Kinda rhymes, no?
Given Rick Pollack’s PLA printing success and temperatures at barrel entry rather than nozzle, I should probably start delineating between temperatures at the barrel and nozzle. If barrel entry temperature is truly a critical factor, then he’s totally right about how going by nozzle temperature is like driving a bus with just the rear view mirrors.
However, this gives me pause. Everything I’ve read about ABS suggests it may be best to melt it right at the nozzle as quickly as possible. I’ve found that ABS cools very rapidly too. PLA on the other hand takes a while to cool (but, then again, I was running my extruder too hot) – with the parts being quite flexible for some time after printing. If barrel entry temperature is that important, then it seems that PLA must need a longer time to melt. If this is the case, should the nichrome be wrapped higher on the barrel? If so, then Rick’s heater cores (which already seem very convenient) are downright brilliant – since you can probably easily adjust where the heat is being applied.
From Nophead’s comment I gather that I was, indeed, running my extruder too hot. I found that at my ABS temperatures (220+) made the PLA just drip right out of the heater – and once that had basically stop it would then intermittently release little amber droplets of plastic. I must have been really cooking it.
With the limited printing I managed with PLA I found that it printed well with my non-temperature ABS Skeinforge settings. So, before I go back to PLA I think I’ll finish tuning my ‘bot using Nick Ames’ Skeinforge tutorial.
That’s what I’m planning on trying as soon as I get the heater core kit from makergear. I want to try putting the heater above the washer (which I think would eliminate the need for a nut also) with thermistor between it and ptfe. My thinking is that it would let me run at a lower target temp, get it to melt near the top and take advantage of the fact that it takes longer to harden so it will keep flowing down and out of the nozzle. Which is the opposite of ABS since ABS hardens so fast, you want to make it melt right at the nozzle.
One problem it might have is that it would make it more likely to ooze up into the ptfe and the heat might warm the ptfe faster. That’s why I’m also getting a hybrid PEEK insulator with a ptfe insert.
With the heater near the nut I would think the PLA would melt up into the insulator and cause a problem.
I really like your idea of using the heater to replace the nut entirely. The only problem I could think of is that it might put too much pressure on the epoxy exterior?
Let me phrase this a little differently using 4042D (ultimachine) not 4032D (MakerBot – has higher melting temp) for the example filament.
As the PLA enters the heating chamber (PTFE insulator/heater barrel/nozzle) it begins to draw heat from the surrounding material…so, the brass at the top of the barrel cools a little due to the PLA entering. This temp drop propagates along the barrel until it is picked up by the thermistor on the other end of the heating chamber. So, the temp drops a little at the top, this propagates along the heater barrel and eventually the thermistor picks up the change in temp and turns on the heater. Well, the heater is between the thermistor and the top of the barrel. How much heat makes it to the top of the barrel before the thermistor warms up? I don’t know but it all seems rather convoluted, maybe someone can model this heat flow. To me, it just seems like an awkward implementation that is difficult to get working and results in too much heat being used…
The nozzle needs to be hot enough that the melted filament can exit but this is more of a “maintenance” issue. The real finesse stuff – transitioning PLA from solid to liquid – happens in the upper section. I’ve found through experimentation that given the right design PLA will extrude if the barrel temp is above 150C (this was not exhaustive experimentation, it was just enough to get PLA working on my system). However, in order to get a MakerBot style extruder working, the temp at the top of the barrel needs to be ~170C and this happens when then nozzle temp is ~220C. Basically I think that 220C as measured at the nozzle is just a brute force method of getting the entry temp to 150C in a sustainable fashion – given the thickness of the barrel, the location of the thermistor, the conduction rate of brass and other variables that I probably don’t understand (yet).
In other words, heating your nozzle to 220C gets the barrel entry temp to 170C. 170C works for PLA (4042D) and ABS. 180C should work for 4032D.
Rick (Pollack)
The temps in the above entry used a 1mm thermistor and the Zach1 table. If you were using 220C with Jet’s table then you were probably at or above 250C based on Tony’s posting in the MB Google group.
Actually, at the time I tried PLA I was using the “Zach 1” table with the 1mm thermistor and was extruding at 220 or so. However, I was really cooking the crap out of the PLA and turning it a deep amber. I’m going to try it again at least 10 degrees lower.
Assuming that you don’t leave it sitting in the barrel with the heater on for a while…it is not going to scorch at 220C (using the above mentioned configuration). Easy enough to replicate when you rebuild…
At 220 the PLA was extremely … drippy. If I left it alone and shut off the extruder for more than about 10 seconds it would just ooze into a pile. Then, if left longer it would start to intermittently drip those amber drops of plastic.
“Above mentioned configuration” being 1mm thermistor + Zach 1 table?