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Maxinai

Prints/Walls too thick

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Hi Ultimaker Community,

I have a problem with my 3D prints and I need your help to fix it.

 

The problem is that my prints are too big but only in X and Y axis. Its not proportional ... a 30mm cube will be 30.5 x 30.5 x 30mm when printed and a 50mm cube will be 50.5 x 50.5 x 50mm when printed. Until recently I just thought my printer was inaccurate and I made things fit by trial and error but this error is too "regular" / "reconstructable" to come from inaccuracy (...I think). I designed a test piece (unable to take&show pictures right now but will do later) that had circular as well as rectangular extrusions and holes of multiple sizes. All holes (circular and rectangular) were 0.5mm to small and all extrusions (circular and rectangular) 0.5mm to big. I don't know what is the cause of this problem (in truth, I don't even know if it has to do with Cura or if it is just my printer).

 

If some of you have any idea what is causing this or how to fix / circumvent this please tell me!

 

 

Specs:

Printer: Anet A8 (Yes... I know... shame on me for being that cheap 😄)

Nozzle: 0.4mm (but I have 0.2, 0.3 and 0.5mm nozzles if that is of importance)

Filament: cheap PLA (most of the time)

Print-temperature: 205°C (extruder) + 60°C (bed) (I found 205°C to work better for me than the standard 190°C (f.e. layer adhesion))

(rest of Cura Settings in the picture)

Cura_Settings.png

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So what you report is two different issues and EXTREMELY common.

 

PLA when it cools it shrinks but stays liquid for a long time (a few milliseconds) while printing so it's basically going down as a liquid rubber band - like snot.  This is pulling inward when making those vertical holes and 0.5mm is about right.  Sometimes 0.4mm.  The most effective solution is to increase the diameter of all vertical holes (but not horizontal holes) by about 0.4 or 0.5mm.  Other people just drill the holes out later which seems silly but works quite well and you don't have to sweat what temperature you printed at or how strong the fan was and so on.  You can also set "horizontal expansion" to a negative value such as -0.4 but then the outer surfaces will also shrink (which may solve your other problem?).

 

Regarding the exterior dimensions - this is not normal - well it's normal at corners only.  So if think of the cube as having 3 edges, 4 of the edges - the vertical ones - are likely sticking out - bulging out.  The other 4 should be fine.  If you measure the cube such that the micrometer doesn't touch any of those 4 edges you should get that the cube is perfect (within 0.1mm anyway).

These bulging corners are caused because the printer slows down for the corners and the pressure in the print head doesn't slow fast enough so it overextrudes as it slows down and then underextrudes as it speeds up again pulling out of the corner.  You can fix these by slowing down the print speed when printing edges and shortening the time spent on corners.  The former is done by lowering the print speed (all the printing speeds).  If you want high accuracy try 30mm/sec.  There are other ways to speed up the print without losing quality - print with larger nozzles and/or print thicker layers.

The other key thing to do is turn off "acceleration control" and "jerk control" if those are enabled.  Those 2 features reduce ringing (which can be quite visible and annoying but I don't care about it and a micrometer does not see ringing at all as it is a very tiny error) but they increase the bulges of corners on cubes.  Removing acceleration control means the printer doesn't slow down as long on corners.

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First off thanks for the help!

The first issue sounds like what i am experiencing, but where can I set the horizontal expansion (In Cura? or in my firmware? I have no idea). I haven't seen any bulging edges in my prints, maybe I just missed them ... but that wouldn't explain why the "poles" are 0.5mm to thick as well. Could it be the same problem as the holes just that this time the PLA of the walls flows outwards? Drilling out my holes works most of the time the problem was that recently i made a lot of "interior tubes" (f.e. i made a ball screw and nut) and interconnecting parts where drilling out just wasn't an option.

Again thanks for the quick response will try the horizontal expansion thing as soon as possible!

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I think of it as "buldging corners" as it's a two dimensional issue.  But it repeats on each layer.  Look for this on any cube you have printed.  Avoid those when taking measurements - or at least be aware.

 

In cura just above the settings there is a search box.  Many people will tell you to make all settings visible.  I don't recommend that.  Instead just type "initial" in the search box and all settings (even invisible ones) will appear that have "initial" in the setting name.  Or search for "horiz".

 

Again with the threading - add about 0.4mm or 0.5mm to any vertical holes in CAD software.  This may seem crazy to have to do but keep in mind that these same sorts of issues happen with injection molding.  You even have to change the angles of corners!  If you want a 90 degree corner I believe you have to make it 89 degrees in the mold!  Plus the whole part shrinks by 0.6% or so with injection molding.

 

Of course usually the mold makers do this kind of thing for you (and charge you $50K).

 

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