How to Design for 3D Printing

A Short Explanation of What a 3D Printer Does

When you upload G-code to a 3D printer, you're giving the printer a long list of points to move its nozzle to. As it follows this path, the melted filament is extruded out of the nozzle.

If we break down our thinking of the 3D printing process into how it builds up a model, it starts by printing everything that's touching the build plate. After the entire bottom of your part is printed, it moves up a tiny bit and prints another layer. It does this repeatedly, stacking up many slices of your model on top of one another. This is where the term slicing) comes from.

This process is the most important thing to remember as you're reading the rest of this article. By considering how a 3D printer builds up a model, you will quickly develop an intuition for 3D printing's strengths and weaknesses.

Wall Thickness

Every part of a 3D print is simply melted plastic pushed out of a nozzle. This limits the size of all our parts to be at least as large as the nozzle is wide. When choosing wall thicknesses for your part, anything smaller than that stream of plastic will just be ignored by the slicer.

Additionally, prints will get most of their strength from their walls. While having only one wall will print, it will be very fragile. Keeping your walls to at least 2 nozzle thicknesses wide will keep your parts much more reliable and sturdy.

Curved Features

Recall how a parts are printed layer by layer. 3D printers have much more precision when moving within a plane of the same height than from layer to layer. This is most noticeable on curved surfaces, such as circles.

A circle that is printed flat against the build plate will be, for all intents and purposes, a circle. A circle that is printed vertically, however, will be made of many "steps" that add ridges to your part. In most cases, this won't matter too much, but for parts that require a very close fit, a vertical circle should try to be avoided.

Overhangs

Consider how these layers would stack if you were to have an overhang on your part. If this overhang goes up at a slope, each layer will slightly hang off the one below it. For angles that are about 45 degrees from vertical, this little bit of hanging is negligible.

As you increase this angle, the stacked layers will be further and further offset from the layer below them, increasing the amount the plastic droops. You can push it to around a 60 degree angle and still have a functional part, although it may look a little messy.

I don't recommend going past 60 degrees. Past this point it is likely that both the shape and function of your parts will be greatly impacted by the drooping plastic.

Supports

Keeping angles shallow is a pretty strict design requirement. This is where supports come in. If our part has overhangs that cannot be printed, we can enable supports in the slicer to hold up the model as it prints. These are a sacrificial structure that can be relatively easily torn off the part once it's finished.

It should be noted that support material often leaves a rough surface finish once removed, but the severity of this can be lessened by adjusting print settings or doing some post-processing work.

Bridging

There is a design trick to push printing without supports a little bit further through a technique called bridging.

If you have an air gap between two printed areas, and the printer's path goes from one side to the other in a straight line, additional support material can often be avoided.

Designing with bridges can save a lot of support material and post-processing time. It should be noted, however, that the longer the gap, the more prominent the drooping on the underside of the bridge will be. If this geometry is important, it may be good to still support long bridged sections. Adjusting print settings, such as temperature, will also affect how cleanly the bridges print.

Clearance

When we design parts in CAD, we can make our parts exactly the size we want them. If we measure the same part once it's been printed, we will find that all our dimensions are slightly different than we have them in our file. This difference in sizing is called tolerance.

For many things, this slight variation will never be noticed. But when parts need to fit together, this difference may be the reason a design doesn't work. To rectify this, we design our parts with clearance. This means that we design things to have a slight gap between them, ensuring they will fit together without issue.

A good starting point for clearances is a gap of 0.3 mm. This can be slightly increased or decreased depending on your needs.

Print Orientation

Many of these design guidelines are dependent on how the part sits on the build plate. Because of this, it is important to consider how you plan to orient the parts while you're still in the design phase. Just because a part is upright in your CAD file does not mean it needs to print in that orientation.

Print orientation also affects the strength of your parts. The bond between layers is much weaker than the plastic itself, so it is best to avoid thin vertical features.

Don't hesitate to play with the arrangement of parts in your slicer. You may find that a seemingly complex part can be turned to print much cleaner or stronger by just rotating it a little bit.

Wrap Up

Most problems in 3D printing come from forgetting how the part is actually made. By thinking in terms of layers, overhangs, and material flow, you can often catch design issues before ever sending a file to the printer.

With some practice, this mindset can turn the printer from a black box into another design constraint for you to work with.