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Thread: Skipped layers

  1. #1

    Skipped layers

    Hello all, yet again a different problem! Occasionally in my prints i observe a "skipped" layer, a layer which seems to be very underextruded. I tried to reprint that model on two more occasions, however to my surprise the skipped layer was in the exact same z-height for all three prints. I just performed a normal maintenance check on the printer. Cleaning rods and lubricating threads, checking if the nuts are still tight etc. I also installed quite some improved plastic parts on the printer. Afterwards i assumed it was over, sighed, and went on with my hobby. 2 Days ago i started to print a new extruder (AndrewBCN's "ultimate wade/gregs accesible extruder") in order to be able to upgrade to a nozzle fan and lighting, and ultimately printing new X axis parts to i can finally fasten my Z-endstop with adjustable endstop "poker"/flag.

    Anyway, the next day the print was finished and i noticed a lot of artefacts in the Z-axis, semi repetitive. (See figure1+2)
    NOTE: The figures also show a repetitive artefact (Most noticeable one) i have no idea what caused this either, i am open to suggestions!


    The skipped layer problem is fairly visible on the second picture.

    (I suffered from random layer misalignment etc before i performed my maintenance check so i did not notice when this problem started to occur. The fact that it also only occurs under specific conditions does not help either.)

    In combination with my previously failed prints i noticed that these skipped layers occur at the start of each hole through the wall!
    The problem occurs as soon as the printer starts printing the infill of the first layer that has a sidewall of the hole. At least, that is one situation in which it happens.
    I have been able to confirm this by printing a (2x1.5x2, XYZ) cube with 1 M3 nuttrap + hole and 1 hole. The skip occurs at both first layers in the hole. (I also tried printing another model which also confirmed this, but i am not going to elaborate on that)

    It appears that either the print head moves down less than 0.75mm after the Z-hop up of 0.75mm or that the printer does not move up far enough each layer and skips to the correct postion at the Z-hop. (I have noticed overextrusion during 100% infill prints lately, this is alleviated after infilling starts. Yes, the extruder is calibrated. But filament diameter seems to have increased drastically to about 2 mm, but i do not trust my calipers, and 2mm filament would cause loads of other issues i do not experience. Even if this is the case this is unlikely related to the missing gap phenomenon.)

    In anticipation of the request for more information, and my pinpointing, the G-code:
    Code:
    G1 Z3.650BLA BLA BLA
    BLA BLA BLA
    
    
    ;LAYER:36
    G1 F3600 E292.31378
    G1 Z4.400                                 //0.75mm Z-hop up
    G0 F9000 X80.082 Y48.262 Z3.750          //GOTO position 0.75mm z-hop down w/o extruding
    ;TYPE:FILL
    G1 Z3.750                  //Z-hop already down?
    G1 F3600 E294.81378              //Start infill - WRONG Z HEIGHT
    G1 F3000 X79.376 Y47.556 E294.82623
    G0 F9000 X79.376 Y46.142
    G1 F3000 X81.496 Y48.262 E294.86362
    
    
    Printer thinks it is at z3.750 (Which was in reality too high!)
    ;LAYER:37
    G1 F3600 E299.07808
    G1 Z4.500                 //0.75mm Z-hop up
    G0 F9000 X79.625 Y48.361 Z3.850         //GOTO position 0.75mm z-hop down w/o extruding
    ;TYPE:FILL
    G1 Z3.850                 // continueing printing as if nothing happened.
    G1 F3600 E301.57808             
    G1 F3000 X85.769 Y42.216 E301.68646
    G0 F9000 X84.355 Y42.216
    G1 F3000 X79.376 Y47.195 E301.77428
    
    
    
    Over the course of 5-10 layers the printer catches up and fills the z-gap with air/plastic.


    Cura Settings (most recent version):
    Code:
    [profile]layer_height = 0.1
    wall_thickness = 0.9
    retraction_enable = True
    solid_layer_thickness = 0.9
    fill_density = 30
    print_speed = 50
    print_temperature = 220
    print_temperature2 = 0
    print_temperature3 = 0
    print_temperature4 = 0
    print_temperature5 = 0
    print_bed_temperature = 60
    support = None
    platform_adhesion = None
    support_dual_extrusion = Both
    wipe_tower = False
    wipe_tower_volume = 15
    ooze_shield = False
    filament_diameter = 1.75
    filament_diameter2 = 0
    filament_diameter3 = 0
    filament_diameter4 = 0
    filament_diameter5 = 0
    filament_flow = 100.0
    nozzle_size = 0.3
    retraction_speed = 60.0
    retraction_amount = 2.5
    retraction_dual_amount = 16.5
    retraction_min_travel = 1.5
    retraction_combing = All
    retraction_minimal_extrusion = 0.02
    retraction_hop = 0.75
    bottom_thickness = 0.15
    layer0_width_factor = 100
    object_sink = 0.0
    overlap_dual = 0.15
    travel_speed = 150.0
    bottom_layer_speed = 20
    infill_speed = 0
    solidarea_speed = 0.0
    inset0_speed = 0.0
    insetx_speed = 0.0
    cool_min_layer_time = 10
    fan_enabled = True
    skirt_line_count = 2
    skirt_gap = 3.0
    skirt_minimal_length = 150.0
    fan_full_height = 0.5
    fan_speed = 100
    fan_speed_max = 100
    cool_min_feedrate = 10
    cool_head_lift = False
    solid_top = True
    solid_bottom = True
    fill_overlap = 15
    perimeter_before_infill = False
    support_type = Lines
    support_angle = 60
    support_fill_rate = 15
    support_xy_distance = 0.7
    support_z_distance = 0.15
    spiralize = False
    simple_mode = False
    brim_line_count = 10
    raft_margin = 5.0
    raft_line_spacing = 3.0
    raft_base_thickness = 0.3
    raft_base_linewidth = 1.0
    raft_interface_thickness = 0.27
    raft_interface_linewidth = 0.4
    raft_airgap_all = 0.0
    raft_airgap = 0.22
    raft_surface_layers = 2
    raft_surface_thickness = 0.27
    raft_surface_linewidth = 0.4
    fix_horrible_union_all_type_a = False
    fix_horrible_union_all_type_b = False
    fix_horrible_use_open_bits = False
    fix_horrible_extensive_stitching = False
    plugin_config = 
    object_center_x = -1
    
    object_center_y = -1


    My setup:

    Prusa i3 - Steel, 12V, NEMA17, igus-drylin bearings Rj4JP, Ramps 1.4SB, DRV8825 drivers, M5 threaded rods
    Wade-style extruder, E3D V6 PTFE-lined hotend (1.75mm), Mk3 aluminium heatbed. 0.3mm nozzle
    Last edited by Reaping miner; 08-24-2016 at 01:31 PM. Reason: small edit

  2. #2
    Senior Engineer
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    This is likely to be an effect from cheap stepper motors or microstepping set too high. With cheap steppers the angle tends to vary a small amount, this is exactly what you pay for with expensive steppers, the angle per step will be more accurate.

  3. #3
    Super Moderator curious aardvark's Avatar
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    that's a godawful print anyway you look at it.
    Do you have print area cooling ?
    That's does look like uncooled pla printed too fast.

    The layers are either all misaligned or your horizontal alignment is out as well.

  4. #4
    Quote Originally Posted by Mjolinor View Post
    This is likely to be an effect from cheap stepper motors or microstepping set too high. With cheap steppers the angle tends to vary a small amount, this is exactly what you pay for with expensive steppers, the angle per step will be more accurate.
    The steppers i have are: Wantai 42BYGHW811

    Quote Originally Posted by curious aardvark View Post
    that's a godawful print anyway you look at it.
    Do you have print area cooling ?
    That's does look like uncooled pla printed too fast.

    The layers are either all misaligned or your horizontal alignment is out as well.
    I checked horizontal alignment before starting the print, this was ok.

    Just one question, how do you explain these suggestions when this problem exclusively occurs at the first layer of a sidewall of a hole in the print? Wouldnt the suggested issues be more regular, like at every Z-hop instead of just that layer?

  5. #5
    Senior Engineer
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    I don't see the problem at the place where holes start at all. If it is the steppers then you will just see a repeated pattern that is related to the steps per mm and you layer height.

    Print a 10 mm square 50 mm high without any holes or sticky out bits or anything, see how that looks.

    Print it solid then hollow then with say 15% infill. Do it as three separate prints, not all on the same print.

  6. #6
    Staff Engineer printbus's Avatar
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    Some thoughts...

    To eliminate one possible issue, try a print without heating the bed. I realize you have an aluminum heatbed and it might make a difference, but multiple people have demonstrated print artifacts being caused by flexing of the bed heater as it cycles on and off using the normal Marlin bang-bang heater control loop. Migrating to SSR and PWM control of the bed heater is the solution for those artifacts.

    What are you using for a filament spool mount? The filament spool needs to unwind very smoothly. If it doesn't (like if you are using the MakerFarm scheme where the filament spool sits on a fixed bar), the filament might be tugging on the extruder until the spool springs free and rotates a bit to relieve the tension in the filament. IMO, the extruder mount on these printers isn't rigid enough to not be displaced by this tugging. Is your hot end a snug fit in the extruder body? A loose fitting hot end would make this worse. Many people have added a thin spacer above the hot end to provide a more solid fit.

    FWIW, I never had much luck with the z-hop. The MakerFarm printers can't move Z very fast due to use of the M5 rods, and my attempts at z-hop led to artifacts being caused by the hot nozzle sitting too long in the same place as it slowly rose up/down, and then there's also oozing from the nozzle. EDIT: My best results were obtained by testing to determine just how fast I could accelerate and then move the Z-axis and adjusting Marlin default settings accordingly.
    Last edited by printbus; 08-25-2016 at 12:27 PM.

  7. #7
    Quote Originally Posted by Mjolinor View Post
    I don't see the problem at the place where holes start at all. If it is the steppers then you will just see a repeated pattern that is related to the steps per mm and you layer height.

    Print a 10 mm square 50 mm high without any holes or sticky out bits or anything, see how that looks.

    Print it solid then hollow then with say 15% infill. Do it as three separate prints, not all on the same print.
    Here are the testprints that show the skipped layers, but not show the most obvious flaw present in the extruder, which i believe is an unrelated issue.



    I am now printing a 15x15x50mm bar at the same settings as before, except for the infill, only 15% instead of 30%. (10x10 is too small, that will print too hot. A nice stepping effect is seen as the area of the layers get smaller, especially in the negative control "pyramid". Again this is why i was printing the extruder body in the first place.)

    It looks like the corners of the objects are curved, this is an artefact of the camera, the corners are straight, only a little bit of elephants foot is present. This is caused by the overextrusion at the start i mentioned before.

    Also one arrow in the right "pyramid" points at a line in the Z axis at an area transition layer.

    Quote Originally Posted by printbus View Post
    Some thoughts...
    To eliminate one possible issue, try a print without heating the bed. I realize you have an aluminum heatbed and it might make a difference, but multiple people have demonstrated print artifacts being caused by flexing of the bed heater as it cycles on and off using the normal Marlin bang-bang heater control loop. Migrating to SSR and PWM control of the bed heater is the solution for those artifacts.

    What are you using for a filament spool mount? The filament spool needs to unwind very smoothly. If it doesn't (like if you are using the MakerFarm scheme where the filament spool sits on a fixed bar), the filament might be tugging on the extruder until the spool springs free and rotates a bit to relieve the tension in the filament. IMO, the extruder mount on these printers isn't rigid enough to not be displaced by this tugging. Is your hot end a snug fit in the extruder body? A loose fitting hot end would make this worse. Many people have added a thin spacer above the hot end to provide a more solid fit.
    I will give printing without a heated bed a try as well. I do want to add that i am actually printing on a piece of glass clamped on the aluminium heatbed (MUCH better adhesion of PLA). Also i do think i am using the fancy mathematical kind of BANG-BANG temperature control (PID?), not the dumb heat ON if its under and heat off when it reaches the temperature. I did notice wobbling/ribbing in the prints of extruder gears. This wobbling/ribbing is entirely different from the problem described in this topic though. That wobbling is much smoother and very regular. (Remarkably this wobbling seems to only be present on the gear surface and not the central structural bit holds the hobbed bolt and the nut.)

    I am using a threaded rod with 2 nuts to lock it in place as a filament holder, mounted on top of the printer. The issues are not caused by the filament spool, i have had that problem before (filament unwound and got tangled), that causes more serious and irregular faults, it is also very audible when sitting next to the printer when it untangles or if it stays tangled the print will fail entirely and the filament will obviously be stuck when you look at it later. The hotend is fairly snugly fit. However the extruder (+carriage) itself is not as rigid as i would like. This will be replaced together with the extruder. I plan to print or design a carriage that can better clamp the IGUS RJ4Jp bearings. (I really dislike the zipties.....)
    Last edited by Reaping miner; 08-25-2016 at 01:11 PM. Reason: added comment

  8. #8
    Here is the bar, in my opinion, no big problems. (yeah layer alignment could be better i suppose. But this is nothing compared to the extruder body i showed before.

  9. #9
    Senior Engineer
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    Well that;s reasonable.

    I think I suspect the slicer but wait until people with more input come along.

  10. #10
    I don't how experienced you are but when I look at your photos they scream of an uncalibrated extruder. Have you calibrated?

    - Jaggy/uneven/ concave/convex sidewalls: uncalibrated extruder
    - Pointy corners on roof: uncalibrated extruder
    - Artifacts: Bad retraction settings
    - Floor wider than sidewalls: bed temp greater than glass temp of material


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