In recent months, we've encountered some cases of hot-end problems with designs that rely on PTFE tubes or inserts requiring a tight seal against the hot end. On such designs, improper sealing of the tube/insert creates a gap that could lead to the formation of a small plug that resists the smooth flow of filament. This sometimes results in a "blow out" of the bowden tube, or poor extruder performance due to the increased resistance.
This image from the Ultimaker wiki is a good reminder on why those tubes or inserts must be installed fully flush against the hot-end to keep the filament feeding smoothly.
Fortunately, if the plug is small or is mostly contained within the tube or insert, it's possible to work the plugged tube out for service and to clear the jam.
In one extreme case, however, we had a customer who had his bowden tube fully sealed against the hot-end and the melted plastic actually worked its way up the inside of the bowden tube for some distance before fully sealing up the assembly. In that case, the stuffed tube section had to be discarded.
When shortening a bowden tube, be sure to use a sharp blade and make a straight, square and clean cut so that the tube can seal tightly against the hot-end. A pencil sharpener to slightly chamfer the tube may be helpful. When reinstalling the tube, spin the tube within the fitting after the tube is in the proper position. This will create a slight groove in the tube that should help the fitting have a better hold on the tube.
