#119 Cooling Time of the Central Part of the Molded Article
While in the last lesson we discussed the experimental equation that predicts the cooling time for the average temperature of the wall thickness of the molded item to be cooled up to a prescribed temperature, in this lesson we explain the experimental equation that predicts the time taken for the temperature of the central part of the molded product to be cooled to a prescribed temperature.
When we want to reduce the so called "post-shrinkage" which is the shrinkage after the molded item has been taken out of the mold, it is necessary to cool the molded article inside the mold until the central part of the wall thickness of the molded item is cooled to a prescribed temperature.
The cooling time tlc for the central part of the wall thickness of the molded article to reach the temperature θe can be calculated using the following equation.
| tlc = s2 / (π2α) ln (4/π ∙ (θr - θm) / (θe -θm)) |
| tlc: The cooling time (seconds) for the central part of the wall thickness of the molded article to reach the temperature θe. |
| Where, |
| s: Wall thickness of the molded article (mm) |
| α: Heat diffusion ratio of the plastic at the cavity surface temperature (mm2/sec) |
| α = λ/ (c ∙ ρ) |
| λ = Thermal conductivity of the plastic resin(kcal/m∙h∙°C) |
| c: Specific heat of the plastic resin(kcal/kg∙°C) |
| ρ: Density of the plastic resin(kg/m3) |
| θr: Temperature of the molten plastic resin (°C) |
| θe: Temperature at the center of the wall thickness of the molded article (°C) |
| θm: Cavity surface temperature (°C) |
* References: "Molds for injection molding" (Keizo MITANI, Sigma Publications (1997))
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