Chapter 5, Problem 5.46P

Circuit boards are treated by heating a stack of them under high pressure, as illustrated in Problem 5.45. The platens at the top and bottom of the stack are maintained at a uniform temperature by a circulating fluid. purpose of the pressing—heating operation is to cure the epoxy, which bonds the fiberglass sheets, and impart stiffness to the boards. The cure condition is achieved when the epoxy has been maintained at or above $170°\text{C}$ for at least 5 min. The effective thermo-physical properties of the stack or book (boards and metal pressing plates) are $k=0.613\text{W/m}\cdot \text{K}$ and $\rho c_p=2.73\times {10}^6{\text{J/m}}^{\text{3}}\cdot \text{K}\text{.}$

1. If the book is initially at $15°\text{C}$ and, following application of pressure, the platens are suddenly brought to a uniform temperature of $190°\text{C,}$ calculate the elapsed time t e required for the midplane of the book to reach the cure temperature of $170°\text{C}\text{.}$
2. If, at this instant of time, $t=t_c,$ the platen temperature were reduced suddenly to $15°\text{C,}$ how much energy would have to be removed from the book by the coolant circulating in the platen. in order to return the stack to its initial uniform temperature?