Basic Rules for STL and OBJ Files

3D printing may seem limitless, but there are a few things that it, or the materials that it uses, can't do. Here is a quick guide of how to avoid a few no-no's when designing your model.

Part 1. Basic Rules

1. Draw in solids bodies, not surfaces, they're easier.

3D printing cannot print surface model CAD models, you must submit a solid body CAD model. The walls can be thin as low as 1mm, but they cannot have zero thickness. Just remember that one of the few things that are no achievable in 3D printing is printing foil.

If you have drawn in surfaces to create a shape, you must ensure that ALL faces are CLOSED off (see tip number four).

Making your 3D model solid and printable using your favorite modeling software;

1. Blender

2. Zbrush

3. SketchUp


2. No shared edges


Your CAD file must not contain any shared edges between objects. It is ok to have a 0.1mm gap between parts, or an overlap if the parts are joined together.

3. Normal normals please


Normals are perpendicular references points to surfaces on CAD models to define whether the surface is an inside or outside face. An inverted normal is a normal that is pointing in the wrong direction, telling the computer that an outside face is actually an inside face when it isn't.

4. Shut your face


Before your model can go to the printer, it must be checked to see if there are any open faces. An open face is where a model is  missing a face from the surface of the part. This error can occur during design when drawing surface models, or sometimes occurs when a file corrupts when saving or converting to .stl or .obj format.


5. Keep your limits


MATERIALMAX. BUILD VOLUME
ABS & PLA Plastics200mmx200mmx200mm
High Detail Resin100mmx100mmx100mm
Precious Metals 100mmx100mmx100mm

Each machine that we use has a set build volume which you cannot build larger than If your model is larger than the build volume, either break it down into smaller individual components or cut join lines through the model.

6. Scaling down


Scaling down your model will dramatically reduce the cost of the print. If you half the dimensions of your model, you will reduce the cost by eight times.

Remember to keep wall thickness to the minimum requirements or thicker once you have scaled down, especially when scaling down large complex objects such as architectural models.

7. Mind the gap


Gaps between moving parts must be at least 0.5mm. Any smaller and the part might stick and prohibit movement. A common mistake when building a model is to make moving parts with zero gaps, make sure you check this before print.

8. Thin arm syndrome


If your model has thin arms with some otherwise unsupported weight at the end, make sure that the arms will be strong enough to hold them, otherwise, the arms could drop or break. This is particularly important when scaling a model down, make sure that after you have scaled a body down that the arms are of sufficient thickness to support the mass at the end.


9. File too large

3D files can be heavy, very heavy. This can be a problem when trying to upload 3D files to websites like merg3d. We suggest keeping your model size to be less than 20MB size per part.

Reducing the polygon count will reduce the files size, but also the smoothness and detail of your model.

The challenge for the designer is to find the perfect trade-off between having a well-detailed, non-pixelated model and a file that is small enough to be easily shared and uploaded. Luckily, this is easier than you might think.

In many 3D modeling programs, you will be asked to define maximum tolerance, precision or deviation of your STL export. It is advised to choose a tolerance of 0.01 millimeters for a good export. Exporting with a tolerance smaller than 0.01 mm does not make sense because 3D printers cannot print this level of detail. When exporting a file with a tolerance larger than 0.01 mm, triangles might become visible in the 3D print.

What to do if your software doesn't allow you to reduce the size of your 3D file?

You can give the free 3D modeling program MeshMixer a try. Just follow these easy steps.


Part 2. Design Rules for Specific Material

We are done with the basics, next is design rules for your material choice.But there are design rules for specific material because they are manufactured using different machines.

1. ABS and PLA Plastics

2. High Detail Resin

3. Precious Metals

4. Colored Sandstone (Soon)

5. Cold-Cast Metals (Soon)

6. Ceramics (Soon)