Successful STL Tips for 3D Printing
This document will help 3D printing users ensure their STL files produce successful 3D printed parts. It will help them become familiar with popular manipulation software, gain a basic understanding of how STL files work, learn to fix common STL faults for successful 3D printing and a few additional actions they can perform on an STL to expand their 3D printing options.
Step 1: STL Manipulation Software
While all CAD software allows the creation of STL files, this process doesn’t always produce a printable file. STL manipulation solutions allow fixing those models to produce 3D printable models, and perform several simple actions to change the model (such as cutting and labeling).
The following table describes popular STL manipulation software solutions. Magics RP is the most extensive solution available. Meshlab is satisfactory as a free tool, although not very easy to use and lacks some functionality. Pricing is not specified, as this greatly varies from region to region. Additionally, most vendors offer several packages. Please consult your local distributor for pricing.
In addition to these STL manipulation softwares, GrabCAD Print can also be used to print natively from your preferred CAD software.
Step 2: Online Services
Online STL-fixing services provide an alternative to purchasing software. A user can upload a file and receive a fixed file, usually within a few minutes. Some vendors offer free automatic services, while others will have a specialist look into more complex issues. Pricing (if applicable) is typically per use, greatly reducing the initial cost. Examples include Netfabb, a free cloud service, and STL- fixing cloud tools by Materialise.
Step 3: Solid Modeling
Most CAD software requires the user to create models that conform to these rules:
- Each model defines a final volume
- Each volume is confined only in one model
- Each volume has a positive value (larger than zero)
When a model conforms to all three rules above, it is considered “watertight.” The remainder of this document will show:
- What makes a model watertight
- Common issues with converting native CAD files to STL files
- Common STL manipulation software suites, which allow converting a faulty STL file into a printable one, and performing operations that are popular with users
Step 4: An Introduction to STLs
The abbreviation “STL” has several related meanings, one of them being “Standard Triangulation Language.” This means that each CAD model can be represented as surfaces made of triangles. This format has long been the industry standard in rapid prototyping. Here is a closer look into the process of approximating surfaces with triangles:
Each 3D form is made out of polygons. A polygon is defined as a at shape bounded by a closed circuit made up of straight lines. Each polygon with “n” sides can be represented using n-2 triangles.
Imagine a box, for example. It has six sides. Each one is a polygon.
Each one of those sides is a square, meaning the entire box can be represented using 12 triangles, two triangles per side, shown below.
Since we are dealing with three-dimensional shapes, each triangle has a direction. This direction is expressed by the normal of the triangle (a line or vector perpendicular to the surface). The outward direction is represented by the normal.
The two triangles below, though they are identical, are facing in opposite directions.
Step 5: Common Errors in CAD-to-STL Conversion
Inverted Normals: An inverted normal is a surface that is pointing in the wrong direction. Occasionally, the interpretation of the surface between CAD and STL results in an inverted normal. Here’s an example: The box below has one face with an inverted normal, so the user will need to fix it to make the model 3D printable. Most STL manipulation software provides an option to fix inverted normals.
Zero Thickness: To be fabricated in the real world, models must have a volume larger than zero. Sometimes, a model is represented on the CAD software using just a 2D model, which has no volume. The part below is a sheet of material that has no volume, though it is three dimensional.
For the file to be printable, it must have some volume, like the part below.
Bad Edges: Per the definition of watertight, all triangles must be connected. Gaps are commonly referred to as “bad edges.” Here’s an example: The two hemispheres are not connected and are marked with a thick yellow line to indicate the bad edge. (Note that each software suite has a different marking.) The gaps are filled in an action referred to as stitching.
There are two types of bad edges: near bad edges and real bad edges. Near bad edges have a neighbor triangle that is closer than a set threshold. Most software solutions close these automatically.
Real bad edges typically enclose a hole in the shape. This is slightly more complex, as it might cause a fault due to zero thickness.
The part above has a real bad edge in one of the faces. As a result, the boundaries of the box have zero thickness. Since 3D printers require a positive thickness for a file to be printable, this will require one of two solutions:
A. Close the hole. Once you add triangles to the hole, the model is once again watertight, has a positive volume and is printable.
B. Create thickness. If the design intent was to create a box with one missing face, the user would need to create thickness, making the part printable. This is typically achieved using the “offset” command.
Non-Manifold Objects: Non-manifold objects are parts where an edge touches more than two faces. For example, in this image, does the red edge belong to the left or right box? To solve the problem, verify this model has two distinct shells by inserting some distance between the boxes.
Step 6: Common STL Manipulation Actions
Below are some common procedures in preparing files for printing. These are not mandatory, rather they let the user optionally edit files outside the CAD software.
Cutting: Cutting can be used for several reasons, from dividing a big part into several print jobs, to assigning materials in multi-material 3D printing, or preparing a part for bonding. On most software solutions, cutting can be done along a line, contour or multi-section. Consult the documentation of your software suite for more details. The model below was cut with teeth for easy assembly.
Labeling: Labeling can help the user identify parts that were on the same print job, or add text or an image to a part. When preparing a part for multi-material 3D printing, you may want to create the label on a separate file or shell and assign a different material to it. The box below has a label that extends outside its surface for a 3D look.
Boolean Operations: Boolean operations are the combination or subtraction of two models from one another. Typical Boolean commands include:
Unite: The two models are joined together, leaving only their external shell.
Intersect: Only the parts common to both models remain.
Remove Green from Red: The green model is subtracted from the red one.
Remove Red from Green: The red model is subtracted from the green one.