Hi, Sue. I've been doing this on a 510 for several months, and mine do too, a little. But it doesn't bother me....I mean I don't taste burnt plastic or anything. You're supposed to match up the long sides of the cart with the little white ceramic thingy at the bottom that sorta looks like a figure 8.....(not sure that's very clear....lol) Anyway, one thing I think helps is that I pull my filler up a tiny but above the top of the cart so that it makes good contact with the bridge.... I don't know, but that seems to help. Also....I use blue foam as my filler. If you haven't tried it, you should. It wicks a LOT better than the stock filler. You can see about it on youtube. Good luck. I still use all my carts even if they got a little melted.
Jan