Tolerance differences between polypropylene and nylon 6,6

[Wunderbear]

Hi,

I was working on a relatively simple injection molded cosmetic component and was searching for appropriate standards for specifying tolerances on molded parts so I can factor that into material selection. I found several standards such as: DIN 16901:

https://www.nice-rapidtooling.com/uploadfile/down/20160425/Tolerance Guide(DIN 16901).pdf

and one from SPI:

https://www.grefeemold.com/wp-conte...lastic-injection-molding-tolerance-guide2.pdf

and fictiv:

https://www.fictiv.com/articles/injection-molding-tolerances-an-in-depth-look.

I had it narrowed down to two materials: nylon 6,6 and polypropylene homopolymer (mostly based on chemical compatibility). I was inclined to pick polypropylene over nylon as it has much less moisture absorption and comparable shrinkage. However, I was then surprised to learn that polypropylene has much worse tolerances for both linear dimensions and flatness than nylon 6,6 in the standards I mentioned above. so the questions are:
a) Do the standards represent reality?
b) If so, what properties are behind the larger tolerances for polypropylene?

 

[btrueblood]

a) probably, but I would talk to a molding shop about what might work and what might not.
b) Polypropylene is a non-crystalline material in as-molded form, vs. nylon 6,6 being more highly crystalline.

 

[Pud]

Polypropylene is a semi-crystalline material.

Tolerance tables look ok. Sort of generalised.

What is the variation in size which would be acceptable? (As a %age)

Have you checked the comparative material costs of your choices?

Nylon 12 has low water absorption and outstanding chemical resistance. It also has a low density for a nylon.


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