Hotends

The purpose of the hotend of a 3D printer is to melt the plastic that is fed into it, and deposit it in a controlled manner.

A hotend consists of a "cold side" which keeps filament as straight and cold as possible until it is fed into the "hot side", where the filament gets heated past its melting point via conduction from the heater block around it. Below the heater block is the nozzle, which reduces the diameter to its final width immediately before extrusion.

Key aspects of hotends to consider are:

Cooling of the cold side
Minimizing conduction of heat from the hot to the cold side
Control of the heater block temperature against the cooling effect of cold filament going in and part cooling airflow
Prevention of leaks on the hot side
Wear resistance
Rigidity
Length of meltzone
What nozzles are available

Structure

Cold Side

The cold side of the hotend is very simple: filament is guided through a narrow tube just larger than the diameter of the filament, and this tube is surrounded by (or itself made from) conductive metal (aluminum or copper) to draw heat away.

These can dissipate heat in three ways: a finned heatsink, a water cooling block, or conduction into a larger metal structure that acts as a heatsink.

At the top of the cold side is often the mounting structure for attaching the hotend to the toolhead, unless the hotend is more deeply integrated into the design of the toolhead.

At the bottom of the cold side, the filament gets delivered to the filament heatbreak, and any additional structural support for the hot side is attached.

Filament Heatbreak

The filament heatbreak moves filament from the cold side to the hot side, with the goal of minimizing heat transfer and preventing leaks.

On many older hotend designs, the filament heatbreak also is the sole support for the hot side, creating a conflict between maximizing structural strength and minimizing heat transfer.

Very old hotends have the heatbreak lined by a PTFE tube that goes all the way down to the heater block itself, which reduces friction and drastically reduces heat creep, but is a wear item and reduces the maximum temperature of the hotend.

The connection from the heatbreak to the hot side is an important consideration for reliability and performance.

Thread-in heatbreaks: these occupy the top few millimeters of the heater block. If the heatbreak is monolithic then this somewhat reduces the effective length of the melt zone. If the heatbreak is bimetallic (a thin low-conductivity tube press-fit into a threaded copper slug) then melting performance is improved but this can potentially be a source of leaks.
Brazed-in heatbreaks: these minimize the thickness of the low-conductivity metal along the filament path of the meltzone, but reduce repairability in the case of failures.
Press-fit heatbreaks: often used in integrated heatbreak nozzles, these are similar to brazed-in heatbreaks but are a leak risk.
Press-on heatbreaks: these do not intrude into the hot side at all and are pressed downward from the cold side onto the top of the hot side. These maximize the meltzone with minimal compromise, but if the pressure is insufficient then they could leak.

If the heatbreak is too conductive, the cold side will warm up too much, causing heat creep. Also if the heatbreak is too short, even if low conductivity, the cold side will also warm up too much, causing heat creep. Conversely, if the heatbreak is too long, the temperature transition between the hot and cold sides will be too gradual, resulting in mild amounts of heat creep that will increase required extruder force for a given flowrate even if it doesn't fully jam.

Structural Heatbreak

Older or more basic hotends use only the filament heatbreak to support the heater block, but this reduces the strength and stiffness of the hotend, increasing deflection of the nozzle tip under high acceleration and making the hotend susceptible to damage when tightening nozzles or if the nozzle rubs on the bed or print.

To mitigate this issue, some hotends are designed with additional structural reinforcement.

There are several methods for this.

Stainless steel tubes in compression with screws in tension (Mosquito)
Standoff screws made of titanium (Rapido 2)
Tensile filament heatbreak with tubes or zirconia columns in compression (Dragon ACE)
Truss (v9, Tricorn)
Titanium tube with cutouts (Chube)

Heater Block

Issues

Heat Creep

Heat creep occurs when filament is allowed to reach its softening temperature on the cold side of the hotend.

When this occurs, as the filament gets pressed from behind by the extruder, the softened filament bulges and jams against the walls of the filament path. No matter how hard it is pushed, flow will completely stop.

Inexperienced 3D printer users may think it's a clogged nozzle but there is actually no obstruction in the nozzle opening.

Heat creep can be caused by insufficient cooling (or actual heating) of the cold side, from a filament heatbreak that is too conductive, too short, or counterintuitively too long.