Causes: During impregnation the layers of felt can slide over each other while passing through the nip rollers. The main force causing the inner layer(s) to pass through the nip rollers is the friction between the inner and outer layers.
Applying too much force by impregnating too quickly will overcome that friction causing the inner to move along inside the outer coated layer and bunch up.
In the extreme examples below (Figures 1 and 2), the inner layer has gathered up as the liner was being impregnated.
The tighter the nip roller and the faster the wet out proceeds, the more likely it is that this bunching problem will occur.
Bunching of this sort can result in circumferential fins and undulations.