3D printing: from aeroplane parts to organ transplants - Part 1

How does it work?

Creating three dimensional objects using printing is a relatively new technology, but one that has already provided value in a wide range of markets. 3D printing is one of several technologies encompassed in ‘Additive Manufacturing’, or AM. This term describes the creation of an object by the deposition of materials in a layer by layer fashion. A good example of this process at work in nature is in the insides of caves, where natural structures (speleothems) are formed by the gradual deposition of minerals over periods of thousands of years. Fortunately for manufacturers, 3D printing is computer assisted, follows a precise set of instructions and carries out runs quickly (single prototype units can usually be produced the same day).

The advent of 3D printing

The first use of 3D printing technology was in the field of rapid prototyping during the late 1980’s. The fast creation of prototypes allows for greater inspection and testing, meaning more efficient production for manufacturers. Tool-path calculation problems were associated with established numerically controlled cutting machines; 3D printing completely eliminated this issue. Since parts are built layer by layer, the depositing machine always has unrestricted access to the object’s surface – this makes writing the computer program for the object’s construction very straightforward.

Competing technologies

The most prevalent technology for rapid prototyping is stereolithography. Typical stereolithographic apparatus includes a clear tank filled with liquid polymer, a vertically mobile perforated platform which moves up and down with the printing process, an ultraviolet laser and a computer which controls the procedure. The photopolymer hardens under exposure to ultraviolet light. The process begins with the creation of a 3D design in the CAD program. The software then digitally divides the model into a series of layers (each usually between 5 and 10 milimeters), giving the laser a sequence for follow to photo-polymerize each layer one by one. As each layer is complete, the platform drops incrementally and “paints” the next until the full model is complete. Multiple objects can be printed at once provided they do not exceed the width and height restrictions of the machine. Sometimes internal bracing is required to support the objects during printing – this can be removed post-completion.

Inkjet technology is utilised in several 3D printing methods.  Direct printing works similarly to a 2D inkjet printer; the main difference being the print head or the surface being printed move vertically as well as horizontally. The nozzles dispense waxes and polymers to construct an object layer by layer. Binder printing also uses inkjet technology. However, two different materials are deposited to create each layer. First a thin layer of powder is spread across the print surface. Next, a binder is inkjet printed on the powder, bonding with it and creating a solid layer of the object. Each 2D layer is stacked on top of one another until the object is complete. Excess powder is then removed and the encased object is retrieved.

Other commonly used methods of additive manufacturing include: selective laser sintering, electron beam melting, electron beam freeform fabrication and fused deposition modelling. 3D printing can be used for final products as well as rapid prototyping; see part two for examples.

Tim Phillips, Catenary Solutions