http://www.ibmwr.org/ktech/saddlebag-repair.shtml:
By Tracy DesLaurier
September 1995
Regarding plastic welding/repairs to your bike parts:

K-bike bags are made from two different types of plastic - the inner sides High Density Polyethylene (HDPE), and the outers Acrylonitrile Butadiene Styrene (ABS). Don't try to repair them using the same materials. HDPE needs to be welded by using a device similar to a hair dryer, but it provides a much hotter and more directed source of heat. The heat gun may force air through it while warming up or cooling down, but it is imperative that the air supply be switched to nitrogen before attempting the weld or it will not stick properly.

The ABS side can be repaired using a "bodied solvent", but this type of repair is probably suitable only for very small holes, or to reinforce an area with another piece of ABS stuck to its backside. ABS can also be hot air welded, and nitrogen is not absolutely required, but better results are achieved by using it.

Three years ago I had a bag fall off on me and repaired it myself using these techniques at my dad's plastic business. The bag is holding up 100%. BTW, for repairs to fairings, do not use these techniques, as they are primarily fibreglass. Use conventional fibreglass repair techniques, but substitute epoxy resin for polyester resin on major repairs.