Rhino Models: Design a printable 3D file
1.1. Before starting to model
Before you start 3D modelling anything using Rhino there are some factors that should be considered.
Key concepts for 3D modelling with Rhino
Rhinoceros works with a 3D modelling category called NURBS which is different than regular mesh 3D modelling. However for 3D printing in Rhino it is still necessary to create a mesh after the 3D modelling process. For this reason it is important to keep in mind and clarify some basic 3D modelling concepts.
NURBS 3D modelling concepts
- Curve: are basically the points, lines and segments that make up the sketch model.
- Surface: is a plane without thickness created between a group of closed curves.
- Polysurface: is a set of multiple contiguous surfaces that build a volume.
- Solid: is a completely closed and joined polysurface that create the 3D model. It is different from a mesh.
Mesh 3D modelling concepts
- Vertices: is a position along with other information such as color, normal vector and texture coordinates.
- Edges: is a connection between two vertices.
- Faces: is a closed set of edges that form a plane without thickness.
- Polygon: is a coplanar set of faces that build an open or a close volume.
- Mesh: is a collection of vertices, edges and faces that define the shape of the 3D model. It is different from a solid.
A solid in Rhino is built with NURBS, which is different from a solid built through mesh 3D modelling. Knowing the difference between these two 3D modelling approaches is important for a successful 3D printable model.
Global 3D printing 3D modelling process with Rhino
Objet dimensions and measures compatibility
An important factor to consider before starting your 3D model is the size of the object relative to the professional 3D printer that will be used to produce the piece. For example, if the model is bigger than the 3D printing area, the model must be adjusted.
Also the desired dimensions of the piece should be coherent with the dimensions used while 3D modelling, as well as the units settings of the file that suits better to the project (inches, centimetres, millimetres). This can be found at ‘File/ Properties/ Document Properties/ Units’.
You can work with the most suitable units for you or for the project while 3D modelling. However before exporting you must always switch the units to millimeters and allow Rhino to scale the model.
Rhino unit settings are always interpreted as millimeters when imported into other programs. So, for example, if you modeled a part of 5 cm × 5 cm × 5 cm using centimeters as a unit, you will obtain measurements of 5 mm × 5 mm × 5 mm when exporting your model in STL. This is why you must change the units to millimeters no matter what unit of measurement you used to begin your model.
Grid adjustment and measurement
It is important to be aware of the measures at all times while 3D modelling as this will allow you to have absolute control over the size of your model. The main tool in Rhino used to visualize the size of your objects is the display grid in the background of the viewports. The grid allows you to work precisely using the “Grid Snap” and the “Linear Dimension” tool.
You can set up and personalize your grid at ‘File/ Properties/ Document Properties/ Grid’.
The “Linear Dimension” tool is Rhino’s technical drawing tool and can be used to find a precise measurement of the model. You can find it at Dimension/ Linear Dimension.
The dimension will appear on the arrows bar that was used to set up the file units, after selecting a starting and ending point for the line.
It is recommended to take a look at the tolerance settings file. This can be found at ‘File/ Properties/ Document Properties/ Units/ Absolute Tolerance’.
The 3D file tolerance should be set up depending on the model size and 3D modelling units. For example a 0.001 mm tolerance for a model with a size of 500 mm × 500 mm × 300mm will be an exaggerated tolerance. Instead a 0.001 mm tolerance for a model with a size of 70 mm × 70 mm × 70mm will be reasonable.
This aspect is also relevant for the mesh creation that will be explained later in this tutorial.
Real time tool verification
It could be also useful to turn on the Check New Objects tool by typing it in the command line “CheckNewObjects”. This command will constantly check every geometric figure created to verify if it is valid (not bad) and pops up a message when the figure created is corrupted.
When starting out with Rhino, as in any other 3D modelling software, it’s essential to keep in mind that the 3D modelling characteristics for 3D printing are different than your average rendering or animation model. Therefore separate guidelines need to be respected throughout the 3D modelling process in order to obtain a printable model.
In this tutorial, various images are used to display the process - these images correspond with models that are most suitable with Sculpteo’s plastic material. It is also important to note that Sculpteo uses Selective Laser Sintering (SLS) to 3D print plastic models. This tutorial takes both of those factors into consideration, however it is important to consider the specific material and manufacturing process that will be used for your project while creating your model. For more information, you can refer to our materials page.
Next, you will see the precautions that must be taken into account to create correct solids during the 3D modelling phase.
While creating a 3D model it is important to understand that a 3D printable model must be “watertight” this means it should be designed to prohibit the entry of water. Most common mistakes using Rhino for 3D modelling that occur with non-printable models are the open polysurfaces, bad geometry and naked edges result in a non watertight model.
Closed polysurfaces and non naked edges
A closed polysurface is a set of multiple contiguous surfaces that build a closed volume called also solids. Namely the volume walls have no holes and they are all attached.
A model made of a closed polysurface will normally look like in the picture below. They are usually built with Rhino’s surface and solid menu operations .
However you can also (voluntarily or involuntarily) create models with an open polysurface which are different than a models made with a closed polysurface. Most of the times, in this kind of model, you can appreciate the interior structure as shown in the picture below (red oval) but models like this are not 3D printable because, even if they exists, the surfaces have no thickness.
On the other hand, sometimes 3D models looks like they are made with a closed polysurface but they are not, this is what is called naked edges. A naked edge is basically a surface edge that is not attached to another surface edge even if both surfaces are contiguous, consequently creating an open object. In the picture below you can see a detached surface selected that, when not selected, may look as a regular closed object.
This could happen involuntary for many reasons, most of the times it's because of an unsuccessful 3D modelling operation like split, trim, rebuilding surfaces or the act of exploding and joining surfaces . This last one is paradoxical but true and it happens when multiple surfaces are joined at the same time. After using these operation it is always recommended to verify the object (as you will see further in this tutorial) because some surfaces may need to be joined separately to avoid creating naked edges which will certainly be a problem during a 3D print.
Valid objects are objects built without bad geometry errors. Bad geometry models or bad objects in Rhino are those who violate certain NURBS rules or have a structural problem. Most often, the bad objects are surfaces in a solid, but bad curves also happen occasionally as well. This happens for many technical reasons, in a surface is usually because the trim curve is bad. This is not something you can make intentionally but it often happens after join operations or other commands that also join objects like booleans operations . These operations split edges and adjust trim curves that in some cases are smaller than the absolute tolerance of the file producing bad geometry.
There are several reasons why digital models for 3d printing are hollow but the main reason is the amount of material that will be used to produce the model. In 3d printing — unlike other production techniques — the cost of fabricating an object is not dictated by the complexity of its shape, but by the amount of material that is required to produce the design. Therefore, making your object hollow will positively affect the manufacturing cost of 3D printing, by decreasing it to less than 60% or 70% of its initial cost.
Another important reason to hollow a model, is keeping your product lightweight. For example if you already have a model and you would like to rescale it to produce a different size of it you can hollow your model to make it bigger but keep it light.
But simply hollowing your model doesn’t affect your product; the hollow interior must have at least two holes to the drain the excess printing material. This is a restriction SLS’ printing process, without the holes to let the material drain it will be trapped within the physical object.
The Sculpteo online uploader has recently been enriched with automated hollowing feature that allows you to handle the hollowing process online. This feature automatically generates the inner shell of your model letting you simply choose the required hole positions on your model. To learn more about Sculpteo’s new hollowing feature and try it now, you can upload your 3D file and click on “Optimize” tab after choosing your material.
Next, you will see how to hollow your model in order to reduce the material used during a 3D print. This will allow for a bigger, lighter, and more economical version of the object using Rhino.
You have to first scale the model to the desire size. Illustrated in this example the model is doubled in the size from 6 cm to 12 cm.
You should keep in mind that the minimum wall thickness for the Plastic material is 0.8 mm and the maximum wall thickness must be coherent with the model size. An overly thick wall in a small object will produce geometry errors depending on the curvature of the interior of the model. It is recommended to try different wall thicknesses to find the best for your object. A rod made with Sculpteo’s polyamide plastic, for example, is rigid at 2 mm thick (check the material guide for more information about polyamide design guidelines).
Once you have determined the right size you should extract a surface to make it an open polysurface. For this you can use the “Extract Surfaces” tool by typing “ExtractSrf” in the command line and selecting the chosen face. Then you can eliminate the face.
After that you can use the “Offset Surface” tool by clicking on it in the left bar menu under Filet Surface/ Offset Surface or typing in the command bar “offset srf”. With the object selected, you turn on the option “Solid=yes”, and set the “Distance” to your desired length (2 mm in this example). If you do not want the external form of the object to change you flip the offset direction to the interior of the model with “FlipAll”. All this can be done using the command bar.
You will result in a solid (or closed polysurface model) with a uniform 2 mm wall thickness.
One single solid part
Sometimes, the 3D modelling parts are made using many different solids that intersect each other which finally compose one single part.
Even if the model is constructed with many different cubes or cylinders, the final piece should be a singular and compact solid in order for a successful 3D print. When you select a single part of your model, you usually should see in the command line “1 polysurface added to selection” this means your part has been added to the entire object.
Also your model can be composed of different parts that do not intersect but are instead extremely, or even entrapped within another object. This manner of 3D modelling is particularly helpful when creating moving parts or other types of inserts, sockets, articulations, etc. In this case you have to keep in mind the 3D modelling tips for this kind of parts and understand that an articulation is made from two single solids and not form an intersected solids.
This is very important when producing a 3D printable file without errors. You can find more information about articulations and design guidelines depending on the material on our materials page.
Rhinoceros includes some great tools for color and texture mapping, but it is not the most suitable software to add colors and textures for 3D printing purposes. It mostly depends on the projects needs and requirements. You can also use color and texture mapping after 3D modelling in Rhinoceros with other powerful CAD software applications such as 3D Studio Max, and the free software. Its features will be described in other tutorials dedicated to it.
Color 3D printing adds color and texture to the final mesh model using color and texture mapping tools. The difference between a regular 3D printing model and a color 3D printing model is that the second one should be exported as a .obj format after the mesh creation instead of an .stl file. A color 3D printing file is made of several files with the color and texture information. When exporting an .obj file, an .mtl file containing the texture mapping informations is automatically created by the software: these two files, and all the textures image files used in the model, should always be kept together in the same folder.
Colors and textures could be added before or after mesh creation. The first thing to do before starting to add color and textures is to go to the right side column at the “Display” tab and switch the “Display mode” to “Rendered” having selected the perspective viewport. This will allow you to see the modifications related to color and textures while applying it.
In order to add colors to the object you have to select the object and go to the right side column at the “Properties” tab select the “Paint tube” button , select the option “Assign material by : Object” and change the color in the “Basic Settings” blank space.
Rhinoceros comes with some default materials for rendering purposes. This materials can also be use for color 3D printing purposes. Before starting you must activate the “Rendered Display Mode” as explaned with the color. You can access the default textures and materials going to the menu “Panels” and select “Libraries”. A new window containing the textures and materials will appear, you will be able to click and drag them from the library to the object in order to add them.
For more precise purposes you can use the textures mapping options in the right side column next to the “paint tube”. This options will allow you to place the textures as you desire on the surface object.
Once you finish, you select the object and you exported as .obj file format. The software will produce the files needed to color 3D printing. You should upload all the files together in a zipped folder to Sculpteo’s website for a successful color 3D printing.