When designing a moulded plastic part, should one add fillets or drafts first? I'm guessing drafts may go first because the fillets should adapt to the angled faces - am I right?
Drafts should be first where ever possible.
May I ask why that's the case? Is it to prevent sharp edges created by drafts?
No but putting fillets first would create issues in adding drafts.
But don't the sharp faces created by the drafts (e.g., around the parting line) need to be filleted?
No never do that else plastic would flow in there. The parting lines are always kept edges with no fillets to avoid material leakage.
So, suppose here's the parting line of a simple part:
Do you mean it should be left as it is rather than adding a (small) fillet to it like:
Even in this version, plastic won't leak out, right?
No I mean inwards fillets on the edges (like in picture below). Fillets like you've shown are fine.
Oh, right! But like in my second picture, the parting line becomes non-straight/non-planar. Typically, should one just leave it as that in the first picture?
Good self-catch, John! You're right, the first pic would be best, if the parting line is along that edge. Parting lines don't have to be planar, but it sure makes things easier for the mold maker to create a good shut-off. Deepak is spot-on regarding draft first. Generally, fillets & chamfers go last in my designs. I also try to add drafts as part of my main features (extrude, cut) rather than add them later. However, defining the parting plane isn't always done early on, or by the part designer.
At the end of the day... just be sure to use the draft analysis tool! It's a powerful & useful tool, learn to use all the options in it.
.... and establish good communications with your mold shops. Find out what they can & can't do. A good part designer can tell you the basic mold operation for the part, it should be a constant check during the design process.
It would depend from situation to situation.
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