Dual Action extruder

I was at a trade show this summer, and saw a design that I haven't seen in these forums. The demonstrators were using the mechanism to give precise measurements of medication, but the principles are the same.

The concept is to send the plastic feed into a syringe type chamber after it gets melted. The benefits were that the extrusion rate could be more closely controlled.
Simply moving the plunger stops the feed to the part. Small dots are easier to make without blobbing.

My bad attempt at a model:

You'd of course need a backpressure valve on the auger to syringe connection to prevent blowback when pressure was applied to the syringe.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

i don't think the main pressure is coming from the plunger at all. but that's where fine adjustments can be made. (including creating back pressure to halt flow from the nozzle.)

andres

Edited 1 time(s). Last edit at 02/03/2009 11:08AM by Andres.

The plunger is used to make sure that there is a constant flow...

Andres is right, the main pressure is coming from the auger, not the plunger.

I forget how they dealt with backflow on the original. They were squeezing out a liquid, not a viscous mass. It might be that the problem of backflow was moot due to the non-compressability of water based meds. An easy work-around would be to make the channel from the main feed into the syringe section longer.

For our purpoes, the syringe tube could be fairly small. Maybe just big enough to deal with the normal stuttering of a build routine. A pressure sensor for continuous feedback shouldn't be too hard to make/get ahold of.

Greetings all,

Interesting notion.
Let me toss in a Blue-sky idea regarding "valving" for melted plastic:

If we could design this extruder such that the auger-to-piston channel was thermally isolated from the rest, then we could heat just that channel when we wanted flow (from the auger into the piston's chamber), and let it cool off otherwise. This wouldn't be instantaneous, but small things tend to have short time constants, so it might be fast enough to cut backflow down to an acceptable level.

Similarly, the "piston" might be done with a bellows, to eliminate sliding surfaces (at the cost of a bellow and limited travel.)

Edited 1 time(s). Last edit at 02/03/2009 05:40PM by Larry Pfeffer.

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Larry Pfeffer,

My blog about building repstrap Cerberus:
[repstrap-cerberus.blogspot.com]