Buckle folder
In a buckle folder, the paper is first passed through 2 spinning rollers, which feed the paper into a pair of guide plates that redirect the paper at a slight angle, bending the paper. At the far end of the guide plates is a "paper stop". As the rollers continue to spin, the paper continues to slide in between the guides until it hits the paper stop, then, having nowhere else to go, buckles at the interface between the rollers and the angled guides. As the rollers continue to spin, the buckle increases, until it is eventually caught by a pair of "nip" rollers, which pull the buckle in and compresses and flattens it into a neat crease.
Adjusting the paper stop in a buckle folder determines where the fold will be placed. Also, a buckle folder may contain only 3 rollers, with one shared by both the input rollers and the nip rollers.[2]
Although buckle folders are fast, simple, efficient, and have small folding tolerances, they are not suitable for substrate of very low (<40g/m2) or very heavy paper grammage (>120g/m2).[5]
Knife folder
In a knife folder, a sheet of paper is fed horizontally over 2 unpowered rollers until it hits a paper stop, at which point a dull blade pushes down on the paper and in between the 2 rollers to create the fold.[2]
Although the knife folder is slower than buckle folders, it is more precise[2] and can handle extreme grammages very well.[citation needed] It also has small folding tolerances, but is a more complex machine.[citation needed]