Injection Molding Cost Analysis: How Material Choice Affects Part Cost

Material Cost Is Only the Beginning

When engineers compare material costs for injection molded parts, the first instinct is to look at the price per kilogram of the resin. PEEK at $80-200/kg looks expensive. Nylon at $4-8/kg looks cheap. But the per-kg price is only one component of the total part cost — and often not the dominant one. The true cost of material choice includes processing premiums, cycle time differences, scrap rates, tooling costs, and the total cost of ownership over the product's service life.

The Cost Breakdown

Total part cost = (material cost per part) + (processing cost per part) + (tooling amortization) + (secondary operations) + (quality/scrap cost). Material cost per part is a function of the resin price, part weight, and scrap factor. Processing cost is a function of the machine hourly rate (which depends on the machine size and capability required) and the cycle time. A high-temperature polymer like PEEK or PPS requires a machine with a high-temperature package (ceramic heater bands, oil-heated mold temperature controller), which adds 30-50% to the machine hourly rate. The cycle time for PEEK is typically 3-5× longer than for ABS or polypropylene due to the high mold temperature (160-200°C) required to achieve optimal crystallinity. The longer cycle time means fewer parts per hour, increasing the processing cost per part. The tooling cost for high-temperature polymers is also higher — the mold must be made from hardened tool steel with oil-heated channels, adding 20-40% to the mold cost compared to a standard mold for commodity polymers.

Break-Even Analysis

For a typical 50g part with a 30-second cycle time in ABS, the total part cost at 10,000 units is approximately $0.85-1.20. The same part in PEEK would have a cycle time of 90-120 seconds (due to the high mold temperature and longer cooling time) and a higher machine hourly rate, yielding a part cost of approximately $8.50-12.00 — 10× higher. However, if the PEEK part replaces a metal component (machined aluminum at $25-40 per part), the PEEK part is actually less expensive. The break-even analysis must always consider the alternative — not just the material substitution but the system-level cost savings. For medical implant applications, PEEK's radiolucency eliminates the need for secondary imaging procedures, saving thousands of dollars per patient. For semiconductor applications, PEEK's purity eliminates wafer contamination that would cost millions in lost yield. The material cost premium is often justified by the system-level savings.

Strategies for Cost Reduction

1. Down-select the material grade: Unfilled grades are 30-50% less expensive than glass-reinforced, and 50-70% less expensive than carbon-fiber-reinforced. Verify whether reinforcement is truly necessary for the application. 2. Optimize wall thickness: Reducing wall thickness from 3mm to 2mm reduces material usage by 33% and cooling time by 56% (cooling time is proportional to the square of wall thickness). 3. Consider regrind: PEEK, PPS, and PEI can use 20-30% regrind without significant property loss, reducing effective material cost. 4. Multi-cavity tooling: Increasing the cavity count from 4 to 8 reduces the processing cost per part by approximately 40% for the same cycle time. 5. Evaluate total cost of ownership: A PEEK component that lasts 5× longer than a nylon component in a wear application provides a 5× cost advantage at the system level, even if the per-part cost is higher.

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
• National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb — Material Property Data. matweb.com