Tool head hot end, robotic end effector. The terms for the next stage of the process differ, depending on your background and what your assumptions are about the configurations of this element of the machine. The part that moves around in the build envelope to extrude plastic can in some cases include both a filament drive mechanism and the subsystem that heats up the plastic, right, in line with each other. That is called a direct extruder and all in one approach that has been the more popular option among Hobbyists 3D printers. When dividing up the anatomy of a direct extruder into its subcomponents and subsystems, you can identify both the cold end and the hot end elements within the tool head design. Bowden strategy such as one we use here for this Ultimaker rinter also popular in most Delta printers. Similarly has a cold end and a hot end, but they are split in the middle with the cod in feeder mount on the frame of the machine and hot in melter on the Gantry carriage. The Bowden connects them together firmly anchored at both halves of the machine. For our purposes, this division is convenient. We have already traced the filament through the feeder along the Bowden tube. So we are ready to focus on the next part, the extrusion drive system, the hot end. Let's tackle the subsystems and components of a hot end. The most important thing to remember when breaking down the elements of a hot end is that while it doesn't heat up plastic as one of its core responsibilities, it is critical that this heat is applied where intended and nowhere else. Most of the refinements involved in the design of hot ends focus on preventing heat creep from propagating up from the thermal block and beyond the heat break, which divides the half of the hot end involved with keeping the filament cool from the half focus on heating things up. The component involved with keeping things cool. It's called the cold zone area not to be confused with the cold end of the extruder. Yes, that's confusing and I'm sorry. The component involved in heating things up actually includes two components. The glass transition piston and the hot zone. So let's tell the story of the hot end subsystem in terms of these zones, ignoring the individual components for a minute. Think back to the glue gun as a point of reference for printing technology. Remember that you need the stick of glue to be cool and solid in the cold zone before it's driven forward into the melt chamber where it will be exposed to the heating elements. Mechanically speaking, you need the force of the solid part of the stick to put pressure on the melted glue to force it out of the nozzle. In an FF style extrusion printer, you need to film it to remain cooler than the glass transition temperature, TG in the cold zone to provide the pressure forward into the melt chamber. As a polymer material is forced past the heat break and into the hot zone, two things happen. First, the material is exposed to enough heat that it becomes more rubbery and flexible. Second, it begins to swell. Bring those two factors together and you have produced a glass transition piston. The piston transmits the forward pressure into the mount chamber and also relieves the pressure during the retraction moves that pull the piston back. And at the forward edge of the piston, material has been exposed for a longer time to the heating element with the hot melted material at the bottom of the chamber that transition continues on from the TG properties to the TM melt properties. The filament becoming much more like a liquid and easier to dispense. The transformation into piston and then into external material is the secret to an FF printers mechanical elegance. The feedstock is also the dispensing tool. And as it completes its task,, it relaxes into the substance to be dispensed. So now let's review this again, but with the components back in place. As the material enters the hot end, it passes through the filament guide through the cold zone on the way to the heat break. As the glass transition temperature of most 3D materials is pretty low under a 100 Celsius, it is important to both insulate the material without and reducing friction and use a heat sink and cooling fan to pull the heat rising up from the heat zone out of the material. After the material moves past the heat brick which can either be a physical gap or the end of the insulating materials, the material is exposed to the first high temperatures of the hot zone and begins to go through the transition phase. The exact place that the melting begins at the leading edge of the piston isn't set in stone, but moves up and down slightly influence by one, the speed and force of the solid material driving down from above. Two, the amount of heat energy being delivered into the melt chamber by the heater cartridge. Three, the size of the opening of the nozzle which determines the amount of material that can be forced out of the chamber at any given time. Four, the speed and force of material dispensed and exiting the system. As melted material passes out of the nozzle and onto the build plate, heat leaves the system. The temperature sensor monitors the current heat and via the PID control loop tuning informs the circuit, which controls the amount of voltage flowing into the heating element. Planning, so that the transition zone and melting zones happen at mechanically useful places within the hot end is a collaborative effort between a 3D slicer software and elements of the control electronics which manage both heating and monitoring heat. The job file instructions map target temperatures to reach in the thermal block to match what material needs to become the piston, to be dispensed into the part. The electronics involved in heating and sensing the heat also have control loop tuning that determines how frequently to sample the temperature, and how quickly, and aggressively to feed voltage to the cartridge heater. Raise the heat too quickly and the melting area rises to quickly, reducing the effectiveness of the piston to deliver and relieve pressure. Raise the heat too slowly and the piston will advance too far into the melt zone no longer melting quickly enough for its leading surface to become the next material to extrude. We will talk next about the nozzle and its role in both venting the melted material forward, and shaping how it is deposited on the build plate, and on to the next layer of the printed object.
