Injection Molding Cycle Time

How is injection molding cycle time calculated?

Injection molding cycle time is the sum of four phases: fill (injection), pack (holding pressure), cool, and mold open/close (including ejection). Cooling time is typically the longest phase, often 50-80% of the total cycle, and is the primary target for optimization.

Cooling time is determined by the part's wall thickness, the thermal diffusivity of the resin, and the temperature differential between melt, mold, and ejection temperatures. It follows Fourier's heat conduction equation for a flat plate: t_cool = (s²/π²α) × ln(4(T_melt − T_mold)/(π(T_eject − T_mold))).

Reducing wall thickness has the strongest effect on cycle time since cooling time scales with the square of thickness. A 20% reduction in wall thickness can reduce cooling time by 36%, significantly improving production economics.

Formula: Total Cycle = Fill + Pack + Cool + Open/Close Cooling Time = (s² / π²α) × ln[4(Tm - Tw) / π(Te - Tw)] Fill Time = Shot Weight / Injection Rate

Example Calculation

A 40 g ABS part with 2 mm walls (α = 0.12 mm²/s, T_melt = 230°C, T_mold = 60°C, T_eject = 100°C). Cooling time = (2² / (π² × 0.12)) × ln((8 / π²) × 170 / 40) = 3.377 × 1.237 = 4.18 s. Fill at 80 cm³/s takes 40 / 1.02 / 80 = 0.49 s, packing is 40% of cooling = 1.67 s, plus 3 s open/close and 0.5 s ejection: Total = 9.84 s ≈ 365 parts/hour.

When to Use This Calculator

• Estimating production throughput (parts per hour) during quoting or capacity planning for a new injection-molded product
• Evaluating the cycle time impact of design changes — especially wall thickness reduction — during DFM (Design for Manufacturability) reviews
• Comparing resin choices by their thermal diffusivity and processing temperatures to select the material with the shortest cycle
• Identifying the bottleneck phase (fill, pack, cool, or mold movement) to target process optimization efforts

Common Mistakes to Avoid

• Using nominal wall thickness instead of maximum wall thickness — cooling time is governed by the thickest section of the part, which may be at ribs, bosses, or gate areas
• Neglecting packing time in the cycle estimate — insufficient packing causes sink marks and dimensional shrinkage; typical packing time is 30-60% of cooling time
• Assuming thermal diffusivity values for generic resin families — filled and reinforced grades (e.g., 30% glass-filled nylon) have significantly different thermal properties than unfilled versions
• Ignoring mold temperature controller limitations — the calculated cooling time assumes ideal mold temperature, but undersized chillers or poor channel design can add 20-50% to actual cooling time

How to Interpret Results

• If cooling time exceeds 70% of total cycle time, focus optimization efforts on cooling — conformal cooling channels, higher thermal conductivity mold inserts (BeCu), or lower mold temperature
• Parts per hour below 100 for small parts usually indicates an opportunity for multi-cavity tooling or a thinner wall design
• If fill time is a significant fraction of the cycle, the injection rate may be too low or the part has thin-wall sections requiring higher injection pressure

Related Standards & References

• ISO 294-1 — Plastics — injection moulding of test specimens: specifies process conditions and cycle time documentation for reproducible property testing
• ASTM D3641 — Standard practice for injection moulding test specimens of thermoplastic moulding and extrusion materials: cycle time and parameter documentation
• SPI Plastics Industry Association Resin Guide — cycle time and cooling rate guidelines for ABS, PP, PE, nylon, and other common resins including thermal diffusivity values
• Rosato & Rosato — 'Injection Molding Handbook' (3rd ed., Springer): comprehensive reference for Fourier cooling time equations, resin thermal data, and cycle time optimization

Frequently Asked Questions

What is the most effective way to reduce injection molding cycle time?

Reducing wall thickness is the most effective approach since cooling time scales with the square of thickness. Other strategies include optimizing mold cooling channel design (conformal cooling), reducing melt temperature, using resins with higher thermal diffusivity, and minimizing mold open/close travel distance. Each second saved per cycle compounds over millions of production cycles.

Why does cooling time dominate the injection molding cycle?

Plastics have very low thermal conductivity (0.1-0.4 W/m·K compared to ~50 W/m·K for steel), making heat removal slow. The part must cool below the heat deflection temperature before ejection to avoid warping. For thick parts (>3 mm), cooling can take 20-60 seconds while fill and pack phases together may only take 3-6 seconds.

How does conformal cooling improve cycle time?

Conformal cooling channels follow the part geometry instead of using straight drilled holes, placing coolant closer to hot spots. This can reduce cooling time by 20-40% compared to conventional cooling. Conformal cooling is typically achieved via metal 3D printing (DMLS) of mold inserts and is most beneficial for complex geometries with thick-to-thin wall transitions.