18. Let the perimeter of a surface development of a $5 \mathrm{~cm} \times 4 \mathrm{~cm} \times 3 \mathrm{~cm}$ rectangular prism be $n \mathrm{~cm}$. Then the minimum value of $n$ is $\qquad$ .
18.50.

As shown in Figure 9, the perimeter of the unfolded rectangular prism is
$$
8 c+4 b+2 a \text {. }
$$

Therefore, the minimum value of the perimeter is
$$
\begin{array}{l}
8 \times 3+4 \times 4+2 \times 5 \\
=50 .
\end{array}
$$