What is 3D Printing?

Generally speaking, there are two different manufacturing strategies to make parts, namely subtractive and additive manufacturing. Both of them have roots in history. Making wooden sculptures, as a subtractive manufacturing technique, dates back to 11000 years ago. Building walls, as an additive manufacturing technique, has its root in 12000 years ago. So, both of the strategies have been well known for quite a long time. But it was only by the advent of the industrial revolution and the availability of industrial machining equipment that the large-scale production of objects evolved. These techniques were based on casting or so-called subtractive manufacturing methods. 

The first industrial additive manufacturing technique of the modern era was invented in 1984 by Charles Hall, based on a technique called stereolithography. This technique used a laser beam to cure a photo-active photoresin and solidify the resin from a liquid state, adding the solid materials layer-by-layer to build a 3-dimensional part.

Light-induced additive manufacturing techniques have a root in UV-curing inks and varnishes used in the graphic arts and printing industry. So, additive manufacturing was called 3-dimensional printing in contrast to traditional printing which forms a 2-dimensional film when observed at a macro scale. 

Stereolithography or SLA belongs to a range of 3D printing techniques known as "Vat Polymerization". SLA printers use a galvano scanner to guide the laser beam to predetermined positions. The laser beam travels on the surface of the resin or its interface with any suitable material such as PDMS and cures the photoresin. After the travel reaches its endpoint, a new layer of liquid resin covers the solidified area and the exposure process starts again. The laser source may project the resin surface or interface from the top or the bottom. Therefore, top-down and bottom-up SLA printers exist. They generally use a 395 nm laser.

The other member of the vat polymerization technique is Digital Light Processing or DLP which relies on the possibilities of DMD chips developed by Texas Instruments.  

In this manufacturing method, a DLP projector is used to illuminate the surface of the photoresin or its interface with any suitable material such as PDMS or FEP. Since projectors can illuminate the whole surface at once, this technique can cure the whole layer by a single projection, and therefore it has a higher printing speed compared to SLA. DLP printers may also be made based on top-down or bottom-up projection concepts. The heart of the DLP printer is its "Light Engine". The light engine generates the light and guides it to the photoresin interface. Light engines may be very simple parts or quite complicated structures. 

A recently developed technique, which is similar to DLP, is called LCD or mSLA technique. In LCD printers, LED sources are used as the light source and an LCD is used to form the mask needed for the selective resin projection. These printers are of the bottom-up type and use an FEP film as the window at the bottom of their vats. Generally, these printers are quite cheaper compared to DLP and SLA 3D printers. Based on the inherent characteristics of LCD panels and LED light sources, they may have a wide range of plate dimensions, from very small to very big.