流程图写好的第二个关键是被动语态的使用。流程图表现的是客观的工作流程或事物发展的过程。因此，我们可以尽量避免用“We should/must/need to do sth.”的主动语态，改为“Sth. Should/must/needs to be done”这样的被动语态。被动语态和情态动词的恰当搭配，可以使文章显得更具有客观、和学术化色彩。

　　The production of cement starts in a mixer, where limestone gravel and sand are convertedinto a mixture, which is then fed into a crusher. Here the mixture is ground into cementpowder, and then is passed through a rotating heater.

　　上文中用到了convert、feed、grind、pass这四个动词的过去分词构造了被动语态，使过程的描写显得非常专业。