Stretch marks on my prints

Hi,

I`ve been getting my printer under control lately and fixing most issues, but one is persistent.

Every 5 layers or so the outside perimeter curls up and doesn't bond with the layer below but it only happens on longer straight part of the print. Around corners the stacking of the layer is normal.



I have no idea how this can happen or where the plastic gets enough space to wiggle out of place, especially because the slight wave pattern caused be the threaded rod is relatively small compared to the gap.

Edited 1 time(s). Last edit at 01/05/2014 08:33PM by sungod3k.

I'm not too sure about this, but parts tend to shrink a bit when they cool and if you're printing a bit on the cold side they may not stick very well. Try raise the temperature a few degrees. For me 5degrees makes a difference.

It could also be that your extruder isn't putting out enough plastic. If the layers are a bit thin after a few layers it'll accumulate and there'll be a gap. Try with a slightly higher extrusion multiplier if you use slic3r or higher e steps value or maybe even recalibrate your extruder to get it spot on.

Interesting pointers, although if true wouldn´t they be more random? Anyway I´m gonna try that and report back.

Edited 1 time(s). Last edit at 01/06/2014 12:41AM by sungod3k.

Make sure your nozzle and tube is clean on the inside.

It looks like the thickness of each layer is changing with z.. What is the printer? Does the spacing between the defect match the thread pitch of the rods? I am guessing that wobble in the z-axis threaded rods is causing vertical movement of the carriage at each revolution.

I just did a test print with higher temp and 10% more extrusion and the gaps got a little shorter but the basic pattern is still there. There now more but shorter gaps.

@brnrd: yes as far as I can measure it the gaps match the pitch.

I´m looking into buying fine threaded rods but the nuts are very hard to find.

Update: I just printed with smaller layerheight and the gap spacing is still the same, that would mean the per rotation of the z-rod, the bed gets wedged up somehow.

Edited 1 time(s). Last edit at 01/06/2014 02:53PM by sungod3k.

I even had bad layer adhesion in prints when the Z-setting for first layer wasn't low enough. The first layer would be too thin and not squashed moderately, this caused a new variable in the whole print of layers not being pushed together hard enough. After adjusting the Z-endstop just a little bit, it all worked fine and didn't have to use the extrusion multiplier or recalibrate E-steps/mm because they were already spot on!

@ohmarinus: how did the adhesion defects show up in the print?

For me it wasn't completely similar to yours, but it would just be a weak part where the layers weren't completely stuck to eachother. However, I am printing with PLA and just getting around. In ABS I haven't had this problem ever.

I took a second look, it might be that your printer is under-extruding? Does the filament sometimes slip in your extruder, or is a a 100% sure extrusion all the time?

I hope you discover the problem, it looks like it's not extruding enough!

If the gaps match the pitch of the threaded rods, then you should focus on fixing your z-axis before messing around with the extruder settings. It would help if you would post a photo of your printer so people here can see where the problem might be. Going to finer threads might not really fix the problem if there's a flaw in the design or the assembly.

Edited 1 time(s). Last edit at 01/06/2014 08:56PM by brnrd.

@ohmarinus: I´m relativly sure that the extruder works correctly and that there is no sliping.

@brnrd: The biggest design flaw with the machine is that the bed is only hold on one side. It also gives a terrible side wobble when not used with proper acceleration settings. I also think that there is something with the z axis but i cant imagine how the vertical movement you mentioned earlier could happen.

Here some pics:





Edited 1 time(s). Last edit at 01/06/2014 09:18PM by sungod3k.

I'm not that familiar with that design and I can't see enough of the z-axis in the photos. When you jog the z-axis, does the threaded rod (lead screw) wobble? If it does, it can be causing the bed to go up and down with each rotation.

When I touch the bed the rod and the two leadscrews stay pretty still. There is one upgrade I´m planning which would make the connection between the mount which holds the leadscrews and the undercarriage of the bed a bit stiffer, we´ll see if that changes something.

The only explanation I can imagine so far is that the threaded rod either wedges itself somehow and relieves tension every rotation or it has a manufacturing error in the threading itself. A tighter threading from fine pitch could actually help here.

try slowing down your entire print and go from there. it could be that rapid infills are too much for your extruder leaving little back pressure when perimeter is laid down, or it could be hardware relayed to the x/y axis. are your belts tight, anything obstructing the carriage, such as wires, gunk , and have you oiled them lately? do you have springs in your prusa z axis, or a weight on the side that has no motor? (i use 4 or 5 heavy magnets because they clamp on, but they add 8oz of weight to the one side of z axis.


does your extruder wobble at all when you try to wiggle it by hand?


can you increase flow rate.....

for whatever reason the visual defect is uneven placement of filament, consistent with the motion of the machine. slow the machine down at least where it visually counts. slow down the perimeter.

Edited 4 time(s). Last edit at 01/07/2014 01:22AM by jamesdanielv.

You've linker to the thumbnail images, not to the actual images I believe, could you relink them, so we can see more of the machine?

try the imageshack link under the thumb thats the bigger pic

@jamesdanielv: your´re good! I slowed it down to 30% of the original speed for the perimeters and the gaps are gone.

Could that mean that the extruder at high speeds isnt able to feed material fast enough? Is it maybe slipping at a certain speed?
But then the gap spacing is still unexplained, the speed is always the same on the longer straight lines, the threaded rod must throw the bed violently out of balance in order for such a gap to happen.

Update: 50mm/s seems to be the magic number. I can see small gaps at this speed. and what I can now also see better is that the material curves upwards.

Edited 2 time(s). Last edit at 01/07/2014 03:49PM by sungod3k.

I don't know for sure, but now you have a good baseline that provides satisfactory results. work from there. good luck!

How did slowing down the print speed reduce the gaps in the wall that matched the z threaded rod pitch?

I cant explain it, other than the extruder working less efficient at high speed and then interacting with some z-rod error.