied by material price.
Part weight comes from design geometry. Lighter parts cost less; wall thickness reduction is the most direct weight reduction approach. A 10 percent weight reduction saves 10 percent of material cost.
Runner waste depends on runner system design. Cold runner systems produce solid runners each shot, adding 10 to 30 percent to shot weight. Hot runner systems eliminate runner waste but increase tooling cost. The break-even volume where hot runner investment pays back depends on material cost and production volume.
Scrap rate accounts for startup parts, rejected parts, and end-of-run waste. Efficient operations run 2 to 5 percent scrap; poorly controlled operations may exceed 10 percent. Scrap rate directly affects material consumption per good part.
Regrind recovery offsets some waste cost. If runners and scrap can be reground and blended with virgin material, typically at 10 to 30 percent regrind ratio, material cost per part decreases. Not all applications permit regrind; some customers require virgin material.
| Material Type | Typical Price Range | Relative Cost |
|---|---|---|
| PP, HDPE | $0.80-1.20/lb | Low |
| ABS, HIPS | $1.20-1.80/lb | Medium |
| Nylon (unfilled) | $2.00-3.50/lb | Medium-High |
| PC | $2.50-4.00/lb | High |
| Glass-filled engineering | $3.00-6.00/lb | High |
| Specialty (PEEK, PEI) | $30-100+/lb | Very High |
Material price fluctuates with petroleum prices, supply conditions, and market demand. Long-term cost projections should account for potential price changes.
Tooling Costs
Mold cost divided by expected production volume determines tooling contribution per part.
Mold cost ranges widely based on complexity, size, and quality level. A simple single-cavity mold in aluminum might cost $5,000. A complex multi-cavity hardened steel mold with hot runners and actions might cost $500,000 or more.
Expected life determines how many parts share the tooling cost. Aluminum molds may produce 10,000 to 100,000 parts before replacement. Hardened steel molds may produce millions. Lower initial tooling cost often means higher cost per part if the tool wears out before production volume is complete.
Amortization method affects how tooling cost appears. Some suppliers include tooling amortization in piece price. Others quote tooling separately. Either approach works, but quotes must be compared consistently.
Why lowest mold cost isn’t lowest tooling cost: A $30,000 aluminum mold lasting 50,000 parts contributes $0.60 per part. A $60,000 hardened steel mold lasting 500,000 parts contributes $0.12 per part. If volume reaches 500,000 parts, the expensive mold saves $0.48 per part, $240,000 total. If volume only reaches 50,000 parts, the cheap mold was the right choice. Tooling decisions require volume forecasts.
Production Costs
Production cost is machine time converted to part cost.
Machine hourly rate includes depreciation, maintenance, energy, floor space, and often allocated overhead. Rates vary by machine size (larger machines cost more), technology (all-electric versus hydraulic), and geographic location. Industry benchmarks suggest:
| Machine Size | Typical Hourly Rate |
|---|---|
| Small (<100 ton) | $30-50/hour |
| Medium (100-500 ton) | $50-80/hour |
| Large (500-1000 ton) | $80-120/hour |
| Very large (1000+ ton) | $120-200+/hour |
Cycle time determines parts per hour. A 30-second cycle produces 120 parts per hour in a single-cavity mold, 240 in a two-cavity, 480 in a four-cavity. Machine cost per part is hourly rate divided by parts per hour.
Example: A $60/hour machine running a 30-second cycle in a two-cavity mold produces 240 parts per hour. Production cost is $60 / 240 = $0.25 per part.
Labor allocation adds direct operator cost. A fully automated cell may require one operator tending multiple machines; labor per part is low. A cell requiring constant operator attention dedicates full labor to that production; labor per part is higher.
Efficiency factors account for real production versus theoretical. Machines don’t run 24 hours at cycle time; there’s startup, changeover, maintenance, and unplanned downtime. Effective hourly output may be 70 to 85 percent of theoretical. Cost calculations should use realistic efficiency, not best-case assumptions.
Secondary Operation Costs
Operations after molding add cost that may equal or exceed molding cost for complex parts.
Assembly of multiple components into finished products requires labor and equipment. Assembly cost per unit depends on complexity and automation level. Manual assembly of five components might take 30 seconds and cost $0.15 per unit; automated assembly of the same components might require $50,000 in equipment but reduce cost to $0.03 per unit. The break-even volume determines which approach makes economic sense.
Decorating through pad printing, hot stamping, laser marking, or labeling adds cost per part. Setup costs spread over production quantity; per-part costs depend on complexity. A single-color pad print might add $0.02 per part; multi-color pad printing with registration requirements might add $0.10 or more.
Machining for features that can’t be molded, such as threads, tight-tolerance bores, or secondary surfaces, adds significant cost. Design for moldability reduces machining requirements. A tapped hole requiring secondary machining might add $0.25 to $0.50 per part; designing a molded-in thread insert pocket with post-mold insert installation may cost less overall.
Inspection beyond standard quality control adds cost for additional measurements, testing, or documentation. Customer-specific inspection requirements may add significant cost to otherwise simple parts. Full dimensional inspection and CMM reports can add $5 to $15 per first article and ongoing per-lot costs.
Packaging costs depend on requirements. Bulk packaging in boxes is minimal; individual packaging in custom trays is expensive. Specialized packaging for cleanroom products, anti-static requirements, or customer-specific containers can add $0.05 to $0.50 per part.
Overhead and Hidden Costs
Cost categories not always visible in quoted piece price affect total cost.
Quality costs include inspection labor, gaging, measurement systems, documentation, and scrap. High-quality programs cost more to administer; low-quality programs cost more in scrap and customer problems. The cost of quality includes both prevention costs (doing it right) and failure costs (fixing or scrapping defects). Industry data suggests total quality cost typically runs 5 to 15 percent of manufacturing cost, with well-run operations at the low end.
Inventory carrying cost affects total program cost even though it doesn’t appear in piece price. Inventory ties up capital, requires warehouse space, and risks obsolescence. Carrying cost is typically 15 to 30 percent of inventory value annually. A $50,000 inventory position costs $7,500 to $15,000 per year to maintain, whether or not those costs appear on any invoice.
Tooling maintenance is ongoing cost beyond initial mold purchase. Molds require cleaning, polishing, component replacement, and eventual refurbishment. Maintenance cost varies by mold complexity and material abrasiveness. Budget 1 to 3 percent of mold cost annually for routine maintenance; glass-filled or abrasive materials may require higher maintenance budgets.
Setup costs spread over production quantity. More frequent orders mean more setups; setup cost per part increases with smaller lot sizes. A $200 setup spread over 10,000 parts adds $0.02 per part; the same setup spread over 500 parts adds $0.40 per part. Order quantity decisions should consider total cost including setup, not just piece price.
Engineering support for process troubleshooting, quality issues, and continuous improvement represents real cost even if not invoiced separately. Suppliers factor engineering overhead into rates; in-house operations should account for engineering time in cost analysis.
Volume Effects
Cost structure shifts dramatically with volume.
Low volume (under 10,000 parts): Tooling dominates. A $50,000 mold adds $5 or more per part. Material and production costs may be secondary to tooling amortization.
Medium volume (10,000 to 100,000 parts): Tooling, material, and production costs are comparable. Decisions affecting any category significantly impact total cost.
High volume (over 100,000 parts): Material and production dominate. Tooling amortization becomes negligible. Cycle time reduction and material cost savings provide the greatest leverage.
Understanding this shift enables appropriate decision-making at each volume level. Low-volume projects should minimize tooling investment; high-volume projects should invest in tooling that reduces cycle time and optimizes material use.
Break-even calculations help compare alternatives. If one approach has higher tooling but lower piece price, the break-even volume is:
Break-even volume = (Tooling A – Tooling B) / (Piece price B – Piece price A)
Volumes above break-even favor the higher-tooling option; volumes below favor lower tooling.
Cost visibility enables cost optimization. Without understanding where money goes, improvement efforts target the wrong areas. Negotiating piece price when the real problem is cycle time, or reducing material weight when tooling drives cost, produces disappointing results. Understanding the full cost picture directs effort toward changes that actually matter.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- Plastics Technology. “Injection Molding Cost Analysis.” https://www.ptonline.com/
- CustomPartNet. “Injection Molding Cost Estimator.” https://www.custompartnet.com/
- Society of Plastics Engineers. “Economics of Injection Molding.”
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.