From DT Online
Thermoplastic materials (e.g. acrylic, ABS, polycarbonate) soften and become malleable when heated. Line Bending is achieved using a Strip Heater which is designed to heat just a narrow strip along its length allowing a sheet of thermoplastics, heated by laying across it, to be folded to the required angle. Once cooled, the thermoplastic sheet will then retain its folded form. The width of the heated area is normally around twice the thickness of the material.
Note: It is almost impossible to create a neat 3D form, a square tube for example, by Line Bending 3 corners and gluing together the 4th - the glued corner will almost always look different. Far better to line bend 2 ‘U’ shaped forms, or even 2 right angles or some other variant, and join them together. The join will hardly ever be invisible so make a feature of it! - e.g. leave a gap by spacing the two parts in some interesting way or join them using some contrasting material.