China flap disc commonly has two types of bend and flat. As the name refers, the flat flap disc has a smooth surface, while the bend flap disc has the surface with an upward curvature. There are indeed big differences between these two types from the appearance.
Some one believe that the curves in the grinding surface can keep bend flap disc better contacting to work pieces, in addition, the cutting speed and surface effects are more uniform, resulting in a better grinding effect than the flat flap disc. But in fact, with the wear of flap disc surface after long time use, there will be some curves in flat flap disc surface as well, leaving the same grinding effect as the bend flap disc.