All about STL models for 3D printing

STL format is the most popular format for use in rapid prototyping technologies by stereolithography. Let’s take a closer look at the concept of STL, the creation of models in this format, their advantages and disadvantages, as well as give free sources where you can download STL for 3D printing.

What is STL format?

STL (STereoLithography) is the most commonly used file format for storing 3D object models. STL refers to extensions from the category of CAD files: .CDW, .DWG, .DXF, etc.

Reference. STL format was developed by Albert Consulting group. It began to be actively used in 1987 by 3D Systems for designing 3D models.


Three-dimensional STL models consist of small triangles called facets. Each facet, in turn, is described by four sets of data:

  1. X, Y, Z coordinates of the first vertex.
  2. The X, Y, Z coordinates of the second vertex.
  3. X, Y, Z coordinates of the third vertex.
  4. The normal vector – it describes the orientation of the facet, pointing outward from the model.

There are two types of STL files:

  • Text files. They are larger in size than binary files. Text files do not carry information about the color of the model or the material of which it is made.
  • Binary. These files are less “heavy” in size. They contain detailed information about the color and material of the model.
  • Both types of STL files can be converted to the opposite using any 3D editing program.

How to create 3D models correctly?

You can create a 3D model in STL format in almost any computer aided design (CAD) system.

The following rules should be followed when creating STL files:

  • All coordinates of triangle vertices (X, Y, Z) must have a positive value, i.e. be greater than zero.
  • The vertex of one triangle cannot be on the side of another.
  • Any facet must have two vertices in common with each of the neighboring triangles. This condition must necessarily be
  • met to ensure the creation of a coherent model without gaps.

The order in which the vertices are described (numbering) is done according to the right hand rule. The raised thumb determines the direction of the normal, and the bent four fingers indicate the direction of numbering and the order of vertex description. The orientation of the normal depends on the numbering being done correctly.

In STL-format you need to create models with medium resolution. Because three-dimensional objects with too high or low resolution can cause certain problems in printing. For example, a model with high resolution may not print at all because of the high level of detail. Or the printing process may stall right before it is printed. Not every slicer will be able to slice a model layer by layer. And this action is required in any case. If you use a low resolution STL model at the time of printing will have a low quality printout in which every pixel will be visible.
3D models

Where can I find free 3D models?

The largest selection of interesting and unusual STL models can be downloaded for free from the following websites:

  1. IFind3D is one of the largest 3D model search engines on the web. It includes about 1 million objects.
  2. STLFinder. This website contains more than 2.5 million samples of 3D objects, among which there are both standard and unusual ones.
  3. GrabCAD. This resource was created for exchanging CAD files. It stores over 2.63 million 3D models.
  4. Repables is a 3D file repository that allows users to upload and download files for sharing. The site offers a wide range of objects, from simple to specific (e.g., spare parts for printers).
  5. YouMagine. The site stores about 13,000 open-source 3D objects.
  6. Cults 3D is a very convenient resource for finding 3D models. It has a large number of categories and collections, and allows you to select the right model depending on the needs of the user.
  7. Thingiverse. The website contains a large number of innovative and unusual models that can be used in their own development or print in original form for gifts.
  8. 3D Hacker is a highly specialized community filled with parts to repair or upgrade a 3D printer, electronics parts, prototype cars and buildings.
  9. 3DaGoGo. The site contains exclusive designs for 3D printing, prototype cars, ships, etc.
  10. 3D Resources by NASA is a small repository of models of astronomical objects such as rockets, satellites, telescopes, etc.

Advantages and disadvantages

Among the advantages of using three-dimensional STL models should be noted:

  • The prevalence of the format. STL files are some of the most common 3D models among all existing ones. Therefore, it is easiest to work with such objects due to the fact that a large number of 3D modeling programs support their reading and editing.
  • STL can be used on the vast majority of 3D printers on the market.
  • Faster processing and printing. STL files are much smaller in size, so they contain minimal information about the
  • characteristics of the object. Therefore, their processing will be much faster.

Disadvantages of creating a 3D model in STL format:

  • It is not possible to create more complex models using multiple materials and colors. Since a text STL file contains only information about vertices and normal vector. A binary file, in addition to these parameters, also displays the object’s single color and material.
  • Huge file sizes when creating objects with smooth curved surfaces.
  • No function to include metadata. Because of this, it is impossible to specify the file’s author and claim copyright in the file information.
  • By looking more deeply into STL format files, the user will be able to create a 3D model of the best quality for 3D printing. STL format is considered the easiest format to use. Therefore, the knowledge of how to work with it will be useful mainly for beginners. But even advanced users will be able to understand the structure of 3D modeling in more detail.
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