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#019 Molding Cycle and Cooling Time

Category : Technical Calculations
November13, 2009

While plastic injection molds are required to have the functions for producing molded parts with the desired quality, at the same time it is also required that production be possible at the lowest possible production cost.

The cycle of plastic injection molding is defined as follows.

Molding cycle t (sec) = t1 + t2 + t3 + t4, where, t1 is the injection time = injection time + dwelling time (sec), t2 is the cooling time (sec), t3 is the time (sec) needed to remove the molded product, and t4 is the time (sec) needed to open and close the mold.

Among the factors determining the molding cycle, the one that is the most important is the cooling time t2. Cooling time is the time from filling the inside of the cavity with molten plastic to the sealing of the gate until the plastic solidifies. From experience, it is known that the cooling time varies depending on the cooling capacity of the cavity of the mold. In addition, it also varies depending on the type of molding material and the wall thickness of the molded product. Predicting what the optimum cooling time is during the mold design stage is a very important matter in estimating the production cost of the molded product. While recently software products have come on the market that predict the cooling time by CAE, in general, the following experimental equation is used for predicting the cooling time.

tla = s2 / (π2•α) ln(8/π2•(θr - θm) / (θe - θm)), where,
tla is the cooling time (sec) related to the average temperature of the wall thickness; s is the wall thickness (mm) of the molded product; α is the heat diffusion rate of the plastic at the cavity surface temperature (mm2/sec), α = λ/(c•ρ); λ is the coefficient of thermal conductivity of the plastic (kcal/m•h•°C); c is the specific heat of the plastic (kcal/kg•°C); ρ is the density of the plastic (kg/m3); θr is the temperature of the molten plastic (°C); θe is the temperature for taking out the molded product (°C); and θm is the cavity surface temperature (°C).

* Reference: "Molds for Injection Molding" by Keizo Mitani, Sigma Publications, 1997 (in Japanese)