wait... but i'm curious. To assemble the headlights back, you don't need to put any additional silicon or anything? You just push it in after it's been heated?
There is no need to put any additional silicon or cement in place as long as:
a) when you pull it apart, you at least try to keep all the cement in it's original place. The stuff becomes stringy, so it's pretty easy to keep it in its place.
b) you don't do this all the time. I have taken one set of headlights and baked them up to 5 times. I never needed to add anything. But if you continue to do this, the cement loses it's strength and is harder to keep in place. You may want to add additional cement then. But for one or two times, there is no need.