How injection cooling time is calculated
Cooling usually accounts for more than half of an injection cycle. The standard flat-plate solution is t = wall² ÷ (π² × thermal diffusivity) × ln[(8 ÷ π²) × (melt − mold) ÷ (eject − mold)]. The important part is that wall thickness enters as a square, so trimming the wall by 10% cuts cooling by about 19%. That is why one heavy rib or boss can dominate the whole cycle.
Effective thermal diffusivity is thermal conductivity divided by density and specific heat. Typical values are about 1.09 ×10⁻⁴ in²/s for PP, 1.24 for ABS, 1.40 for nylon 6, POM and PMMA, 1.86 for HDPE and 2.02 for PC. Semi-crystalline resins cool more slowly than conductivity alone suggests because of the heat of fusion, so check the resin supplier data sheet for an effective value.
Running the mold colder shortens cooling but can add residual stress, warpage and gloss problems, while ejecting hotter speeds the cycle at the risk of distortion during ejection. Treat the result as a design starting point and tune it during sampling while watching dimensions and appearance. Review cooling channel layout and water temperature with your mold designer.
Frequently asked questions
Use the thickest section. That area governs when the part can be ejected, so an average value returns a short time and invites ejection distortion.
Most often the mold surface runs hotter than the setpoint because cooling lines are too far away or too few. Latent heat in semi-crystalline resins, trapped heat on the core side and thick sections near the gate all add time as well.