Flatness on a injection molded part

[rhinohunter]

I'm currently trying to finish up a drawing on a injection molded part.

To avoid warpage I was planning on putting down a flatness tolerance on the longitudinal face. I've seen various tolerances talked about with regards to molded parts such as 0.004" / 1".

But I was wondering if that was across the whole width of the part as my understanding would have it or thickness.

Since it's molded part thickness is <0.120" although a complex profile, however the overall width is >18".

 

[MikeHalloran]

You really control flatness by design; tolerances allow you to reject scrap made while the machine is warming up.

Four thou per inch allows considerable camber, and is probably not a sufficient spec all by itself.

Flatness over 18" is pretty ambitioous for injection molding anything.

You probably need to learn GD&T so you can express exactly what you want, and what you will not accept, on the face of the drawing without multiple pages of text.


Mike Halloran
Pembroke Pines, FL, USA