I have a Joye 510 and yes the bottom of the carts do melt just a bit after a day or two. I don't think it would ever totally destroy the cart though, they only melt a little bit. Here I took a picture for ya:
BTW, most of what you see on that cart is just moisture. The only melted part is where it looks indented on the very end.
Also, I'm glad I found out via this and another thread that the 901 carts work on 510s. I've really been wanting those whistle tips, and the closest I can find in an offical 510 size is the "flat tips" which are still pretty thick, compared to the 901 whistle tips.
P.S. enjoy the pic, here what I had to do to get it lol:
1) tried my phone camera, not sharp enough
2) busted out my old Sony DSC-S50, took the pic
3) Realized it's USB driver is not supported on either my Win7 or 2008 box
4) Downloaded and installed Virtual PC 2007 on my Server 2008 box to run XP so I could connect the camera
5) Virtual PC attempted to install a network driver that BSOD'd the machine
6) Several safe-mode reboots and some troubleshooting later I figured this out and got the driver uninstalled
7) Realized my Acer netbook has a card reader which supports the old-*** Sony SD card, and I didn't have to do any of the above.
8) Face-palm'd
Moral of the story, don't ever install Virtual PC 2007 on Vista, 7, or 2008. I think they have a patch that works with these OSes now though.