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I've been printing PLA on a Pegasus 12" for 2 years. Print quality has been great but in the last few months the quality of the prints has deteriorated. After looking into it, it seems when the head moves away from the spool and puts the filament in tension (and pulls on the spool) the extruder cog slips on the filament. That results in traces where no plastic gets extruded the furthest away the head moves from the spool (the spool is on the left in the picture below):

See diagonal traces where plastic didn't extrude

The cog slipping can be seen between 0:22 and 0:27 on the video below: the cog on the right spins continuously but the ball bearing on the left stops spinning for 5 seconds. https://photos.app.goo.gl/cTfySUgXDy1XKXGv8

The end result is a part with multiple gaps, especially on the side farthest away from the spool.

enter image description here

I've cleaned the teeth of the cog (removed plastic dust) to improve traction but that didn't solve the problem. I've also tried to clean up the extruder by inserting a wire from the hot end side, no improvement in print quality.

Any ideas on how to fix that slipping problem?

Edit with solution:

To complement Oscar's answer below, here is what I've done that fixed the problem: it turns out the issue was coming from a lack of friction on the filament. To increase the friction, I've slipped a pair of 0.25mm thick pieces of plastic sheet between the bearing axis and the mount. See pic below.

enter image description here

That increased the force on the filament enough to fully solve the slippage problem. The print quality is perfect now (see pic below).

enter image description here

The plastic sheet trick is just a short term fix. I'm going to try swapping the drive gear for a slightly thicker one and contact MakerFarm to see if there is any long term modification they recommend.

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    $\begingroup$ does the same happen on a fresh spool of filament? it could be too humid or brittle, depending on the bend angle, which explains why it works closer to the spool. If it were just an extruder issue, it wouldn't likely work well anywhere. $\endgroup$
    – dandavis
    Oct 17, 2018 at 16:42
  • $\begingroup$ Yeah I ran a comparison test with a new spool of filament, freshly opened but the problem still persists. $\endgroup$
    – clem
    Oct 17, 2018 at 21:05

2 Answers 2

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These symptoms are generally caused by friction in the extrusion path.

First, you should make sure the spool of filament can unspool freely.

Second, in order to prevent slipping of the filament, you should consider increasing the force of the roller bearing onto the filament and extruder wheel, or increase hotend temperature. A (partial) clog can also be causing this. You could try a "cold pull" or "atomic pull" where you heat up the filament to print level, push a little through by hand and let the nozzle cool to about 60 % of the printing temp and then steadily pull the filament out of the hotend, this should remove all gunk inside, repeat if necessary. A too low of a layer height can also put the filament extrusion under pressure.

As an aside, slipping of filament is often accompanied by an audible clicking noise. There are a few questions on this topic.

It also looks as though your bed is not centered in the middle as you are printing outside the bed limits in the front and have a little room left at the back!

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  • $\begingroup$ I've tried a cold pull. The filament broke inside the heat break during the pull so I disassembled the hot end and cleaned the nozzle which had significant deposit inside. Unfortunately that didn't solve the problem. $\endgroup$
    – clem
    Oct 17, 2018 at 21:02
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In addition to the answers from 0scar, you might check your hot end temp. I was having a similar problem with slippage and prints that would literally fall apart in my hand even if they looked ok.

It turned out that the filament I was using which claimed to be PLA actually wanted a much hotter temperature than most PLA. Once I bumped it up to around 216-218 °C the slippage stopped and the prints came out beautiful. Might be a possibility if this is a new filament for you.

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  • $\begingroup$ I've been printing at 225deg C consistently for the last 2 years with good results but just to be sure I did a print at higher temp (231C) and one at lower temp (207C). That didn't solve the issue. $\endgroup$
    – clem
    Oct 17, 2018 at 21:04

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