An outer case includes an upper case, a lower case, and a plurality of guiding structures. The lower case includes a bottom wall defining a plurality of through holes. The guiding structures are arranged on the bottom wall. Each of the guiding structures includes a first guiding plate and a second guiding plate, the first guiding plate includes a first connecting part and a first blocking part connected with the first connecting part, the second guiding plate includes a second connecting part and a second blocking part connected with the second connecting part, the first connecting part and the second connecting part face each other. The first blocking part is suspended on the bottom wall through the first connecting part, the second blocking part is suspended on the bottom wall through the second connecting part, the first blocking part and the second blocking part are staggered.