3D models are the foundation of 3D printing. Every print begins with a digital model designed using CAD software, which is then converted or “sliced” into instructions that can be understood by the 3D printer. The outcome of the print is a physical representation of the digital 3D model.
Knowing how to make a 3D model for printing is very important, since there is no other way to create printed objects. This isn’t the case with all manufacturing processes. Compare 3D printing to machining, for instance: machines like mills or lathes can either use computer numerical control (with a 3D model providing the instructions) or be operated manually, without any digital foundation at all. But 3D printers can only work with digital instructions.
3D models for printing are made using computer-aided design (CAD) software. CAD applications are powerful tools that allow for the creation of very complex 3D designs. Some users make their own 3D designs by drawing and modifying geometric shapes on-screen, but models can also be algorithmically generated. Typically, users can choose between 2D or 3D design views. Overall, contemporary design software offers a huge variety of features that help users create stunningly detailed 3D models.
Recommended reading: How to find free STL files
However, knowing how to make a 3D model for printing requires more than just artistic talent. After all, not every 3D model is suitable for 3D printing: a perfectly rendered model of an airplane might look great on screen, but its protruding wings and ultra-fine propeller blades might be very difficult to extrude using molten plastic. 3D designers must therefore be realistic about the possibilities and limitations of the printing process. Furthermore, 3D printer users need to know how to convert their digital models into machine-readable instructions in the form of G-code. To do this, they will need a basic grasp of slicing software.
This article goes over the basics of how to make a 3D model for printing. It discusses the best CAD software for 3D printing, basic design guidelines for 3D printing, and the best slicers for turning 3D models into printable code. Note, however, that this article focuses on the printability aspect of 3D modeling; CAD newcomers looking for a comprehensive introduction to 3D design should consult the documentation and tutorials for their specific software.
CAD software for 3D printing
All 3D models for printing start with computer-aided design software. CAD applications allow for the design of complex 3D designs consisting of various geometric shapes. Such software is used to design parts for digital manufacturing technologies, including 3D printing and CNC machining, in fields such as aerospace, healthcare, and consumer products. CAD software can also be used to create 3D models for digital-only applications such as 3D animation and video games.
Most 3D modeling tools fall into one of two paradigms: direct modeling or parametric modeling. However, many applications offer both options. Direct modeling, which involves adjusting the 3D geometry (by dragging and dropping, stretching, etc.), is faster and simpler and is typically used during early-stage design. Parametric modeling is a mathematical and “history-based” paradigm where parts are designed by determining features and constraints. Changes are made step by step in a way that conforms with the original constraints.
Some professional-oriented CAD software is expensive, with most packages now sold with a yearly license rather than a perpetual license. However, there are many excellent free CAD programs suitable for 3D printing, such as those listed below.
Free CAD software for 3D printing
Blender: A free and open-source tool used by animators and 3D printer users alike, Blender is a mesh-based direct modeling application favored for its artistic tools. It is therefore suitable for 3D models with organic shapes, such as figurines and gaming pieces, though less suitable for functional or mechanical parts. It is less intuitive than some other applications.
TinkerCAD: Beginner-friendly and browser-based (it does not need to be downloaded onto a computer), TinkerCAD works using drag-and-drop 3D shapes and saves files to the cloud. The online 3D software is intuitive and a great starting point for CAD novices. As such, it is slightly limited in scope but nonetheless suitable for a variety of printable models.
SketchUp Free: Trimble’s free, browser-based version of SketchUp contains the basic tools for 3D modeling. It uses cloud storage, which can be helpful when accessing projects across multiple devices. One advantage of SketchUp is its integration with 3D Warehouse, which contains a large library of free-to-download models.
Fusion 360: Autodesk’s powerful CAD application is available on a free license for non-professional users. Some features are omitted, but the software contains enough tools for the design of mechanical engineering parts, via direct or parametric design. Though well-designed and fairly intuitive, Fusion 360 is recommended for more advanced users rather than absolute beginners. It is more suited to 3D printing than Maya, one of Autodesk’s other CAD applications.
Onshape: Though the full version of Onshape is aimed at professional and industrial product designers, Onshape Free is a great browser-based tool for 3D printing hobbyists. One caveat is that your designs automatically become public; only users of the paid version can keep their models private.
FreeCAD: Popular 3D modeling software FreeCAD is a versatile and free 3D design package suitable for a range of 3D printing applications, including architecture and product design. Notable features include an advanced geometry engine, a parametric environment, and compatibility with a wide range of file formats.
Design for 3D printing
Learning how to manipulate CAD software in order to create complex 3D shapes takes time. Every application works in its own unique way, and there are many features to explore.
Needless to say, 3D design is more multifaceted than 2D design: not only is there an extra dimension to consider, but designs must always have manifold geometries (they must be “real” 3D shapes with fully joined-up edges, faces, and vertices). Many 3D printing errors stem from non-manifold CAD models, which are not 3D printable since they do not represent objects that can exist in physical space.
3D printer users need to remember that their 3D models won’t just exist on the screen. They will be turned into physical 3D objects, and designers must therefore create their 3D models with the FDM 3D printing process in mind — even if that means making compromises (such as removing overhangs, thickening up fine features, or adding a wide base). In the professional sphere, this printing-focused approach to design is called Design for Additive Manufacturing (DfAM). But the rules of 3D printing design apply equally to industrial designers and hobbyists.
Bridging is when a 3D model features a suspended horizontal section supported by two vertical sections, like a raised bridge supported by two abutments. Bridging can cause issues for 3D printers: since the horizontal section is not supported from below, it can sag or even collapse entirely.
The issue can be solved in different ways. Keeping the bridge less than 5 mm should prevent sagging, otherwise a support structure may be needed underneath it. Support structures are generated automatically by the slicing software (they do not need to be added to the CAD model), printed along with the rest of the part, then cut or broken away manually.
Overhangs are a little bit like bridges. They are protruding horizontal sections, but they are connected on just one side to a vertical section (rather than on each side). An example of overhangs would be the two horizontal “wings” of an upright “T” shape.
Like bridges, overhangs can cause issues if they are not supported from underneath by support structures, and can therefore sag or collapse. Generally, overhangs will support themselves at an angle of up to 45°. The “wings” of an upright “Y” shape, for example, can be supported by the vertical stem. If the angle is greater than 45°, supports should be used.
Recommended reading: Cura support settings, from angles to Z distance
When designing your own model using CAD software, it’s easy to create boxy shapes with sharp corners and edges. But 3D printer nozzles are round, so the lines of material they extrude cannot produce perfectly right-angled corners; designers should keep this in mind, especially when designing components that are meant to fit snugly together. (A similar limitation applies when designing for CNC machining, in which the cutting tool is round).
Fortunately, rounded corners can be a blessing as well as a curse. Adding fileted (rounded) interior or external corners to the 3D model can improve strength and reduce stress, especially on bridges and overhangs.
Because the FDM printing process prints in layers, it is often possible to see “layer lines” where the different layers meet. These lines are especially pronounced — sometimes critically so — when printing round surfaces (on a 3D printed ball, for instance). This issue is sometimes called the “stair-stepping” effect, because the noticeable jumps between layers resemble a staircase.
When making a 3D model with round surfaces, designers should know that the printed surface may look much more jagged than the digital surface. One solution is to use a very low layer height when slicing the model, so the steps are smaller and less visible; another is to use a 3D printing service to get the model printed using Stereolithography (SLA); another is to treat the printed part with post-processing techniques like sanding and smoothing.
One advantage of 3D printing is the ability to design holes directly into the part, rather than drilling them out later. However, designers should bear in mind that FDM printers can’t always replicate the exact specifications of the CAD model.
Issues can arise with vertical axis holes (e.g. a hole in the top face of a part). This is because when the nozzle pushes down on material on the hole perimeter, it squishes it towards the hole aperture and therefore reduces the diameter of the hole. This can be a problem if the hole is meant to accommodate a fastener, for example. Holes may therefore need to be slightly oversized, with trial and error often required.
The first layer of a print is the most important, because good bed adhesion prevents the part slipping during printing. One of the best ways to ensure good bed adhesion is to design a 3D model with a wide and flat base; more points of contact with the build surface means greater adhesion and stability.
Unfortunately, this can sometimes mean compromising on part design. Printing a ball, for instance, is difficult because the part of the ball that touches the build surface is just one small point. But a half-ball (a dome) with a flat base would print much more successfully. Note that parts can also be split into sections (e.g. two separate domes) then assembled using glue or fasteners to make the complete object.
One of the most important things to remember about 3D printing is that printed parts are anisotropic. That is, they are stronger in one direction than another. So when designing functional parts that require some degree of strength, it is important to consider the printing and layer orientation.
In short, printed parts are weak where the layers meet. This means that a printed part has low tensile strength along the Z-axis (you could more easily break the part by pulling it top to bottom rather than side to side). Parts are strongest in planes parallel to the build surface.
A practical example would be a thin printed shelf designed to support heavy objects. The shelf will be strongest if printed “flat” or parallel to the build surface, but weaker if printed upright or perpendicular to the build surface.
Once the 3D model is complete, the CAD software can export the design as an STL file. However, the 3D printer cannot process the STL file as it is; it needs to receive instructions about the part in the form of G-code. Knowing how to make a model for 3D printing therefore requires a basic understanding of slicing software.
The core functionality of a slicer is to convert a 3D model into machine-readable instructions. Specifically, it translates the 3D model into a sequence of flat layers and determines the appropriate linear movements of the 3D printer for creating those layers. The slicer also sets a number of printing parameters (printing temperature, infill pattern, support structures, and much more) and writes those settings into the G-code.
As with 3D design software, there are several free slicing applications for turning 3D models into 3D printer instructions.
Free slicers for 3D printing
Cura: Ultimaker’s free-to-use and open-source slicer is popular with professionals and hobbyists alike. Offering a simple workflow and downloadable plugins, Cura divides the slicing process into three stages: Prepare, Preview, and Monitor. Helpful tools include print time and filament usage estimates.
Slic3r: Developed and maintained by a group of RepRap community members, the open-source Slic3r is regularly updated by its active community of open-source developers. With features like auto-repair and multi-extruder slicing, Slic3r is a good tool for those interested in pushing the boundaries of their 3D printer.
OctoPrint: Primarily a tool for remote 3D printer management (including webcam monitoring of prints), the free OctoPrint software also offers slicing features.
Figuring out how to make a 3D model for printing requires three general areas of understanding: familiarity with your CAD software of choice, an understanding of 3D printing design constraints, and a grasp of the slicing process. Once these foundations are in place, making your own models will become a fast and natural process that results in functional, error-free printed parts.
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 Cacace S, Cristiani E, Rocchi L. A level set based method for fixing overhangs in 3D printing. Applied Mathematical Modelling. 2017 Apr 1;44:446-55.