Top of hot-end too hot?

Can anyone provide feedback about their experience with a simple hot-end design using an aluminum block heater like this one from mixshop? [www.mixshop.com]

I just rebuilt it with a new brass barrel, but before I try it with filament and have another "Millenium Falcon" moment, just thought it would be good to hear about others' experience with this design (which I suspect is a pretty common design). My main concern is that the top of the hot-end seems to get too hot, and the filament will probably melt / expand at the top and will cause jamming. In my previous version I tried a heat-synch arrangement between the aluminum block and the PFTE barrel, but there were a bunch of other problems with my construction, so I decided to use the KISS principle this time and just build it as suggested.

Any thoughts or comments about the simple (?) hot end construction with an aluminum block heater as shown in the link above will be appreciated.

Note: I'm using 3mm PLA filament and am using a temp of 185C.

Oh, and one other question: How far down should the filament be advanced before starting a printing operation? Should the tip of the filament start just above the brass tube, or should it be advanced down into the brass barrel near the tip? It seems that the g-code instructions dump a bunch of filament down into the tube just before the actual printing starts, so perhaps the filament should start out above the brass tube.

Edited 2 time(s). Last edit at 02/06/2012 07:06PM by canam.

I'm currently happily working with this design.

Of course that is after breaking and rebuilding it 7 times.

I've destroyed 1 nozzle. heated too much trying to clean...

ruined 3 PTFE tubes and have 2 suspect brass.

I do love the design though because of its simplicity, cost and I've learned SO much from it.

Others will tell you to get something better.

Proper assembly is tricky.

Do buy a tap to make the threads. My first go was drilled wide enough to thread the brass in by hand. This worked fine for a few weeks of testing before I had a wire on the heater come off and the extruder push the brass out. It wouldn't stay in after that.

On the units I taped the threads into, I only tap to the same distance as the drill. 10mm. As the tap is tapered this causes the last quarter or about of the PTFE to not have threads. This makes it a little harder to thread the brass all the way into the hole, but as it is cutting its own deep threads, it's more sturdy. I've actually put as much force on the filament as I could trying to clear previous jams and have yet to dislodge the brass. The extruder hasn't either. It just strips the filament if the nozzle is clogged or I'm pushing too much filament down.

I also use a drill after threading the brass into the PTFE to ensure smooth connection where the Brass and PTFE meet inside.

I've only experienced jamming when I:

a) Ran the brass too far up the PTFE causing molten plastic to reach the top and pool or adhere to the extruder.

b) Overheated my PLA and left burnt plastic in the hotend.

So, I keep no more that 10mm of brass in the PTFE

I've used both ABS and PLA now and have no problems restarting/heating after some time off.

Starting:

You need to pressurize the print head/nozzle before printing.

This means either extruding a length of filament out the nozzle before hitting print or (apparently in your case) having the gcode 'purge' the head before starting.

The filiment should be all the way down the barrel before heating. Watching for the ooze to know you are close to the right temp for the plastic you are using. Different pla's or abs's melt at different temps.

Hope this helps!

Edited 3 time(s). Last edit at 02/06/2012 09:40PM by Komb'.

Thank you for the notes. Sounds like your experiences have been similar to mine. I was telling one of my kids a few weeks ago about how much I've learned from the reprap experience -- your comments certainly support that view. Lots of fun!

I gave up on that design after breaking a couple trying to get them apart again, for some reason for me they always leaked out of the top. I now use a Jhead for convenience because I do a lot of printing for other people. My printer has been running consistantly 8 hours a day 5 days a week for nearly a year now.

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Experimenting in 3D in New Zealand

I have no experiance using this design but would stay away from any hot end design that uses a joint between brass and PTFE to hold it all togather. Because PTFE softens due to the heat, you are always on the edge of pulling the threads out. Also the purpose of that long section between the actual hot end and the extruder body is to insulate hot temperatures from reaching the extruder body. They best way to insulate the extruder body is to cool the tube. PTFE on the outside acts as an insulator keeping the heat in the brass, allowing it to travel up to the extruder. So in my opinion, any hot end that uses PTFE on the outside, and as a structural element of the hot end, is a poor design.
There are many desciptions of good designs in these forums. Good luck.

I was able to get this design of hotend to work for about 10hours before the pressure deformed the PTFE and made the bore larger than the brass barrel. This resulted in a complete failure. I (and many others) improved the design by using PEEK rod as a structural component with a PTFE tube that is press fit into the centre. PEEK is an excellent insulator and is structurally sound at extrusion temperatures. Increasing the bore of the barrel to 3.5mm and reaming the hole after drilling greatly reduced the amount of force required to extrude PLA. I leave the bore of the inner PTFE tube at 1/8th or ~3.18mm. The insulator is 70mm long and I have no problems with overheating or up-flow of molten plastic. It also runs fanless.

Also I'm finding that a 6.8 ohm resistor doesn't have the power to extrude at high speeds. I'm working on using wound nichrome elements in parallel to produce a more even and higher wattage heater.

I must admit, I haven't had my rig running for that long at a stretch.... yet.

Would a plate holding the brass, mounted just below the PTFE solve or at least delay this problem?

It may delay it, but it would not solve the failure I experienced. IMHO PEEK is the way to go.

Another way to delay the inevitable is the good old hose clamp, round the low end of the PTFE barrel.
I have one of these "classic" designs, and with the wrap-around "girdle" have had no hint of movement in the brass bolt,

I had to add a fan on the upper part of the barrel to reduce the PTFE "grabbing" on hot/expanding PLA, plus a simple heatsink, (and a wind-shield on the hot-end of the hot-end). It was getting to 60 deg on the outside where the PTFE disappeared into the X-carriage, now it's well below that.

And after a couple of jam-drillouts, I think there must now be enough slop down near the junction with the brass to allow molten up-flow, which then reaches the nicely cooled part of the PTFE, and again grabs and stops the PLA.

(I like the simplicity of the design, but to broaden my exposure to designs, will be next trying probably a Stainless one (a la North90) , and although I will continue to experiment I also have a J-head coming for those times I really want prints-without-hassles)

Edited 1 time(s). Last edit at 02/09/2012 01:56AM by nb99.

I like the wrap-around "girdle" idea!

I think I'll implement it.

Regarding the PTFE/Brass connection I HIGHLY suggesting trashing the idea of threading the PTFE. Rather install a thread insert (heli-coil) [en.wikipedia.org] that has m6 ID threads.
I had a simular issue where heat and pressure caused the brass bolt to press out. Since installing a heli-coil I have had no issues with the brass pressing out.