How do 3-D printers work?


Additive manufacturing has extended past manufacturing and into prototyping, fashion and even medicine. So how do different 3-D printers work? Jake Port outlines seven 3-D printing devices and the pros and cons of each.


3-D printers: they can convert digital models into real-life objects and make it possible to build objects beyond most other constructive processes – and often in the comfort of your own home. But how do they work and what are the different types available?

At its core, a 3-D printer works on the basis of additive manufacturing. This is where material is deposited or manipulated to form a platform for another layer to be deposited above. The process repeats until the full model is constructed.

Construction often occurs on a stage – a flat platform that may be locked in place or move around, often on a horizontally plane.

As we will see, layers can be stuck together in different ways. Layer thickness is determined by the printing process and can be as fine as 0.003 millimetres.

Apart from rapid prototyping, 3-D printers have been used in the production of custom prosthetics, jewellery, film props and art. They are even finding their way in engine part production. General Electric, for instance, is incorporating 3-D printed parts in jet engines.

There are several different types of 3-D printer, each with its own advantages and disadvantages, suited to particular roles and applications. Let’s take a look.

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Fused deposition modelling

Advantages: Is inexpensive and widely used as the acrylonitrile butadiene styrene plastics are widely available and fairly strong.

Disadvantages: The model quality is determined largely by the nozzle radius. Speed and accuracy are low compared to other 3-D printing technologies.

Materials available: Polymers and plastics including nylon and acrylonitrile butadiene styrene (also known as ABS).

If you have a 3-D printer at home, it likely builds via fusion deposition modelling. Otherwise known as material extrusion, these printers use a heated nozzle to extrude material across a stage in the desired pattern.

In some circumstances, a support structure may be needed, but often printers first make a mesh structure on which they build.

Each layer cools and fuses to the layer below, but sometimes chemical agents such as epoxies are used to add strength.



Material jetting

Advantages: Can be very precise and multiple colours can be used at the same time.

Disadvantages: A supporting structure is needed and there's a limited range of materials. Post-processing using sodium hydroxide and water is often needed to remove excess material.

Materials available: Polymers and plastics such as acrylonitrile butadiene styrene and polypropylene.

Think of these as 2-D inkjet printers that have evolved.

Using an electrically charged deflecting plate and an electromagnetic field, a stream of drops can be precisely positioned on a platform. The next layer is deposited above and the process repeats until the final model is completed.

A variant of this known as "drop on demand" allows individual drops to be placed on a surface rather than a continuous stream.



Binder jetting

Advantages: Different colours can be used for the same print and is generally faster than other printers. Unused powder becomes the support structure and can be recycled with a wide variety of powder and binder combinations possible.

Disadvantages: A lot of post-processing can be needed to clean off excess powder. Wax may be added to increase structural strength.

Materials available: Stainless steel, ceramics and polymers such as acrylonitrile butadiene styrene.

If you ever made sandcastles at the beach, you would have noticed that mixing sand with water formed a stronger structure. In a simplified way, this is the principle behind 3-D printers that use binder jetting.

Powdered material such as ceramic or polymer is loaded into a container. A nozzle precisely applies a small amount of an adhesive binder in the shape of that particular layer before it is covered with powder and the process repeated.

The process can take place in a heated chamber to increase the build speed as it makes the materials more viscous and easier to apply.



Powder bed fusion

Advantages: Relatively inexpensive. The forming powder acts as the supporting structure. A wide variety of materials can be used.

Disadvantages: The laser needs large amounts of power, structure size is limited compared to other methods and it can be quite slow.

Materials available: Stainless steel, titanium, aluminium, nylon and more.

Unlike the processes discussed so far, powder bed fusion does not involve the deposition of a heated material or adhesive to form the print. Instead, these machines, of which there are a wide variety of subtypes, may use a high-power laser or electron beam to melt and bind powdered material.

Once a thin layer has been melted, powder is reapplied and the cycle starts again. This may need a vacuum and the powder is supplied by a hopper and reservoir.

The powder is spread evenly over the surface of the stage using a large blade or roller. The whole chamber is also heated close to the melting point of the powder to reduce the laser’s energy use.

This technology has been widely used in the production of custom-made prosthetics, jewellery and prototypes. It can be exceptionally precise but this is largely determined by the size of powdered grains. Small grains will yield a more precise structure while larger grains produce more imperfections.



Directed energy deposition

Advantages: Can build strong structures to a very high degree of precision.

Disadvantages: Post-processing may be needed depending on the material used. The range materials available is quite limited.

Materials available: Metals such as titanium and cobalt chrome.

Most 3-D printers construct an object in a chamber from scratch. Sometimes, though, a job may need a 3-D printer to build upon or repair an existing structure.

In these cases, directed energy deposition can be used.

A nozzle feeds a wire or powder towards its tip, where a powerful laser beam or plasma arc heats the powder to its melting point, binding it to the structure’s surface.

The nozzle or stage is mounted on a highly mobile arm rather than a horizontal plane, allowing it to move with a high degree of flexibility. Operators can increase the speed at the cost of precision, or slow it for more precise tasks.



Stereolithography

Advantages: Can produce large models quickly and with a high degree of precision.

Disadvantages: A supporting structure is needed along with substantial post-processing involving alcohol cleaning, support structure removal and ultraviolet curing. There are also few types of resin available.

Materials available: Resins and ultraviolet-curable photopolymers.

Stereolithography, also known as vat photopolymerisation, uses liquid polymer resins that harden under ultraviolet light.

The process begins with a stage sitting only thousandths of a millimetre below the surface. An ultraviolet laser beam is directed, with a mirror, across the surface of the resin, converting it to a solid on contact.

When the first layer is complete, the stage slightly lowers and the process begins again.

Once built, the whole model is raised with the stage, cleaned and cured under ultraviolet light.



Sheet lamination

Advantages: Multiple metals can be used for the same structure. Has the ability to build overhangs and can be quite inexpensive, needing generalised tools for post-processing.

Disadvantages: Post-processing may be needed and structures made with paper are not structurally strong. Excess metals may be difficult to remove.

Materials available: Paper, plastics, sheet metals and almost any other material that can be rolled.

Unlike any other process listed so far, sheet lamination can use regular A4 paper as its building material. A subtype, known as laminated object manufacturing, involves layering of paper that is stuck with glue and cut using a sharp knife – no heat or melting needed.

Each piece of paper is cut slightly differently and the process can produce highly detailed colour models.

Other printers may use another process known as ultrasonic additive manufacturing, which uses an ultrasonic welder to bond sheets or ribbons of metal.

Each metal layer is rolled onto the growing structure. Because the metal isn’t melted but bonded at specific points, these printers use significantly less energy than other types of 3-D printer.

The one major downside of ultrasonic additive manufacturing printers is they need cutting machines to separate and finish the model.

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