The tried and tested way of bending ABS pipe (and some other plastic pipes such as PVC) is via heat bending. When undertaking any type of heat pipe bending make sure the area is well ventilated. Ensure you use protective equipment such as gloves, goggles, and apron and so forth to prevent damage or injury. At no point should ABS pipe be exposed to excessive temperatures or naked flames.
To bend ABS pipe correctly requires that the requisite amount of heat be applied uniformly to the length of pipe that is to be bent. There are several common heating methods in use these are hot air ovens or guns, electric pipe heating blankets and flameless hot gas torches. The temperature and amount of heat required will vary according to the size and thickness of the pipe and the angle of the bend required. Excessive exposure to heat can also cause the pipe to distort; it can take a great deal of time and practise to get this method correct.
With small bends up to about 30 degrees maximum you can attempt to bend the pipe without any internal support. For bends over this you will need to provide internal support to the pipe it prevent collapsing. This is generally done by plugging or capping the ends of the pipe after filling it with fine grain sand or perlite (cat litter). Ensure it is compacted as much as possible to prevent air pockets. These materials provide both support and distribute heat evenly. An alternative (and lighter for long lengths of pipe) is to consider using air pressure though this method is more involved and skilled.
If making a number of identical bends it can be beneficial to create a forming rig or plywood jig to ensure each bend is as uniform and identical as possible. Clamps can also be useful to hold it in form as it cools. As a rule of thumb any bend should be at least fives times the outer diameter of the pipe to prevent flattening of the pipe whilst bending. Once the bend has been achieved cool the bend down via the application of water to “set” the pipe in place.
Take heed that any form of heat bending will induce stress in the pipe that can be retained after the bend has been made. Therefore heat bending ABS pipe should only be used in applications where it is installed in ambient temperature and is not subjected to its maximum pressure. Also bear in mind the fine grain sand or perlite can cause slight pitting to the interior of the pipe.