Government invitation letter?

Our Company Managing Director and the Director needs to travel to China on a business trip by 7th of this month. We import Tea Colour Separator machines from China is the sole agent for Hefei Meiya Optoelectronics Technology Inc, Hefei, China (Manufacturer of "Meyer" Tea Colour Separators machines). Our Directors have visited China last in December 2015.

Please let us know; 1.whether they need Government invitation letter for this visit.
2. How long it will take for them to obtain their VISA