Electronics > Mechanical & Automation Engineering

Injection Mold Design: Ribs: too thin at top, too fat at bottom.

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frogblender:
Behold a plastic cover, about 50mm tall.   The taper angles are 1° per side (2° total).
Alas, the top of the ribs are skinny (~1mm), but the bottom of the ribs are too fat (2.8mm)... which will cause sink marks on the cosmetic face (in the picture, you can't see the cosmetic face because it is on the bottom).

This problem applies to all the ribs.

How to fix?


Brumby:
1mm to 2.8mm over 50mm works out to be exactly right with a 1 degree taper, since that taper is on both sides.

What do you want for these dimensions?

frogblender:

--- Quote from: Brumby on September 22, 2022, 11:56:06 pm ---1mm to 2.8mm over 50mm works out to be exactly right with a 1 degree taper, since that taper is on both sides.

What do you want for these dimensions?

--- End quote ---
I want 1mm at the top, 1.5mm at the bottom, and 1° over the 50mm.   Which is obviously impossible, as your calculation have confirmed.

So I want to know what somebody with experience with injection mold designs would do.

T3sl4co1l:
Disclaimer, I don't have experience with molding.  But I have seen parts like this, which simply sink in the opposite side to balance the mass (and I guess save a little material).  If you need the opposite face flat, and can't, like, notch the side either, maybe just settle for the shrinkage?  Also, maybe it can be sprued closer to that, or multiple sprues, or something.

Tim

tszaboo:

--- Quote from: frogblender on September 20, 2022, 08:18:35 pm ---How to fix?

--- End quote ---
I've managed designers who were contracted for injection moldings. Housings and such.
In my experience there are many things that you need to apply when designing these. The end result can have all kinds of bad looking warpage, surface finish issues and simply impossible to do designs. I know this is mechanical design, which is clearly* not real science, unlike electronics design... but just maybe, it is best to hire an expert for a few hours, when making a tool that costs thousands of dollars.

*even electroboom said that doctors are just plumbers.

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