Blender notes to self: 3D Printing

This post was written by eli on October 5, 2018
Posted Under: Blender and 3D

As I use Blender only occasionally, I’ve written down quite a few hints to myself for getting back to business. If this helps anyone else, so much better.

I’ve also written two similar posts on this matter: A general post on Blender and a post on rendering and animation.

Printing methods

See a summary chart on this page.

  • Fused deposition modeling (FDM/FFF): Melted plastic (ABS/PLA/Nylon) coming out from a nozzle. Layer thickness ~0.2mm. Cheap, but the geometry is limited to self-supported models, or the result literally drops. Also relatively limited accuracy and minimal thickness.
  • Selective laser sintering (SLS): Laser sinters or melts a powdered material (typically nylon/polyamide). Layer thickness ~ 0.1mm. No restriction on geometry,  but the printer parts have surface porosity. The only plastic-like material out there is nylon, typically PA12 (or PA11), coming in dull colors. PA2200 is a very common (and good) powder in use, which produces PA12.
  • Stereolithography (SLA/SL/DLP): Based upon curing of a photopolymer resin with a UV laser. Layer thickness ~ 0.05mm. High quality but expensive manufacturing.

Preparing for printing

  • When the object has fine details on a larger object (e.g. a funnel made to match a certain geometry at its top), consider setting up the larger structure first, create a dense mesh with subdivision surface, and do the adaptions on the final mesh (or partly subdivided?), possibly with a modifier (e.g. Curve). It’s otherwise extremely difficult to get a sane mesh, and it bites back with overlapping faces and whatnot.
  • The mesh must be manifold = no holes. Also, it should have no vertices, edges or faces that don’t enclose an volume, no intersection of bodies, no overlapping of edges or faces. Double vertices and edges are not good, but since the mesh is translated into STL, they go unnoticed as long as the duplicates are accurate. If they’re not, this causes warnings that can be ignored, but can lead to missing the important warnings.
  • Watch the model with Flat shading (click button in Tools) at the toolshelf to the left. Smooth shading is misleading.
  • When resizing in Object Mode, be sure to apply (Object > Apply > Scale), so that the measurements in Edit Mode (and otherwise) are correct. Same goes for applying rotation and possibly location.
  • The result is like at rendering. Bends done by bones are exported.
  • Export to .stl, which is a format consisting of just a list of triangles. The file doesn’t include units, which is why it’s required to state units when uploading a file.
  • In properties / Scene (third icon from the left), set the Units to Metric and Scale to 0.001 for millimeters (these units will go to the STL file, which is unitless).
  • Also, in the “View” part of the properties pane (keystroke “n”), under Clip, make sure “End” is significantly larger than the objects involved, or there will be weird cut-out effects as the view is rotated and moved around. This property sets the “global cube”. What’s outside this cube becomes invisible — faces become partially cut.
  • In the same pane, under Mesh Display, consider enabling Length for “Edge Info”, which displays real-life measures of each edge. Only in Edit mode, only for selected edges. These length are subject to scaling, so it’s wrong if the object has been scaled.
  • Consider lock the scaling to unity of relevant objects to prevent confusion.
  • The 3D printing add-on should be enabled. At the left bar, there will be a 3D Printing tab, allowing for a volume calculation.
  • Before uploading, do some cleanup: Mesh > Vertices > Remove Doubles, as well as the Cleanup/Isolated and Cleanup/Non-Manifold in the 3D printing toolbox.
  • If the 3D toolbox spins forever when pressing the “Volume” button, it’s not a good omen, obviously.
  • Once uploaded, odds are that a lot of warnings on non-manifold edges and intersected faces. These can be checked with Blender’s 3D Printing Toolbox. In particular note that in Edit Mode, there’s a button saying “Intersected Face” which selects the faces marked as intersected. The underlying reason is can be the use of the Boolean modifier, which may create a lot of double edges (two adjacent faces have separate edges instead of sharing one). These double edges occur a lot more than those causing warnings by these tools, probably only when there’s some difference between the two edges. If this is the reason for these warnings, there’s no problem going ahead printing (saying this from first-hand experience).
  • Pay attention to the “Infill” percentage, which means how much of the internal volumes contain with plastic vs. filled with air cubes by the printing software. The layer height also influences the precision and finish.
  • Matching parts: If one part is supposed to go into another, there is no need for an air gap, but there will be friction (my experience with a 2 mm blade into a groove with the exact width, ABS 200 um printing).
  • Checks and export into STL include the active object in Blender only. No need to remove supporting objects before exporting.

Printing online

Online printing is an ugly business, and it’s a bit difficult to blame the service providers. People upload inherently flawed models, made with modeling software that produces output files with geometrical ambiguities, end up disappointed and then blame the printing shop. Those running these services get used to angry customers who write bad reviews on them everywhere, and eventually adopt a strategy of “the customer is always wrong”. It’s impossible to work on a good reputation when a lot of people get angry on them, and there’s nothing they can do about it.

But some companies take this to extreme. See my notes on Shapeways below.

Another thing is that 3D printing is unrepeatable. There is always some human intervention in the process for achieving the best results, as perceived by the operator. With FDM printing, there are several parameters that affect how the layering is done. With SLS printing, this may involve rotating the 3D model, in particular if the same model is printed several times. Such rotation can allow more pieces in each round. And with any technique, human intervention is often there to fix flaws in the mesh, if such exist. Each time, these fixes may be different, or not made at all.

So printing is a bit of a roulette game. The best strategy is therefore to minimize the risk. Simply put, go for the service provider that appears legit and offers the lowest total cost. That’s the only parameter you really have control of.

Printing services charging higher aren’t necessarily better. There is very little indication of who will provide you with the result you wanted. If they ship with UPS, it’s not a good sign, as this courier provides lower prices and has less emphasis on how happy the receiver of the package will be. It seems like serious vendors turn to DHL. But this doesn’t help much, as most vendors work with UPS.

If larger quantities are required, test printing is a good idea to spot problems with the model. However if the design is sensitive to tolerances below 0.5 mm (even with SLS), testing doesn’t guarantee anything. In particular, the transition from a small amount to a larger one can make the same printing service provider choose another machine, a different orientation, or a different post processing chain.

There is no way around this gamble. Neither does it help much to stay with a specific vendor. It makes perfectly sense to make the test printing with one provider, and the larger amounts with another. Stick to the one that gives the better price for each phase. Try to remain with the same material, and on a really good day, with the same machinery.

In short: 3D printing isn’t a long-term relationship. The service providers would of course rather have you stay, but they will typically do nothing but some sweet talk for that purpose.

Where to look for print shops

This is the market situation as I understand it as of July 2019. It’s a dynamic market.

Today’s bazaars of printing companies seems to be Printelize and Treatstock, which allow uploading a design and get offers from a lot of companies. Nice offers for the lower range of printing quality, in particular the former, but pick those enlisting the shipping costs, or it typically ends up with 30 USD added. Actually, for a really low-cost item I didn’t manage to get any deal closed through Printelize.

For SLS and such, it’s basically same as the the lower end of market prices.

As of January 2020, Treatstock doesn’t allow selecting the printing method, and offers service providers that only have FDM for a job that is way too complicated for that method. So only for low-end projects.

Quite recently, I found Craftcloud, which is a bit vague about materials and processes, even though it’s more-or-less written in the offers. And I got the impression that more companies from China are represented there.

In the past, I recommended place to find a print shop is 3D Hubs. However they’ve changed to offering a single deal, and other places seem to give better offers. Their tools for analyzing a model are still great (X-Ray view and graphically highlighting problematic places), so they’re still worth a visit. Even though some of their complaints on my designs were clear false alarms.

And their minimal order is $35. Lower orders are simply raised to that sum.

I’ve had good experience with 3D Print UK, who performed professional SLS printing of 110 small pieces with PA2200: Their price is unbeatable, their black dye is really nice, and the polish finish (plain type, done for free) is the smoothest I’ve seen so far. They did however rotate the printing orientation without asking, which I wasn’t all that happy with. So it’s recommended to request locking the orientation if that makes any difference. But even if I had to reprint everything because of this, and pay for it fully — it would still have been cheaper than any alternative I had.

My experience with Shapeways

I’ve had a good experience with them regarding a non-professional SLS job (“Versatile plastic”, which is a nice name for PA2200, a powder for Nylon 12), which didn’t require much accuracy. The 3D model they got was flawless, so they printed it, sent it, and all was fine.

And then I needed some professional printing with the same process and material. When I say professional, I mean a Kit-Kat sized plastic part with a groove into which a PCB is pushed. It it’s too narrow, it won’t go in. Too wide, and it won’t hold the PCB in its position. Plus some 2mm holes fitting an M2 screw and matching nut. And there were also issues with mechanical strength and flexibility. In short, every 0.1 mm counts.

As I was under the illusion of printing repeatability at the time, I made a round of test prints of my model. I won’t get into the technical details of how the printing results were different from the 3D model, because the crucial inaccuracy turned out to be inevitable, as I learned later by experience. And it was also quite easy to fix on the 3D model. However Shapeway’s response to my complaints (“the customer is always wrong” mixed with “the customer always measures wrong”) made me abandon them temporarily for Sculpteo, which turned out even worse (see below).

So after the Sculpteo detour, I went back to Shapeways with a model I hoped would work. It turned out that they don’t reduce the price at all for larger amounts, even not for 110 pieces. But I was ready to put up with that, as I had reasons to believe that the result would turn out OK.

But then they made the move which surely broke some kind of record, and I’m not sure if it’s about being obnoxious or plain stupidity. And it was all about what to write on the packet, containing a $379 print job: Immediately after issuing an order, I asked that the “Sold to” on the UPS waybill would be the same as “Bill to” in the order and invoice. This has to do with taxing, customs and in particular who owns the goods officially. It matters if it’s a company making the order. Anyhow, this is what they should have done by themselves, since the shipping address is just where the package goes physically. They buyer is whoever pays.

I got the answer that it’s impossible, and that the waybill on the parcel is printed automatically. So I asked to delay the shipping until this issue was sorted out, and got the answer (from more than one person) that it can’t be stopped. Indeed they shipped it, it got the wrong custom declaration, and it went down to some trashcan in UPS’ offices. I couldn’t use any of its content the way it was declared (company bureaucratics).

So this is something one must know about Shapeways: Once you’ve placed the order, the train can’t be stopped. No human is any control anymore. It just happens by itself.

Tactically speaking, they were right: Once they send the parcel in my direction (more or less), the credit card company can’t cancel the deal. But if it doesn’t arrive in time, I do have a case with the credit card company. So they did the right thing, given that their underlying attitude was to grab the money, and tomorrow doesn’t exist.

My experience with Sculpteo

I made quite a few different tests 3d models and sent them for printing with different materials and finish. It ended up with a complete fiasco. Here are a partial list of things that went wrong.

  • One of the test models was printed twice instead of another which wasn’t printed at all.
  • Some models were requested with color, some without. They got it mixed up, and dyed the wrong models.
  • Tons of residual powder inside the plastic.
  • There was this 1-2 mm thick cavity for containing a PCB. It was filled with stuck plastic, so the PCB couldn’t be inserted. I wasn’t able to clean that up.
  • The screw holes ended up too narrow to fit the screws.
  • Plastic parts with 0.3 mm spacing between them melted together on the test on 60µ printing, but not with the 100µ-120µ printing (yes, the finer printing is the one that failed).
  • Put several pieces from different models in one bag.

The worst part was of course that plastic parts had melted together. The warnings you get from the web tools are always about small details which might break (don’t worry, some did, but I accept that since I was warned about it) but nothing about melting. And that’s on a gap of 0.3mm relative to 100µ-120µ printing.

Their response read as follows (early March 2020, this isn’t Covid affected yet):

I would like to point out that Sculpteo prints hundreds of objects daily and your objects look very similar and can easily be mixed up between the models, This is also why the incorrect objects were dyed.
For the space between parts like the stems that are fused together need a spacing of 0.5 mm and not 0.3 mm as you have made, this is why the objects are fused together, normally you can use a Stanley knife to cut these areas.
You have stated that the holes are incorrect, I would like to remind you that there is an average tolerance of +/- 0.3 mm. this is why there could be a slight issue but it is part of 3D printing technology.
I can have the parts that were incorrectly dyed and the parts that were dyed when they should not have been reprinted and sent to you as soon as possible.
[ ... ]
In regards to the objects being filled with powder, it is due to the 1 to 1 ratio (example: 1 mm in width for 1 mm in depth). Our high-pressure air jet is not able to remove the powder in an enclosed area and it can be easily be removed with a paperclip.
We regret if you think that we are not able to provide a professional service but we do give lots of information that our customer can read before placing their order.

Bottom line: Sculpteo may be nice for playing a bit with 3d printing, but if you have professional intentions, you probably want to stay away from them. One may think that the ±0.3 mm tolerance is a general statement to keep bottoms covered, but then it happens in actual 60µ printing.

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