A Dot Matrix extruder

Hi All,
One of the most frustrating things about the extruder is the inability to completely shut off the flow of plastic at will - without drips.
Are there any ideas out there on how this can be done?

I keep returning to the idea of a powder bed melted by a fine laser to produce very fine melt patterns, rather than an actual extruder.

Are there any ideas out there as to how our current extruder design can turn on and off without drip or stringing?

Nathan

Using a laser would move the design *far* away from replicabiity -- a low cost machine that can "print" the parts to construct a working laser would be an amazing device indeed :-)

Jonathan

Wiper?

I've heard, mostly from Forrest I think, you just reverse the motor a little and that would tend to pull the filament back in. Now that's theory and how well it might work for various extruders at various temps and speeds may be another matter.

I would think of an e-magnetic shutter instead.
Shouldn't be that much a controllerProblem (open if extruderbarrel is turning, shut if not should do I'd guess)
But I see a problem about cold material that keeps the shutter from opening

Maybe something like a needled nozzle, if the needle is heated it'll be okay.

'sid

sid Wrote:
-------------------------------------------------------
> I would think of an e-magnetic shutter instead.
> Shouldn't be that much a controllerProblem (open
> if extruderbarrel is turning, shut if not should
> do I'd guess)
> But I see a problem about cold material that keeps
> the shutter from opening
>
> Maybe something like a needled nozzle, if the
> needle is heated it'll be okay.
>
> 'sid


I still think there would be a problem with threading - all the prints I've seen so far have had to have the fine threads cleaned off. I'm not sure if a wiper method would completely solve this either.
Maybe the extruder can deposit power (somehow...) then melt it due to a high temperature needle in close proximity?

"then melt it due to a high temperature needle in close proximity?"

I was thinking a jet of hot air... but both of these solutions would result in a secondary heating element. That's a big mark against 'em.

I wonder what effect vibration might have on a (semi?) molten plastic thread. Might liquify it enough that it gives up... Might have no effect at all.

~D

"I was thinking a jet of hot air... but both of these solutions would result in a secondary heating element. That's a big mark against 'em."

Extruder-type plastic welding machines work on this principle.

"I wonder what effect vibration might have on a (semi?) molten plastic thread. Might liquify it enough that it gives up... Might have no effect at all."

Stratasys original patent grant mentioned the use of ultrasound to achieve a molten state in plastics a few degrees lower than one would ordinarily expect to see happen. I do not know if they actually used this approach in commercial product.

what about the famous spitbag-style?
If the extruder should extrude, theres a spitbag placed under the extrudernozzle ( that may be a spoon or whatever)
everything that will drip will not drip onto the print but into the spitbag

That is not elegant nor beautiful, but it can help reducing the unwanted threads.

*justthinkingaloud*
'sid