(Solved): Little plastic widgets are made by extruding a polymer through a shaped die and slicing it off with ...
Little plastic widgets are made by extruding a polymer through a shaped die and slicing it off with a knife every time it reaches a certain length. These widgets then fall through the air and cool off as they do so, hopefully reaching an acceptable temperature when they land in the bin. Each widget has a maximum dimension of D=5mm and an initial temperature of 160∘C, where the ambient air temperature is 20∘C. (Use V/A=D/3 if needed.) Other useful properties include: - Polymer thermal conductivity =8.0Wm⋅K - Polymer density =900kg/m3 - Polymer specific heat =2500J/kg⋅K - Heat transfer coefficient =100W/m2K (a) The centerline temperature of the widget has to be below 150∘C when it hits the bin. How much time will that take? Is this a practical way to cool these parts? (b) Will the time be longer or shorter if a different polymer is used with one-third the thermal conductivity, but the other parameters approximately the same? Explain why.