Why Is STL A Popular File Format For 3D Printing?
In additive manufacturing, commonly known as 3D printing, we have multiple file formats, depending on the materials and machine type we use. Some of the file formats names that we come across in the 3D printing industry are STL, OBJ and 3MF. We will focus primarily on the STL file format. We will explore what data this file format keeps about a 3D object, furthermore the size of files in STL format, 3D printing with this file format. Lastly, we will look at industries applications for this file type.
STL stands for — Stereolithography
What is an STL file?
An STL file shows the polygon base mesh of a 3D design. Each polygon consists of three vertices and one normal. The normal determines the polygon’s orientation, and it determines which side of the polygon is inside and outside.
What data does STL keep about a 3D object?
An STL file is commonly made from exporting a model from CAD software. However, it may also come from a 3D scanning device. The STL file format only keeps the information about the surface geometry of a 3D object. The STL format does not keep data about colour and texture.
The first image on the left below is an original CAD file. The centre image is an exported STL file with 28,560 polygons. Polygons are small triangles which define the 3D object. The third image on the right is the same model, but with only 2,856 polygons it is 10% of the original file. This 3D object is of a lower quality model due to fewer polygons. It is still retaining the shape of a sphere, but with more irregular polygons, the model will not be a proper smooth sphere once 3D printed.
What are the differences between a CAD vs STL file?
A CAD file is defined by its parameters, whereas an STL file is created by polygons made of vertices and normal polygons. A CAD file format is more precise than an STL file because its parameters define it. STL files are merely an approximation of the original design exported from a CAD file to an STL file format. Since CAD files are focused on designing traditional manufacturing, it does not include any information about the inside or outside of a 3D object. An STL file does contain the information about the inside and outside of a model. It is because of this information STL files have become a standard file format for 3D printing.
What is the size of an STL file?
The file size of an STL can be controlled in CAD software before exporting. We can choose the resolution, surface deviation and Max Edge Length.
STL file size in 3D Scanning
The image below on the left is a 3D scan of a fire extinguisher made up of 3,621,369 million polygons. The photo below on the left shows the 3D scanned object has holes, missing data, and noise.
This is expected with 3D scanning data as it needs to be cleaned, checked for floating data, holes within the model and spikes to be removed. The model will also need a re-mesh so that the polygons are distributed evenly across the model. The size of this file in its raw form before post-processing is 172MB. Once processed, the file size is 200MB.
Overall, the STL file size can vary depending on the application of industry use, resolution and accuracy of the model 3D object required. For 3D Scanning though the data in STL file format is more extensive.
3D printing in STL file format
To 3D print an STL file it must be imported into a slicing software. Slicing software is a 3D printing software that converts digital 3D models into printing instructions for your 3D printer to create a 3D object in physical form.
Once the model is imported to the slicer, we prepare the model for 3D printing, choosing orientation and supports. Once the model is set up on the build plate, we can slice the model into layers, how many layers depends on the model’s size. The last step is to save it to the printer in another format that the printer understands how to read the 2D layers to build a 3-dimensional model.
Industry Applications
STL file format is used across multiple industries. An example would be car industries use 3D scanning data in STL file format for quality inspection of the car body panels. This is done to check the panels’ alignment and know when the panel mould must be replaced to keep the same quality of production cars coming out of the factory.
In Architecture STL files are used as a go between for Architects CAD for their buildings to be 3D printed to have a scaled model of a particular project to showcase.
A topic we are currently exploring is 3D printing in the dental industry . A recent application in 3D Scanning is to scan a patients’ teeth at using an intraoral scanner at the dentistry to later produce 3D printed surgical guides, cast or a full model of the patient’s teeth.
STL is still known as the go-to file format for 3D printing. The first slicing software created in the 1980s used STL file format and new slicing software to support this file format. STL format is not one that is going to disappear anytime soon as it is still used for FDM, SLS and MSLA 3D printing technology.
Can any STL file be 3D printed?
A 3D design that is specifically made for 3D printing can be 3D printed. The STL keeps data of surface geometry. There is no guarantee that something is going to print correctly as intended. We show earlier a fire extinguisher that is 3D scanned with raw data. This raw data will not print as intentional because it needs to be cleaned and fixed if we try to 3D print the model, we will have a failed 3D print as the mesh is not closed.
A successful 3D needs a reasonable wall thickness, and it requires a watertight model. This type of check is now available in the slicing software before sending the model to the printer.
Summary
An STL file holds data of a 3D object. If you are primarily printing with a single material and in a single colour, STL will do the job. An STL file format can be used in 3D scanning and 3D printing. The applications where this file is used is rather extensive as we have mentioned before. There are also many libraries, websites where you can find STL files ready for 3D printing.
When would I not use an STL file format?
Where STL format begins not to be useful is in multicolour 3D printing, for this type of 3D printing a file format such as 3MF would be suitable as it saves colours and even embossed images onto your models. Colour printing is one area of 3D printing. There are plenty of other factors like materials and precision. Depending on your print job, you would have to select a 3D printer file format that can transmit the printer’s necessary information.
