The hotend. Operation and thermal performance

Operation and Thermal Behavior of a Hotend

The extruder pushes the filament into the hotend, creating pressure inside it. When reaching the nozzle, the filament melts and exits due to the pressure generated by the extruder. For the filament to extrude correctly, it is essential that it only melts in the nozzle area and remains cool throughout the previous path.

Image 1: Hotend Temperature Scheme. Source: Filament2print

That's why all hotends are thermally divided into two zones:

• The hot zone, which must always remain above the filament melting temperature.
• The cold zone, which must always be below the material's glass transition temperature (Tg).

Keeping the hot zone above the filament melting temperature is simple, as it depends solely on the heat applied by the heating cartridge. However, keeping the cold zone below the Tg can be challenging. The fundamental element for this is the heatbreak or barrel, the only element that is part of both the hot and cold zones.

Types of Heatbreaks and Heat Dissipation Optimization

The heatbreak serves as a thermal break, physically separating the hot zone from the cold zone. There are fundamentally two types:

• All-metal: Made entirely of metal. They usually use metals with low heat transfer coefficients, with steel and titanium being the most common. Within the all-metal heatbreaks, there is a subtype that provides better thermal performance, the bimetallic heatbreaks. In this type of heatbreaks, two different metals are used for the inner and outer zones, one with a low heat transfer coefficient that acts as a thermal break and another with a high heat transfer coefficient that conducts heat to the heat sink.
• Non all-metal: These types of heatbreaks have a PTFE insert inside that serves as a thermal insulator. They have the limitation of not being suitable for printing temperatures above 240°C, but they are the most suitable for printing PLA.

Image 2: Design of a bimetallic all-metal heatbreak. Source: Slice engineering

For the heatbreak to have adequate thermal behavior, it must always be in contact with a temperature dissipation system, generally consisting of a finned heat sink and a fan. In this case, it is essential to maximize heat transfer from the heatbreak to the heat sink, so thermal paste should be applied at their junction and ensure maximum contact. Likewise, heat transfer between the heating block and the heatbreak must be minimized, so thermal paste should never be applied at their junction.

Closed Printers and Heated Chambers

There is a particular case where thermal control of the hotend becomes complicated. These are printers with closed or heated chambers. Through typical heat dissipation systems based on heat sinks and fans, the lowest temperature that can be reached is the ambient temperature.

When using closed printers, especially those with heated chambers, the ambient temperature inside is close to the material's Tg, so the hotend's cold zone will often be above this. To solve this problem, it is common for printers with heated chambers to have heat dissipation systems using liquid cooling, capable of extracting heat from the hotend to the outside of the printer. Liquid cooling systems add some complexity to maintenance, requiring frequent checks of the pump, pipes, and coolant levels.

Image 3: Hotend with heat sink based on liquid cooling. Source: Dyze Design.

In the case of closed printers without heated chambers, the temperatures reached are usually not excessively high for most materials, with PLA being the only problematic one. To print PLA on closed printers with hotends without liquid cooling, it is important to keep the printer open during printing.
Problems caused by poor thermal performance of the hotend

The main problem resulting from inadequate thermal performance is jams caused by filament softening in the cold zone. This is why failures mainly occur with filaments with a low Tg, such as PLA. This problem is known as heat creep and is one of the most common in PLA printing.

When jams and extrusion problems are observed with PLA, which disappear when using other materials such as PETg or ABS, it is usually a symptom of a problem in heat dissipation. Generally, this is solved by reapplying thermal paste at the junction between the heatbreak and the heat sink.

Hotend Limitations

One of the main limitations to consider with a hotend is its ability to melt a certain volume of plastic per unit of time. This is known as the maximum volumetric flow rate and mainly limits the maximum printing speed. The volumetric flow rate is obtained from the product of the layer height by the extrusion width and by the speed. That's why the maximum speed at which a specific hotend can print is lower the higher the configured layer height or the larger the diameter of the nozzle used. Some manufacturers, especially those of high-quality hotends, provide the maximum volumetric flow rate among the specifications.