Injection Molding Cooling Time Calculation Method
Cooling time in injection molding refers to the duration required for the molten plastic to cool from filling temperature to a temperature at which the part can be safely ejected. It is one of the most critical factors determining cycle time and part quality. Cooling time is calculated using theoretical and empirical formulas, and adjusted based on actual molding conditions.
1. Theoretical Calculation Formula
The theoretical cooling time is derived from one‑dimensional heat transfer theory and is suitable for thin‑walled parts with uniform thickness. The formula is expressed as:
Cooling time = (Maximum wall thickness² × 3.14) ÷ (4 × Thermal diffusivity) × ln[(Melt temperature − Mold temperature) ÷ (Ejection temperature − Mold temperature)]
The maximum wall thickness is the most influential variable, as cooling time is proportional to the square of the thickness. Thermal diffusivity values are obtained from material datasheets. Typical values range from 0.8×10⁻⁷ to 1.2×10⁻⁷ m²/s for amorphous plastics and from 1.0×10⁻⁷ to 1.5×10⁻⁷ m²/s for semi‑crystalline plastics. Melt and mold temperatures are set according to processing requirements. Ejection temperature is the temperature at which the part maintains its shape, typically below the glass transition temperature for amorphous plastics and below the melting point for semi‑crystalline plastics.
| Material Type | Material Name | Empirical Coefficient | Applicable Wall Thickness (mm) |
|---|---|---|---|
| Amorphous | ABS | 1.3 | 0.5-6.0 |
| Amorphous | PC | 1.4 | 0.5-6.0 |
| Amorphous | PMMA | 1.5 | 0.5-6.0 |
| Semi-crystalline | PP | 1.7 | 0.5-6.0 |
| Semi-crystalline | HDPE | 1.9 | 0.5-6.0 |
| Semi-crystalline | PA6 | 1.6 | 0.5-6.0 |
| Semi-crystalline | PA66 | 1.8 | 0.5-6.0 |
| Material | 1.0mm | 2.0mm | 3.0mm | 4.0mm | 5.0mm | 6.0mm |
|---|---|---|---|---|---|---|
| ABS | 1.8 | 7.0 | 15.8 | 28.2 | 44.0 | 63.4 |
| PC | 2.1 | 8.2 | 18.5 | 32.8 | 51.5 | 74.2 |
| PP | 1.7 | 6.8 | 15.3 | 27.2 | 42.5 | 61.2 |
| HDPE | 2.9 | 11.6 | 26.1 | 46.4 | 72.5 | 104.4 |
| PA6 | 1.5 | 5.8 | 13.1 | 23.2 | 36.3 | 52.2 |
| PA66 | 1.6 | 6.4 | 14.4 | 25.6 | 40.0 | 57.6 |
| Correction Factor | Condition | Coefficient | Adjustment Range |
|---|---|---|---|
| Wall Thickness Uniformity | Uneven/Complex | 1.2 | +20% |
| Cooling Channel | Optimal | 0.9 | -10% |
| Cooling Channel | Poor | 1.3 | +30% |
| Mold Temperature | 10℃ Higher | 1.4 | +40% |
| Material Property | Crystalline | 1.3 | +30% |
| Dimension Accuracy | High Precision | 1.2 | +20% |
2. Empirical Calculation Formula
For practical production, a simplified empirical formula is often used for quick estimation:
Cooling time = Empirical coefficient × Maximum wall thickness²
The empirical coefficient ranges from 1.2 to 1.5 for amorphous plastics such as ABS, PC, and PMMA. For semi‑crystalline plastics such as PP, PE, and PA, the coefficient ranges from 1.5 to 2.0 due to the latent heat of crystallization released during cooling. This method provides a fast and sufficiently accurate way to determine initial process parameters.
3. Determination of Calculation Parameters
The maximum wall thickness must be accurately measured from the part design. If the part contains ribs, bosses, or other thick sections, those dimensions are used as the basis for calculation. Melt temperature and mold temperature are adjusted based on part appearance and molding stability. Ejection temperature is determined from material specifications or verified during mold trials to ensure the part does not deform or shrink excessively after ejection.
4. Correction Factors for Actual Production
Theoretical and empirical calculations are based on ideal conditions and must be adjusted for real‑world factors. For parts with uneven wall thickness or complex geometries, cooling time is increased by 10% to 30% due to uneven heat dissipation. Efficient cooling systems with large‑diameter channels, small spacing, and high flow rates can reduce cooling time by 10% to 20%. Poorly designed or blocked cooling channels require longer cooling times.
Higher mold temperatures increase cooling time by 20% to 50%. Semi‑crystalline plastics require additional cooling time of 20% to 50% due to latent heat release. Parts requiring high dimensional accuracy may also need extended cooling to minimize shrinkage errors.
5. Practical Application in Production
In actual production, an initial cooling time is calculated using either theoretical or empirical methods. During mold trials, the cooling time is initially set to 90% of the calculated value. The part is then evaluated for deformation, shrinkage, warpage, and surface quality. Adjustments are made in small increments until the part quality is stable and the cycle time is optimized.
By combining theoretical calculation, empirical estimation, and practical adjustment, the optimal cooling time can be determined to achieve both high product quality and efficient production.
