Cans are usually aluminium (and a fairly cheap form at that as you can rely on internal pressure) so you can melt them quite easily as aluminium melts at low temperatures.
"tin" cans are usually nickel plated steel (it is fairly obvious but use a magnet to test).
To answer your question cast iron does have a fairly high melting point (certainly more than aluminium but it usually has a composition around the eutectic point of iron and carbon (quickly it is a point where metal freezes/melts without an intermediate stage and is usually a low temperature compared to the rest of the alloys of a metal). It would probably be higher than 1150 degrees C but cast iron is a somewhat generic term (the range of alloys covered by the term is huge compared to the rest of the basic steels) and with the temperatures you are playing at it will not do it wonders.
Phase diagram for iron carbon system:
http://www.calphad.com/graphs/Metastable%2...e%20Diagram.gif
Original page:
http://www.calphad.com/iron-carbon.html
Secondly baking trays for moulding, maybe 4 hits if you are lucky and cool them during it.
The other problem is the melt "sticking" to the mould on repeated use; iron alloys are used to cast aluminium but you tend to have to coat it with sulphur or something to stop it which I would not suggest you go looking for if you do not have a good reason (backyard chemistry is not one). See die casting for more on this.
To this end I would look at a ceramic "mould" and a throwaway melting pot before doing it all over a short space of time (industry has a continuous process).
You probably also want a way of removing as much paint as you can from the can (an acid bath should do it but you can also look to organic solvents) for you are not going to generate the best ingots and leaving that in is even worse.
Have a look at
http://www.backyardmetalcasting.com/ while you are at it too.
re thermite: it is a good reaction, get some rust (powder form is good so just scrape it off something rusty), some magnesium or some other method to generate heat fairly quickly (a proper sparkler or two is nice as it doubles as a fuse) and the aluminium (powder is good too).
Mix it, light it and get out of there (it is not an explosive but it is hot and aluminium oxide powder is not the nicest stuff to inhale).
Only problem here is that you are using your aluminium to do it and while it is a good cutting/welding reaction it is not a good heat source.
