Wait for the writeup! 
I'll wait for the write up.
]]>Wait for the writeup!
As you can see, increasing f_b increases pressure, decreasing f_s decreases pressure.
It looks to me like increasing fs decreases pressure.
What are some reasonable values for all of those numbers? Do you have them for the filastruder?
]]>How much coupler internal thread should be exposed (internally exposed) between nozzle and barrel
It isn't about exposed thread, you just need to tighten it a bit. 5-10ft-lbs is plenty for the plastic not to leak.
I am worried about the inside of your barrel now being too smooth, but if worse comes to worse I can send you another.
I dropped two pellets in my cleaned and sanded barrel and they moved by way of the auger all the way to the coupler threads - nozzle was not on.
I have the black pipe black coating cleaned out my barrel - used a 9" sanding spindle from a floor style oscillating spindle sander and some lacquer thinner (thinner did virtually nothing by itself).
]]>Once I get my CNC mill I am going to make my own barrel and play with it.
Ralph
]]>The coefficient of friction and other math involved is for the melt zone, not the solids conveying zone. The solids conveying zone (where the pellets are still pellets) doesn't build much pressure, the pressure is built in the melt zone.
]]>However, one would think that the pellets would feed until they met resistance at the end of the barrel, and then, once melted and coating the inside of the barrel, this friction would be self perpetuating through the pellets sticking to the layer of molten plastic.
Just curious if the test was conducted for any length of time with a fully heated barrel, or whether it was done cold. I'm assuming the former, but I'll ask anyway
2.) The other is the coefficient of friction between the polymer and the heater barrel. You want this to be high, and you want the COF between the polymer and screw root to be low. Think of it as rotating a bolt while holding the nut. The more friction between the nut and the bolt, the greater the torque required (increased motor load). If there was no friction between your fingers and the nut, the nut would not advance on the bolt. Unfortunately this isn't really a specification on a datasheet. It does mean the barrel needs to have some internal roughness, otherwise the polymer to barrel COF will drop and melt pressure will drop with it, resulting in reduced output. Warning: math ahead!
The relevant variables here are P(z) (pressure profile), f_b (barrel to polymer dynamic COF) and f_s (screw to polymer dynamic COF). As you can see, increasing f_b increases pressure, increasing f_s decreases pressure. [1] In fact, some screws are fluoropolymer (PTFE or similar) coated. Too bad that's way out of our little Filastruder's price range.
]]>elmoret wrote:One thing I meant to add - the internal surface of the barrel needs to be rough. I'm doing a write-up right now that will elaborate on this point. Basically, you want the polymer to barrel COF to be high.
I understand the need for this with an injection press, where shear contributes most of the heat, but I'm not convinced that COF is terribly important in a design such as this one, where the heat is produced by a heating element, and extrusion is occurring through forcing highly viscous material into a zone where the material is much less viscous and has an exit point.
Is this something you learned through a failed experiment, or is this a theoretical conclusion?
We found this out by trying a stainless barrel. The auger would spin the pellets around in the pipe without moving them forward.
]]>elmoret wrote:One thing I meant to add - the internal surface of the barrel needs to be rough. I'm doing a write-up right now that will elaborate on this point. Basically, you want the polymer to barrel COF to be high.
I understand the need for this with an injection press, where shear contributes most of the heat, but I'm not convinced that COF is terribly important in a design such as this one, where the heat is produced by a heating element, and extrusion is occurring through forcing highly viscous material into a zone where the material is much less viscous and has an exit point.
Is this something you learned through a failed experiment, or is this a theoretical conclusion?
Both.
It isn't about heat generation. Trust me, and wait for the write-up.
Hint: You say "forcing highly viscous material". How is that force generated? Create a FBD (free body diagram) of the of the screw, barrel, and polymer.
]]>One thing I meant to add - the internal surface of the barrel needs to be rough. I'm doing a write-up right now that will elaborate on this point. Basically, you want the polymer to barrel COF to be high.
I understand the need for this with an injection press, where shear contributes most of the heat, but I'm not convinced that COF is terribly important in a design such as this one, where the heat is produced by a heating element, and extrusion is occurring through forcing highly viscous material into a zone where the material is much less viscous and has an exit point.
Is this something you learned through a failed experiment, or is this a theoretical conclusion?
]]>To reiterate: It does would just fine with a normal cleaning.
]]>