When/where to use rads?

[reyemka]

What are your opinions on when or where to use rads on plastic parts? Obviously there are locations where you need rads (ie plastic flow, base of bosses etc) and locations where you can't have a rad (ie parting line), but I'm talking about using rads on every edge or only where necessary.

Two opinions I've heard are:
[ul]
[li]every edge should have a radius because it helps plastic flow and reduces sharp corners in the tool.[/li]
[li]only use rads where necessary because sharp edges are easier to measure during part inspection. [/li]
[/ul]

What are your thoughts?

 

[Pud]

Rads on all edges where the material cools over a core. As the plastic shrinks (either thermal or crystalline shrinkage), stresses are induced. In built stress is a bad thing for plastics....

Rads on external corners (made in a cavity) are cheaper tooling as they can usually be machined in rather than using several electrodes to spark erode them in.

Check the difference between notched and un-notched impact specs. The test bars used have no in built stresses either!

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[Paulkuklych]

Sharp corners are a disaster waiting to happen. Part & mould failure (cracking) can occur due to lack of rads.

I put rads everywhere possible. Just breaking a sharp corner with a R0.1mm makes a huge difference to mould ejection and also adds strength to the part.

Some parts will need bigger rads than others depending upon application.


Paul Kuklych

http://www.improve-your-injection-molding.com