Editor's Note: This is the first of a series of posts excerpted from Jason Griffey's Library Technology Report "3D Printers for Libraries."
Fused deposition modeling defines 3D printing for most people, as it’s by far the most common and in many ways the simplest technology for 3D printing. Fused deposition modeling uses a variety of plastics that fall within a range of melting points and that fuse when melted and resolidified, the most common of which are ABS (acrylonitrile butadiene styrene) and PLA (polylactic acid). We’ll discuss the specifics of these and other print substrates below.
The most common arrangement for an FDM printer is called Cartesian print engine, because it uses basic Cartesian coordinates (X,Y,Z) to create the printed objects. Even this general category comprises multiple types of printers and two are most common: the Makerbot style, which relies on a fixed plane X and Y print head and moveable Z print bed; and the so-called “RepRap” style, which relies on a fixed plane X axis, while the Y axis is controlled by moving the print bed itself and the Z axis is accomplished by moving the entirety of the print head system vertically upwards.
Makerbot Replicator (above)
RepRap style 3-D Printer (above). Photo by John Abella.
Alternatively, with a Delta printer, a significantly different geometry for a FDM printer, the printhead is suspended from 3 arms that are controlled along vertical supports, while the print bed is completely stationary. This arrangement allows the printhead to “float” above the print bed and be located at any physical point in 3 dimensions simply by altering the relation of each of the three arms to the other. This is the same sort of control geometry in the flying cameras used in NFL games, applied to a robot.
SeeMeCNC's Rostock Max (above), a delta printer.
Regardless of the control geometry used, the method of printing is the same for both types of FDM printers. The printhead for both is a metal tube with a heating element and thermistor to control the temperature, and the plastic substrate is melted by the high heat of the printhead. Pressure is applied by forcing in more plastic, causing some of the liquid plastic to extrude through a small nozzle that ranges from .2-.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 said 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 follow, building the object over time along the Z axis. You can think of layer height as the equivalent of the DPI 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 lower the layer height, you’re adding layers to the overall build.
For example, lets 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 .3mm, you’ll end up printing a total of 167 layers. If you printed that same cube at a fine resolution (for most printers around .1mm) then you’d end up printing 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 an object being printed slowly from the bottom up, if the object 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.
All FDM printer software has built in the ability to include supports for printing, when issues like this arise. Printing an object with supports means that the software builds in vertical towers whose only purpose is to give the object a structure upon which to print. The best case for a support structure is that it 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 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 (discussed in next week’s post) is high impact polystyrene or HIPS, which allows a printer with dual extruders to print support structures that can be dissolved off of the actual print.
As with any sort of specialty product, a vocabulary of 3D printing has sprung up , and if you’re new to it, some terms are inscrutable without research. One example would be 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, and the other is known as the Bowden extruder. On a direct extruder FDM printer, a motor on the moving print assembly includes the hot-end and the nozzle, and the 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 hot-end and nozzle, and takes it off the moving printhead altogether. 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 time the Bowden setup will have with pushing it into the print assembly.
A few other good-to-know FDM terms (and some of these I’ve already used without explaining, forgive me, dear reader) are: hot-end, build plate, nozzle, spool. The hot-end of an FDM printer is the metal piece with the heating element inside that melts the filament. Usually they are made of some form of non-reactive metal, such as aluminum, brass, or stainless steel. The nozzle is the very small diameter (.2-.5mm) 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 .3mm, and you attempt to print at a .4mm layer height, there’s .1mm 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 75-80% of the nozzle diameter. So for a .4mm diameter nozzle, your maximum layer height would be around .3mm. 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 which 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 vary widely, but fall into a few basic categories, the primary of which is heated or non-heated. A heated build plate adds cost to the printer, but is absolutely necessary for printing certain types of filament (ABS, Nylon, and more).
Another aspect of the build plate is its composition, and whether you print directly onto the plate, some covering such as tape, or 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 print heads. Glass is a very popular build plate material for this reason, although many FDM printers ship with alumninum 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 non-heated 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 makes feasible specific kinds of printing or the use of specific plastics. 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 could 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 3D printer issues.