Last updated: 2026-09-05 · 5 min read
Material Cost per Part — How Do You Calculate the True Resin Cost for an Injection Molded Component Including Regrind, Runner and Scrap?
Material cost per part includes runner weight, regrind ratio, scrap rate and drying loss — not just resin price times part weight. For a 20g ABS part in a 4-cavity cold runner mold: shot weight is 120g (4 parts + runner), resin $2/kg, cost per part $0.06. With 3% scrap and 20% regrind, effective cost rises to $0.063 — a 5% increase most quotations ignore. Hot runner eliminates runner waste, cutting material cost by 33%.
Key Facts
- Formula: cost/part = (shot weight × resin price) / (cavities × (1 – scrap rate)). Shot weight = parts + runner (cold) or just parts (hot runner). For a 4-cavity cold runner mold: 20g parts + 40g runner = 120g shot. At $2/kg ABS: $0.24/shot, $0.06/part. Hot runner: 80g shot, $0.16/shot, $0.04/part — 33% savings.
- Regrind reduces cost but adds hidden expenses. At 20% regrind ratio, resin cost drops 20% — but handling (grinding, blending, inspection) costs 5–10%. Net savings: 10–15%, not 20%. Regrind also degrades properties: 10–20% impact loss for PC, 5–10% for nylon. See the regrind guide for safe regrind ratios by resin family.
- Scrap rate is the biggest variable. 1% scrap adds 1% to cost; 10% adds 11%. Scrap includes startup shots, dimensional and cosmetic rejects. Mature process: 1–3%. New mold or difficult resin: 5–15%. If the quotation does not state scrap, assume 3%.
How Does Material Cost per Part Compare Across Resin Families?
| Resin | Price ($/kg) | Part Weight | Cost/Part (4-cav) | Cost/Part (hot runner) |
|---|---|---|---|---|
| PP | $1.5–2.5 | 20g | $0.012 | $0.008 |
| ABS | $2–3 | 20g | $0.015 | $0.010 |
| PC | $3–5 | 20g | $0.025 | $0.017 |
| PA66 GF30 | $4–7 | 20g | $0.035 | $0.023 |
| PBT GF30 | $3–5 | 20g | $0.025 | $0.017 |
| POM | $3–5 | 20g | $0.025 | $0.017 |
| PPS GF40 | $10–18 | 20g | $0.090 | $0.060 |
| PEEK | $80–120 | 20g | $0.600 | $0.400 |
Shot weight — the total weight of material injected per machine cycle: parts + runner (cold runner) or just parts (hot runner). Shot weight determines material cost per cycle. For a 4-cavity cold runner mold with 20g parts and 40g runner: shot weight = 120g. Hot runner: shot weight = 80g — 33% less material per cycle.
For commodity resins (PP, ABS), material cost is a small fraction of the total part cost — the mold amortization and cycle time dominate. For engineering resins (PPS, PEEK), material cost becomes significant — at $0.60/part for PEEK, the resin cost for 100,000 parts is $60,000. At that level, hot runner conversion (which eliminates runner waste) pays for itself within 10,000–20,000 shots.
What Three Levers Reduce Material Cost per Part?
- Switch from cold runner to hot runner — eliminates runner waste entirely. A cold runner for a 4-cavity mold weighs 30–50% of the total shot weight. The runner is either scrapped or ground for regrind (at reduced value). A hot runner keeps the melt in heated channels — no runner, no regrind, no waste. The hot runner system costs $5,000–30,000 depending on cavity count and nozzle type. For a 4-cavity ABS mold at $0.015/part (cold runner) vs. $0.010/part (hot runner): savings of $0.005/part × 1,000,000 parts = $5,000 — the hot runner pays for itself in the first production run. The hot runner vs. cold runner cost guide covers the full payback calculation.
- Increase cavity count — spreads the runner cost across more parts. A single-cavity mold has no runner sharing — the runner weight per part is the full runner weight. A 4-cavity mold shares one runner among 4 parts — the runner weight per part drops by 60–75%. An 8-cavity mold drops it further. The trade-off is higher mold cost ($15,000 for 4-cavity vs. $8,000 for single) and higher clamp force (larger mold requires a bigger machine). The break-even is typically 50,000+ parts — below that volume, a single-cavity mold is more economical.
- Optimize part weight — reduce wall thickness where the design allows. A 20g part with 3.0mm wall can often be redesigned to 14g at 2.0mm wall without losing stiffness (if ribs are added). The 30% weight reduction saves 30% on material cost per part. The trade-off is a more complex mold (ribs require additional machining) and tighter process control (thinner walls freeze faster, requiring higher injection speed). For high-volume parts (1,000,000+ shots), the wall reduction pays for the mold complexity within the first 100,000 parts.
Regrind — ground sprues, runners and rejected parts that are blended back with virgin resin at a specified ratio (typically 15–30%). Regrind saves 10–15% net material cost after handling, but degrades impact strength (10–20% for PC, 5–10% for nylon). Each thermal cycle shortens the polymer chains — regrind tolerances vary by resin family.
Material cost/part = (shot weight × resin price) / (cavities × (1 – scrap)). PP $0.008–0.012/part; ABS $0.010–0.015; PC $0.017–0.025; PEEK $0.40–0.60. Hot runner saves 33% material vs. cold runner. Scrap 1–3% mature, 5–15% new mold. Regrind saves 10–15% net after handling costs.
- Shot weight confirmed — parts + runner (cold) or just parts (hot runner). Runner can be 30–50% of shot weight.
- Scrap rate stated in quotation — 1–3% mature process; 5–15% new mold. If not stated, assume 3%.
- Regrind ratio and handling cost included — 20% regrind saves 10–15% net, not 20%. Impact loss varies by resin.
- Hot runner payback calculated — savings/part × annual volume vs. hot runner system cost ($5k–$30k).
- Material cost vs. total part cost understood — material is 10–30% of total; cycle time and mold amortization dominate.
Frequently Asked Questions
Should I include regrind value in the material cost calculation?
Regrind from cold runners has value — it can be sold to regrind brokers at 30–50% of virgin resin price, or blended back into production at 15–30% ratio. If the regrind is blended back, the material cost savings is the regrind ratio (20%) minus the handling cost (5–10%) — net savings of 10–15%. If the regrind is sold externally, the revenue offsets the material cost by $0.30–0.50/kg of regrind — typically $0.001–0.003/part for a 20g part.
For engineering resins (PPS, PEEK), regrind is more valuable ($5–30/kg resale) and the savings are significant. For commodity resins (PP, ABS), regrind value is negligible ($0.30–0.50/kg) and the administrative cost of selling it may exceed the revenue.
How does drying loss affect material cost for hygroscopic resins?
Hygroscopic resins (nylon, PC, PET, PBT, TPU) lose 0.1–0.5% weight during drying as moisture evaporates. For nylon dried from 0.5% to 0.1% moisture: 0.4% weight loss. For 1,000 kg of resin, this means 4 kg of water evaporated — the effective resin cost increases by 0.4%. For PEEK dried from 0.1% to 0.02%: the loss is negligible (0.08%). The drying loss is a small factor — typically 0.1–0.5% of material cost — but for high-cost resins (PEEK at $100/kg), even 0.5% adds $0.50/kg to the effective cost.
What is the difference between material cost and total part cost?
Material cost is typically 10–30% of the total part cost for injection molding. The rest is: mold amortization (10–20%), machine time/cycle cost (30–50%), labor and overhead (10–20%), secondary operations (painting, assembly, packaging 5–15%) and profit margin (5–15%). For a $0.50 ABS part: material is $0.015 (3%), machine time is $0.20 (40%), mold amortization is $0.10 (20%), labor/overhead is $0.10 (20%), profit is $0.085 (17%). Reducing material cost by 33% (hot runner) saves $0.005/part — only 1% of the total part cost.
Key Takeaways
- Cost/part = (shot weight × price) / (cavities × (1 – scrap)).
- Hot runner saves 33% material vs. cold runner. Pays back in 10k–20k shots.
- Scrap 1–3% mature, 5–15% new mold. Always state scrap in quotation.
- Material is 10–30% of total part cost. Cycle time and mold dominate.
Send the part drawing with the resin, annual volume and target cost — the quotation breaks down material, machine, mold amortization and overhead line by line, with the assumed scrap rate and regrind ratio clearly stated.
Sources: Plastics Technology — Injection Molding Cost Calculation and Material Cost per Part · Plastics Technology Online — Hot Runner vs. Cold Runner Cost Comparison · Society of Plastics Engineers (SPE) — Cost Modeling for Injection Molding Technical Papers · MatWeb — Resin Price Database by Material Family · ICIS — Weekly Engineering Resin Price Index and Market Trends
— Arthur, Tooling Engineer at aCatMould