3D Printers are machines that produce physical 3D models from digital data by printing layer by layer. It can make
physical models of objects either designed with a CAD program or scanned with a 3D Scanner. It is used in a variety
of industries including jewellery, footwear, industrial design, architecture, engineering and construction, automotive, aerospace, dental and medical industries, education and consumer products.
3D printing is a strong developing technology which is considered to be a very important method for making products in the future. It has certain advantages to conventional production techniques en makes it possible to make product that couldn’t be built in other ways.
A promising method is 3D DLP printing. This is one of the vat polymerisation SLA techniques. It works by
projecting images to a resin in layers, partly masked. This technology uses a light source that projects light on a Digital Micro mirror Device, which reflects the light with millions of microscopic mirrors that together form an
image. On the spot where the image projects, a photo polymerisation process is initialised and thus cures. By moving a build platform, a new layer can be cured and a 3D product appears. Big advantages are the possibility to cure
fast and with high accuracy, with a clean appearing surface finish. One of the disadvantages is that they are limited to a single material. This is why it is not possible to print a product that has soluble support material.
DLP technology was invented in 1987 by Larry Hornbeck of Texas Instrument and became extremely popular in projectors. DLP uses a computer-controlled, micro-mirror grid, laid out on a semiconductor chip. These tiny mirrors tilt back and forth. When a mirror is tilted, it reflects light, creating a bright pixel. When the mirror is tilted the other way, the pixel is dark. The technology is used in movie projectors, cell phones, and also for 3D printing. One of the benefits for 3D printing is its speed: You can print layers in an instant with this type of 3D printer.
Hideo Kodama of Nagoya Municipal Industrial Research Institute made the first published account of a 3D printed solid model in 1981 using photopolymer technology.
DLP (Digital Light Processing) is a similar process to stereolithography in that it is a 3D printing process that works with photopolymers. The major difference is the light source. DLP uses a more conventional light source, such as an arc lamp with a liquid crystal display panel, which is applied to the entire surface of the vat of photopolymer resin in a single pass, generally making it faster than SL. Also like SL, DLP produces highly accurate parts with excellent resolution, but its similarities also include the same requirements for support structures and post-curing. However, one advantage of DLP over SL is that only a shallow vat of resin is required to facilitate the process, which generally results in less waste and lower running costs.
In this process, once the 3D model is sent to the printer, a vat of liquid polymer is exposed to light from a DLP projector under safelight conditions. The DLP projector displays the image of the 3D model onto the liquid polymer. The exposed liquid polymer hardens and the build plate moves down and the liquid polymer is once more exposed to light. The process is repeated until the 3D model is complete and the vat is drained of liquid, revealing the solidified model. DLP 3D printing is faster and can print objects with a higher resolution.
Because of the nature of the SL process, it requires support structures for some parts, specifically those with overhangs or undercuts. These structures need to be manually removed. In terms of other post processing steps, many objects 3D printed using SL need to be cleaned and cured. Curing involves subjecting the part to intense light in an oven-like machine to fully harden the resin.
Bottom-up or top-down printing
In DLP printing there are basically two construction types. Most often they are referenced with bottom-up or top-down. Both techniques have their own benefits, but also their own problems. Since a system with changeable resin vats needs to be constructed, the only suitable type will be bottom-up.
In top-down DLP printing, the object that is printed is on top op the build platform and is moved downward during printing. The depth of the vat limits the maximum height of the printable object since the object sinks deeper and
deeper. The printer cures the top layer of the fluid. While for printing an object only a small amount of resin might be needed, the vat needs to be filled completely with resin. Since the resin is not stored in perfect conditions this way, degeneration of the resin will occur. Top- down printers have one big benefit compared to bottom up printers: They are easier to build and they don’t need a mechanism to break the vacuum between the resin vat and the object. While the platform moves deeper into the fluid, it changes the fluid level of the resin. Making the support of the build-platform slim and the area of the surface relative big can solve this problem.
In bottom-up DLP printing, the printed object hangs upside-down on the platform. The object is pulled out of the resin. The biggest advantage is that only a small layer of resin needs to be in the vat. The biggest disadvantage is
that the cured layer is directly placed on the bottom of the resin vat. The resin vat needs to be covered with an optic-transparent anti-stick layer and the vat needs to be made of a material that is also transparent to the wavelengths where the resin is sensible to. Since the layer is cured directly to the bottom of the vat, a vacuum between the wall and the bottom of the product is formed. A force is needed to pull the product from the bottom of the vat. The vacuum can be broken by using by peeling, or by moving the object to a deeper place in the vat. This makes the design of a bottom-up printer more complex compared to a top-down printer.
The biggest advantage of DLP over other techniques is that a whole layer is cured at once. This reduces the positioning errors in the XY-plane due limited mechanical movement. The mechanical system is less complex then other techniques, since it only consist of a single axis of movement. The resolution of 3D printed parts using DLP printing is considered to be very high and the surface finish is smooth. This makes DLP printing suitable for the production of masters for injection moulding. Since in DLP printing the minimal diameter of vertical oriented objects is smaller, the supports can be a lot thinner then in FDM.
This article builds on the following works :