Regardless of the control geometry used, the method of printing is the same for both types of FDM printers. The printhead is a metal tube with a heating element and thermistor to control the temperature. The plastic substrate is melted by the heat of the printhead, and pressure is applied by forcing more plastic in, causing some of the liquid plastic to extrude through a small nozzle that ranges from .2 mm to .5 mm in size.

A print from an FDM printer begins with a single layer of plastic applied very thinly to the print bed, the nozzle moving across the print bed and depositing the plastic in the shape of the object it’s creating. This initial layer is the base layer of the object, and the second layer will be deposited directly on top of the first and will fuse due to the properties of the plastic involved. Once the second layer is completed, the third, fourth, and so on will be done, building the object over time along the Z axis. You can think of layer height as the equivalent of the DPI (dots per inch) of a printed page. It’s the resolution of the object in the vertical dimension, and the smaller the layer height, the smoother the final product will appear. It will also take significantly longer to print since as you reduce the layer height, you’re adding layers to the overall build.

For example, let’s imagine you’re printing a 5-cm-tall cube. If you print that cube at what would be considered a fairly rough layer height of .3 mm, you’ll print a total of 167 layers. If you print that same cube at a fine resolution (around .1 mm for most printers), then you’ll print 500 layers, tripling the number of overall layers and the time necessary to print the object.

Because FDM printers rely on building objects vertically in the open air, they have issues with specific geometries of objects. If you imagine a thing being printed slowly from the bottom up, if the thing has a significant overhang or free-hanging part like a wide doorway or something like a stalactite, it won’t be printable without supports on an FDM printer. The plastic has to be deposited on something: otherwise the print won’t work.

All FDM printer software has the built-in ability to include supports for printing when issues like this arise. Printing an object with supports means that the software builds vertical towers whose only purpose is to give the object a structure upon which to print. In the best case, a support structure would be easily removable from the rest of the model, either by just peeling them apart or, in a slightly more advanced process, by printing the supports in a type of plastic that is soluble in a solvent, while printing the object itself in a plastic that is insoluble. The most popular of these support structures (discussed in more detail below) is high impact polystyrene, or HIPS, which allows a printer with dual extruders to print support structures that can be dissolved off the actual print.

As with any sort of specialty product, there’s a vocabulary that has built up around 3-D printing, and if you’re new to looking at these printers, some of the specific terms are inscrutable without research. One example would be the terms for the two types of extruder setups found on FDM printers. The extruder is the part of the FDM printer that forces the plastic filament into the hot-end and through the nozzle onto the build plate. One is simply called a direct extruder, while the other is known as a Bowden extruder. On a direct extruder FDM printer, the moving print assembly includes the hot-end and the nozzle, and a motor pulls filament off the spool and drives it directly into the hot-end. The majority of FDM printers have a direct drive extruder. The Bowden extruder removes the motor assembly from the moving printhead. In a Bowden setup, the motor pushes the filament from the spool through a tube connected to the hot-end and nozzle. The advantage to the Bowden is that it significantly reduces the weight of the moving print assembly, which means that it can move more quickly and can change directions without serious jitter problems. The disadvantage is that it is, in some sense, pushing a rope, and the more flexible the filament is, the harder a time the Bowden setup will have with pushing it into the print assembly.

A few other helpful FDM-specific terms (and some of these I’ve already used without explaining, forgive me, dear reader) are hot-end, build plate, nozzle, and spool. The hot-end of an FDM printer is the metal piece that contains the heating element and melts the filament when the filament reaches it. These are normally some form of nonreactive metal, either aluminum, brass, or stainless steel. The nozzle is the very small diameter (.2 mm to .5 mm) that the melted plastic is forced through under pressure on its way to the build plate. There is a relationship between the nozzle diameter and the possible layer height of the output from the printer. Because you are extruding tubes of melted plastic and they need to be pressed together in order to fuse, the layer height can’t be any larger than the diameter of the nozzle. If it were, you would be extruding into thin air, without the new layer pressing into the old layer. To help visualize this, if the width of your extruded plastic is .3 mm and you attempt to print at a .4 mm layer height, there’s .1 mm between the plastic and the layer below it—not good. In practice, a good rule of thumb is that the maximum layer height is somewhere between 75 and 80 percent of the nozzle diameter. So for a .4 mm diameter nozzle, your maximum layer height would be around .3 mm. Generally speaking, the goal is to have lower and lower print heights, as that makes for a smoother and smoother final product. But for rough prints, or demos, having a higher maximum layer height can speed up prints tremendously.

The last couple of FDM-specific pieces of terminology are build plate and spool. Spool is easy, as it’s the way that filament is generally purchased and used. A typical purchase of ABS or PLA would be a kilogram (2.2 pounds) of plastic, wrapped onto a plastic or cardboard spool that hangs on the printer and plays out filament as needed. In an FDM printer, the build plate is the surface upon which the plastic is extruded. The specifics of the build plate vary widely but fall into a few basic categories, the primary of which is heated or nonheated. A heated build plate adds cost to the printer but is absolutely necessary for printing with certain types of filament (ABS, nylon, and others).

Another aspect of the build plate is what it’s made of and whether you print directly onto the plate, onto some tape or other covering, or onto a glue or other adhesive. Heated build plates are usually made of either aluminum or tempered glass, although occasionally stainless steel shows up. Unheated build plates can be composed of the same things, as well as acrylic. The important thing with build plate construction is that you want something that will not warp or deform over time, since if the plate itself isn’t flat, it’s impossible to level it appropriately to the printheads. Glass is a very popular build plate material for this reason, although many FDM printers ship with aluminum plates that are then covered with a replaceable printing surface of some kind, most commonly PET tape or Kapton tape for a heated bed, or painter’s tape for a nonheated bed.

The price points for FDM printers are typically determined by size, more specifically print volume or the size of the print bed, and a variety of upgrades that make specific kinds of printing or printing with specific plastics more easily done. Print bed sizes range from very small (no more than 3 inches by 3 inches or so) to massive (over 12 inches by 12 inches). The print volume determines the maximum size of a single object that you can print or, conversely, the number of smaller objects that you can print at the same time. Printing larger objects is also more difficult because as you print larger things, there’s more opportunity for a small error to creep into the print due any number of common 3-D printer issues.

Most of the issues with FDM printing are related to the fact that it’s a very mechanical process, and tuning the printer is key. The most sensitive aspect of the process is the relationship between the extruder/nozzle and the build plate. Because the printhead has to extrude an even layer of plastic onto the build plate, it’s necessary that the build plate be perfectly flat relative to the nozzle. If there is any warp or unevenness, you’ll get uneven attachment to the plate or other forms of print failure. This is the most common issue with FDM printing, especially with new operators. The first question that should always be asked if a print fails is, “Is my build plate level?”

And prints will fail. FDM printing is a complicated mechanical process, and while you can tune a FDM printer to be very reliable, at some point you will have a failure and will come back to a print that looks like someone poured plastic spaghetti on your build plate. This is normal. Recalibrate, relevel, and try again.