Monday, 6 June 2011

My nozzle is too big

sOK, that is not something that I say often.

On irc the other day I said something like "If I put glue on the bed and then extrude will that bugger up my nozzle" which is one of those phrases that only works in a very specific context.

Anyway - to explain - the hot end is the bit the heats up the plastic. The extruder forces the plastic in to the hot end. My hot end is a brass tube with a fine 0.5mm diameter nozzle on the end with a heater and a thermistor.

When I started I had some issues with it and one suggestion was to clean it out - which I did. Part of this was using a 0.5mm needle to clear the nozzle. I don't have needles of that type lying around, and the suggestion of checking out the nearest crack den did not help much. So I found a very fine pin and used that. It was a tight fit but with true brute force and ignorance I managed it.

What I now know, thanks to some nice new digital calipers, is the pin is 0.60mm diameter. Oops. Since then the nozzle itself has broken off leaving a hole which is not ideal. I have a new hot end on its way from Germany this week.

In the mean time I have found what happens when your nozzle is too big, and it is a tad bizarre.

Basically you extrude too much plastic at a time or too thick or something, but the upshot is that after a few layers printing the hot end is fighting with the previously printed layer which is too high. It is pushing down on it, and well. This takes a few layers as my hot bed is a PCB and flexible, and only when it has flexed as far as it can does it really start pushing down.

One effect of this is obvious - the nozzle can push the whole piece sideways and ruin it - thankfully the 60C hot bed with PLA sticks well so that does not happen.

Secondly the plastic cannot get out as the nozzle is jammed up against the previous layer. This means the extruder cannot feed. Now this works by a knurled bolt pushing down on the filament with a sprung bearing the other side. What happens when the hot end gets stuck is it strips the filament cutting a nice groove in it meaning it does not move any more. When the hot end gets to a bit with space under it, hardly any plastic comes out. You get a few layers of fine string and then nothing - job ruined.

The solution is simple - you configure the software to know the nozzle size - it then makes necessary allowances in the layer spacing and so on, and you end up printing a courser image but it does not foul up half way through.

What is odd, is that should all be allowed for in the amount of plastic extruded, which means setting the nozzle bigger should send more plastic. So maybe it is another setting I am missing. Certainly it is a lot better running on a thicker layer setting.

When I get the new hot end I can go back to 0.5mm setting and see if that works.

P.S. It is not all about nozzle size. I had filament diameter wrong (2.8 but in fact 2.7), and packing density wrong (0.85 for ABS when PLA is 1) and E steps per mm wrong. Get these right else you get bad prints! Even a few percent wrong anbd you are screwed...

P.P.S I think my hot end is buggered so waiting for the new one to arrive.


  1. Adrian, are you trying to compete with this?

  2. Cool - custom fittings one hopes :-)