# The mold that is used in an injection molding process is constructed of metal (p = 7800

## Question:

The mold that is used in an injection molding process is constructed of metal (p = 7800 kg/m3, c = 450 J/kg ∙ K). The mold, to be heated to 190°C prior to injection of the thermoplastic material, must be subsequently cooled before ejection of the finished part. Pressurized water at 30°C is available for cooling. The mold has dimensions 50 mm x 100 mm x 40 mm and the mold designer must specify inclusion of N cooling passages, of diameter 5 mm, to be machined into the mold. If one passage can be placed every 10 mm along the length or width of the mold, the designer can therefore specify either five 100-mm-Iong passages or ten 50-mm-Iong passages. The total mass flow rate of water, divided equally among the channels, is 0.02 kg/s. Which configuration (N = 5 long passages or N = 10 short passages) should the mold designer specify in order to cool the mold faster and, in turn, increase the number of parts that can be manufactured daily? What is the initial rate of cooling of the mold (oC/s)? The velocity profile in each channel is fully developed prior to entering the hot mold. Neglect the mass of the thermoplastic part.

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