I decided to set up a company of my own after graduation. 这里的graduate为什么不是用ing,

问题描述:

I decided to set up a company of my own after graduation. 这里的graduate为什么不是用ing,
介词后不是用动词ing吗

这里应该用名词,所以是graduation,如果应定要用graduating(动名词可以用),因为graduate是及物动词,一定要用graduating from school是。我觉得上面那位的回答也对