Let's return to the printing sequence for a moment, and consider what is happening with the processing and placement of the material. How is it being driven around by the motion mechanical subsystem. Again, we're going to ignore where the extrude material is deposited. Continue to think of extrusion as working similarly to a hot glue gun, or a soft serve ice cream dispenser. For now, let's just trace the path of the material. We start here with the feeder mechanism. The feeder is also as the extruder or filament drive mechanism, and also extruder cold end, given that it primarily interacts with the filament plastic as a room temperature solid. This sub system uses a stepper motor, extruder drive gearing, and material guide, in and out of the feeder. A filament tensioner solution and optionally a flow sensor. This simple mechanism is the means that the 3D printer uses to move the material, by translating the rotational force, the shaft of the stepper motor, into the linear force forward and reverse. Happily, the use of thermoplastics grants the system some linearity, in the amount of force applied to the amount of material extruded out to the tool head. In other words, it's possible to plan the degree to which you move the filament forward or back, and achieve the precise amount of plastic exiting the other end of the system. But given the frequency and exacting links to these movements, it's critical to have a feeder mechanism that can accurately execute these planned moves without breaking, grinding, slipping, or buckling the filament. There are a number of strategies for how to produce an extruder drive gear, that will grip the filament firmly enough to push it forward and back, without slipping. Machining methods such as knurled, hobbed, and other serration patterns are schemes for extruder drive gears, in direct contact with the filament to clinch it reliably. Some extruders are direct drive, i.e drive gear is mounted directly on the motor shaft. Other strategies, such as geared feeders, make the drive gear that is in contact with the filament a secondary or tertiary element in the gear train, using the gear ratio to adjust the amount of force and rotational distance required for the stepper to achieve the desired linear movement of the filament. So when we talk about a successful extruder drive gear, we're talking about the extruder feed gearing, which firmly grip the filament, and the tensioner subsystem. Also known as an idler, which maintains an even pressure of the filament up against the extruder feed gearing. These mechanisms are responsible for ensuring that single-digit degrees of rotational movement from the extruder stepper will result in the correct amount of movement for the filament in the desired direction. When tracing the movement of the filament feed on to the hot end portion of the extruder system, it's important to remember that no matter how successfully you translate instructions into movements within the feeder subsystem, the amount of movement in the feeder and the amount of filament deposited on the object are not identical values. Part of the process of designing and tuning a desktop 3D printer is measuring and experimenting with a whole host of factors, that will be accounted for in the extrusion systems path planning, from amount of movement to speed of movement, to time offset compensation, to matching the transition of the plastic in the tool head from piston to molten material. Tracing forward from the feeder mechanism, the next element that you encounter is a Bowden tube, mechanically a part of the extrusion system, an extension of the mechanism of the feeder itself in fact. Fixed on both ends with very little play, within its tight low-friction tunnel, the Bowden tube translates the linear force moving upwards out of the feeder into linear force moving downwards into the top of the hot end. By using a Bowden solution, the mass and jerking vibration of the feeder mechanism can be mounted on the case. It doesn't need to be driven around and compensated for, on the hotend tool head carriage itself.
