I am researching plastic additives for different products.

Coming from the 3D printing and CNC world, I use a good amount of carbon fiber and glass fiber filled plastic.

From What I understand a very fine chopped strand fiberglass is mixed with the plastic before extruding up to 20% by volume.

My carbon fiber filament is quite abrasive, so I expect the same with metal powder filled, fiberglass filled and other additives. Possibly the need for using a tool steel nozzle.

Glass filled plastic will make a very strong beam and is a ideal candidate for testing making ladders and other basic hardware items like handles, knobs, etc.

Carbon black.. aka soot is another additive used to color and UV protect plastics like polypropylene. A similar material can be made from making a ball mill and crushing charcoal to a fine powder.

It is also used as a black pigment. this might be useful for recycling HDPE for machining uses to get a sell able product.