The problem with extrusion
October 03, 2007 06:40PM
The problem, in general, with extrusion is that you need to extrude a substance which undergoes a rapid decrease in viscosity between the inside of the extruder and the outside of the extruder, AND which adheres to the previous layer well, AND which does not gum up the extruder exit.

Actually, and I'm not sure I can think of how to say this concisely, there's a tradeoff between the pressure you're willing to exert inside the extruder and the viscosity change needed.
Re: The problem with extrusion
October 03, 2007 07:11PM
Hi Russ! You might want to take a look at the conversation on this topic that is happening over at hydraraptor.blogspot.com right now. Nophead (Chris) is doing some work extruding HDPE from a Mk II extruder that directly addresses what you are thinking about. There are some good bloggings for you there. Best, Forrest
Re: The problem with extrusion
October 04, 2007 06:04PM
I don't think viscosity change is the point. The hot plastic is very viscous inside the extruder and is the same after it leaves it. It doesn't vary much as it cools until it gets close to the point where it solidifies. If it was less viscous it would be easier to extrude but it would tend to slump away after it was deposited.

I think whether it adheres to the layer below is simply a matter of whether the junction between the two gets hotter than the melting point or not.

My thoughts on the matter are here [hydraraptor.blogspot.com]

I am certainly not speaking from authority here, it not my domain of expertise but I enjoy experimenting and theorising.


[www.hydraraptor.blogspot.com]
Re: The problem with extrusion
October 05, 2007 03:03AM
I'm trying to derive a general theory for extrusion so that I can predict what materials will extrude well and which will not. For example, water very close to freezing would probably extrude reasonably well into a supercooled surface.
VDX
Re: The problem with extrusion
October 05, 2007 03:30AM
... we had to bring to work a CNC-system with a dispenser-head for automated glueing clutch-tubes in aluminium-rings.

With a air/vacuum-dsispenser and the special heat-curing glue there was a lot of trouble with controlling the heating of the dispenser and the nozzle.

In particular we had to deal with the room-temperature and humidity at the workplace, so in the end i inserted a 3-staged heating and temperature-control for the dispenser-body, the nozzle and the table, on which the aluminium-rings were fixed.

With the extruding dispenser there is not so much trouble with viscosity, as with the air-pressurized one, but it's thinkable too, that with the room-temp the friction an extruding-force/-speed will vary too!

Viktor
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