Q&A for PORTION of scheduled webinar

Hi there!

I've been hosting some webinars for some time. In our webinar format, we start with a 30 minute presentation before heading into an hour long Q&A. The challenge is the audience often asks questions during the presentation, whereas we're prefer for them to save them until the end.

Can we set up the Q&A so that it opens at a specific time? (ie after the 30 minute presentation is complete). 

Or is there a way to turn the Q&A on after the presentation?