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How a 3D Printer Works

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15 thg 4 2020

All of the personal 3D printers that we will look at in this chapter share many similarities with one another, at least in principle, although they might each approach things a little differently. Let’s take a closer look at how these 3D printers work using the example shown in Figure 1-1.

Figure 1-1. How a 3D printer works (original model courtesy Gary Hodgson, 2011)

Cartesian Robot

Central to a 3D printer is the idea of a Cartesian robot. This is a machine that can move in three linear directions, along the x-, y-, and z-axes, also known as the Cartesian coordinates. To do this, these 3D printers use small stepper motors that can move with great precision and accuracy—usually 1.8 degrees per step, which translates to resolutions in the fractions of a millimeter range through the

unique way that these stepper motors are controlled. The three-axis robot is like any other computer numerically controlled (CNC) robot and is able to position its thermoplastic extruder along each of these linear axes of movement to lay down layer upon layer of hot plastic. All of the 3D printers in the book use timing belts and pulleys along their x-and y-axes to provide fast yet accurate positioning, and most use threaded rod, or lead screws, to position the z-axis with even greater precision.

While it might all sound complex, it is really not all that complicated, as nearly all of these DIY 3D printers use standardized off-the-shelf components that are put to use in many different industries. In part, it took a lot of hard work over the years to figure out what worked well and what didn’t to end up with such outstanding printers today. Thanks to an open and sharing community, these designs and improvements have been shared freely, further improving the technology.

Thermoplastic Extruder

With our Cartesian system providing accurate linear positioning, we need an extruder capable of laying down thin strands of thermoplastic—a type of plastic that will soften to a semiliquid state when heated. The extruder (see Figure 1-2), arguably the most complex part of a 3D printer that is still seeing intense development, is actually the marriage of two key elements: the filament drive and the thermal hot end.

Figure 1-2. Complete extruder with filament driver and hot end 

The filament drive pulls in plastic filament often bundled in spools of either 3mm or 1.75mm diameter filament using a geared driver mechanism. Most, if not all, contemporary filament drivers use a stepper motor to better control the flow of plastic into the hot end. These motors are often geared down with printed gears or an integral gearbox, as shown in Figure 1-2, to give the filament driver the strength needed for continued extrusion.

The filament, after being pulled into the extruder by the filament driver, is then fed to the heater chamber or hot end. The hot end usually is thermally insulated from the rest of the extruder and is made up of either a large block of aluminum with an embedded heater or some other heater core, along with a temperature sensor. When the plastic reaches the hot end, it is heated to somewhere around 170°C to 220°C, depending on the plastic to be extruded. Once in a semiliquid state, the plastic is forced through a print nozzle—with an opening somewhere in the vicinity of 0.35 millimeters to 0.5 millimeters in diameter—before laying this thin hot extrusion onto the printbed drawing lines that outline that layer of the shape to be printed or fill that layer using some type of infill pattern.

Printbed

The printbed is the surface that your 3D prints are built on. The size of a printer’s printbed will vary from one printer to the next, ranging from 100mm² to

200mm², somewhere between 4 inches and 8 inches or larger. Most, although not all, personal 3D printers on the market offer a heated printbed (see Figure 1-3), either as standard or as an option, although it is also easy to build one from scratch if needed. The printbed is used to prevent warping or cracking of prints as they cool and to create better adhesion between the first layers of the print and the printbed surface.

Figure 1-3. Heated printbed

The surface of the printbed is often made from either glass or aluminum to better spread the heat across the area and to make for a smooth and level surface. Glass provides the smoothest surface to print on while aluminum conducts heat better for a heated platform. To prevent the object from lifting off the surface in mid-print, these surfaces are often covered in one kind of tape or another to provide a surface that is inexpensive to replace periodically. These materials include Kapton or polyimide tape, PET or polyester silicon tape, or even hardware store–variety blue painter’s tape, depending on the type of filament used.

Linear Motion

The type of linear motion system (or the mechanical assembly that allows each axis to move) that is used by the 3D printer will often determine how accurate the printer is, how fast the printer can print, and how much or how little maintenance will be needed by the printer over the long term. Most personal 3D printers use smooth, precision ground rods for each axis, and either plastic, bronze, or linear ball bearings to glide across each rod. Linear ball bearing systems have gained a lot of popularity lately for their longevity and smoother operation over the life of the printer; although they are often louder during operation than bronze bushings like the self-aligning variety shown in Figure 1-

4, which are generally quieter but often require a little more work to align during the build process.

Figure 1-4. Self-aligning bronze bearings on a Prusa Mendel

The “best” type of linear motion system for a 3D printer is about as personal a decision as the kind of car you drive. 3D printed bushings are very cheap, but don’t last that long. Machined plastic bushings (called Igus bushings) are very smooth and work well for the slow-moving z-axis, but they tend to deform under heavier use. On the other hand, the reliability of linear bearings discussed earlier depends on the quality of the smooth rails they ride on and they cost more. Other, more exotic materials, like felt, have also been tried with mixed results. Some printers even use industrial linear glides that have the potential for greater accuracy and longevity at increased cost and mechanical complexity.

Endstops

The length of travel for each linear axis is limited by usually mechanical or optical endstops (see Figure 1-5). Basically, these are switches that tell the printer’s controller electronics when it has reached a limit in one direction of movement in order to prevent the axis from moving past its limits.

Figure 1-5. Mechanical endstop on y-axis

While endstops are not strictly needed for operation, having at the very least one endstop in the minimum position on each axis will allow the printer to home itself at the beginning of each print for repeatable and accurate prints every time. We will discuss the electronics in greater detail in Chapter 2.

Frame

Holding everything together is the 3D printer’s frame. This frame forms the structural element of your 3D printer and its material and construction determine a lot about the final accuracy of your printer. All of the RepRap style designs will use 3D-printed components along with threaded rod and other hardware to make the frame structure. On the other hand, Box Bots like the MakerBot or the MakerGear Mosaic use laser-cut plywood that is bolted together to make the frame, as shown in Figure 1-6.

Figure 1-6. MakerGear Mosaic plywood frame

A laser-cut plywood frame uses slot-and-tab construction, where two pieces of plywood are held together with tabs from one piece fitting into slots in the other, and connected with nuts and bolts. This type of frame is generally easier to assemble and offers better up-front precision so that printer calibration is easier; however, these frames are often louder during operation and all those screws will eventually need retightening later. Conversely, threaded rod frames make for quieter robots, but add to an even more complex assembly and calibration. If you are sourcing all the parts for your 3D printer, then you will often need to cut the various lengths of threaded and smooth rod again, adding to the complexity of the overall build. Finally, some of the more recent printer designs are using commercial aluminum extrusions to build a frame that is rigid and easy to assemble, although it can cost as much as $100 to $200 more.

So that’s a brief overview of the main components in a 3D printer. We will look at the workflow of moving from a digital 3D model to printing a 3D design in the next couple of chapters; but for now let’s talk about some of the more popular and affordable 3D printers on the market today and how they compare with one another.

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