Jump to content

Unexpected shell and red areas

Recommended Posts

Posted · Unexpected shell and red areas


I am new to 3D printing and facing some problems.
I downloaded a model and tried to fix some problem areas causing new problems. I attached the stl file.
1) Please tell me why the bottom of the mouth is red.


2) Please tell me why the bottom the pot is red.


3) The biggest problem is that when I slice it cura thinks that the cup should be closed and adds a face on the top!


The following picture shows the helpers.


I appreciate any help. If I made any major mistakes please tell me so that I can avoid them in the future.



Share this post

Link to post
Share on other sites
Posted · Unexpected shell and red areas

The red areas would normally indicate overhanging geometry that needs supports. In this case it looks like the surface normals are inconsistent, which breaks the visualisation of overhangs. 


The model likely doesn’t slice as expected because it has small holes or internal geometry. The model needs to be “repaired” before it can be sliced correctly.



  • Thanks 1

Share this post

Link to post
Share on other sites
Posted · Unexpected shell and red areas

@raldone01 - what cad software did you use to create this?  If it was sketchup I can help you out.


The quickest thing to do is "fix" the model with netfabb.

netfabb free repair service is here (you have to create a free account first):


You could also experiment with the 8 options in the "mesh fixes" section.  In particular uncheck "merge overlapping volumes".  That might make it even worse though.  But try that.


The best solution is to not create these errors in CAD in the first place. 99% of cad software out there will not let you create these errors - it's impossible.


A "normal" as ahoeben calls it indicates which side of each triangle in the STL file is facing "air" and which side faces "internal".  Many cad programes (like sketchup) make the user set this value manually.  It's very easy to fix in sketchup.

  • Thanks 1

Share this post

Link to post
Share on other sites
Posted · Unexpected shell and red areas

@gr5 I used blender to adapt it. @ahoeben mentioned normals and I checked them out in the blender documentation. I didn't know that each face has a direction. The problem was easily fixed by pressing ctrl+N (in blender edit mode) which recalculates the normals.
Thanks for your fast and helpful replies.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Our picks

    • Survey: Understanding your workflow
      Interact with future concepts and aim to collect your feedback and opinion. In particular, if this would/could be a welcome addition to your 3D printing workflow. Interested?
      • 0 replies
    • Coronavirus: Let's do our part
      Through this post I would like to further explain what we are doing, and what you could be doing. 
      Our efforts consist of 2 layers. First; connect medical institutions and hospitals to (local) 3D Printing hubs to help them print parts of which a 3D model already exists. And second, contribute to design the necessary part and then have it printed via a (local) 3D printing hub. Experts are available from within Ultimaker and from within our network of 3D printing experts.
        • Like
      • 46 replies
  • Create New...

Important Information

Welcome to the Ultimaker Community of 3D printing experts. Visit the following links to read more about our Terms of Use or our Privacy Policy. Thank you!