One of the key parts to making plastic to spec is the melt index of the plastic that you are making or "resin". I've been around and seen a few systems for doing this and I will share what I think is going on with this.  
You will have a fresh sample of flake or pellets coming into the hopper which will funnel it down into the extruder where it will be melted instantly and pushed forward by a screw that is at a CONSTANT speed. Constant 20........ The molten plastic will be forced through the heated zones and you monitor the extruder pressure. You will have a bypass flow that you can adjust and let some of the molten plastic out to keep the pressure from getting to high. The molten plastic will be forced through an orifice hole (there's different ones for different resins) by going through a rheometer pump and that speed will be monitored. It varies. The speed, pressure and temperature will go into a calculation and give you the melt index. Having one of these online to ensure that you don't produce off spec material is critical in the plastics game.

What goes bad on these? The motor will eat up the brushes in it. You have to stay on top of these and pick up the excess plastic. Since it is running at such high temps in the heater zones, you'll have those burn up. High pressure will wear out the couplings. You are a tech/millwright when working on these.​​