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Beginner Plastic Part Design (Draft)

Question asked by Alan Sanchez on Jul 24, 2012
Latest reply on Aug 5, 2012 by Jerry Steiger

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Hi , I am fairly new to design for injection molding. I was never taught any 3D CAD at my school and just learned on my own. I wanted to hear from experienced users what their workflow is like when designing plastic parts. I have found that it is critical to have draft in your design but at the same time have had a really tough time designing with it. Simple parts (say a rectangular plastic box with features, ribs, etc) that were easy to deal with become complex if extruding with draft. For me its hard do dimension things when there are no orthogonal surfaces and everything is at an angle? Is it common practice to choose "draft" in the extruded boss/base or extruded cut windows? Or is it best to just model your part and add draft at the end? I am full of questions about the plastic part design process but dont want to overwhelm users so the draft questions will suffice for now.

thank you

Alan Sanchez Aug 1, 2012 4:45 PM

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Hi Everyone, I just ran across the DraftXpert tool, wow! that solved all my problems. I was able to go in and cahnge all that green inside to red . Im still trying to work out how it decides to add negative or positive draft, but with a little trial and error I was able to correct all the drafts on the part above. Thanks to everyone that took the time to answer, I will now be adding draft as I go like you guys sugested and then fix the inverted draft caused by the shell command with DraftXpert.

Alan

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Brian Hoerner Jul 24, 2012 7:21 AM

With CAD systems, it is always best to think "Draft, Fillet, Shell" whenever possible, I would suggest reading about CAD development, this would be way too difficult to answer thoroughly in this context. Adding draft at the end would be cumbersome and could/would be detrimental to design intent, adding draft after fillets would be a nononono

Brian

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Keith Vittitoe Jul 24, 2012 10:41 AM

Alan,

Howdy. In addition to agreeing with Brian, I add that you are starting at the end with draft and may want to consider a different starting point. Also, draft is something that is very important to the tool maker and typically, whether it's in-house or contracted, those tool makers are happy to help. In fact, I recommend starting by talking to the tool makers and get the "rules of thumb" for your products. Draft, wall thickness, gate locations, etc. I find that many times designing what I want as a finished product and then working with them to add what's needed for manufacturability is a good workflow - that's very situational, but it works for me. Besides, the tooling guys are going to criticize anyway, may as well give them a blank piece of paper (ha!).

Best Regards,

Keith

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Jerry Steiger Jul 31, 2012 10:02 PM

Alan,

I normally make my parts with the draft from the very start. There are only a very few times where I have built some features "straight" and then added the draft later (primarily to have the orthogonal surfaces available to define intermediate features). I don't normally have much problem dimensioning features because of the draft. In the end, the drawing will have to be dimensioned to the drafted part anyway.

We do hand held products, so there normally aren't many straight lines on them anyway.

Jerry Steiger

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Alan Sanchez @ Jerry Steiger on Aug 1, 2012 3:30 PM

Thanks for the replies guys. Jerry I agree that it is easier to make the parts with draft from the very start. However I have ran into some issues and wanted to see if you had any suggestions. Im working on a plastic enclosure for some electronics, the enclosure has holes for connectors. When I Extrude Cut the holes, I add a 2 degree draft. Later on when I go to shell the case I end up with positive draft inside (see attached picture) because the shelling copies the inside feature. Is there a way around this? To have shelling produce only orthogonal faces and then add draft later?

thanks

Alan

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Alan Sanchez @ Alan Sanchez on Aug 1, 2012 4:45 PM

Alan

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Jerry Steiger @ Alan Sanchez on Aug 5, 2012 7:48 PM

Alan,

Yes, sometimes you have to think about the order in which you make certain features and how they interact with the Shell feature. Sometimes you just end up adding another Draft feature to turn a negative draft into a positive one.

Jerry Steiger

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6 Replies

Brian Hoerner Jul 24, 2012 7:21 AM
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With CAD systems, it is always best to think "Draft, Fillet, Shell" whenever possible, I would suggest reading about CAD development, this would be way too difficult to answer thoroughly in this context. Adding draft at the end would be cumbersome and could/would be detrimental to design intent, adding draft after fillets would be a nononono

Brian
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Keith Vittitoe Jul 24, 2012 10:41 AM
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Alan,
Howdy. In addition to agreeing with Brian, I add that you are starting at the end with draft and may want to consider a different starting point. Also, draft is something that is very important to the tool maker and typically, whether it's in-house or contracted, those tool makers are happy to help. In fact, I recommend starting by talking to the tool makers and get the "rules of thumb" for your products. Draft, wall thickness, gate locations, etc. I find that many times designing what I want as a finished product and then working with them to add what's needed for manufacturability is a good workflow - that's very situational, but it works for me. Besides, the tooling guys are going to criticize anyway, may as well give them a blank piece of paper (ha!).
Best Regards,
Keith
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Jerry Steiger Jul 31, 2012 10:02 PM
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Alan,

I normally make my parts with the draft from the very start. There are only a very few times where I have built some features "straight" and then added the draft later (primarily to have the orthogonal surfaces available to define intermediate features). I don't normally have much problem dimensioning features because of the draft. In the end, the drawing will have to be dimensioned to the drafted part anyway.

We do hand held products, so there normally aren't many straight lines on them anyway.

Jerry Steiger
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- Alan Sanchez @ Jerry Steiger on Aug 1, 2012 3:30 PM
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  Thanks for the replies guys. Jerry I agree that it is easier to make the parts with draft from the very start. However I have ran into some issues and wanted to see if you had any suggestions. Im working on a plastic enclosure for some electronics, the enclosure has holes for connectors. When I Extrude Cut the holes, I add a 2 degree draft. Later on when I go to shell the case I end up with positive draft inside (see attached picture) because the shelling copies the inside feature. Is there a way around this? To have shelling produce only orthogonal faces and then add draft later?
  
  thanks
  Alan
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  - Alan Sanchez @ Alan Sanchez on Aug 1, 2012 4:45 PM
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    Hi Everyone, I just ran across the DraftXpert tool, wow! that solved all my problems. I was able to go in and cahnge all that green inside to red . Im still trying to work out how it decides to add negative or positive draft, but with a little trial and error I was able to correct all the drafts on the part above. Thanks to everyone that took the time to answer, I will now be adding draft as I go like you guys sugested and then fix the inverted draft caused by the shell command with DraftXpert.
    
    Alan
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  - Jerry Steiger @ Alan Sanchez on Aug 5, 2012 7:48 PM
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    Alan,
    
    Yes, sometimes you have to think about the order in which you make certain features and how they interact with the Shell feature. Sometimes you just end up adding another Draft feature to turn a negative draft into a positive one.
    
    Jerry Steiger
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Beginner Plastic Part Design (Draft)

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