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 Post subject: Gallery of print flaws
PostPosted: Mon Jan 20, 2014 11:22 pm 
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It seems like every once in a while somebody gets a print that has some interesting flaw they don't know how to describe the issue, what to call it, or if anyone else has ever seen the problem before. Each year this community orders prints of at least a few hundred thousand parts from Shapeways so interesting errors are bound to pop up all the time.

So, I figured it's time we had a thread with photos of the various print flaws we come across.

Bubbles:
Attachment:
print_flaw_bubble.png
print_flaw_bubble.png [ 1.55 MiB | Viewed 1204 times ]
I think this issue is caused by moisture in the powder preventing proper sintering.


Bulging layers:
Attachment:
print_flaw_bulging_layers.png
print_flaw_bulging_layers.png [ 1.21 MiB | Viewed 1204 times ]
I don't know what causes this. It's actually really common but usually the bulges aren't very pronounced. I usually clean these up with a scalpel whenever I think they'll affect turning. If I scalpel them off before I polish, they're completely invisible by the end of the polishing stage.


Aliasing:
Attachment:
print_flaw_print_aliasing.png
print_flaw_print_aliasing.png [ 1.02 MiB | Viewed 1204 times ]
This issue is most pronounced when a face or curved surface forms a shallow angle with the powder layers. Some people call this "stair stepping".


Raised faces:
Attachment:
print_flaw_raised_faces.png
print_flaw_raised_faces.png [ 2.79 MiB | Viewed 1204 times ]
I'm not entirely sure what causes this. Sometimes a flat face gets raised up. Sometimes only a part of the face gets raised up. Sometimes it causes a sharp lip to form all the way around the face. This could be related to aliasing. Carl has a photo of this here as seen in this post.


Rough face / surface:
Attachment:
print_flaw_rough_face.png
print_flaw_rough_face.png [ 1.33 MiB | Viewed 1204 times ]
I really have no idea what happened here. It might just be many bulging layers. The pentagon visible in the center of the part is a different print flaw (underside feature distortion).


Shredded piece:
Attachment:
print_flaw_shredded.png
print_flaw_shredded.png [ 1.06 MiB | Viewed 1204 times ]
This might be a bubble. It makes the pieces look like it went through a garbage disposal for a few seconds.


Sunken faces:
Attachment:
print_flaw_sunken_faces.png
print_flaw_sunken_faces.png [ 3.41 MiB | Viewed 1204 times ]
This is a lot more pronounced in person than it looks on the photo. The flaw surface of many parts is bowed in. This flaw was apparent in the original print, it wasn't caused by the polishing or dyeing process.


Truncated pieces:
Attachment:
print_flaw_truncated.png
print_flaw_truncated.png [ 1.05 MiB | Viewed 1204 times ]
Looks like the piece was sticking outside of the printable bounds.


Underside feature distortion:
Attachment:
print_flaw_underside_feature_distortion.png
print_flaw_underside_feature_distortion.png [ 1.7 MiB | Viewed 1204 times ]
There is a post underneath this face. It probably shrank when it cooled and pulled part of the face down with it. This is a common print flaw but is usually very minor. Rarely is it so pronounced as in this image.


There are other flaws like a completely unsintered layer which causes the parts to simply break into two. There are also parts that got shifted mid-print. I don't have any pieces like this handy to take photos.

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Last edited by Brandon Enright on Sat Jan 25, 2014 3:06 am, edited 1 time in total.

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 Post subject: Re: Gallery of print flaws
PostPosted: Mon Jan 20, 2014 11:37 pm 
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These pieces didn't print with their footers.


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 Post subject: Re: Gallery of print flaws
PostPosted: Mon Jan 20, 2014 11:39 pm 
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Zem wrote:
These pieces didn't print with their footers.

That's pretty unusual. Do you think they were all truncated in the same way? Like were they all located on one side of your model so that if your whole model was truncated their mechanism portion wouldn't print?

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 Post subject: Re: Gallery of print flaws
PostPosted: Mon Jan 20, 2014 11:41 pm 
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Brandon Enright wrote:
Zem wrote:
These pieces didn't print with their footers.

That's pretty unusual. Do you think they were all truncated in the same way? Like were they all located on one side of your model so that if your whole model was truncated their mechanism portion wouldn't print?


It was actually a problem with the way shapeways interpreted my STL. Apparently the footers and the piece showed up for them as having an invisible gap between the two rather than them being one solid object. I supplied my client with a new stl of the pieces and that seemed to fix the issue.

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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 2:02 am 
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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 7:19 am 
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I hate to say it, but it's topics like this that COMPLETELY deter me from buying from Shapeways. I really don't want to spend a huge amount of money on something to get flaws and then struggle with the company to get them sorted and wait more weeks to get replacements which even then may not be right.

From what I read, flaws like this are almost normal and not the rarity you would hope.

I have a massive list of puzzles I want to get but I'm too frightened to try it. I just don't have time to sort out problems. The extra cost buying ready made from the designers means that my list tends to stay as just that, a list and not a collection! Shame really! :shock:

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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 8:02 am 
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Don't be put off, I've ordered hundreds of pieces from Shapeways and less than a handful have had (minor) flaws. The worst thing is if a piece is missing, and SW have always sorted this out, no questions asked.

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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 12:58 pm 
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Puzzlemad wrote:
From what I read, flaws like this are almost normal and not the rarity you would hope.

I've ordered dozens of puzzles from Shapeways, and the only flaw I've experienced that was listed here was stair stepping. Even then, stair stepping can be avoided by not arranging parts such that faces are at shallow angles to the X, Y, and Z axes of the model. The only issue I've had that warranted contacting support was when I had a few pieces missing. This is more common than print flaws listed here, but I've started including extra pieces in all of my models, so this becomes less of a problem. Additionally, I've never had any issues contacting Shapeways support; they are always quick to fix any and all issues that may come up in your order.

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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 1:08 pm 
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Puzzlemad wrote:
From what I read, flaws like this are almost normal and not the rarity you would hope.

Most certainly not... :lol:
*Ahem* I personally have gotten more print flaws in my designs than I have in the past. I used to seldom have issues, but nowadays every other order of mine has some issue. Most often it's missing pieces or bubbling on some surfaces. Luckily these are minor and I've always been able to sort them out.

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 Post subject: Re: Gallery of print flaws
PostPosted: Sat Jan 25, 2014 2:14 pm 
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I find many shape-bag :(

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 Post subject: Re: Gallery of print flaws
PostPosted: Fri Mar 07, 2014 12:29 pm 
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So I just got my 2nd shapeways puzzle today, got some print flaws, but I won't show the aliasing as this has already been shown. But of 5 centre pieces and the 5 corresponding caps, 3 are perfectly fine.. Where as the other 2.. Ended up like this:


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 Post subject: Re: Gallery of print flaws
PostPosted: Fri Mar 07, 2014 12:32 pm 
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Looks like they were too close and printed partially fused and Shapeways dyed them while they were together.

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 Post subject: Re: Gallery of print flaws
PostPosted: Fri Mar 07, 2014 12:47 pm 
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Yeah that's what I thought. I've updated the stl now so there won't be any errors like that.

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 Post subject: Re: Gallery of print flaws
PostPosted: Sun Mar 09, 2014 10:42 pm 
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Brandon Enright wrote:
Bubbles:
Image
I think this issue is caused by moisture in the powder preventing proper sintering.

Shredded piece:
Image
This might be a bubble. It makes the pieces look like it went through a garbage disposal for a few seconds.


The EOS heats the chamber to 170C, well above the boiling point, so it couldn't be caused by moisture. Besides, moisture couldn't fuse the plastic together.

These could be artifacts caused by rasterization of a faulty STL mesh. Your data might be the source of the problem, but it could just as easily be was caused by someone else's model nearby. Shapeways rejects non-watertight models because they will cause this problem, but sometimes it can occur with data that's theoretically-watertight if it contains certain hard-to-detect artifacts. It causes random threads of fused material aligned with the machine's X axis, forming a bridge between the faulty part and anything else nearby.

Do you see these particular artifacts often?

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 Post subject: Re: Gallery of print flaws
PostPosted: Sun Mar 09, 2014 10:53 pm 
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VeryWetPaint wrote:
The EOS heats the chamber to 170C, well above the boiling point, so it couldn't be caused by moisture. Besides, moisture couldn't fuse the plastic together.
According to my testing, that's above the initial softening point at around 150C. I'd be surprised if the chamber got this hot since I'd expect the powder to start clumping at around 150C.

Initially I was imagining moisture being vaporized by the laser and causing these bubbles but thinking about it more, I'm sure you're right that the chamber is heated above 100C. That doesn't rule out moisture though. There is no reason why moisture in the powder couldn't evaporate and form a bubble. As the machine lays down a layer of powder, imagine a clump of powder sticking together because it is wet being deposited and then evaporating and forming an uneven surface. This surface would also cause issues with several future layers laid down on top of it.

VeryWetPaint wrote:
These could be artifacts caused by rasterization of a faulty STL mesh.
I've ruled that out. The bubbles are inconsistent errors. That is, they never appear the same way in the same place twice.

VeryWetPaint wrote:
but it could just as easily be was caused by someone else's model nearby.
That's certainly a possibility.

VeryWetPaint wrote:
Do you see these particular artifacts often?
I don't see bubbles often. The most common structural issue with prints that I run into is the "Bulging layers" flaw. It's the only common print flaw that causes turning issues.

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 Post subject: Re: Gallery of print flaws
PostPosted: Mon Mar 10, 2014 2:05 am 
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Brandon Enright wrote:
VeryWetPaint wrote:
The EOS heats the chamber to 170C, well above the boiling point, so it couldn't be caused by moisture. Besides, moisture couldn't fuse the plastic together.
According to my testing, that's above the initial softening point at around 150C. I'd be surprised if the chamber got this hot since I'd expect the powder to start clumping at around 150C.

That was off the top of my head, so I might be remembering the wrong temperature. But the temperature is kept very close to the melting point in order to minimize the temperature rise required for the laser to fuse the particles together. (Parts are built oversize to compensate for thermal shrinkage.)

Brandon Enright wrote:
VeryWetPaint wrote:
These could be artifacts caused by rasterization of a faulty STL mesh.
I've ruled that out. The bubbles are inconsistent errors. That is, they never appear the same way in the same place twice.


Actually, that inconsistency of the 'bubbles' doesn't rule out problems with the data. Your data is undoubtedly error-free or Shapeways would have rejected it, but it could still contain problematic encodings that cause intermittent rasterization issues. These types of problems occur due to random chance, depending on precise part placement in the build cavity.

To illustrate, here's a hypothetical group of triangles that might be an 'excerpt' from an STL mesh, a group of 4 triangles covering a pentagonal region. Let's assume this is just part of a larger model, and we're just omitting the rest of the surface outside this region.

The region inside the pentagon is fully covered, so what's wrong with it? The green, yellow, and blue triangles all share a common vertex (red) that isn't a vertex on the pink triangle--it's located in the middle of an edge!
Attachment:
triangles.png
triangles.png [ 2.08 KiB | Viewed 542 times ]

Due to the limitations of floating point precision this mesh is only intermittently watertight in practice, even though it should be thoroughly watertight. Consequently, rasterization vectors near the red point sometimes don't resolve as intersecting any of these triangles -- they escape through gaps between the triangles. Those gaps don't really exist, but they creep in due to very tiny computational imprecisions. These 'gaps' wouldn't occur if all triangles had shared the same vertexes because all objects would incur the same imprecision; any error that pushed a point out of one triangle would push it in to another if they used the same vertexes.

ImplicitCAD is one example of a program prone to generating this kind of problematic STL due to a faulty implementation of the Marching Cubes algorithm; the problems concentrate where flat surfaces transition to curved ones.

Even so, I'm not suggesting your data is at fault; I'm merely insisting that you can't rule out the possibility yet.

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