Solvent welding using acetone is the best technique I've found for repairing ABS. I've done it successfully on several computer cases and car parts. I've even been able to salvage hopeless looking screw holes that shattered into three or four pieces. Don't worry about the verbs melt or weld - it's really not any harder than applying CA glue.
I'd practice on a piece of junk ABS first. I use a q-tip (aka cotton swab, cotton bud - a little piece of cotton on a stick) to apply a thin coat of acetone to each side of the broken piece, then press them together. If it's a piece that will be visible, try to bias the acetone toward the inside corner, leaving the outside corner dry. The pieces still stick together within 10 seconds, then just let it sit to evaporate. After the acetone evaporates, I usually go back and add another coat or two to the inside face of the crack to get the plastic to flow into the crack a little more.
Acetone will immediately change the appearance of the surface, so be careful to keep it toward the inside face.
I agree that the CA residue may interfere with the weld. If there's room to affix a plate on the inside, as suggested earlier, I would try to weld a plate cut from some junk ABS.
You might be able to sand back the glued surface to reveal clean ABS, bevelling the wall toward the inside of the case and leaving the outer face untouched. After applying acetone, add some bits of pulverized ABS to fill the gap. I haven't tried this technique. It might be tricky to get a bevel on the thin wall section, and it would leave a thin point on the outside face that may be easier for acetone to leak through and make a visible mark.
In my experience, the most common things to break on plastic parts are tabs and clips that hold pieces together. They tend to break right where they flex. Due to the need to flex, those will probably always be a marginal repair. Every time I've tried to repair a tab with CA, it breaks as it flexes on reassembly. Solvent welded tabs will usually go back together at least once.
Super glues also seem to be a one-shot deal in my experience. The first repair is reasonably strong, but subsequent repairs fail quickly. Since you're not adding foreign material to the joint, solvent welding works about as well on later repairs as on the first.
Robert
