>>I plan on building quite a few cages from pvc sheets and would like some input on the different thicknesses. Average cage size would be 5ft x 30inches x 14 or 16 inches high.
First, I'm assuming you mean expanded PVC, not regular PVC. If you mean regular PVC I have no experience with that.
1/4" at a minimum. 1/2" would be easier to work with but is more than twice as expensive and is sometimes more than twice as heavy (at least two brands of expanded PVC go to a higher density for their 1/2" product).
If cost and/or weight are most important to you then consider 1/4". 1/2" would be better for ease of construction and if you don't mind a heavier cage.
>> Also building ladder racks for 41 qt sterilites.
Well this may change my opinion. When you say ladder racks I assume you mean racks where sections of the shelving pieces run unsupported by the sides. In this case I'd recommend 1/2" material. If the racks will be more than 4' tall then I definately recommend 1/2".
It might be easier for you to just order a large quantity of 1/2" material and be done with it. But the 1/2" material is probably not absolutely necessary for the cages, so you might also mix and match.
Also, you should find a source of 5'x10' material rather than 4'x8'. You'll likely waste a lot less material for both 5'x30" cages and 41 qt. racks.
>> Would chemical bonding or welding be better? Or a mix of both?
Chemical bonding is a type of welding, also called solvent welding. Regular welding is called thermal (heat) welding in this case.
For do-it-yourself projects thermal welding is not necessary. Unless you have the welder, of course.
For the 1/4" material joining panels with solvent welds and PVC angle is all you need. For 1/2" material the joints can be solvent welded and then reinforced with rivets or screws. The ability to take rivets and screws is what makes the 1/2" material so easy to work with.
It is possible to use rivets with the 1/4" material but it is tedious and precise work. Even with all the care in the world you can still split the material. I don't recommend it.
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Current snakes:
1.1 Gonyosoma oxycephala - (Silver/Yellow)
3.4 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)