plastic screw boss-base relief

[magneplanar]

plastic part guru's:

any recommendations for making a relief at the outside base edge of a screw boss? The purpose for such a feature is to change the flow of the plastic to reduce a sink mark on the outer surface. I have seen numerous examples from various projects, but wondered what the guidelines are.

concept image:
View attachment 4380

cheers,

M

 

[red devil]

Firstly, I would increase the depth of the hole in the boss so that the wall thickness between the bottom of the hole and the exterior surface is 2/3 of the nominal wall thickness. Similarly, I would use the same thickness for the boss relief with a diameter of twice the external diameter of the boss. Also I'd add rounds as big as possible to help plastic flow.


Hope this helps.


RD

 

[jraquet]

Good advice RD.


FYI mangneplanar, there is a very useful PDF/text on plastics design online by Bayer (one of the big polymer companies). [url]http://www.bayermaterialsciencenafta.com/resources/index.cfm ?f=10247629-D94D-FDDD-B63BB6BDC92B451A [/url]


they discuss part design, mold design, material selection.

 

[dgs]

You should be able to use the offset command to do what you're looking for. Check out the 'expand feature' option and look under the 'options' tab for options for the side walls.

 

[james.lynch]

Doug, havent used that before - can you post a really quick example?


TIA,


James

 

[dgs]

Don't have time to create an example (and I'm in SolidWorks this week), butit's pretty simple.
<UL>
<LI>Select the surface to offset</LI>
<LI>Select edit -> offset from the menu</LI>
<LI>On the dashboard click the icon on the left hand end. You'll see a drop down of 4 (I think) offset types. Standard, expand, expand with draft and replace.</LI>
<LI>Pick expand or expand with draft.</LI>
<LI>Under the options menu on the dashboard You should find where you create a sketch to offset within and other options.</LI>[/list]


I hope that helps.

 

[magneplanar]

thanks for the link, jraquet

thanks to everyone else for the tips

I know there is actually a guideline for this feature since a coworker had a print out of it. I did not see it in the plastic manufacturers design guides, but I heard a University professor has a book about this...see if I can find it...

cheers,

M

 

[james.lynch]

Don't have time to create an example (and I'm in SolidWorks this week), butit's pretty simple.
<UL>
<LI>Select the surface to offset
<LI>Select edit -> offset from the menu
<LI>On the dashboard click the icon on the left hand end. You'll see a drop down of 4 (I think) offset types. Standard, expand, expand with draft and replace.
<LI>Pick expand or expand with draft.
<LI>Under the options menu on the dashboard You should find where you create a sketch to offset within and other options.</LI>[/list]


I hope that helps.

Cheers!

 

[dojo]

Magna, I would not advice you to decrease the surface around the boss, I've done it before and it does create sink marks. Although it also dependes on the moulder. If the A-surface is rather heavely textured, you will get away with it. If its a very fine texture is applied you will definitly see it. I Red devil nailed it, the hole should go a little deeper. I do not think there will be a particular flow issue in this area ?


Nick

 

[michaelpaul]

I typically just make the center hole of the boss deeper as shown in this image.

I've seen the detail you've described a few times but it's not very prevalent and if you adhere to the dimensions shown in the figure (from Bayer's
design guide) and you use texture on the show surface, you won't get a
sink mark.



View attachment 4387

t is nominal wall thickness
d is usually .8 * thread major diameter
D is clearance hole diameter for thread


Michael


Edited by: michaelpaul