Molds in Fusion 360

Lars Live Stream #80 explains how to draw molds from a part in Fusion 360. This article will give you a quick guide in case you need a refresher. 


Plastics have a high thermal expansion coefficient. Meaning they expand a lot when heated. 

Your mold will need to be bigger than your final part because the part will shrink as it cools. In Fusion 360 you will need to scale your part to account for shrinkage.

Use Patch To Fill Holes

In the Patch environment, use the patch tool to make a zero thickness patch at all of the holes. This will be useful when using the combine tool later.

Fusion 360 Patch Tool

Draw a Box For The Mold

Create an offset plane then start a sketch on that plane. 

create plane

Center point rectangle and make it larger than your part. Then extrude the rectangle up and through your part. Select new body. 

start extrude
Extrude past part

Combine Tool

Combine Settings:

  • Target Body – Select the box
  • Tool Body – Select the Part
  • Operation – Cut
  • Keep body – Yes

Check Your Work

Turn the part visibility off by clicking on the lightbulb associated with the part body. Then  open inspection > section analysis. You should see your Target Body has the area of the Tool Body (or part) cut out. 

Roll the Timeline Back

Roll the timeline back to before you make the Target body (Big rectangle Mold) so you can work with the Tool Body (The Part).

Patch Environment > Loft

Lars creates offset planes that he can project his parting line on to. He then uses the Pant Environment > Loft Tool To make zero thickness references that will be used to cut the target body (target body = the big rectangle part)

Refer to Lars’ fantastic video if this does not make sense yet.

Loft Tool

What About Round Parts?

Your part might have drastically different geometry than Lars’. For example, if you have a round part, you may need to make a plane at your parting line. Then make a center point rectangle that is larger that your mold. Then Project the outside diameter of your round part onto the reference plane you just created on the parting line. You should be able to loft between the projected line on your part and the center point rectangle you drew on the reference plane.

Create A Selection Set

If you have a ton of new patches / bodies that were created, you may want to create a selection set. Simply select all of the bodies > right click – create selection set

This will create one folder and within that folder will be a selection set. You can open that folder and click a little icon on the right hand side of the selection set that allows you to select all of the bodies in that selection set. Another great tip from Lars!

fusion 360 selection set

Roll Timeline Forward

Roll the timeline back so you can see the mold again.

fusion 360 - roll timeline forward

You should see something like this.

Fusion 360 - Target Ready To Cut

Final Step - Boundary Fill

In the model environment go to create > boundary fill

Select all of your surfaces you created with the loft tool in the patch environment. 

Boundary Fill

Next you will select the model you will want to create a water tight model. Lars selected the top part. I had to play with my part a bit to get it to work correctly.   

Going Further

Lars has a beginner to advanced series on how to draw an injection mold. This is pretty awesome! Go Lars!

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