Empathy List Archives

U

ugwick

Mon, Dec 25, 2017 9:01 PM

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

--
Sent from: http://forum.openscad.org/

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 -- Sent from: http://forum.openscad.org/

EN

Ed Nisley

Tue, Dec 26, 2017 12:21 AM

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 ...

--
Ed
https://softsolder.com

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 ... -- Ed https://softsolder.com

NH

nop head

Tue, Dec 26, 2017 1:31 AM

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 ed.nisley@pobox.com 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 ...

--
Ed
https://softsolder.com

OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

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 <ed.nisley@pobox.com> 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 ... > > -- > Ed > https://softsolder.com > > _______________________________________________ > OpenSCAD mailing list > Discuss@lists.openscad.org > http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org >

U

ugwick

Tue, Dec 26, 2017 5:53 PM

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 !

--
Sent from: http://forum.openscad.org/

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 ! -- Sent from: http://forum.openscad.org/

RR

Robert Rapplean

Tue, Dec 26, 2017 10:12 PM

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".

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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Extruder Head Power Clearing