Parts of a Filament Printer

A filament printer is pretty simple—a common analogy is to call it a computerized hot glue gun. However, there are several critical parts that affect reliability, print quality, and what kinds of materials you can use.

Stepper Motors

Consumer 3D printers usually have four or more stepper motors , commonly called steppers. As the name implies, these are precise motors that move their shafts in predefined angular steps. The availability of very reliable, precise, yet cheap steppers like those in Figure 2-4 has been an enabler for many consumer goods.

Typically, one motor drives each axis (sometimes two, on the vertical, or z, axis) by being coupled through pulleys to a belt or cable, or to a drive screw for an axis (often used for the z axis). Another motor drives the extruder gear. The steps per millimeter of your gears (how much an axis, or the filament, moves as the motor turns) is one of the things limiting how accurate a 3D printer can be, although it is not usually the principal limitation. Motors for 3D printers typically have 200 steps per revolution.

Extruder

The extruder is the part of the printer that melts and moves the filament. It is made up of several parts. We have already mentioned the extruder drive gear and its motor, which push the filament into the hot end. The hot end in turn is comprised of a heater, a nozzle, and a sensor (called a thermistor) to sense how hot the nozzle is.

The nozzles are very precise. They typically measure 0.35–0.50 mm in diameter and can be made entirely of metal, or they may be lined with a material called polytetrafluoroethylene (PTFE) to minimize jamming. All-metal hot ends are required for printing many materials.

The hot end includes a heating element and a sensor to regulate temperature, a thermal transition zone (often with a heatsink and fan on the cold end), and a nozzle that is usually made of brass. Recently, hardened-steel and ruby-tipped nozzles have appeared—some filaments are very abrasive, and these are more resistant to abrasion. Figure 2-5 is a close-up of a nozzle. It is the triangular object under the “O” in “HOT.”

How Printing Works

First, filament has to be pulled from the spool into the printer, usually with a gear driven by a stepper motor. There are a variety of geometries for this. Sometimes a gear is right next to where the plastic will be melted (the hot end), and sometimes it is separated by a flexible tube, called a Bowden tube .

Extruders with a gear pulling in filament without an intermediary tube are called direct-drive extruders. Confusingly, the same term is used to distinguish extruders with the drive gear mounted directly to the motor from ones that have a gear reduction. In either case, the gear gripping the filament is called the extruder drive gear . Figure 2-6 shows the drive gear on a Bowden-style printer.

Some manufacturers like direct-drive extruders; many have moved now to some form of Bowden extruder. The main advantage of having the gear right at the extruder is simplicity, though it also means that the motor is being carried around with the hot end. This can in turn mean slower printing and other issues.

A print starts with a 3D CAD, which is turned into a series of commands for the stepper motor and the temperature controls. This process is discussed in detail in Chapters 3 and 7. Once those commands are loaded onto the printer, via cable, SD card, or wireless connection, the printer will first heat up the nozzle and the platform (if it has a heater). Once everything is up to temperature and the printer performs whatever self-checks it is capable of, printing begins.

The first layer of the print has to stick well to the platform. There are various techniques for ensuring this. Some involve software settings (Chapter 3), and others are related to using the correct platform surface for the material you are printing (Chapter 4). Assuming that the print sticks, as a layer builds up, there are areas that do and do not have plastic within one layer. This means that the nozzle will be extruding during some parts of the layer, and not during others.

During the time that the printer is not laying down plastic, the extruder gear will first pull the filament back a bit to relieve pressure in the nozzle. This is called retraction , and getting it right is something of a black art that involves a process of trial and error for particular combinations of printer hardware and filament material. Poor retraction settings can make a print stringy or blobby, like the one in Figure 2-7.

If the print fails to stick, most printers do not know that and will happily continue to print. This will result in a big pile of plastic (Figure 2-8). It is something of a rite of passage to leave a printer for a while and come back to a big hairball. Take a picture, sigh, and take comfort that you are now an official member of the 3D printing tribe.

Suppose that you were printing something that had a part sticking out, like someone reaching out an arm. As you built up your print layer by layer, you would eventually reach the first layer of the bottom of the arm. But as you laid out plastic to form the first layer, it would just fall down since there was nothing to support it. To address problems like that, we use support material. Support is extra material that is added to the model to allow it to print a layer at a time. Figure 2-9 shows a print with support. The process of creating support is discussed in Chapter 3.

Printers can handle overhangs of about 45 degrees—the subsequent layers overlap a bit and allow you to print these moderate slopes. Printers can also bridge over open areas. If you are printing a hollow cube, for instance, the top will be over open space, but supported on all four sides. Many of the support and bridging decisions that need to be made are somewhat automated these days, but in Chapter 3 we talk about the human intervention that is still required.

Assuming that everything went well, you will have a print that builds up from the platform layer by layer. The layers will be more visible in some filaments than others. Layer height is something that most printers allow you to set; common heights run from about 0.1 to 0.3 mm. Figure 2-10 is a close-up of the output from the Part Daddy giant printer in Figure 2-3, which is a little easier to see than some. (This printer uses plastic pellets rather than filament, which is why the colors are variable.)

Filament Choices

There are many choices for 3D printer filament. Your printer, however, will likely only be able to handle a subset of them, and will either use 3 mm or 1.75 mm diameter filament. The quality of your 3D printing filament will make the difference between endless frustration and a good experience, so let us go over what “filament quality” means first.

Consumer 3D printers, as we have just seen, are really not all that sophisticated. They have, in a way, outsourced their complexity to their filament. Consistency and quality is better now than it was a few years ago, but can still be uneven.

If you are 3D printing, you need a pair of digital calipers to check your filament’s diameter. A “3 mm” filament will typically be about 2.85 mm. A 1.75 mm filament will usually be about that. If you have a printer that uses cartridges, you will need to assume that the extra money you pay for your filament is taking care of this step for you. Be sure to adjust the filament diameter setting in your software (Chapter 3) to the actual value of your filament diameter. Typically, we just leave the value at 1.75 or 2.85 without adjustment most of the time these days. Figure 2-11 shows checking filament diameter with a pair of calipers.

There are a lot of different filaments available. We discuss some of the commonest ones, and what 3D printer hardware and software you need to use them.

Filaments need to be kept in a cool, dry place. Do not open a spool of filament out of its shrink wrap until you are ready to use it, because keeping it sealed will keep moisture out of it. After a while, you will have a bunch of partial spools on the go. We keep ours in airtight plastic boxes—a 5-gallon paint bucket with a good lid works well; Joan is partial to 22-liter stacking plastic boxes with good seals. Resist the urge to use your spools as a colorful wall display, since that is asking for both dust and moisture contamination of your filament.

Nylon is particularly sensitive to humid conditions. If nylon gets damp, the water will pop and sizzle out as it heats up, leaving pits and gaps in the print. If you live someplace with routinely high humidity, you may want to explore reviews of the various filament-drying systems out there, or DIY equivalents.

Dust is an issue, too. Keep the filament clean; in Chapter 5 we have some suggestions on how to do that. Do not try to do 3D printing and other fabrication that creates dust (ceramics, woodworking, machining) in close proximity.

PLA

By far the commonest filament material is PLA, polylactic acid. It is a biodegradable, corn-based (sometimes sugarcane-based) plastic. It melts at a relatively low temperature and will stick to a variety of platform surfaces. PLA usually requires an extruder temperature of around 210 degrees C.

One of the reasons it is so popular is that PLA does not require a heated platform. Low-cost printers usually suggest that you put blue painter’s tape (1.88-inch wide 3M ScotchBlue 2090 works well) and use that as your surface. If you do have a heated platform, follow your manufacturer’s suggestions for what to put on the bed. Often it will be plain glass, or you might smear on some glue stick first. The adhesive found on painter’s tape is not suitable for use on platforms heated much beyond room temperature.

The downside of PLA is that, well, it melts at a low temperature. A PLA print on a hot car dashboard will warp and creep. (Creeping is a tendency to flow slowly under pressure—for example, a PLA print might develop a dent if something is pressing against it in a warm room.) If that is not an issue, PLA is a very good material for quick prototypes, student projects, and the like.

There are specialty filled PLA mixes that contain stone, wood, metal, or glow-in-the-dark fine particles. Objects made with this can look surprisingly like they are made of the respective substance, with a bit of polishing. However, these mixes are hard on nozzles and tend to abrade them; hardened or ruby nozzles can get around this problem. There are also formulations of PLA that have a nice sheen without any post-processing. Experiment a little (many filament sellers have sample packs) to see what you might like to work with.

Silk PLA is PLA mixed with lignin fibers. The fibers expand a bit when printed and make the layer lines nearly invisible, which gives a sheen to the material that can look like burnished metal. Figure 2-12 shows a piece printed in silk PLA.

Figure 2-13 shows the bad side of PLA: a garden sign that sat out in the California sun for about two years gradually sagged under its own weight.

ABS

Acrylonitrile butadiene styrene (ABS) is the plastic used for LEGO bricks, among many other things. It is durable and far less vulnerable to warping in warm temperatures. The flip side of that is that it requires a high nozzle temperature—from about 220–260 C. A heated bed is also an absolute requirement for ABS, at 90–110 C. People often use PET or Kapton tape on a heated bed for ABS. As ABS cools, it wants to shrink and will pull up from the bed, as you can see in the print that was stopped after a few layers shown in Figure 2-14. Some trial and error is required to get it to stick and lay flat.

Caution

Although we recommend ventilation with any 3D printer, ABS fumes in particular are an issue. However, blowing a fan directly on the print will tend to make it fail. Arrange your ventilation if possible to be pulling air off the print rather than blowing on it.

ABS can be smoothed and post-processed in a variety of ways. We talk about some of those in Chapter 6, where we focus on what happens after the print comes off the printer.

Multimaterials vs. Multiple Extruders

The other reason you might buy a printer with multiple extruders is to print objects in more than one color at a time. With only one extruder, 3D printers print with just one color of filament for the entire print. In essence, you develop two interleaved computer models, one for each color, and the printer alternates extruders in each layer. Because the extruders would interfere if they tried to print at the same time, typically dual-extruder prints take longer than single-extruder ones do because one extruder prints, then wipes itself off (so it does not dribble on the layer once it is done), and so on, back and forth.

Alternatively, some machines now come with a device that can take filament from multiple spools and splice a mixed filament to mimic what multiple heads would have done, but with one nozzle. This is an interesting development to watch; there are some aftermarket devices available as add-ons to existing printers.

Aftermarket Upgrades

If you want to be able to print some materials that will not stick to your original print bed material, there are now a variety of aftermarket materials you can stick on to your platform. A commonly used one is BuildTak (www.buildtak.com), which can be glued on to your existing platform. The only issue is that some materials stick too well to BuildTak , and getting the prints off it is challenging. You will need to recalibrate your printer if you add a clip-on or stick-on platform on top of your original. See “Calibrating Your Printer” in Chapter 5.

Similarly, there are replacement third-party nozzles. Here, however, the replacement process is more sophisticated. If you are not a tinkerer who is comfortable with building a printer from a kit, you probably should not go that route.

Advanced Filament Printers

As the consumer and higher-end “prosumer” markets expand, some specialty printers have emerged. Some are very large, having a build volume of a cubic meter or more. Some have up to five extruders, or one of the multimaterial extruders we mentioned earlier.

Another niche is occupied by Markforged (www.markforged.com). This company has a printer that lays down nylon in parallel with another head that lays down one of several continuous fibers. Thus, they are 3D printing carbon fiber or other composite parts; Markforged has stated in webinars that its early adopters often use their printers to make tooling. Some Markforged printers use a filament with chopped carbon fiber filler, which increases the stiffness of the base nylon. This evolution of materials with custom printers is probably a niche that will grow as specialty applications emerge; novel materials are now driving a lot of the 3D printing universe as the printers become more commoditized.

Markforged is also one of several companies developing printers that are designed to print with filament (or, in some cases, rods) with a high percentage of metal filler. Metal-filled filament is available for many printers, but while most of these filaments can only give the appearance of metal, these machines are designed to create real metal parts by allowing you to put the printed parts into some combination of a chemical bath and furnace that will remove out the plastic matrix material and sinter the metal particles together. Chapter 11 has more detail about 3D printing with metal.

3D Printing Pens

3D printing “pens” (Figure 2-15) heat up 3D printing filament (usually 1.75 mm) and allow you to freehand draw with the melted plastic. They are useful for welding together broken prints, since you can use a thin line of the same material as you are using for your 3D print. In other words, they are more or less hot glue guns that use 3D printer filament.

The pens can also be used to do 3D freehand drawing. However, unless you have really steady hands, it is not as easy as it looks. Cover a piece of cardboard with the same blue painter’s tape you would use on a cold 3D printer platform and use it as your work area.

Vendors sell short strips of PLA filament to use with these devices, but you can use 1.75 mm regular 3D printer filament, which is much cheaper. We usually cut a few meters or so off at a time to use because the regular spools can be annoying to use with a pen. You can use as many colors as you want in one design, since you will just load strips of filament one at a time to create an object. We usually save the last few meters of filament on a spool to use with pens.

Some pens can use both PLA and ABS, with an ability to set temperatures appropriate for both, and most have a speed control to manage how much plastic you are using at a time. Using one of these pens can be a good exercise in building your intuition about what a 3D printer is doing, as you will most likely make a stringy mess the first time you use a pen.

Concerns about ventilating an area where you are melting plastic, ABS in particular, still apply with a pen. And of course the pen tip gets very hot, just like the extruder on a 3D printer. If you are using PLA, you will need to have some sort of airflow anyway to cool the material as it hardens out of the pen if you want to build in 3D.

There are stencils and such things too; search online for “3D art pen” for examples and videos. Eiffel Tower models are the aspirational thing to build, but you obviously should not start there. No computer modeling is required to use a 3D pen, but manual dexterity and a bit of artistic ability help. Pens with good temperature control and ability to use multiple materials cost about $50–$100.

Printing Process

Resin 3D printers have an ability to create far finer detail than filament-based printers. A resin printer is limited by its laser spot size (SLA) or the pixel size (DLP and LCD). This means that resin prints can have very smooth surfaces relative to those created by a printer using filament.

Resin printers still need to create support, but the nature of the support is a little different. Typical printers are “bottom-up” and build a layer at a time, which is peeled off the bottom of a tray. To survive the peeling process with every layer, support is required. However, the object prints upside-down and support primarily withstands the forces associated with peeling off the bottom at the end of each layer. Figure 2-17 shows a print building upside-down on a Form 2. The greenish glow is the laser.

Printers that use a laser to cure resin use a series of coordinates that the laser spot moves through, philosophically similar to the commands that control a filament-based printer. They can generally move the laser spot much faster than a printer can move a heated nozzle, though. Although most have proprietary software, some of them have adapted software developed for filament printing.

Printers that use a projector need a series of 2D images, one for each layer. Some of these printers have an HDMI port as an input, and their software treats the printer as a second monitor on which it displays those images. A popular way to run these machines is to connect a Raspberry Pi running NanoDLP to avoid tying up a larger computer. This avoids potentially ruining a print or even damaging the printer if the computer tries to display something else on that “screen.”

Post-processing

Post-processing is required on SLA prints, and typically involves washing off the print with isopropyl alcohol to remove uncured resin and exposing the print to UV light to ensure that the resin is fully cured. This can be a challenge in environments that are not set up for disposing of chemicals.

Each manufacturer will have a process for its own resins; read the process on the manufacturer’s site before purchasing your printer so you are sure you will be set up to handle it. Typically, prints emerge from the printer a little sticky and need to be washed off and light-cured. Figure 2-18 shows a resin print just after it has been washed and cured and is ready to snip off supports. You can also see how much more open support is on a resin printer.

Notice that this process requires wearing nitrile gloves to keep the resin off your skin. We talk a little more about resin printers in Chapter 4.

Materials

Resin printers use a variety of light-cured resins. Most of them are sensitive to UV light so that you do not have to use them in a dark room, since visible light does not harden them. But because by definition you cannot see UV light, you might be surprised if you do not control the environment around the printer. For example, having a resin printer in direct sunlight can destroy the printer (or at least its resin tank) by hardening a brick of resin into it.

Much of the development in resin 3D printing at the moment is in proprietary resins with special properties. Formlabs, for example, has a high-temperature resin that it says can be used for injection molds. Biocompatible resins for dental work have been an active area of study as well. Because the materials are so crucial, many manufacturers have closed systems with materials cartridges and proprietary software. These may have great properties, but they will be correspondingly expensive.

Formlabs (www.formlabs.com) has been an innovator in the materials space. If you want to know more, you can read up on its proprietary materials on the website, which includes case studies.

SLS

One of the oldest technologies in this space is selective laser sintering (SLS). Typically, a very fine powder is spread on a build platform, and a laser is used to sinter the powder together. More and more powder is added as the print grows. SLS can be used to make very fine, detailed prints. SLS prints do not need support, because the unfused powder acts as a support. However, the powder is very fine and hard to deal with, and SLS has been an expensive technology. Some “desktop” SLS machines are beginning to come on the market, and this may be an area to watch.

Many machines that print metal use a process called direct metal laser sintering (DMLS), which is the SLS process used for metals. Metal powder has to be worked in an inert atmosphere to prevent fires, so DMLS machines are very expensive since they need to be completely enclosed and filled with an inert gas. Other processes like selective laser melting (SLM) and electron beam melting (EBM) use even higher power to more fully fuse metal powder. We discuss metal printing more in Chapter 11.

Binder Jetting and Material Jetting

There are also processes that use a powder bed that do not require a laser (or other highly directed heat source). These processes typically use an inkjet head similar to what you would find in a desktop photo printer. Instead of heating the powder to fuse it, they deposit glue or other binding agents onto the powder. Binder jetting machines typically use gypsum powder and can deposit ink along with the binding agent to create full-color prints.

Binder jetting can also be done with metal, but that requires a second step in which the loosely bound particles are infused with a lower-temperature metal, or a wash and sinter stage to remove the binder. Some machines even sinter nylon powder by using inkjet heads to deposit sensitizing and/or inhibiting agents to the powder before heating it with a more diffuse heat source.

Other processes do away with the powder entirely and use inkjet heads to deposit liquid resin. These printers deposit tiny droplets of UV-sensitive resin and then quickly expose it to UV light to solidify it. This process requires that multiple materials be used—a build and support material at least—and can be used to produce prints with gradients of color and even of material properties like flexibility.

Bioprinting

Printing with biological materials has become fairly mainstream now in lab environments. In some cases, the printer is using some sort of paste to create an object for a biology lab. In others, the printer is very precisely squirting a liquid. The lower-cost “bioprinters” are basically a robotic pipette that can move in three dimensions; there are several competitors in this space already, and there will probably be more. Search for “bioprinter” to see the huge range of capabilities and price ranges.

Organovo (www.organovo.com) has been printing human tissue, as have other researchers. Right now, most people are looking at projects like skin or ear repairs, but there is long-term interest in building entire human organs.