Ways to cast a rod shape out of resin?
So assuming you want it solid and not hollow. I really think the comment Chris H made about the pre-cast acrylic rod would be a much better option financially and structurally.

If you do want to go down the route of casting however, the pipe option is one you could do with. If you ensure there's mold release all down the inside of the pipe, giving it a push at the end providing it doesn't decide to fuse with the pipe - lots of people de-mold with air guns too. However, you have to allow for the fact that when resin is curing, it heats up. So even if you make sure you buy the straightest pipe imaginable, you can still run the risk of it warping and trying to fuse with the pipe.

Heat and general casting also create the other issue - air bubbles. If this is a life size model, then you probably wont find a vaccum chamber to remove the bubbles. You Might get away with it if you have a lot of time to let the air bubbles out - like a resin that cures over a few hours, but if any remain, they will catch any light you throw up though the saber and will stick out like a sore thumb.

I cast a huge chimney before for a giant replica of the Battersea Power Station. To do this, the blank was made - originally turned by one of the woodworkers on the team, and then detailed ribbing was added. This blank was then cast in silicone rubber using a large pipe it could sit into. This also involved a plug being made too, as the top half was hollow on the inside to reduce weight. Once the silicone jacket was made, before removing the chimney, we removed it from the pipe and created a 3 part (two sides and a bottom) fiber glass jacket which could be bolted together to make sure it kept it's shape. Something that large will not stay straight on it's own. This is the kind of method I think you'd need to use if you want a smooth rod that will be straight and usable. 

The other thing you need to consider is the structural integrity of the rod. A lot of clear resins would not be very shock absorbing if they're cast that long and thin. One bad smack could cause it to break at the hilt - which if you make the mold wouldn't be too bad. An Acrylic Rod would be able to stand up to a bit of abuse, and you can chose to frost it if you want to catch the light better. You can also shape the top so it has a nice dome finish like lightsabers would have.

Lastly, if it's something that's going to take a lot of abuse, no just for display etc. I would recommend getting a Ludosport saber, or at least looking into what kind of polycarbon their lightsabers are made of, as they can withstand a lot and still look rigid.

I'm sorry I know it sounds fairly negative, and I don't mean to put you off casting if that's what you want to try. I'm just trying to highlight what you're trying to cast is not normally something that would be done when there are other cheaper less time consuming options available. If you want me to go into more detail of how you could use a silicone jacket to make it I can do.