3D Printing Part 2 (Troubleshooting)
- pennarchtank
- Oct 30, 2018
- 6 min read
So you’ve crawled all the way up to the fourth floor of the FABLab, fought your way through the throngs of panicking grad students, and finally handed over your file to the 3D print operator – but wait! you’re print isn’t ready for printing because it has (GASP!) naked edges (or something equally nefarious). Never fear, the upper class men are here! We’ve listed a few tips and tricks below to (hopefully) help you fix your file and get you into the queue so that your last minute model making can be as stress free as possible.
The Issues and How to Fix Them:
Let’s start with some of the basics – you’re going to hear a lot of terms like NURBS, mesh, and STL in the next section. Almost all 3D software make your 3D forms out of a bunch of polygons that are used to create a composite mesh that forms the solid shape of your object. NURBS is a type mesh that Rhino uses to create your 3D modeled shapes. It takes the average curve between two points to form of a rectangular mesh of the 3D shape. STL is a different type of mesh that uses triangles to form the mesh skin of the 3D shape. A lot of problems usually occur when you’re switching from one mesh form to another and can include the following:
Minimum Wall Thickness
3D printers have a certain width at which they can print; sort of like how hot glue guns have different size nozzles for glue extrusion. Additionally, each material that the printers print with have different strengths which can also affect the thickness with which you can print. Certain models might have a wall thickness that is impossible for the printer to make or that will fail when it is printed (meaning that either the printer or material cannot physically produce the thickness that you want because it’s too thin usually resulting in holes in your model or broken parts where thin legs or structures fell apart during the printing process). Alternatively, things such as surfaces in Rhino have infinite thickness in the program, which if we remember calculus, infinity goes to 0 which means that the printer won’t be able to read it to print it, or if it does print it, it will come out as a single layer of filament or material which tends to be extremely fragile.
How do I fix this?
Well for starters, make sure you check the minimum thickness for the printer or material that you intend to use. The exact minimum changes by type of printer your using and the type of material so be sure to check as your designing your print or going back to offset the edge/surface/etc in your final model. The FABLab has very detailed instructions about what each printer can produce and what materials that they print with and if you can’t find there are plenty of outside web resources you can use to get a general sense of how thick you should make the walls of your model. Also, if you’re using single surfaces in Rhino that you intend to print, it might not be true to your actual dream plan but to ensure that things print make sure you “EXTRUDE SURFACE” to a reasonable thickness before your take your model to print.
Intersecting Forms
In Rhino you can mash 3D forms into each other without properly joining them and they will stick together without a problem. And by properly joining them I’m not referring to the commands “JOIN” or “GROUP” I mean commands like “BOOLEAN DIFFERENCE” or “BOOLEAN UNION.” If a 3D printer sees the thin cylinder sitting inside of a box, even if you make sure that they intersect or touch in the model, it’s not good enough for the computer because it needs to be told exactly how to fill the intersecting space in between them. To the computer, your two objects sitting inside each other are like those people in that Grey’s Anatomy episode that got stuck together with a pole (if you haven’t seen it you probably get my point – it’s not good).
How do I fix this?
If you have two shapes that you want to join, I would try “BOOLEAN UNION” so that it takes out the parts where the two objects intersect that look like two outsides and creates one part with a single interior that the printer can read easily as a single object to print together. If you have a shape that you want to subtract from another shape but you still want there to be a closed surface where the subtracting shape is carving out, use “BOOLEAN DIFFERENCE” instead of a command like “SPLIT.” Boolean commands can be pretty tricky because they require the object to be completely closed in order to work properly. There’s a link below to the Rhino bible on Boolean commands but for the issue about holes see the next topic on our list: Non-Manifold/Non-Watertight/Naked Edges.
Non-Manifold/Non-Watertight/Naked Edges
Our favorite! Naked Edges, non-manifold objects, or non-watertight objects basically mean that your 3D model has a hole somewhere and like that mouse that just ran under your bed, it might not be visible but it’s there. Sometimes holes form when your model is converted from NURBS to STL, the way the two meshes work don’t always play nicely together and you can get holes along the seams of polygons when the smoother curves from Rhino change over to the triangulated meshes of STL files. They could also be leftovers from where you were cutting into your shapes to throw in some rad stairs in 301 or some crazy spines on that blow fish based seating arrangement you’re making for 201.
How do I fix this?
This is one of the more tedious ones of the mix. Essentially you need to find the hole and then go plug it, easier said than done. One way that you can do it, is to go into your STL model and use the “SHOW NONMANIFOLD EDGES” or “SHOW NAKED EDGES” and then “ZOOM NONMANIFOLD” or “ZOOM NAKED EDGES” to find where your holes are and then use “JOIN EDGES” to close them. For more detailed instructions on these commands see here:
Inverted Edges/ Inverted Surfaces
This means that part of your model is inside out. This creates a problem for the printer because it doesn’t know which way to fill the model.
How do I fix this?
First, select your object and type “DIRECTION” to show which direction each face of your object is oreinted towards. Once you find the offender, sorry the reversed side, you can explode your solid object and use the command “FLIP” to flip that particular surface right side in so that it matches the rest of your model and then “JOIN” your objects together. Alternatively, if you’re already in the STL, you can try “UNIFY MESH NORMALS.” For more on surfaces see here:
Non-Supportable
The printer is not ready for your jelly. A non-supportable model is something that the printer either can’t build support for because it’s too complicated of a shape (think some crazy spirals or a Zaha Hadid building) or its far too thin and skinny for normal supports to work.
How do I fix this?
Well, we can’t all be Frank Gehry, so the easiest way is to change your model a bit to make it something less wild and crazy that the printer will be able to support as it prints. But wait there is hope! If you remember from the last 3D print post we talked about some materials that are self supporting like powder printing and SLA or SLS printing where your print lies in a bed of powder as its being made rather than has external supports built onto it as its printed. These generally are better for very delicate prints with long and thin rod structures or big loops, the only downside is that the extra support material of these prints can make them exponentially more expensive.
That’s our list of the main issues we have with 3D printing, but if you want to more detailed information check out these links here:
The Desperate:
While it is best to fix your parts in the program that you originally made them in, there are some alternative programs that you can use if all hope is lost. For starters, my personal favorite is NetFabb from Autodesk (free with a student account) because it has a preset standard fix that it can run that usually fixes most errors. However, this software works only on Windows so unless you are able to bootcamp your Mac you might want to try MeshMixer which is also from Autodesk that allows you to work with your file once it is in a mesh (.STL) format in order to fix places where there might be holes or inverted faces.
To find out about these programs and more, see here:
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