Are you interested in plastic for 1) building cages for profit, 2) weight savings, or 3) ultimate ease of cleaning and durability, or 4) some combination of the above three?
Many people assume that light weight and durability are inherent to all plastic building products. They are not. In fact, some plastic products are heavier than wood and are several times the cost.
Regardless, I think there are plastics that can be used by the do-it-yourselfer without the steep learning curves associated with thermal welders and thermal benders/forming rods.
If I were to build my own plastic cages I would only pay extra cost to save weight. If I wanted ultimate durability I would use plywood and epoxy and save money over plastics.
For weight savings 1/4" Expanded PVC is the way to go. It is a difficult material to join, however. I have experimented with different solvents and mechanical fasteners (mainly rivets) but have never found a combination I was 0 comfortable with.
In my opinion the best way for the do-it-yourselfer to join this material is with the use of 1"x1" PVC corner angle. This product can be solvent-welded along all the edges, thereby creating a very strong joint.
If you're looking to produce cages/racks in large numbers this changes things a bit.