Basics of 3D design for FFF technology
In the 3D printing process in FFF technology it is possible – in a big simplification – to distinguish the following stages:
- 3D computer design,
- preparation of the model for printing,
The foundation of 3D printing is always correctly created CAD model and its proper preparation for 3D printing, which takes place in software compatible with the printer, eg. Simplify 3D. The thickness of the walls and holes or supports are determined at this stage and the stability of the 3D print, ultimate durability or aesthetics depends on it.
3D design involves a lot of variables that are difficult to summarize in one article. Below we introduce only an outline of basic concepts, from which it is worth to start searching for knowledge in specialist works and articles. Separate and wide problem is a material shrinkage. Remember that design is just one of the points on the way to the perfect 3D print. Many factors should also be taken into account in further processes, such as slicing or choosing the right 3D print parameters – although the calibrated settings in professional devices are a big help here.
Wall thickness and hole size
“When designing, remember about the minimum wall thickness of the model, which should not be less than the width of the material path to be extruded. For a good strength model we recommend using walls with a width of at least 2 mm,” says Piotr Maslej, Mechanical Engineer at OMNI3D. An important issue is the design of holes that will match the cooperating element. Two possibilities are indicated here: design of holes smaller than the assumed diameter, intended for correcting in the post-production process or designing the diameter with regard to variables, including the fact that due to the specificity of FFF technology, the holes usually have a diameter smaller by about 0,1 mm from the assumed one.
While modeling an element, it is also worth remembering that the fewer supports, the easier implementation and post-processing of the 3D print we get – with more accuracy of it. The number of supports depends on the angles of walls inclination relative to the table. Wherever the angle is smaller, we can expect that a program compatible with the 3D printer create more supports. Sometimes we can not avoid it, but in all cases when it is possible, it is worth re-checking the structure of the model.
The accuracy of the exported file
The last of the basic design principles is the very path of processing the model into a file ready for 3D printing. STL (Standard Triangle Language), a standard file format in 3D printing, is based on the CAD model. Using the triangle mesh, it maps the walls of the model. As the grid density increases, the size of the file and the time it is processed by the cutting program grow, but at the same time the model accurately reproduces the shape we care about. If the detail consists only of simple shapes (cubes, prisms), it can be exported with less densities of the net without losing the specificity of the model. Details with curved planes (round) are exported with the greatest possible density of the triangle mesh.
Layers with poor and good export quality (simplification).
A balance must be found between the high density and the size of the file. “If we sink the net too much, the file will take up more disk space, and thus, its processing by the program preparing for printing will last longer, without the increase of the print quality,” says Piotr Maslej.
The three-dimensional STL model is then transformed into a G-Code, a software language that issues commands to a 3D printer.
Diversified durability – setting the printout on the work platform
When designing a part that is supposed to transfer loads, you need to remember that 3D printing is an anisotropic body. This means that it has different strength properties in different directions. For example, when analyzing tensile strength, the printout has a different strength parallel to the print layers from the perpendicular strength to the print layers. These differences depend mainly on the material used, 3D print parameters and the way the details are cooled.
In order to achieve the optimum strength of a 3D printed element design for tensile work, it is necessary to set the detail so that the layers are applied in parallel to the acting tensile forces.