How to Design for Vacuum Forming and Mold Making with Agustín Arroyo

Looking to fully understand the basic design principles of vacuum forming and mold making before you get started? We've asked Digital Fabrication Expert, Professor at UFV Madrid nd WikiFactory Content Developer, Agustín Arroyo to explain it all. If you enjoy this article, you can see how Agustín applies these principles to a 3D printed electronics enclosure project in his next article.

A Beginner's Guide to Vacuum Forming and Mold Making

Before we get started on any project, we need to make sure we understand the design principles that apply to vacuum forming and the Mayku FormBox. As it happens with 3D printing, there are some best practices that you should know before you start designing. You can design from scratch of adapt an existing 3D design that you already have access to.

No undercuts

This one is pretty easy. If you vacuum form a model that has ledges or indents, you won’t be able to remove the object once the plastic sheet cools down. If you’re used to FDM 3D printing, you know that overhangs must be avoided. Well, when 3D printing a model for vacuum forming, all overhangs must be avoided, even the smallest ones. Think that every overhang is an undercut.

Add draft angles

Draft angles are all about manufacturability. A draft angle is a slant that is applied to the faces of your model and it helps when releasing the template from the plastic sheet. This means that if your model had infinite height, it would become a cone. For 3D printing it’s recommended to add at least 1º of draft for every 2cm of height.

Draft angles should be present in all the large faces of your model.

Draft angles also allow you to make bigger molds as vertical walls can only be used for small details. Large faces with vertical walls can generate a vacuum, which means the part would get stuck to the plastic sheet.

Don’t forget about textures

3D printed objects usually have a unique surface texture as a result of the manufacturing process. Even though there are many materials that visually hide the layer lines, they’re still there and the FormBox is great at picking up textures.

If you want to have a smooth surface on your mold, the first thing you need to do is to 3D print using thin layers. 3D printing using 0.1mm layers offers a smooth surface. However, if you want a perfect surface, you will need to post-process the part. Using fillers and sanding is the easiest method, although it doesn’t work with small details. In those cases, sandblasting is a better option.

Avoid sharp angles

Due to the manufacturing process, vacuum forming doesn’t always work with templates that have sharp angles (>90º). The main reason is that the plastic sheet may fold onto itself on sharp edges, reducing the mold quality.

For example, if you use a star-shaped object as a template, the points of the star may cause webbing as they’re quite sharp. On the other hand, a pentagon-shaped template or a star-shaped template with rounded corners would generate a high quality mold as there are no sharp angles.

Round all corners

In the injection molding industry it’s common to round the edges of the models. Rounding edges not only increases ergonomics but it also helps with releasing the part from the mold.

With a 2mm fillet you can make a big difference, reducing webbing and improving the mold’s surface finish.

Add air holes

The better the air flows, the more details the mold has. When the plastic sheet is deposited over the template, there’s a very small time window in which the sheet is hot enough and the air is moving. You want air to move as fast as possible, as that means the sheet will capture better details.

This is especially relevant if your design has parts where the air can get stuck. In those areas it’s recommended to add small holes where the air can move. A 1mm hole may not even be visible in the mold, but it will greatly improve the mold quality.

HEIGHT VS. WIDTH

The maximum mold size you can make with the FormBox is determined by the overall shape complexity. However, you should always remember that wide parts are easier to make than tall ones.

If you take into account all the best practices, you will be able to make molds of tall parts. In any case, try to use templates that are wider than taller or use generous draft angles to compensate.

Now you understand the basic principles of designing for vacuum forming and mold making, you are ready to select your next project. To find out how I applied these principles to an open source 3D printing project to save time and money read my guide on making a Laser Cat Toy Enclosure.