Wednesday, December 24, 2014

J-Head Mk 8 Prototype

A J-Head Mk 8 hot-end prototype has been machined using concepts borrowed from the Mk IV, Mk VI, and Mk VII J-Head hot-ends as well as the mini J-Head Mk II.

In order to accommodate 3mm filament, and use the axial thermistor, I would have to re-visit the larger heater blocks as used with the Mk IV-B and earlier hot-ends.  Since the market is moving towards 1.75mm filament, I decided to design the new Mk 8 for 1.75mm filament only.  I'll continue to make Mk V through Mk VII variants for users of 3mm filament.

The goals of the Mk 8 hot-end were to fix two main problems.  The first problem is that sealing the threads with ptfe tape still results in a minimal number of failures due to leakage.  So, I re-added the tapered internal sealing surface between the brass and PEEK as was used in Mk IV and earlier hot-ends.  In theory, no ptfe tape should be required.  However, I will still use it so as to provide for a second seal.

The second issue to resolve was related to the thermistor.  While dual thermistors will allow for a safe shut-down in the event that one thermistor falls out, it would be better to make for an easier thermistor installation.  Therefore, I decided that the Mk 8 would use an axial thermistor like the 1.75mm MK VI hot-ends.  Unfortunately, there is not enough space to properly install an axial thermistor in 3mm hot-ends. 

Since this hot-end is for 1.75mm only, and 2mm/4mm tubing had been tested with the mini J-Head, I decided to switch over to using the smaller tubing as a liner.

The cooling vent design has been borrowed from the Mk VII design, which was derived from a modified Mk II.  Originally, the fluted design was used to simply lighten an old Mk II hot-end.  However, it also helped considerably to cool the core of the hot-end.  So, the fluted design was adapted for use on the Mk VII design as well.  For the Mk 8, the flutes were modified so as to be closer to the core of the PEEK nozzle holder.

A set-screw stack of a solid set-screw and hollow set-screw has also been borrowed from the Mk VI and Mk VII designs so as to secure the heater cartridge.  By using a hollow set-screw, as the top set-screw, it is possible to loosen or tighten both set-screws without having to remove them.  The top, hollow, set-screw is used to jam against the bottom set-screw and lock it in place.

In the near future, I am going to run off a limited production run of these hot-ends for both field testing and to explore additional minor design improvements.

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