Now we're going to talk about the nozzle component, both the inside and the outside. Most nozzles are made of brass because brass is easier to machine and excellent at transmitting heat. There are limitations, however, when using brass to make nozzles. The same qualities that make them easy to produce means that they're soft enough when processing abrasive materials. The materials will then wear on the nozzles over time. The material exiting widens the opening as it moves out of the nozzle. As a result, there's a third-party market for hardened nozzles such as tool steel and ruby-tipped varieties. These won't suffer when even extruding materials that contain sharper elements like metal powders, carbon, and glass. The function of the nozzle is two-fold. It mounts into the thermal block and extends the melt chamber, and typically most of the melted material is in the reservoir inside the nozzle's tip. It also constraints the amount of material that comes out and mechanically influences the placement of plastic on the bill pipe. Nozzles and thermal blocks are typically suited to specific filament diameters. It doesn't mean you can't melt 175 diameter filament in a 285 millimeter diameter hot in. But the process works better if you're using the right internal hot in diameter to match your chosen filament. The closer it hugs to the filament, the more precision you'll have in controlling the amount of extruded plastic. The most important factor when selecting a nozzle is the size of the opening diameter. The most common size used is 0.4 millimeters, and it has been so for years. 0.4 millimeter is a good compromise between the size of the two most common diameters of filament, in terms of pressing down without producing too much back pressure, and the width of the bit of the plastic that is extruded from a 0.4 millimeter nozzle is somewhere pretty close to 0.4 millimeters, maybe a little bit less depending on the material, and offers enough X, Y resolution to satisfy most prototyping needs. As there are more materials and more applications for 3D printing than ever before, there are now a range of nozzles available. Some machines ship standard with a 0.5 millimeter nozzle or wider, typically for printing large objects, and nozzles as small in diameter as 0.25 and 0.15 millimeter are available for very fine detail. A wider or smaller nozzle doesn't change the accuracy of placement in X, Y plane, but changes the diameter of the bit of plastic leaving the nozzle. Think about frosting a cupcake. If you want to make a small delicate feature like leaves or flowers, you want a nozzle with a small opening. Even if you have a really wide nozzle, you are still the one moving the bag of frosting. Certain materials are better processed with particular nozzle diameters. For example, filaments containing wood or metal powders can clog more easily with really small openings. The powder content might pack into the tip of the nozzle and block extrusion. Other materials are more viscous and require too much force to exit a small opening effectively in any case. There are a number of different terms for features of internal geometry within the nozzle. I will use the terms external tip, cone, shoulder, and throat to describe the space the material travels from the melt chamber to the outside. As the material moves forward, it shrinks down for matching the width of the filament to the width of the nozzle diameter. Let's call this area the cone. It's essential for directing the forward pressure when extruding the material, ensuring that the plastic exits the tip in a straight line and not at an angle. The external tip both establishes the outer diameter of the extruded material and has a shape of its own. Typically, there's a flat ring surrounding the opening and it has an additional mechanical function. It irons down any material that has risen up on the top surface of the print. The shoulder is a flat internal ring around the throat. It's less of an essential part of the nozzle, and more of a clue as to how the nozzle was originally machined. Most thermoplastics are forgiving enough not to be affected by the shoulder. Thanks to the shear thinning properties. But some materials have more difficulty, and this is the reason for the availability of specialty nozzles in terms of their internal geometry. For example, for the Ultimaker cores, in approach to the hot end that combines the heating element with a nozzle, there are specialty BB cores that have a more gradual slope and eliminate the shoulder. This is better suited to PVA material, which can compress and cross-link when encountering the shoulder and so doesn't typically behave well with a typical nozzle. It is much more costly to produce the internal geometry of a BB core nozzle because it doesn't match typical machining processes. The throat is the tiny tube that extends from the edge of the cone and out to the external tip of the nozzle. A throat isn't typically very long. But with some materials, it can be helpful when evening out the pressure as the material emerges. In the early desktop printers, the nozzle was a fixed part of a hot end. These days, operators want the flexibility of switching between nozzles based on the task at hand. There are many more options for modularity. Take the N6 threaded nozzles used by a number of systems from E3D to the Ultimaker 2's Olsson block. In the Ultimaker 3 as in is as 5, the core system offers a variety of nozzle diameters and ones that are suited to specific material properties.
