Okay, finally we get to the well, 3-D printer part of the 3-D printer. At first glance, not much to see. Looking at the hot end from the outside, there isn't that much to look at that grabs your interest beyond the active cooling fans and shroud and the molten plastic that may or may not be squirting out of the nozzle at the moment. The extrusion system manages the filament delivery process from coil to drip. In the case of the printer we're looking at specifically, you'll notice that the exterior design itself has been split between the filament drive on the back of the machine and the hot end part moving through the build volume. This split, necessary for Bowden-based printers, will be a helpful division for addressing how extruders function using any of the available design paradigms. The extrusion system is the key activity for the machine as a whole. The whole machine exists to drive that tool head through the build volume. There's a lot to explore here, let's get right to it. What goes into the extrusion system? This system involves the entire extrusion train, from filament spool, through the filament drive mechanism that draws the material forward and back. Through the Bowden tube, into the hot end, down through the hot end, and finally out of the nozzle. The analogy our industry tends to use for introducing novices to desktop 3D printing is of a hot glue gun duct taped to a CNC. It is a bit tired, this metaphor, but I think it's worth revisiting in greater detail. With a hot glue gun, you squeeze the glue stick feed trigger, and solid, cylindrical glue stick advances forward into the melting chamber. If this is the first squeeze, this could take a lot of force. But as soon as the heating chamber is flooded with melted glue, it usually requires less force than you apply, at least in my experience from burning myself. What's happening inside the chamber is that the front circular face of the stick begins to swell in the heat. And the main body of the still solid part of the stick acts as a syringe plunger, to translate the forward pressure from the feed mechanism, right upon the molten material already melted in the chamber. The body of the glue stick near the melted material remains solid enough that the action of forcing it into the chamber, applies enough pressure upon the melted glue, for it to squish forward and out of the way. As this material needs to go somewhere. In this case, the best place for it is to go right out the tip of the glue gun, and right onto the craft paper, or tongue depressors, or furniture joinery where you wanted to guide it. But the story of the glue stick isn't over, and this is the part to remember when you apply this analogy to FFS-style 3D printing. The front of the stick, now further in the chamber and unshielded from the heating element, becomes the next goop of molten glue that can be forced out of the glue gun at the next squeeze of the trigger. It is important to reiterate that this means of driving feedstock forward and using it mechanically to force the next payload of molten material doesn't work because that's how nature works in general. This works because of the odd engineering properties of the special class of materials that do behave this way. Most materials in nature, you apply heat to them and then they melt, combust, vaporize, disintegrate. Applying heat doesn't usually make them easier to mold and shape. It fundamentally changes them. Likewise, most materials, when you apply significant pressure on them, they compress, squash, rebound or otherwise translate that force into something a bit harder to control. What you need is non-Newtonian behavior. You need a material that inherently can pass through a few mechanically different stages without inherently, chemically undergoing a real state change. Polymers, plastics, are already a type of material that are a bit more friendly to these sorts of manipulations, but still within this huge range, most of them would likewise burn, compress, break or otherwise change. Such that you wouldn't be able to predict precisely what force would be required to squish workable material out of an extrusion train like the hot glue gun and onto your printed part. We will talk in greater detail about thermal plastics and thermal elastomers in another lecture, the materials particularly suited to 3D printing. But for now let's remember that the mechanics of how an extruder functions depends on types of materials compatible with a hot glue gun type experience. What goes into the extrusion system? Let's review the entire extrusion train from spool to part. Filament spool, this is the material source. The feeder, this is the element that draws in the material from the filament spool and delivers it to the hot end. In the case of a direct drive extruder, the feeder is on the tool head itself, just above the hot end. In the case of a Bowden extruder, the feeder mechanism is mounted on the frame of the machine with a tube, fixed at both sides, that translates the motion up through the feeder. It's a movement at the other end of the tube down into the hot end. The hot end, the part of the tool head that heats up the material itself. The hot end has these subcomponents. The cold zone, the area where it is important for the material to be cold and solid. Both to be advanced by the feeder and to be forced into the hot end to apply forward pressure on the molten filament in the reservoir. The heat break, the key to prevent the heat from propagating up into the cold zone area. The transition zone, this is a little bit nebulous but this is the area where the solid material swells and softens on its way, semi-molten, into the material reservoir. The hot zone, the area where the material liquefies and can be squirted out of the tool head. The thermoblock houses the hot zone where the heater and temperature sensor are typically mounted. Cartridge heater, the means of delivering heat to the thermoblock in the hot zone. Temp sensor, the means of detecting changes in temperature within the tool head, used to monitor whether targeted temperatures have been met or exceeded. The nozzle, the end of the bottom of the reservoir where the liquified material is directed down and out into the build area. And external to the hot end, but a critical component, are the fans, both the active cooling on the part and cooling the heat break and hot zone on the tool head. The surface of the build plate plays a critical role in encouraging successful print adhesion. There are options for materials such as glues, tapes, films and detachable plate adhesion systems, to apply to the surface to improve adhesion. What is critical for the extrusion system to function? There are a few elements that are truly critical for the extrusion system to function well. I'm mentioning them here to give you a head start on some of the lectures that we will have later in this section. First, established in the mechanical system, the nozzle needs to be the same set distance from the build plate, everywhere within the build area. This way, when the extrusion system is squirting out plastic, the plastic can be deposited in the correct place, bonding with the build plate or next layer down in the part. Build adhesion in general is critical and tactics, both with a build surface and in software, with tricks to help hold the part down, are called adhesion strategies. Because you need the flow of material from the nozzle to match anticipated flow rate established in the job file. You often need a dump or purge stage to advance enough material through the nozzle in a non-critical place, such as back corner or the front lip. That by the time the tool head is in place, reduce your part, the material is flowing consistently. This is also critical when switching between tool heads, layer by layer, in a dual extruder machine. Or when changing materials to introduce a new one into the same extruder. You may need a special type of nozzle, depending on the type of material used. Either in terms of internal geometry, in the case of a PVA nozzle, or in terms of abrasive resistance, so that you can print out composite materials that include sharp, abrasive elements such as carbon, metals, glass or wood. And if you print with composite material or fussy thermoplastic such as PVA water soluble filament, you may need to experiment with means to clean out your tool head when you're done. Such as purging with a neutral material, or using cleaning sticks to pull out remaining residue from a previous print job. We will discuss these topics and others in the extrusion system focused lectures for the course. It would be easy to construct an entire course around just this one system and still leave things out. Nozzle design and flexible filament systems alone are well worth exploring at greater detail for those interested. Thanks to the many open source desktop 3D printer designs still In active development. A great way to learn more about the extrusion system design is to check out some of the sites I shared in the course materials. Catch glimpses into the constant evolution of extruder and hot end design, not to mention a handful of fascinating new layers added to this process. Such as advanced filament detection and machine vision to grant printing system more information about successes and failures when printing.
