Last updated: 2026-09-02 · 5 min read

How Do You Benchmark Injection Molding Cycle Time — and What Should the Number Tell You?

Cycle time benchmarks compare the actual seconds per shot against the theoretical minimum based on part wall thickness, resin thermal properties and cooling efficiency — and the gap between actual and theoretical reveals whether the process is optimized or whether cooling, mold design or machine settings are adding cost to every shot. A cycle time that is 30% above the theoretical minimum means 30% more cost per part — over a million-shot program, that is a significant sum that no quotation should accept without justification.

Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. See also related guide.

Key Facts

  • The theoretical minimum cycle is set by cooling time — approximately 80% of the total cycle for thin-wall parts. Cooling time scales with the square of wall thickness: doubling the wall quadruples the cooling time.
  • Benchmark formula: cooling time (seconds) ≈ wall thickness² (mm²) × thermal diffusion factor — for ABS the factor is ~2.0, for PP ~1.8, for PC ~2.5 (higher = slower cooling).
  • The remaining 20% of the cycle is fill, pack, mold open, ejection and close — typically 5–15 seconds regardless of wall thickness. Optimization focuses on the cooling 80%, not the fixed 20%.

    • Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Cycle Time Benchmarks by Wall Thickness

    Wall (mm) ABS (sec) PP (sec) PC (sec)
    1.0 10–12 9–11 12–15
    2.0 16–20 14–18 20–25
    3.0 25–30 22–27 30–38
    4.0 35–42 32–38 42–52

    What the Gap Between Actual and Benchmark Reveals

  • Cooling channel design: conformal cooling (channels that follow the part contour) reduces cooling time 15–30% vs. straight drilled channels — the benchmark gap reveals whether the mold was designed for production efficiency or just for function.
  • Mold material: beryllium copper inserts in hot spots conduct heat 3–5× faster than steel — the benchmark gap at thick sections reveals whether high-conductivity inserts were used.
  • Process optimization: higher melt temperature improves flow but extends cooling; lower mold temperature speeds cooling but may cause flow defects — the benchmark shows where the process is balanced.
  • Machine response time: slow mold open/close or ejector cycles add seconds that compound over millions of shots — the benchmark gap in the non-cooling portion reveals machine efficiency.

    • What This Means for Your Quotation

    The mold quotation should include the expected cycle time based on wall thickness and resin — with the benchmark comparison showing whether the mold design (cooling channel layout, high-conductivity inserts) achieves the theoretical minimum or adds cost to every shot. When the cycle time is benchmarked, the per-part cost is transparent; when it is not, the buyer pays for every unnecessary second over the program’s life without knowing it.

    Frequently Asked Questions

    Can cycle time be reduced after the mold is built?

    Marginally — process optimization (melt temperature, mold temperature, cooling water flow rate) can recover 5–10%. Structural changes (adding cooling channels, installing high-conductivity inserts) require mold modification and are expensive. The time to optimize cycle is during the mold design, not after.

    How does cycle time affect per-part cost?

    Machine cost is charged per hour; if the cycle is 20 seconds instead of 15, the machine produces 180 parts/hour instead of 240 — a 25% reduction in output. Over a million-shot program at $0.05/part machine rate, the difference is $0.0125/part × 1,000,000 = $12,500 in additional machine cost alone, before material and overhead.

    What is the role of mold flow analysis in cycle optimization?

    Mold flow analysis predicts the cooling time based on part geometry, cooling channel layout and resin properties — it shows hot spots where cooling is inadequate and predicts the cycle time before steel is cut. The analysis lets the mold designer optimize the cooling layout for the minimum cycle, not discover the problem at trial.

    Key Takeaways

    1. Cycle time benchmarks compare actual seconds against the theoretical minimum based on wall thickness and resin — the gap reveals cooling design and process optimization quality.
    2. Cooling is 80% of the cycle; conformal cooling and high-conductivity inserts close the gap between actual and theoretical — but must be designed into the mold, not added after.
    3. The quotation should include the benchmarked cycle time with the per-part cost impact — so the buyer knows whether they are paying for optimized cooling or for every unnecessary second.

    Send aCatMould the part drawing and resin, and the quotation includes the benchmarked cycle time — with the theoretical minimum, the expected actual cycle and the per-part cost at both, so the mold design is optimized for production cost, not just for function.

    Sources
    Plastics Technology — cycle time optimization in injection molding
    SPE — cycle time benchmarking and cost analysis
    MoldMaking Technology — cooling design for cycle reduction

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet