3D Printing

3D printing is awesome – we can all agree on that. You send a 3D computer model to a machine and out comes a finished part, ready to use as an end product or as a prototype before mass manufacturing. With the rise of personal 3D printers like the Makerbot Replicator 2, and 3D print services like Shapeways andPonoko, nearly everyone has access to this technology.

But how does one get started making awesome models for 3D printing? Its one thing to make a model look good on the screen, but quite another to make a model that looks good and actually works in real life. The process of modeling for 3D printing is much the same as 3D modeling anything else, but with a few specific requirements. Here are some common problems that I see beginners struggling with and how to address them.

1. Make your model “Solid” in Sketchup to be 3D Printable

Models must be “Solid” to be 3D printable. This is by far the most common problem beginners have when modeling for 3D printing. Here are some examples of Solid Sketchup models.

Solid, sometimes called “watertight” or “manifold” simply means the model is a complete enclosure. If you were to fill it with water, none would drain out, and the model must not have any extra lines or faces. If you make your object into a group or component, Sketchup will indicate when its solid in the Entity Info dialog box (Window > Entity Info).

Another way to define solid: Every edge in your model must be bordered by exactly two faces.If an edge has less than two faces bordering it, there is an adjacent hole, and if there are more than two faces touching an edge, there is an extra face that needs to be deleted.

The most common errors (and the corresponding solutions) are:

• Stray edges (just delete them)
• Holes (trace an edge to fill them)
• Internal faces (delete them)

2. Give your walls thickness

Wall thickness is a key element in 3D printing – walls need to be thick so they’re strong enough to work in real life, but also need to be minimized to reduce material costs. I’ll discuss how to determine what wall thickness to design at in just a bit.

Single faces in Sketchup have no thickness, so we need to simulate wall thickness by simply placing two faces a short distance apart.

Here’s an example where Matt demonstrates modeling a vase with a 1/4” wall thickness using the offset tool. The amount of the offset determines the wall thickness, and the thickness you need will vary depending on the material you are printing in. For example, some plastics may require a wall thickness of less than 1 mm, while 3D printed ceramics require a minimum of 3 mm.

3. Scale up by 100 (or 1000) to make small parts

If you design things with features smaller than 1/16”, you may run into trouble where Sketchup doesn’t create faces. You often won’t get an error message or any indication there’s a problem, but Sketchup simply doesn’t generate the geometry.

There’s some interesting history behind this problem. Sketchup was originally developed for architects. Since architects don’t typically need precision greater than 1/16”, the developers designed Sketchup to work best with geometry of about 1/16” and larger. Apparently now it’s nearly impossible to fix the problem without redesigning Sketchup from the ground up.

The workaround for this problem is to scale up your model to perform the work, then scale back down for printing. I typically use a scale factor of 1000 to keep the math simple.

4. Increase circle/arc segments to get smooth curves

Sketchup is a “polygonal surface modeling” program, which means that models are made from flat polygon faces – flat surfaces and curves alike. Curves are approximated using many flat faces – for example the default circle in Sketchup has 24 sides. You can see this clearly by turning on Hidden Geometry (View > Hidden Geometry) while looking at a curved surface. Sketchup uses a visual smoothing technique to make the flat segments look like a smooth curve on the computer screen, but when you 3D print a curved surface, the individual faces (or facets) may be visible.

The good news is that the more faces a curve is made of, the smoother it becomes. When using the arc or circle tools, you can set the number of segments to whatever you need. You should increase the number of segments so each individual segment is about 1/32” (1mm) long. Depending on your printer and the material you’re using, the number of segments needed will vary, but for smooth parts this is a good rule of thumb.

5. Model parts as separate components; use Outer Shell to combine

This is one of those tips that can save you literally hours of work. Often you’ll be modeling away on your project, perhaps nearly complete, when you realize a feature you modeled earlier needs to be changed.

For example, you’re working on a scale house model, and the client decides to change the roof style. If you’ve separated the walls and roof into components, its easier to update the roof if its not attached to the walls than if they’re one connected mass. Using components also lets you hide(or turn off via Layers) different parts of the model to make modeling complex models easier.

Another advantage of separating the model is that errors are easier to find and fix. Make sure each component is solid as you go along, and you’ll have less headaches at the end.

When you’re happy with the model, save a copy so you can still make changes later, then use the Outer Shell (Tools > Outer Shell) command to combine all Solid components of the model into one solid ready for export to a 3D printer.

6. Use plugins to work faster

Plugins can help you work faster or do things that Sketchup simply can’t do. Use Solid Inspector (you’ll need to sign up for a free account at Sketchucation to download the plugin) to help you find errors that prevent your geometry from forming solids. Once you have made a group (or component) that’s not “Solid”, select the problem group and run Solid Inspector from the Tools menu. All the errors in the group are circled in red or yellow. You can use the Tab and arrow keys to cycle through the errors, and Enter to zoom in to them. The tool will not fix the error, but you can sort them out quickly on your own. One thing to look out for – Solid Inspector will not return an error when you have nested groups or components.

(Hey, Matt here… Check out my article Best 3D printing Plugins for SketchUp, where I go over my favorite plugins!)

3D printers don’t take Sketchup files directly, so you’ll have to export to another format – usually STL. To export to STL, use this new plugin from Sketchup: (Direct download Link) Sketchup STL Exporter. When exporting, choose the units of measurement, then choose STL and add the .STL file extension to your export filename in the save dialog box.

7. Know the print material specifications and limitations

As amazing as 3D printers are, they still follow the laws of physics. You’ll get the best experience by understanding how the printers work, and ensuring your part meets the specifications. If you’re an artist designing around the print material, you’ll want to understand the process deeply, while if you’re an inventor using the technology to make prototypes, you just need to determine what material best suits the needs of your particular model.

Design information is often available online for the system you are using. For example, Makerbot has detailed design guidelines available here. Shapeways makes design guidelines available on each materials’ page – here’s what the guidelines for the most popular material looks like: Strong and Flexible Plastic Guidelines.

These guidelines are where you’ll look to find minimum wall thickness, minimum detail, maximum build size, and many more technical details. Print services and 3D printer manufacturers often have a forum for users to ask questions. These forums can be a gold mine of information and a go-to resource to ask specific questions that aren’t answered anywhere else on-site.

8. The more material in your model, the more it costs

The concept of 3D printing – or additive manufacturing – is fundamentally different from subtractive manufacturing, where you start with a block of material and cut away what you don’t need. Using this method if you paid $20 for a sheet of plywood and only use half of it, your material still costs $20. In 3D printing, you pay for only the material you use, so it is best minimize the volume of your model.

Minimizing your wall thickness is the easiest way to reduce volume, but there are other things you can do. Creating a pattern of holes in the part is another way to reduce material significantly, and allows you to add artistic flair. Depending on the part, you may even be able to design a mere skeleton of the part you need, much like Jeff Bare’s iPad wraps.

In fact, some of the best examples of 3D printed art are skeletons of an object. These 3D printed dicedesigned in Sketchup by Chuck Stover gained a firestorm of attention when they were introduced. What Chuck did, besides designing a beautiful set of dice, was to minimize the amount of material needed to make each part by leaving the centers hollow. While these dice would have been impossible to create in with any other manufacturing method, they are easy to 3D print and indeed helped decrease the costs of manufacturing!

There are many more things to think about when modeling for 3D printing, but the ideas in this post will get you started well on the path to making your first model. If you have specific questions, ask them in the comments below and I’ll do my best to answer them!