North90's hot-end

This hot-end is designed to be build with simple tools and without the need for great engineering skills.

About the design

WHY another design? The complexity of most hot-end designs (lathe needed) and the use of hard to obtain and expensive materials (Peek) make it hard for the average person to build their own hot-end. This design is great for everyone who wants to build a hot-end but who doesn't have access to a lathe, cnc-machine and drill-press.

Thermal design

Each hot-end consists out of three main parts:

• hot end where the plastic is melting (usually brass tube + nozzle)
• insulator which keeps the heat away from the extruder (usually PTFE(=Teflon) rod)
• cold end the cool part of the hot-end which mounts to the extruder (can be any kind of material)

This hot-end uses stainless steel as an insulator. Stainless steel conducts heat less easily than normal steel, brass or aluminum, but much better than PTFE.

MATERIAL	THERMAL CONDUCTIVITY W/(M*K)	NOTES
PTFE	0.25	will deform when hot: can't be used as structural part
PEEK	~0.25	expensive+hard to obtain
normal steel	45
stainless steel	18
brass	120
aluminium	220
dry wood	0.05-0.12

from http://en.wikipedia.org/wiki/List_of_thermal_conductivities

The thermal conductivity of stainless steel is about a 100 times larger than that of PEEK or PTFE. To have a similar insulation as a PTFE insulator the cross-section of the insulator has to be a 100 times smaller.

A normal 16mm PTFE insulator (with a 3mm hole) has a cross-sectional surface area of 194mm2. The heat transfer for a PTFE insulator that is 25mm long at a temperature difference of 200C is 0.38W.

The stainless steel insulator of this hot-end is 6.35mm outside and an inside diameter of 5.7mm. The heat transfer for this insulator (25mm long and 200C) is about 1.1W. This is almost 3 times as high as the PTFE insulator of the same length.

The extra heat transferred through the stainless steel insulator has to be dissipated with a heat sink.