Is my hot end too long?

Hi again,

Okay, when my hot end decides it wants to work and gets up to temperature, i can manually feed with ease a good 30 to 40mm of filament quickly through, but then it becomes increasingly harder in the space of a couple of seconds, until i can no longer push it through, then retracting the non melted filament and trying again 30 seconds later i get the same effect.

I've currently got not heatsink or fan aimed towards the brass of teflon area, so i was wondering if the heat has spread too far up through the ptfe, melting the plastic too early?

The whole hot end comes to around 60mm in length, so im not sure.

Ollie

PLA has this problem as it gets soft at low temperatures. After a while heat travels up the barrel and softens the filament, and then it's like pushing a cooked noodle through a drinking straw.

You don't specify which hotend you use, but a small fan pointed at the top of the barrel should do the trick.

It could also be you use a too high temperature (I use 150-180C for my PLA, depending on print speed and type of PLA). The short fix is to turn off retract and print very fast, not letting the filament get time to soften, but it's not a long time solution I'd recommend.

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-Nudel
Blog with RepRap Comic

Ah thanks, my hot end is of um "unique" design, basically what I had to hand tool wise with a bit of scap brass and sloppily coated in fireplace cement. I had the idea of watercooling the top of the barrel, much like that in my computer, but never got round to doing it. In time, I'm sure it'll get done. But for now, I'm sure I can find some spare fans around here.

Thanks, Ollie

You can also put a heatsink (eg some tapped aluminum) as close to the hot end as you can to sharpen the hot->cold transition. I was doing some experiments and found that only letting 5-7mm more barrel warm up caused jams that heatsinking that same area fixed. Symptoms were exactly what you described: New filament fed fine, but a long enough pause caused a "jam" which was easily fixed by reversing for a moment.

Pushing filament through at high speed is not something I consider as a good benchmark of your printer performance.

See, the thing is, when you expell filament at high speed, you are essentially using the filament as a coolant to the system. That heat can only be replenished at the rate at which your heater block conducts heat. If the cooling rate exceeds the heating rate, you get exactly what you have observed.

There is valuable information in your experiment. Namely, you now know how much filament over time is required to cool the heater block to the point where it will no longer extrude. Divide this rate in half, and it should serve as a good starting point for determining the extrusion capability of your hot end.

All this is out the window, of course, if your problem is actually related to PLA travelling back up the insulator. If this is the case, I suggest you modify your design to include some of the ideas I've implemented in tappering the hole in the PTFE.

See [reprap.org] and look for the illustration I made showing the PTFE cross section.

-Wildseyed-