Hot End Design Theory
The hot end is arguably the most complex aspect of 3d printers as it deals with the tricky business of melting and extruding plastic filament. To understand the design features of hot ends, you must have a basic knowledge of the thermal properties of thermoplastics, specifically the way they behave at their glass transition temperature (Tg).
At temperatures below Tg, thermoplastics retain their hard, solid consistency (as we see in plastic filament). As the temp rises above the Tg of the thermoplastic, its consistency changes from solid to rubbery and it begins to expand.
If you continue to increase the temperature to the melting temperature (Tm) the consistency will change from rubbery to liquid. The plastic filament enters the hot end in its solid state and exits in its liquid state. Within the hot end there is the crucial thermal transition from solid to rubbery to liquid.
In its rubbery state, the plastic will expand and grip the inside of the hot end and will resist extrusion/retraction and thus increase the likelihood of the hot end jamming. As a result, the hot end developer makes an effort to mitigate this problem by reducing the area that the rubbery plastic can grip and cause jams (by shortening the transition zone), and by reducing the friction between the rubbery plastic and the interior walls of the hot end (by polishing the internal pathway within the hot end). This rubbery filament problem is more apparent when extruding PLA which has a very low Tg (about 60C).