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Cautionary tale on nozzle changing technique  

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djholm
(@djholm)
Active Member
Cautionary tale on nozzle changing technique

I am quite new to FDM printing, having built my first (MK3s kit) about a month ago.  When I was slicing the parts for the Lack enclosure, I decided to heck with printing for 24+ hours with a 0.4mm nozzle and changed to both 0.6mm and 0.8mm nozzles.  So far so good!

I should note here that there are contradictory instructions for nozzle changes within the Prusa web pages and YouTube videos, and indeed in the wider Internet E3D V6 discussions.  The Prusa video and their 3D Printing Handbook both advise to heat the nozzle to 285ºC, use a spanner and unscrew the heat block a bit (45º max), then use a 7mm socket to unscrew the nozzle.  Re-assembly is the opposite, tighten the new nozzle with the 7mm socket, then screw in the heat block (undo the prior 45º movement).  In the Prusa Help pages, the procedure does not specify moving the heat block, rather just holding it with a spanner and using the 7mm socket to remove and install the new nozzle.

After a number of nozzle changes over the past 4 weeks, two days ago I was printing PETG and horrors, big boogers on my print!  I was using a heat sock, and discovered that filament was leaking both between the nozzle and the heat block, and above the heat block (between the heat break and the heat block).  And thanks to the sock, everything was now nicely entombed in PETG.

What to do?  Well, the heat block seemed hopeless, so I entered an order at PrintedSolid for replacement parts.  But that’s going to take a few days, and I want to play!  Since the heat block was such a mess anyway, I threw caution to the wind,  heated it up, loosened the screws for the heater cartridge and the thermistor, pulled them out (while hot) and immediately shut down heating, and then went to work on disassembling everything.  An hours’ work with a heat gun, plus cleaning threads with an M6 die (heat break) and and M6 tap (heat block), and everything was ready to put back together.  The heat cartridge and thermistor had PETG on them, but heating  them for 20 seconds (just a minimal set point of 40ºC - meaningless since they weren’t in thermal contact) was enough to soften the PETG and allow me to insert both into the heat block.  Success, and now I’ll have some spare parts arriving.  The only weird glitch was that initially the thermistor was reading about 5ºC lower than ambient when cool (so about a 200 kOhm rather than 100 kOhm thermistor) and that resulted in mintemp errors.  After a couple of hours of printing the thermistor is back to normal.  

Now for the diagnosis.  I discovered that my final nozzle change before the bad leak had resulted in no gap at all between the nozzle and the heat block.  This means that the heat break had moved (unscrewed) upwards over the course of my various nozzle changes.  Because the nozzle was not tightened against the heat break, there was no seal between the two parts and filament leaked.  I believe the problem is with the 45º rotations of the heat block.  Because the heat sink (the upper part of the E3D) is not tightly held by the extruder body, it’s possible for rotations of the heat block to not screw or unscrew the heat break, but to instead rotate the heat sink.  A combination of heat sink and heat break rotations can result in the heat break moving up relative to the heat block.  As long as the nozzle still reaches the heat break (as evidenced by any gap between the nozzle and the heat block when the nozzle is tight), no problem.  Note that z-height (i.e., your first layer) isn’t affected by this relative movement of nozzle and heat break to heat block.

So my strong conclusion is that it’s probably best to follow the version of nozzle changing instructions that doesn’t include rotating the heat block.  Probably if the heat sink fits tightly in the extruder body (so it doesn’t rotate) this won’t happen, and maybe some teflon or scotch tape should be added to the top of the heat sink so that it does fit tightly.  Also, after tightening the nozzle there must be a gap between the top of the nozzle and the heat block, otherwise a leak is bound to happen.

Posted : 18/07/2019 4:04 am
Dave Avery
(@dave-avery)
Honorable Member
RE: Cautionary tale on nozzle changing technique

actually the top of the lower section of the heat break should be about flush with the top of the heat block and the nozzle should end up about 1/4 turn from being tight against the heat block and torqued against the heat break after heating the assembly to around 280C. the top of the heat break should be seated in the heat sink and that threaded section should end up about flush with the bottom of the heat sink. this leaves a gap the height of the narrow section of the heat break between the heat sink and heat block.

one trick to help replacing the heat break is to leave the nozzle and heat break tight in the heat block and use that assembly to apply torque against the heat sink to loosen the heat break from the heat sink. then you can heat up the heat block and easily loosen the nozzle from the heat break 

the nozzle height is set by the stacked heights of the heat sink,heat break and nozzle

 

 

Posted : 18/07/2019 3:52 pm
Bunny Science
(@bunny-science)
Noble Member
RE: Cautionary tale on nozzle changing technique

Thanks for the cautionary tale. 

 

Posted : 18/07/2019 5:23 pm
djholm liked
scott.b47
(@scott-b47)
Trusted Member
RE: Cautionary tale on nozzle changing technique

Can someone share the reason for the 45 degree rotation of the heat break in the Prusa instructions? It seems to be introducing an additional variable in the procedure, as you could end up inadvertently rotating the heatsink as the OP describes.

Posted : 30/07/2019 2:22 pm
djholm
(@djholm)
Active Member
Topic starter answered:
RE: Cautionary tale on nozzle changing technique

I’m guessing the strategy behind the 45º rotation is to relieve the heat pipe to nozzle compression (that is, unscrew the nozzle and heater block from the heat pipe, lowering both relative to the heat pipe).  That would make the nozzle easy to remove from the heater block, and minimize the risk of bending or breaking the heat pipe.

I’m finding that for me at least, the heat pipe is actually pretty rugged.  At high temperatures probably it is susceptible to damage from bending motions, but that’s pretty easy to avoid with a well-positioned spanner on the heat sink.

I’m in the middle of building a second mk3s via parts and the Bear upgrade frame.  I’ve attempted to better secure the heat sink within the R4 extruder body by wrapping it with Teflon tape, but I don’t think that’s enough.  Some sort of mechanical stop is needed.  I’m going to avoid any intentional heater block rotations on all future nozzle changes.

Posted : 30/07/2019 2:50 pm
--
 --
(@)
Illustrious Member
RE: Cautionary tale on nozzle changing technique

I'm pretty sure the idea behind the 45 degree rotation is to loosen the heat break from the heat sink; some earlier discussions indicate that was once a best practice.  But in the MK3/S, I suspect the heat sink simply rotates in the extruder so the action of twisting 45 does nothing constructive.

Personally, I now just heat, unload, loosen the nozzle, replace nozzle, heat, tighten to spec. Load. Print.  

 

Thinking out loud... I'd like to think E3D would put a flat on the heat sink grip zone to aide in location/alignment and rotational strength. Makes only a bit of sense that it is not indexed in any way... but everything lower is threaded, no real way to ensure the heater block is square with the heat sink flat (if it had one).

 

Posted : 30/07/2019 9:49 pm
Dave Avery
(@dave-avery)
Honorable Member
RE: Cautionary tale on nozzle changing technique

my new favorite tool for easy nozzle changing

 

https://www.thingiverse.com/thing:3441470

 

 

 

Posted : 31/07/2019 4:29 am
djholm liked
ben_r_
(@ben_r_)
Trusted Member
RE: Cautionary tale on nozzle changing technique
Posted by: david.a66

my new favorite tool for easy nozzle changing

 

https://www.thingiverse.com/thing:3441470

 

 

 

Um... Because a crecent wrench / adjustable spanner doesnt work?!

This post was modified 2 years ago by ben_r_
If at first you don't succeed, redefine success!...
Posted : 02/09/2019 8:03 pm
bobstro
(@bobstro)
Illustrious Member
RE: Cautionary tale on nozzle changing technique
Posted by: david.a66

my new favorite tool for easy nozzle changing

 

https://www.thingiverse.com/thing:3441470

The website for the requisite parts to build that thing is non-functional. When last I checked a few months ago, the owner indicated he'd be away for a bit. Do you have any updated info?

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 02/09/2019 8:35 pm
Dave Avery
(@dave-avery)
Honorable Member
RE: Cautionary tale on nozzle changing technique

https://www.printedsolid.com/collections/hardware/products/heater-block-holder-for-3d-printer-nozzle-the-wrench

 

Posted : 02/09/2019 9:40 pm
ben_r_
(@ben_r_)
Trusted Member
RE: Cautionary tale on nozzle changing technique
Posted by: david.a66

https://www.printedsolid.com/collections/hardware/products/heater-block-holder-for-3d-printer-nozzle-the-wrench

 

Looks like from the images and the reviews that version is poorly finished and not coated so it will rust.

If at first you don't succeed, redefine success!...
Posted : 02/09/2019 9:52 pm
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