Scientific Calculation and Reference Basis for Injection Molding Cooling Time
Cooling time accounts for the largest proportion of the entire injection molding cycle, which directly determines product quality and production efficiency. Excessively long cooling time will waste production capacity and increase production costs, while insufficient cooling time will cause residual internal stress, product warpage, shrinkage depression and ejection deformation. Most on‑site adjustments rely on experience, lacking scientific quantitative basis, resulting in unstable product quality and unreasonable cycle setting. Mastering the scientific calculation method of cooling time can effectively balance molding quality and production tact time.
1. Theoretical Calculation Formula of Cooling Time
The core of cooling time calculation is based on the maximum wall thickness of plastic parts and the thermal diffusion performance of materials. The classic cooling formula is widely used in the injection molding industry to calculate the minimum cooling time required for the melt to cool from molten temperature to safe demolding temperature. The cooling time is in a square proportional relationship with the wall thickness, which means that the thicker the product, the longer the cooling time grows exponentially. Material thermal diffusivity, melt temperature, mold surface temperature and demolding temperature are the four core calculation parameters. The theoretical calculation value is the minimum cooling limit under ideal conditions, which needs to be superimposed with safety margin in actual production to avoid quality risks caused by environmental and equipment fluctuations.
2. Material Thermophysical Parameter Reference Standard
Different plastic materials have different thermal diffusion coefficients, which determine the heat dissipation speed. PP material has high thermal diffusion efficiency and fast cooling speed, while PC and ABS have low thermal diffusion and slow cooling. The demolding allowable temperature of different materials is different, which is close to the material heat distortion temperature. If the demolding temperature is too high, the internal heat cannot be completely dissipated, resulting in post‑mold shrinkage and deformation; if the demolding temperature is too low, the production cycle is too long. Modified materials and glass‑filled materials have changed thermal properties, so the parameters need to be based on the official material data sheet rather than conventional base material data.
3. Structural and Mold Correction Factors
The theoretical formula is only applicable to flat products with uniform wall thickness. In actual production, plastic parts have thick structures such as reinforcing ribs and boss columns, which are prone to heat accumulation. The local thick glue position becomes the key factor determining the cooling time, and the theoretical value needs to be increased by 20% to 50% safety factor. The mold cooling water circuit layout, water temperature and water flow will affect the actual cooling effect. Uneven water circuit layout leads to inconsistent mold temperature, resulting in different shrinkage of each part of the product. At the same time, the cooling time cannot be less than the gate freezing time, otherwise backflow and dimensional instability will occur.
4. On‑Site Verification and Parameter Determination Method
The theoretical calculation result is only the initial reference parameter, and the final production parameter needs to be verified by gradient test molding. Gradually reduce the cooling time, observe whether the product has deformation, shrinkage and blister after demolding, find the lower limit of cooling time, and retain 10% to 20% fluctuation margin for mass production. For precision products, mold temperature sensors can be used to detect the core temperature of the product in real time to ensure that the internal temperature meets the demolding standard. Long‑term continuous production will cause mold temperature to rise, and the cooling parameters need to be rechecked regularly to ensure stable production.
To sum up, the scientific setting of injection cooling time needs to combine theoretical formula calculation, material thermal performance parameters, product structural characteristics and actual mold cooling conditions. Theoretical calculation provides a quantitative basis for parameter setting, and on‑site verification ensures the practicability of parameters. Reasonable cooling time setting can not only eliminate cooling defects such as warpage and shrinkage, but also optimize the production cycle, maximize production efficiency and ensure stable mass production quality.
