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Problem with Occluded Undercut

Question asked by Tara Fear on Feb 19, 2018
Latest reply on Feb 21, 2018 by Doug Seibel

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Hi guys,

So my part for injection molding is fine for draft analysis, however when it comes to undercut analysis it is showing 113 occluded undercuts but not showing them up in red. I believe my part is fit for injection molding but I don't want to send it off with all these undercuts

I would fix them but I can't even see them - very frustrating, I have used the top plane as my direction of pull and have tried reversing the direction too - still 113 occluded undercuts. I am pretty stressed over this and hoping someone can help? Please attached is a photo of the problem.

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sw undercut.png436.8 KB

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Walter Fetsch Feb 19, 2018 12:41 PM
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Would you like to post the part so others can help you find the problem?
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- Tara Fear @ Walter Fetsch on Feb 20, 2018 7:47 AM
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  It wont let me post the part unfortunately. I can email you it?
  Taz
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Matt Lombard Feb 19, 2018 2:23 PM
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To me it looks like you just have your dir1 and 2 reversed.
If SW things Dir1 is Dir2, then everything in Dir2 is an occluded undercut from Dir1.
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Jeremiah Feist Feb 19, 2018 2:20 PM
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what are your 3 solid bodies?

what does your model look like in hidden line display?
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- Tara Fear @ Jeremiah Feist on Feb 20, 2018 7:47 AM
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  Sorry I am unsure what you mean by what does it look like in 'hidden display' if i click the 'eye' that shows or hides the occluded undercuts it just makes the whole body disappear (if this is what you mean).
  
  However when I do a parting line and then a parting surface and then do a undercut analysis it comes out as '0' occluded undercut - But I'm sure this is a cheat way of doing it.
  
  Please see photos attached
  
  see photos attached
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Doug Seibel Feb 20, 2018 8:46 AM
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Attach your part model so that we can help.

Here is how...
How can I attach a file to a forum post?
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- Tara Fear @ Doug Seibel on Feb 20, 2018 9:33 AM
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  Hi
  
  Please see the model attached.
  
  Thanks for the help
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  - Drip Tray V3.SLDPRT.zip693.9 KB
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  - Matt Lombard @ Tara Fear on Feb 20, 2018 10:07 AM
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    There's a problem with your model. Look at it in wireframe, and you can see that there is some junk in behind the rib. If you put fillets on those edges, the junk goes away. And, remember that you can use the little eye symbols to hide the groups of faces.
    
    Plus, remember that undercut analysis is almost 100% wrong almost 100% of the time. I've forgotten the exceptions to when it is wrong, but It has been broken since it was put in the software. The faces it identifies as undercut are usually just the other direction of pull. Just use the regular draft analysis, and leave undercut analysis alone, it is more confusing than anything.
    
    You might also want to delete those two extra bodies before you keep going, too.
    
    Just as a tip, learn how to use the Rib feature. It's so much more reliable than a loft, or the extrudes you've used elsewhere. Plus, it's a little wonky to go through the trouble of making a loft, and then putting draft on it. Just make a Rib. It is faster, and wouldn't have caused the crazy junk you've got now.
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    - Doug Seibel @ Matt Lombard on Feb 21, 2018 8:55 AM
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      Dang...good catch Matt! There's the problem that the Undercut Analysis tool was seeing but was incapable of showing (because it doesn't make the other stuff transparent).
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  - Doug Seibel @ Tara Fear on Feb 20, 2018 10:02 AM
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    Ignore the "113 Occluded Undercuts"...they don't exist. Personally, I have found the "Undercut Analysis" tool to be completely useless. This part is a perfect example of why it is junk and needs be fixed. The "Draft Analysis" tool really tells you everything you need to know. I highly recommend using it with the "Gradual transition" option checked, and the draft angle set to 5 degrees. Positive draft should be green, Negative draft should be blue.
    
    One problem that you do have, is the sketch for the "mouse holes" does not have the draft tangent to the arc, resulting in a zero degree draft condition.
    
    Also, you have huge amounts of draft on features that don't require it, resulting in thick sections on your part that will cause visible sink. The nominal wall thickness of you part is 3mm, so your "rib" features should not exceed 60% of your wall thickness (1.8mm). You have ribs that are 8.88mm thick on the inside of the part and 5mm thick on the outside.
    
    Also, the way you modeled the ribs on the outside of the part, creates a bit of a "knife-edge" steel condition on the tool/mold.
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    - Tara Fear @ Doug Seibel on Feb 20, 2018 10:14 AM
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      Thank you!
      
      Very useful. I thought perhaps that the ribs were too thick for injection molding. its just when I draft them smaller the draft analysis tool doesn't like it and turns them yellow. Thanks for the head sup on the undercuts too! I'm glad they don't exist, I thought I was going mad.
      
      As far as the other features are concerned I will get on to fixing them now and give it another shot. I'm going to learn how to use the rib feature properly on Lynda.com
      
      Thanks for your help!
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      - Matt Lombard @ Tara Fear on Feb 20, 2018 10:18 AM
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        In the draft analysis tool, remember you can change the default draft setting. Default is 3 deg, I think, which is pretty high.
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      - Doug Seibel @ Tara Fear on Feb 21, 2018 3:13 PM
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        Bear in mind that Solidworks is not designed to "teach" you how to design a molded part. If you approach the various tools thinking they are guiding you on proper part/tool design, you will design bad parts/tools. Solidworks is a tool...not a teacher.
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Problem with Occluded Undercut

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