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csdragon

Gaps at prints

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Hi!

At many prints I've got gaps between the first and the second wall or between the first wall and the filling (- it depends if i print with 0.4 or 0.8 wall).

It's not that dramatic but it's the last step to perfect prints...

Most of the time the first (sometimes the 1. to 5.) layer is perfect and after some time the gaps appear.

Here are some pictures:

IMAG4297IMAG4307IMAG4308

I already added some axis end caps (http://www.thingiverse.com/thing:54075) to reduce the free play at the axis, aligned the axis, tensioned the belts or tried prints at different temperatures (from 207 to 218°C).

I even tried to set the diameter at Cura to 2.83...

Nothing really helped to close the gaps.

What can I try to avoid these gaps? Is this a problem at my config at Cura or do I have to search at the printer?

 

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I looked at your 3 pictures and I know exactly what it is but you might not believe me so first I have to tell you that i've seen this a lot. You have 2 symptoms that are the same problem: The infill isn't quite reaching your perimeters. #2: your diagonal infill has two close together passes, followed by a gap. Then 2 close together, then a gap. Both of these symptoms are caused by something called "backlash" or "play". There are many things that cause play on the UM. The most likely is loose belts and the most likely problem is the short belts (those to the motors) so slide those motors down again. Here is a link to an explanation:

http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474

Finally if you still don't believe me after looking at the diagram, scroll up two posts from the above one and look at the photo of the 5 cubes. Click on it and read the explanation by the original poster.

It could also by your long belts. There is a video in the UM assembly instructions that if you play it will show you how to play your belts like a guitar string and what pitch you should hear and how to tighten the long belts. All on a short video.

 

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I already tuned the long belts as shown in the video before my post... They sound exactly like them.

First it wasn't possible to tighten it such strong but I installed some belt tensioner to do it.

The only thing that can possible be are the short belts...

Oh and thanks for the link... Sounds interesting! I'm about to read it!

 

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Thanks for your last posts! I tensioned the short belts and proofed the long belts again and now the gaps disappeared completely.

But I have one problem that didn't disappear (printed out a new one after tensioning the belts): I printed a calibration tool for an old model helicopter (20 year old "Schlüter Magic") and the size should be 40x40x15mm... But when I measure the printed part the worst results in the middle of the object were 39,62x39,78x14,67mm

With a difference to almost 0.4mm at all axes its not the best calibration tool in my opinion! Do somebody know from what this departure appears?

 

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That's typical - .4mm shrinkage. Not sure what causes it. I think it's because PLA shrinks as it cools. So now you calibrated. Just make the object .4mm larger on all 4 sides, lol.

 

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It's the nature of the beast I'm afraid. If the plastic didn't shrink we wouldn't have any problem with parts lifting off the platform either :) Even when working in the much more refined area of injection moulding you have to take shrinkage into account.

 

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There's a scale factor in cura. It's pretty easy to just print .5% bigger or whatever size you want. It's the center of the 3 icons that scale/rotate. So you could increase just X and Y without scaling Z (which tends to not need scaling).

Unfortunately it's more complicated. The bottom layer shrinks the least because the tape holds it well and you get "elephants foot" unless you do a raft. The bottom layer is also under more stress after the print is done. Each layer shrinks as the next layer is being applied. Walls on the side of the part closest to the fan shrink faster. Each layer is placed correctly but the layer below may have already started shrinking. Then there are other issues: if the part isn't cool enough it can sag due to gravity or other forces. I don't understand all the forces. So instead I just print once and adjust any critical dimensions as necessary. Usually there are only a few critical dimensions such as the diameter of a hole or width of a part. The Z dimensions tend to be spot on so never need adjusting.

 

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Since the printer starts every layer at 'exactly the right height', there's less scope for shrinkage in the z direction, although I guess it can happen a little bit.

The height is measured out a layer at a time, starting from the home position. So if your home position is fractionally off, that will affect your overall print height. Also, if you set a different first layer height in Cura, that will also affect you overall object height - I believe it slices the first layer as if it was the right height, and then just prints it fatter - causing everything else above it to move up slightly.

 

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Well not in my case... It should have been 15mm and the print was 14,67mm

 

Clearly we have a different definition of "spot on". .33mm off, eh? That's like 3 sheets of paper. That's like a thin layer of paint. How thick are your layers? Don't expect it to be more accurate than 1/2 layer height because cura either makes a layer or it doesn't. It can't make a half layer.

Yet it should! The top layer should always be a custom thickness don't you think? Or maybe the top 2 layers. Or maybe all the layers should be as close to the setting you requested but in such a way to finish perfectly on the final layer. I guess this is only helpful if the top is flat. If the top is spherical maybe that wouldn't be helpful.

Anyway, if the print was 500mm tall I bet it would still be within .5 mm. The Z position of the nozzle should be much more accurate than .1mm but the plastic itself has a mind of it's own.

 

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