Fauxpa46,
The sizing die is the first one that squeezes the outside of the case back to size and pushes the primer out. I'm thinking you are referring to a powder-through expander die that flares the case to accept a bullet.
Expanders sticking to cases are usually one of a couple of things. One is that the brass is too clean. The powder residue normally in an expander actually helps lube the expander action. If you are cleaning by wet tumbling with pins, the cases can get so smooth that they contact the steel intimately enough to not release easily from steel. Taking a bore brush on a small handle and turning it inside the case mouth a couple of times to rough it up a little can correct that.
Another source is brass gradually building up on the expander. Friction between two rubbing parts is greatest when they are made of the same metal alloy. So when brass builds up on the expander it behaves as if it is made of brass, and that sticks. The cure here is to clean the expander with a copper-dissolving bore cleaner until it is back to bare steel. Then you can take a patch with some powdered graphite on it and burnish the graphite into the steel. That will cause it to release easily for some time, and dipping the mouth of every tenth case or so in powdered graphite will keep enough there to continue to work well.
Some folks prefer to treat the expander with NuFinish and let it dry or to use another long-lasting automotive wax.
