3D Printing
Contents
Introduction
3D printing explained
A 3D print starts off as a virtual design of the object to be printed. Virtual designs are created using CAD (Computer Aided Design) software. These 3-dimensional designs can be turned into physical models using a series of techniques called additive manufacturing. In additive manufacturing processes, subsequent layers of material are created to create a structure, resulting in a 3-dimensional object. These layers (slices), used to build the object, are thinly sliced horizontal cross-sections of the object. To prepare the design for printing, software creates these slices, which allow the printers to create the object layer by layer.
Methods of 3D printing
Not every printer used the same technology to construct its prints, several ways of building a print exist and all of those technologies as of 2012 are additive. Differences between technologies are mainly in the way layers are added together in order to create the model. SLS (Selective Laser Sintering) and FDM (Fusion Deposition Modelling) are the most common technologies using melting or softening materials in order to allow for the layers to be built up. Another commonly known technology cures layers of liquid material to build a model, this is known as SLA (Stereolithography).
SLS (Selective Laser Sintering)
This technology uses a high powered laser to fuse powder made up of plastic, metal, ceramic or glass granulate to create the model. The model is built up out of layers of powder, which is placed in the printer. The material is brought up close to its melting point by a radiant heat source, prior to the laser crossing over the bed of material. When the laser crosses the bed, it tips the temperature of the material just over the melting point, causing the powder to fuse together. After a layer of the model, or ‘slice’, has been completed, a new layer of the powder is added on top of the powder bed. Due to the relative cold temperature, this layer cools down its underlying layer in the process. After this, the entire process starts again, creating the model in slices and fusing it together as the process progresses. The big advantage of SLS is that no support material is needed in order for the print to be created, as the remainder of material around the print supports the print itself.
FDM (Fused Deposition Modeling)
Fused deposition modeling works by using a plastic filament or metal wire to feed an extruder. The filament is heated up in the extruder and a molten or heated up filament will come out of the extruder’s nozzle, allowing it to deposit this molten filament wherever needed. By controlling the flow of filament, layer thickness and printing speed, differences in print quality, connection between layers and ability to construct different geometries can be controlled. The filament starts to cool down and solidify immediately after extrusion.
SLA (Stereolithography)
The technique of stereolithography uses a UV-laser to cure layers of UV-curable photopolymer resin to create layers of the object. For each layer, the laser projects a cross section of the object onto the surface of the liquid resin, curing the layer of resin and joining it to the layer below (as seen from the object’s orientation, most non-professional SLA printers print upside-down, so the printer’s bed actually moves up.) This technology has the advantage of being relatively accurate (0,05-0,15mm).
Model creation
When designing an object for 3D printing, a program used for 3D modeling is used to generate the geometry of the design. This geometry is needed to generate the information needed for 3D printing. However, a couple of hints can be given when generating this geometry:
- Be precise; when the model has holes in the model, creating single face geometry, the slices generated by slicing software will act out and as a result will not be able to create nice slices.
- Think about your design; read about the technology used in 3D printing and you will be able to design objects suited for 3D printing without too much post-processing.
Creating a model is possible in a wide variety of CAD programs, in this case we will focus on Rhinoceros.
Rhinoceros uses NURBS geometry to design form, this allows for a less constricted workspace and less limitation on form, in comparison to non-NURBS design programs.
When the object is created, the workflow to a printable file is clear and short;
- Create a solid model in Rhino
- Convert the model to a polygon mesh ( the 3D printsoftware can't read Solids or NURBS geometry )
- Save the file in Rhino as a STL model ( which is a polygon mesh format )
- The last step is generating a G-code in the 3D print software
It’s just four steps! HOWEVER;
Generating mesh from a NURBS form which is not a closed polysurface, can create a non-uniform model. When, after meshing the object, the object is still correct, but the printing program has problems with non-uniformity. Gaps in the mesh may appear.
The reason for it is that if you didn't convert a polysurface or a solid but a NURBS to a mesh geometry the computer doesn't know how to correctly join the edges into one single mesh geometry. When you convert a NURBS surface to a Mesh all the seperate surfaces are converted independedly. This can lead to gaps at the edges. If you convert a polysurface or a solid the mesh will be created of a single closed surface insted of a series of seperate surfaces.
Sometimes however there can still be problems with the mesh. For this a quick solution is possible:
Autodesk had a program called Meshmixer which has a fix option under analyse.
Creation of a model for 3D printing, using Rhino
A step by step example of creating a printable model.
- The first steps consist of defining the model in NURBS geometry. Reminder: There are a lot of ways to create your own model, this is just an example.
- Depicted above is the creation of the NURBS model used in this walkthrough. There is three steps in the creation of the model; draw the outline of the object, extrude the polyline, cap the extrusion.
If the extrusion is able to be capped, it should be a closed polysurface. You can check this in your own model by exploding the model and joining it again. Resulting in a report of the generated model in the command line output of Rhino.
- After checking if the model is indeed a closed polysurface, it can be transformed into a mesh and be exported as an STL-file. This file type is most commonly used for the model to be loaded into the 3D-printer’s program (supported file extensions are: .stl, .afm, .obj and .3ds).
This can be done by selecting the “from NURBS Object” function in the drop down menu under Mesh at the top of the screen.
- The Mesh model now needs to be exported as an STL-file in order to be loaded into the Repetier program. This is done by selecting the Mesh model and selecting the “Export selected…” function under the File menu at the top of your screen. Be sure to save the exported STL-file somewhere you can find it later.
Cam Lab gypsum printer
The 3D printer creates gypsum plaster models directly from digital data. The Faculty of Architecture has one 3D powder printer. The 3D printer builds the model layer by layer like multiple cross sections. Fixed parts of the model are treated with binding agent and the rest is loose plaster. After 3D printing the model is dug out and cleaned with archeological carefulness.
In this short manual we will explain how to create your model for the 3D printer.
Preparation
Models for the 3D printer can be created with 3D modelling programs like Maya and Rhino. The model has to be 'watertight' and build with polygon geometries. All normals of the faces must point outwards.
Export in Maya
Software of the 3D printer does not 'understand' other type of geometry like NURBS. Export your model in Maya as a VRML file. When exporting keep the settings as it is. Select vrml2Export from the plug-in manager if the VRML exporter is not loaded.
Export in Rhino
Save the whole model or export a part of the model as a ZPR file. This file format is native to the software of the 3D printer.
Export in other programs
Any other program which can export ZPR (ZCorp), STL (stereolithography), VRML, 3DS (3D Studio Max), can be used to create the model for the 3D model
Example of modelling in Rhino
Preparation for 3D printing in Rhinoceros
Sizes and Units
The maximum sizes for the models are:
- 20,3 x 25,4 x 20,3 cm (8 x 10 x 8 inch)
The 3D printers use centimetres or inches as units, but it is always possible to scale the model to the desired scale.
Modelling
In general the gypsum plaster models obey the laws of physics. Designs with spans defying the force of gravity may need extra support in the model to prevent a collapse of the model. The same is valid for columns. Very long and thin columns can break just like real-life columns which can also break or bend.
We have some rules of thumb:
- Minimum thickness of 3 mm
- Embossments are already visible with only 0.1 mm difference.
