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3D printing

Lesson 3:

Main 3D printing techniques.

I this section will explain the three most widely used 3D printing techniques.

a) Stereolitography (SLA).

This was the first 3D printing technique to be invented. It is used to print with a very specific type of material: resins. As indicated in the previous section, resins, also known as thermosetting plastics, are a group of non-recyclable plastics that cannot be melted into a new shape as many times as we want.

To make an object with any of these plastics it is necessary to carry out a chemical reaction known as curing. For this reaction to occur temperature (and sometimes pressure) must be applied to the plastic.

In stereolithography 3D printers it is a beam of ultraviolet light the one that heats the resin to provoke the curing reaction. There are two different models of SLA printers, but the operation of both is quite similar:

In some cases, it is the platform where the piece rests the one that moves down. In that case, the process is as follows:

  • A very thin layer of resin that has not yet been cured is added.
  • The beam of ultraviolet light that strikes from the top cures the parts of the resin layer that must become solid. A mobile mirror is in charge of sending the beam of light in the right direction.
  • The platform moves slightly downwards to continue printing the next layer.

This process is repeated over and over again until the piece has been completed, as you can see in the next video.

In other cases, the platform moves upward and the ultraviolet light beam strikes from the bottom of the printer. Of course, in this case, the resin tank must be transparent to let the light beam pass (as can be seen in the video).

In this type of printer, each time a layer of resin is cured, the platform rises, allowing the next layer to be printed. In the following video, you can see an SLA printer of this kind.

b) Fused deposition modeling (FDM).

It is probably the technique that comes to mind when it comes to 3D printing. This type of printer is the one that has spread the most throughout the world because they are the most affordable and easy to assemble. In addition, the materials they usually use are among the cheapest for 3D printing.

The most commonly used materials in this type of printer are plastics from the group of thermoplastics, that is, plastics that can melt and change shape over and over again (although if the operation is repeated many times they can lose certain properties). Among the most widely used thermoplastics are the following two:

  • Polylactic acid (PLA).
  • Acrylonitrile butadiene styrene (ABS).

Both plastics are sold in the form of coils like the ones in the picture, which can currently be purchased for around 20 euros:

However, fused deposition modeling is a very versatile printing technique that can be used with a lot of different materials. In fact, the food printers, the building construction printers or the and stem cell printers seen on the first page of the unit belong to this group.

The following image shows the main parts of a common FDM printer. On the right you can see a detail of one of the main parts of the machine, the extruder:

*You can download the image with blank fields here.

a) The plastic coil: the plastic to be printed is placed rolled up on a support and the end of the filament is introduced into the extruder.

b) The extruder is the part of the printer where the plastic is melted, it is made of several parts:

  • The gears: are toothed wheels that introduce the filament into the fuser.
  • The hot end: it is a small chamber where the plastic melts.
  • The heater: is an electrical resistor that generates heat to melt the plastic.
  • The temperature sensor: it is in charge of measuring the temperature in order to know when to stop heating it.
  • The nozzle: it is a piece with a small hole through which the plastic is released.
  • The fans: whose function is to cool the plastic so that it solidifies quickly after leaving the nozzle as well as to cool the hotend to prevent it from overheating. It can be activated or deactivated since its use may or may not be convenient depending on the type of plastic used.

c) The bed: it is the base, usually made of glass, on which the printed piece rests. Sometimes they have a heater (hot bed) that makes it easier for the piece to adhere to the surface.

d) Stepper motors: are a special type of motor that can be controlled with great precision. Unlike typical (continuously rotating) motors, stepper motors can rotate a certain angle, allowing small offsets. The number of motors depends on the printer, but typical 3D printers have a motor to activate the gears that introduce the filament and three more to move the extruder and the base in the 3 directions of space.

e) Worm drives: move the bed up and down depending on the direction of rotation.

The printing process in this kind of machines is the following:

  • The material that is going to be printed enters the hotend where it is melted by the heater.
  • Once it is melted, the material is released in small quantities through the nozzle.
  • Typically, a pair of stepper motors is responsible for moving the extruder in the plane of the printer and another motor is responsible for vertical movement.
  • As it happened with SLA printers, there are two models of FDM printers. Once a layer of the part has been printed, two things can happen, depending on the model: that the base moves downwards (leaving space to print the next layer), or that the extruder moves upwards so that it can proceed to print the next layer.

The following video explains the operation of a large FDM printer that can print in two different colors, since it has two separate extruders:

c) Selective laser sintering (SLS).

It is the most used technique for metals 3D printing. It is important to make a difference between the printing of metals (tipically using SLS) and the printing of parts with a plastic and metal composite.

In the first case, a completely metallic part is obtained, while in the second case, the printed object will have a metallic appearance, but will be made up of plastic (normally PLA) with tiny metal chips. therefore, in the first case, the mechanical properties (hardness, resistance…) of the object will be those of a metal, while in the second case the properties will be the ones of the plastic. Printing all-metal parts requires a special technique, such as selective laser sintering which will be explained below, while printing with a plastic plus metal composite can be done on any FDM printer.

In fact, selective laser sintering is very similar to stereolithography.

The process is at follows:

  • A roller spreads a thin layer of very fine metallic powder over a container.
  • A laser beam strikes from the top, causing the metal particles to sinter. Sintering is a process that consists of melting just the surface of the metallic particles, in such a way that they remain linked with each other, without having completely melted. A moving mirror is in charge of directing the laser in the right direction (just like in STL).
  • The platform on which the printed part rests is then lowered to continue printing the next layer.

This process is repeated until the piece is completed, after which it is removed from the container.

The main differences compared to the stereolithography technique are: 1) the material used and 2) the power of the laser beam, which must be higher than that of the ultraviolet light beam necessary to cure the resin in the SLA technique.

In the following video you can see the whole process:

There is a variation of this process in which the metal powder is not just sintered but completely melted. This technique is called selective laser melting (SLM). The printing procedure is exactly the same as in the SLS technique.

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