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My New Hotend design

Posted by terramir 
My New Hotend design
February 26, 2012 02:53PM
Well I liked wildeye's hotend as well as some of the others, but I thought I would make some changes, some out of necessity, example here being I don't have a 5ohm 5w round resistor that can be substituted here, and some because I thought it could be a bit improved here is my sketch.


Notes: the thermistor is a honeywell that is put through wrap the leads with kapton tape to insulate place the hole as close to the copper as possible inside the aluminum.
#2 as said above you can replace the square resistor with a round one. Place the hole for the resistor as close as possible to the copper nozzle inside the aluminum.
#3 the second aluminum sheet is to have a many as possible fins cut out this is to reduce the heat transferred to the PTFE
#4 the Peek bushings are made out of 1/4 peek rod (about 4 bucks a foot) 4 mm threading on the inside 6mm on the outside into the heat sink. My hope is that much of the heat will have already been removed by the fins and the peek will further insulate the mounting screws.
#5 the nozzle is made out of a lincoln 0.025" 0.6mm welding tip which is hammered to reduce the size then drilled, however instead of compression fitting (see wildeye's work) A 6mm thread is cut out of the brass coupling this way tips can be exchanged from 3mm (1/8 3.1mm drill to a smaller size).
#6 one of the reasons for this design is that I read about problems structurally when the ptfe heats up.
#7 one or two layer's of kapton tape on the heating block may improve performance I still need to determine this, because it would keep the heat inside the block the goal is by the time the heat get's to the threading of the PTFE because of the heatsink it won't be hot enough to cause major damage.

Transfer rates of the material's involved
Brass 109 W/m.K
Aluminum 250 -255 W/m.K
Copper 400 W/m.K
PTFE 0.25 W/m.K
PEEK 0.25 W/m.K
Steel 16-19 W/m.K

Something I am Thinking of is adding small little threaded heatsinks on the 4mm steel screws from the extruder because that may prevent damage the the extruder and carriage, however I don't know if they would help or even be counter productive will have to run some tests.
both the heatsink and the heating block are compression fitted on the brass nozzle.
What do you guys and gals think feedback is appreciated

terramir

Edited 1 time(s). Last edit at 02/26/2012 02:56PM by terramir.
Re: My New Hotend design
March 04, 2012 06:55PM
Any comments folks?
about to finsh this in the next few days and take it for a test run what do you guys/gals think before I run onto the virgin voyage?
terramir
Re: My New Hotend design
March 04, 2012 06:58PM
I think you will find that it gets to warm up in the cold zone which really impedes printing in PLA.


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Re: My New Hotend design
March 04, 2012 07:03PM
well That is one opinion I would like some more any suggestions in regards to the pla issue?
terramir
Re: My New Hotend design
March 04, 2012 08:16PM
With most of the current hotends available (J-head, Makergear, Adrians) it is highly suggested to have a fan blowing across the cold zone. This is to keep the filament stiff for as long as possible. When the filament gets soft way before the tip (should be below 60c about 10mms above the bottom of the PTFE/PEEK) it starts to act like a soft noodle and requires more force to extrude and you end up with lots of hysteresis (delay from extruder turning to output). This being said the less insulating material used in the zone between cold and hot the better.

So basically I would suggest cutting the threaded portion off of the air fitting so it is not insulating the PTFE so high up.

Also the small taper you have at the top of the welding tip will cause you all kinds of issues as the filament expands and jams.

Nophead describes the issue here
And he shows how serious it is in this video (notice no nozzle)


I made this picture for someone else a while ago.



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Re: My New Hotend design
March 05, 2012 12:30AM
so if I ensure that there is no gap there I should not have a problem especially because the m4 screws holding the aluminum heatsink would cause the thermal expension to close that gap. Thanks for the input I understand what you are saying not I just need to fix my extruder so I can run some test runs, however I will not know about the PLA thing until next month or so because I can only afford to get abs this month.
terramir
Re: My New Hotend design
March 05, 2012 12:42AM
It should work really well for ABS, it seems to like being pre-heated in the barrel.

Also make sure you have the PTFE under compression from top to bottom so the pressure of the molten plastic does not push it apart. You will want to check this after having it heated for a few minutes as the PTFE gets soft and you can compress it more (careful the hole does not get smaller).


FFF Settings Calculator Gcode post processors Geometric Object Deposition Tool Blog
Tantillus.org Mini Printable Lathe How NOT to install a Pololu driver
Re: My New Hotend design
March 12, 2012 05:13PM
Here is a trick that I don't think I've shared anywhere. When the hot end is assembled, but before you run any plastic through it, take a 3.5mm drill bit down through the PTFE rod, and ream it out while the block is hot. This will remove a little bit of material that has expanded into the cavity, and reduce the likely hood of an expansion jam.

The designs like mine do well if they are not left to heat unattended for long periods without printing anything (getting hotter and hotter in the insulator). As long as you are printing, the PTFE will remain at a safe temperature.

If you do allow the hot end to sit for a while, don't try and extrude material until you have let it cycle through a cooling cycle.

-Wildseyed-
Re: My New Hotend design
March 12, 2012 09:40PM
Overheating when idling is probably an issue with a lot of hot ends.
Maybe we should add a function to existing host software to cool the heater down to a safer temperature if the motors are idle for too long?
Re: My New Hotend design
March 16, 2012 03:54PM
@ lincomatic2 My thought was to include a separate thermistor in the heat barrier for this.
Re: My New Hotend design
March 16, 2012 05:34PM
I revised the design since I started it I'm now using a threading on the outside of the copper nozzle, 7mm and I'm making a new heater block that will screw right on and not contact Oh yes another revision from brass to steel for the air hose coupling. otherwise the heatsink and the peek bushings will stay the same will update the pic up there as soon as possible.
terramir
Re: My New Hotend design
March 16, 2012 10:26PM
We think alike. I bought a steel coupling to see if it would reduce the heat transfer to the PTFE. Measured the temperature at the threads. For some strange reason, it was actually hotter than with my brass coupler, and the hot end takes longer to warm up with the steel, too.
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