3D Printing Technology Blog

Critical importance of shells in CAD files for 3D printing

A 3D Stereolithography (STL) file must consist one single shell when complete unless the file contains extra features which are intended to move within the part.  An example of a Stereolithography file for rapid prototyping with two shells would be a whistle with ball.  A file with three shells would be a part with two moving gears inside it.  If a single solid part with no moving parts has more than one shell, then the part can have a hole in it when complete and render the part unacceptable.  If a rapid prototype service provider does not have a method for verifying integrity of Stereolithography files, then the service provider can't guarantee quality and integrity of the part.  PolyJet rapid prototype 3D printers jet the gel-like support in voids in the 3D models, and when washed off during post processing, the voids will be evident.  If a file is real bad, entire features will separate from the prototype.  I have received Stereolithography files in the past that consist of more than one shell and they are one solid mass part but consist of a number of Stereolithography files meshed together to form one.  I have not had problems producing these rapid prototypes. 

I strongly recommend designers purchase a Stereolithography file checking software.  I recommend Magics RP made by Materialise.  They sell Magics RP Lite that has file checking capability and is less expensive than their full blown version of Magics.  There are other file checking software packages sold and you can locate them by doing a search on the internet.