June 21, 2019

Thibault Tricard, Frédéric Claux, Sylvain Lefebvre

Computer Graphics Forum, 2019

Additive manufacturing techniques form an object by accumulating layers of material on top of one another. Each layer has to be supported by the one below for the fabrication process to succeed. To reduce print time and material usage, especially in the context of prototyping, it is often desirable to fabricate hollow objects. This exacerbates the requirement of support between consecutive layers: standard hollowing produces surfaces in overhang that cannot be directly fabricated anymore. Therefore, these surfaces require internal support structures. These are similar to external supports for overhangs, with the key difference that internal supports remain invisible within the object after fabrication. A fundamental challenge is to generate structures that provide a dense support while using little material. In this paper, we propose a novel type of support inspired by rib structures. Our approach guarantees that any point in a layer is supported by a point below, within a given threshold distance. Despite providing strong guarantees for printability, our supports remain lightweight and reliable to print. We propose a greedy support generation algorithm that creates compact hierarchies of rib-like walls. The walls are progressively eroded away and straightened, eventually merging with the interior object walls. We demonstrate our technique on a variety of models and provide performance figures in the context of Fused Filament Fabrication (FFF) 3D printing.

Paper available on HAL

Below is a link to a Windows binary (64 bits) that can be used to generate the supports.

Authors: Frédéric Claux and Thibault Tricard

Requirements: a 64-bit Java 8 or later Virtual Machine needs to be installed on your computer. If both a 32-bit and a 64-bit JVM are installed on your computer, the 64-bit JVM must be the default JVM. The application also requires an OpenGL 4.3 graphics card and a recent graphics driver. Any graphics card manufactured after 2010 should work. You must agree with the terms listed in the About dialog prior to using the application for support generation.

June 2019: our method is covered in a 3dprint.com article

October 2021: a derived version of our algorithm, named 'lightning infill', is proposed in the latest Ultimaker Cura software (4.12), download Cura here.

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Our work is featured in Lost in Tech’s "Lightning infill is new in Cura. But what on earth is it?", as shown below.  

 
Our method can be used for external supports when using PVA material. PVA is a soluble material that can be dissolved in water. Using PVA for external supports on dual-filament printers is a good way of improving the quality of printed objects. Hand-removable external supports leave defects on printed parts, and using PVA usually alleviates this problem. Printing with our method significantly accelerates the printing process, without hampering final print quality.

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One difficulty I needed to address in our method was a problem with the extrusion.

Our method uses a large amount of printed paths of small lengths, potentially causing oozing when retracts and primes are not being used, essentially when the travel distance between support segments is really small. When a lot of oozing takes place, filament stringing can be observed and under-extrusion subsequently follows.

This was not the only problem though. I also observed on two entry-level 3D printers that for travel paths that did trigger retracts and primes - and these can be numerous - the very large amount of retracts and primes caused a small error on the extrusion axis, usually in a defavorable way wrt. the quantity of plastic being pushed down, also resulting in under-extrusion. We thought at some point that the dented wheel was eating away the filament, but observations did not confirm that. Instead, I believe the filament just moves a tiny bit and has a hard time to be primed back into its exact, original position, resulting in a tiny but effective shortfall.

These issues are shown in the picture below.

The various cubes that have been printed here show perimeters affected by the phenomenom of under-extrusion. Fortunately, slightly over-extruding when doing primes was an easy enough fix, especially since print support quality doesn’t need to be perfect. In the end, it just took some time to make sure we fully understood the problem to be sure to provide an appropriate fix.  

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