Extruder Head Power Clearing

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Extruder Head Power Clearing

ugwick
Merry Christmas all !

Still playing around with the custom build of a multi color printer at the
moment . Leads to a question as to clearing the melt chamber inside the
extruder head in favor of a new color one . As I DON'T know how to set up a
dump tower or simimlar , I was thinking of modifying my print head to accept
a compressed air line which would be tripped on to clear the head by a
solenoid . THe pressurized air would push the melted plastic thru the head
to clear , shut off and infeed the next color filament . This option could
still be used with retractions but should be a much quicker clearing of the
mix chamber without the need of the waste tower and the movement times
involved .

Thoughts ?  I would appreciate ANY feedback on this one fellas .   Rick



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Re: Extruder Head Power Clearing

ednisley
On 12/25/2017 04:01 PM, ugwick wrote:
> pressurized air would push the melted plastic

Molten plastic has roughly the viscosity of cold molasses and requires
absurdly high pressure (*) to make it move. Hot plastic also sticks like
glue to the side of the hot end, so (hot?) air would eventually force a
channel through the center to the nozzle, after which it wouldn't move
any plastic and leave a conical plastic plug with a small hole along the
middle.

As an example, you can run cleaning filament (basically nylon) through a
hot end until it comes out clear, let it cool, pull it back out, and
you'll get a mold of the hot end + nozzle in nylon coated with whatever
plastic you were using before. Even gummy nylon can't push all the
previous filament out of the nozzle.

Rheology is hard!

(*) Back of the envelope calculation. An extruder motor with 0.4 Nm of
torque and a 12 mm dia direct-drive gear applies 6.8 kg = 15 lb to the
filament. Across the end of a 1.75 mm filament, that's 4 k psi ...

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Re: Extruder Head Power Clearing

nophead
The best way to get PLA out cleanly is to heat it above the glass transition but below the melt point, e.g. say 80C, and pull it out backwards in its rubbery state.

On 26 December 2017 at 00:21, Ed Nisley <[hidden email]> wrote:
On 12/25/2017 04:01 PM, ugwick wrote:
pressurized air would push the melted plastic

Molten plastic has roughly the viscosity of cold molasses and requires absurdly high pressure (*) to make it move. Hot plastic also sticks like glue to the side of the hot end, so (hot?) air would eventually force a channel through the center to the nozzle, after which it wouldn't move any plastic and leave a conical plastic plug with a small hole along the middle.

As an example, you can run cleaning filament (basically nylon) through a hot end until it comes out clear, let it cool, pull it back out, and you'll get a mold of the hot end + nozzle in nylon coated with whatever plastic you were using before. Even gummy nylon can't push all the previous filament out of the nozzle.

Rheology is hard!

(*) Back of the envelope calculation. An extruder motor with 0.4 Nm of torque and a 12 mm dia direct-drive gear applies 6.8 kg = 15 lb to the filament. Across the end of a 1.75 mm filament, that's 4 k psi ...

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Re: Extruder Head Power Clearing

ugwick
In reply to this post by ugwick
Thanx fellas . I was avoiding going thru the total force calcs for this idea
and am pleased to see that one of you did . I never thought about the
conical breakout of the material by the air pressure applied . This makes
total sense . Also , the semi molten state of the material in the melting
chamber may not be completely ins its 'plastic state' . THe rubbery state .
Better brush up on my material sciences stuff I guess . hahahahaha

Another idea shot down for me BUT with great advice to save the grief along
the way of pushing a bad idea .
You guys are my new heroes !



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Re: Extruder Head Power Clearing

Mythobeast
In reply to this post by nophead
On 12/25/2017 6:31 PM, nop head wrote:
> The best way to get PLA out cleanly is to heat it above the glass transition
> but below the melt point, e.g. say 80C, and pull it out backwards in its
> rubbery state.

I do this every time I change filament. My bowden extruder motor will slip while
the head is cool, so I can let it run backwards while the head heats up. As soon
as the contact point gets warm enough to separate, it pulls it free, along with
anything that might clog the head. I've heard this called a "cold pull".

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