How to calculate cooling time in injection molding?

Cooling Time in Injection Molding

Cooling time is typically 50–70% of the total cycle time and is calculated using the following formula:

Core Formula (Jansson's Equation)

Where:

Thermal Diffusivity

α=ρ⋅cpk

• k = thermal conductivity (W/m·K)

• ρ = density (kg/m³)

• c_p = specific heat capacity (J/kg·K)

Typical values for common materials:

Step-by-Step Example

Given: PP part, wall thickness = 3 mm, T_m = 230°C, T_w = 30°C, T_e = 90°C, α = 0.77 × 10⁻⁷ m²/s

1. Convert thickness: s = 0.003 m

2. Calculate the log term: ln(4/π × (230−30)/(90−30)) = ln(1.273 × 3.33) = ln(4.24) ≈ 1.44

3. Apply formula: t_c = (0.003²) / (π² × 0.77×10⁻⁷) × 1.44 ≈ 17 seconds

Key Rules of Thumb

• Thicker walls = longer cooling — time scales with the square of wall thickness (doubling thickness → 4× cooling time)

• Lower mold temp → faster cooling but risk of sink marks/warpage

• Target ejection temp is typically 20–30°C below the material's heat deflection temperature (HDT)

• Conformal cooling channels can reduce cooling time by 20–40%

Practical Tips

• Use uniform wall thickness to avoid differential cooling and warpage

• Cooling time is dominated by the thickest section, not the average

• Simulation tools (Moldflow, Moldex3D) give more accurate results for complex geometries