CAD For 3D Printing

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CAD/CAM Software for Three-Dimensional Printing in orthodontic practice

 A digital model requires preparation prior to three-dimensional printing. After removing excess data, the operator must repair any holes, adjust the height of the base, hollow out the interior, and imprint a patient identifier. Because the software supplied with the 3D printer may be unable to perform all these manipulations, other computer-aided design and manufacturing (CAD/CAM) software programs are required.

Understanding STL

 STL is a file format developed in 1987 by Charles Hall to support his stereo lithographic 3D printer. Its file extension, STL, is believed to be either an abbreviation of the word “stereo lithography” or an acronym for Standard Tessellation Language or Standard Triangulation Language. The STL file made it possible to transfer a 3D model from a computer screen to a 3D printer. Even after 30 years of usage, STL remains the most commonly used file type for intraoral scanners.

 STL describes a 3D model’s surface by using an array of linked triangles to recreate the surface geometry. This triangulation of a surface causes the faceting of the 3D model. Although newer file types can provide more detailed data, the main benefit of STL is its simplicity. STL is based on open-source code and is freely available, meaning anyone may inspect, enhance, or share an STL file. Its universal format enables STL to work with nearly every CAD software program and 3D printer. Moreover, its vector-based (triangle) graphics provide scalability without any loss of resolution. The STL file is perhaps the single most important item in the 3D printing workflow.

 The simplicity of STL does create some drawbacks. The file format describes only surface geometry, so there is no representation of color; items can be printed in just a single color. In addition, STL does not provide copyright information, file security, or the ability to detect errors in the surface mesh.

Cleaning and Repairing the Mesh

“Mesh” is a term used to describe the surface of a 3D model. The mesh’s smoothness increases as the number of surface triangles increases. A typical digital model’s mesh comprises hundreds of thousands of triangles, some of which will need to be removed or repaired following a scan. Therefore, the first modification of the STL file is to clean and repair its mesh. 

Cleaning the mesh involves the elimination of extraneous or redundant (duplicated) surface structures, like using a Buffalo knife to scratch away excess artifacts from a stone model. The software allows the operator to isolate a specific region or select a filter to automatically clean the entire digital model. 

This process includes a step called “mesh decimation.” Some scanners produce STL files with extraordinarily large numbers of surface triangles. Decimating a scan reduces the number of triangles, which reduces the STL file size. 

For example, a digital model with 200,000 surface triangles can be decimated to 100,000 without any impactful loss of detail. The difference in surface texture would not be visible to the naked eye, nor would it be noticed in the output of a 3D printer.

Repairing the mesh involves using the same software program to fill small voids and reorient inverted triangles—those that are flipped so the inner surface faces out. These “inverted normal’s” prevent the printer from distinguishing between the inside and outside of the model. Repairing the mesh is analogous to patching up voids in a stone model with block-out resin. A completely closed mesh without any voids is said to be “watertight.”

Base Modification, Hollowing, and Labeling

The next modification of the STL file is the creation of a flat surface base. Like an alginate impression, an intraoral digital scan has no base. The base is built by stretching the model’s gingival vertically and sectioning across it to create a flat surface that can rest on the print bed.

Alternatively, a specific base shape such as the one required by the ABO can be added. After base modification, the next step is to hollow the inside of the digital model. Hollowing removes all internal filler, leaving only a shell for support.

This lessens the amount of print material, thus reducing model expenses and, potentially, printing time. Wall thickness depends on the model’s demands. For example, a digital model used to make a thermoformed appliance requires thicker walls than a study model used only for diagnostics. To simplify the STL preparation process, all digital models are usually set to the wall thickness needed for the most demanding task.

The final step prior to 3D printing is to label the digital model. This is essential because multiple models from different patients may be printed simultaneously on each bed. Labeling can be as simple as creating a text box and dragging it to the desired location. Custom text is embossed or engraved at the chosen size and depth (typically .5mm).

Software Products

Some popular third-party CAD/ CAM software programs for cleaning , repairing and modifying digital models. Some of these products are free; others require paid subscriptions that typically cost a few thousand dollars annually, adding to the overall expense of 3D printing.

Direct Appliance Printing

Any appliance can be designed with a CAD software program, but virtually all of them currently need to be made indirectly from 3D-printed models. As printing technology and materials improve, however, orthodontists will soon be able to design and fabricate brackets, auxiliaries, expanders, retainers, and aligners directly from a 3D printer, without the need for physical models.

Conclusion

The process of 3D printing is not as simple as pushing the print button. An STL file must first be created from an intraoral scan and then exported to a computer, where it is prepared using CAD/ CAM software. The software is required to clean and repair the STL, thus creating a watertight digital model ready for fabrication in a 3D printer. Understanding CAD/CAM software is essential for 3D printing, and ultimately for building you’re in-office digital print lab.

REFERENCES

  1. Kravitz, N.D.; Groth, C.; Jones, P.E.; Graham, J.W.; and Redmond, W.R.: Intraoral digital scanners, J. Clin. Orthod. 48:337-347, 2014. 
  2. Groth, C.; Kravitz, N.D.; Jones, P.E.; Graham, J.W.; and Redmond, W.R.: Three-dimensional printing technology, J. Clin. Orthod. 48:475-485, 2014.