Obviously dull shears will fold instead of cut hair. But what causes new or recently sharpened shears to periodically fold instead of cut hair.
The most common cause is that the blades are not properly adjusted. When the blades are too loose the cutting edges of the blades will separate during the cutting action. Properly adjusted blades hold their contact through the full cutting stroke.
Shear blades are design with a slight curvature so that when screwed together there is a degree of spring force that keeps the cutting edges in contact throughout the cutting stroke. This is called the “set” of the blades. If shears are too loose, this spring force is lost and the edges lose contact with each other. When this happens the cutting action is compromised and the hair bends rather than cuts.
Shears that are simply too loose and not properly adjusted get mistakenly accused of being dull. Even new high quality professional shears will bend hair and appear dull if the blade tension is too loose.
The blade tension is too loose if after being fully opened and allowed to fall closed, the blades fall completely closed on their own.
The ideal tension should be such that when shears are fully opened and allowed to fall closed on their own, they should just catch right before closing completely. This is the ideal tension that optimizes the shear’s performance. As the shears begin to dull with use, increasing the tension a bit can help.
If you can tighten your shears a bit and do not feel any additional resistance, this is a distinct sign your shears were too loose.