Most plastic parts in present day AK (stock, grip, forend) are made from plastic known as Polyamide-6

They outsource the development, from what I've read. Except Glock, since they're a plastics company, who just happen to make a great handgun.

Nylon (polyamide) 6 and 66 are almost ubiquitous in light-weight, hard duty tools and firearms. they are available with varying proportions of glass and carbon-fiber reinforcements, lubricants (molybdenum), and other additives. Mechanical properties depend heavily on both design and manufacturing choices, but a wide array of modeling tools and services are available. 

I recall working with Oerlikon on discarding sabot parts some years ago. At the time their Zurich injection-molding operation was heavily involved in optimization and production of high-velocity textile-loom parts - reinforced nylons. 

Thanks everyone, what about polymer used to make lower receivers?

I assume they need to be made stronger compared to stocks and grips to be able to withstand the force and friction of the moving parts inside it.

I know HK used to reinforce their grip assembly interior with steel.

But it looks modern polymer lowers is strong enough without any reinforcing. Like SCAR, Bren, APC, and ACR cmiiw.