How to Solve Extruder Skipping

If you hear clicking when filament is moving in your printer, it may be that your extruder is skipping. This isn’t always a big issue, but it does upset the flow rate so your prints are probably not as good as they could be if you are skipping. In extreme cases, it can ruin the print and could even damage your extruder. So what causes it?

The short answer is that the extruder skips because the stepper motor can’t produce enough torque to move the filament the requested amount over the requested period of time. Practically, though, the question is why does this happen and how do you fix it.

There are four basic reasons you might get an extrusion skip: jams, mechanical issues, electrical issues, and software issues. Some of these issues are interrelated so you might divide them up differently than I do.

Jams are usually caused by some obstruction in the path of the filament. Sometimes, it is the filament itself getting kinked. Binding in the feed (for example, a tangled spool) is another common cause. Cheap filament may be bulged out. However, there is another source of obstruction: your misaligned bed. If the nozzle gets too tight against the bed, plastic can’t flow, pressure builds up, and skips will occur. If you can hear skipping when the nozzle is not all the way down, this isn’t your problem (or, at least, it isn’t your only problem). If you get skips uniformly on the first layer, but not later, the whole bed may be too high. If you get skips only on the first layer in a certain area, the bed may be tilted (although some printers automatically deal with that).  The solution is to clear the jam, where ever it is. If it is bed related, get the bed height correct and flat. If you are extruding at too low of a temperature, the plastic is harder to flow and again you may build up enough pressure to skip.

Another possible cause of bulging filament can occur if the plastic gets squashed at the point it is being grabbed. Pretty much all extruders have some kind of gear or hobbed bolt or pinch roller that grabs the plastic and pushes through some kind of hole or tube. If this pinch point is too tight, you can flatten the filament making it too fat to feed. Another possible issue is if the filament is getting hot over time because your hot end isn’t insulated well or your extruder motor is getting hot and the plastic is becoming soft enough to get pinched even though the pinch pressure isn’t very high. Some extruders can even be adjusted so the pinch is too tight for the motor to feed through. If skipping occurs after a long time, you could be getting a soft spot.

Mechanically, you can have a few issues. I mentioned that there is usually some kind of pinch mechanism and a path for the filament to flow. All of this stuff has to be aligned pretty closely or you get binding (which is a kind of a jam). Your hot end needs to line up with the other end of that hole or tube, too. One thing that is easy to overlook is the speed of the extruder. If you try to move plastic too fast, the gear may just slip. Printing fast, printing high volumes (e.g., thick layers), or having too high of a setting on E steps can lead to this. I had this problem recently when I switched from a geared extruder to a direct drive one. Even though I recalibrated the extruder for printing, I forgot the same number is in the printer’s default EEPROM settings. Starting the printer up and manually extruding was a disaster, but prints worked well and after a print, the manual extrusion would be ok. The default speed was just too fast for the direct drive extruder to handle (the geared one needed the faster motor turn to get a slower speed through the gear box).

Electrically, you need to be sure the servo has the right amount of current flowing. Most printer boards have some way to adjust the servo current. If the servo is cool to the touch after a good print setting and you are skipping, you may need to increase the current. If the servo is very hot after a few minutes of printing you may need to decrease the current. A hot motor can cause skipping, so this is a case of where more is not more and less is not more. Just enough is more.

The same holds true for software. You need to have an accurate extruder calibration. Like I mentioned before, spinning too fast will cause the extruder to slip. High acceleration can be the culprit as well.

So how can you bundle all this up into a reasonable check list?

1. Start with the head well away from the bed. If you manually extrude filament do you get skipping? If so, check the temperature, the filament path, or the calibration.

2. Notice if the skipping only occurs at certain places consistently. This could be where you bed has “bumps” or (in a case it took me a while to figure out) that the extruder stepper motor cable is intermittent and those locations put stress on the wire in the right way to break the connection. If it skipping mostly on one corner or one side, suspect bed leveling. Level the bed if necessary. Note this doesn’t really mean to get the bed level to the ground. It means the bed must be parallel to the movement of the print head.

3. If you don’t have extrusion on the first layer at all, then almost certainly the Z height is too low. Readjust your Z height.

4. If all else fails, try raising temperature and lowering speeds. Try a different material. Adjust the drive current to the extruder stepper, if necessary.


A Cheap 3D Printing Spool Holder

DSC08268Do you need a simple way to hold a spool of 3D printing filament while printing? A common 12″ bar clamp will do the trick for many workspaces. Just clamp it to your work bench so that the bar is parallel with the ground and slip the spool over the remaining bar. You can use a piece of filament through the hole on the end or a clamp (as shown in the photo) to keep the spool from walking off the end of the bar.

To reduce the friction on the spool, I printed a small tube that I put over the bar so the spool rests on the tube instead of the rectangular bar but, honestly, I don’t see that it makes much difference.