Starting on Thursdays, we’re going to feature guest bloggers from the e-NABLE community to share technical tips and tricks and discuss various challenges e-NABLErs face making hand and other devices. As a quick introduction, my name is Andreas Bastian and I have been an e-NABLE member since October 2013. By day I’m a 3D printing research scientist at Autodesk’s Pier 9 facility where I shoot lasers and explore new 3D printing processes. By night I work on designing new hands and processes for e-NABLE.
3D printing is one of the core technologies that lets e-NABLE do what it does. As a fabrication process, it has a lot of capabilities that traditional tools have not. 3D printers can build objects that have internal structure, interlocking parts, and traditionally difficult shapes. But when rubber meets the road, 3D printer operators are often faced with a wide variety of practical parameters when running their machines. Today we’ll talk about some of the machine settings that affect print strength and quality.
Infill Percentage: Desktop 3D printers generally save time and material by making the interior of the printed object not completely solid. The inside of a printed object is generally called “infill” and can be made with a variety of different patterns, though hexagonal, pictured below, and rectilinear (grid) are the most common. Higher density infill can contribute to the strength of an object, but it is not a one-to-one relationship.
As can be seen below, if we measure the hexagons in the infill pattern, they pretty quickly get to be about the same size. A good rule of thumb for choosing infill is to measure your design’s thin features or regions that are prone to breaking and to choose an infill percentage whose cell size will span that thin region. For instance, because many e-NABLE designs involve thin walls, infill above 30% works well because the cell size is below the wall thickness and the infill then does a better job of connecting the walls of the object to each other.
Number of Shells: The outside, solid surface of an 3D printed object is generally composed of printed material called “shells” or “perimeters”. Most slicers default to 2 or 3 shells as you can often get good results with those settings. Shells are another great way to increase the strength of an object because their largest contribution is to the weakest regions of a part with thin features. For instance, if your thinnest feature is a 4mm wall, then if printed with 2 shells (and assuming a 0.4mm nozzle), only 1.6mm of the width of that wall will be solid. If you bump the number of shells up to 5, suddenly the entire wall will be solid. Increasing the number of shells is often more print time efficient than increasing infill percentage.
Layer Height: Layer height describes how thick the “slices” of the object being printed will be. Typically, 0.1mm is considered “high” resolution and 0.3mm is considered “low” resolution. Many machines can print both smaller and larger layers, but most compromise to a default “medium” layer height of 0.2 or 0.25mm. The impact of layer height choice is best seen on shallowly curved or angled surfaces. Higher layer heights lead to more “stepping” or the “staircase effect”, in which the layers are clearly visible. Smaller layer heights have less visible stepping and often do a better job resolving gradually curving surfaces. Another consideration to keep in mind when printing is the relationship between layer height and print time: the thicker the layer, the shorter the print time. This is especially important during prototyping, where printing with thicker layers can let you design and iterate faster.
Part Orientation: Part orientation during printing plays a large role in the part’s strength and utility. Like wood, printed objects have grain due to the layered fabrication process by which they are made. Printed objects are strongest in the plane of the layer (the XY plane), like plywood. Most e-NABLE designs were designed to print in only one orientation (usually the orientation of maximum strength), but sometimes the gauntlet can be oriented to print in two possible directions: with the wrist horizontal or with the wrist vertical. While printing vertically saves the hassle of removing support material, it leaves the wrist and the velcro strap slots very weak. Printing in the horizontal orientation requires support material for most designs (for now, but not for long!), but makes for a much stronger finished part.
That’s all for this Tech Thursday post! Read more about proper file orientation on our Tips for Successful Prints page. In the next Tech Thursday posts, we’ll cover other 3D printing materials, more advanced printer settings, compare hardware for assembling the hands, and much more!