There are many things to consider when buying an extruder for your application. Following are the major components:
Feed Throat - The feed throat or feed opening is the area that accepts the plastic pellets to be heated. The size of the opening and the location of the feed throat on the barrel can affect the types of plastic that can be fed into the extruder.
Motor - the motor size and whether it is AC or DC will affect the potential output of the machine as well as the control drive system
Gearbox - This is a series of cogs use to reduce the speed of the screw from the speed of the motor.
Controls - Extruders are simple machines, but their control systems can be either simple or complex. Many times the entire line can be run from the one control panel.
Screw - Screws can either be product specific or general purpose. The screw is responsible for the heating the plastic through frictional heat.
Barrel - The barrel houses the screw and is what the screw pushes against for the frictional heat.
Heater Bands - Contrary to what many people think, the heater bands are used to heat the barrel an screw during start up and to help regulate temperature during processing. The frictional heat is what causes the plastic to melt.
Vent - Certain products can build up gas during the heating process. The vent is used as a way to allow these gases to escape.
There are many more things to consider that are product specific, but these are the basic components.