Basic rules of thumb when dealing with PVC is to not let it turn to carbon and release the nasty stuff. Up to that point it can be heated and shaped or hot air welded. Being dissimilar materials will probably be a bigger issue when trying to join PVC and ABS together. While they both have similar "melt" points and burn points, they tend to stick together like oil and water. It why plastics areby ficoce - Reprappers