The all roller and skatewheel type curves.
If boxes must remain oriented you should use a taper roller or a skatewheel curve section. If you use roller type curve sections, boxes should be spaced so the rollers are allowed to function properly. If boxes go through in a group or in a train, they more than likely will not stay oriented. The reason for this is that boxes behind are pushing those ahead of them.
The majority of a box or package must be on the curve section before it will turn properly. Long boxes can be a problem since they may not feel the effect of the curve until enough of the package moves onto the curve. If boxes are moving at a high rate of speed, they will not negotiate the curve properly.
In a gravity flow type system, it might be necessary to decline the curves. This is somewhat difficult since the curve must be twisted.
Material Flow & Conveyor Systems, Inc. carries and sells an excellent selection of conveyors!