As noted in earlier posts in the series, FDM (fused depostion modeling) printing is by far the most common inexpensive method of 3D printing. In this post, we’ll look at alternatives.
We are starting to see stereolithography (SLA) printing move downmarket into the affordable-for-libraries zone. I’m aware of a couple of libraries that have already purchased stereolithography printers.
SLA involves a light-sensitive resin and lasers. Liquid resin is contained in the body of the printer, with a build plate that moves up and down inside the resin. The resin solidifies when exposed to a specific wavelength of light, usually in the UV spectrum, and the printer has a laser or lasers tuned to that specific wavelength. The build plate starts near the top of the resin, and the lasers sweep across, solidifying the resin in the appropriate areas. The build plate then lowers, and the lasers repeat their sweep, building layer after layer, one after the other as the object is built. You can also have this process occur upside down, as in the FormLabs Form 1 printer, where the build plate is actually above the resin, and as layers are added it pulls the completed layer out of the resin.
SLA printing has several advantages over FDM. Because the print is always encased in liquid resin during the process, it is much more forgiving as to geometry of design. Not completely-- there still has to be some connection to the base layer. You couldn’t print a “floating” horizontal piece, for instance. But in general, the resin provides substantially more support for designs than those available from FDM printers. The other major advantage is that the detail level is limited by the crystallization of the liquid and the size of the lasers, which means that you can have very fine details in an SLA print. It’s possible to achieve .025mm (25 microns) layer heights with SLA prints.
Stereolithography printing has its limitations. The first is that the resin is only available in a very limited number of colors, generally a clear or translucent material and white. When compared to the rainbow of colors available for FDM printing with ABS or PLA, it feels limiting. The second, and far more worrisome, is that most vendors of this type of printer manufacture their own resin. The printers are designed to tune the wavelength of the lasers to the specific resin they sell, thus making it very difficult for anyone to compete with them on consumables for the printer. This would be the equivalent of buying a printer from HP, and having to then buy paper and toner from them as well in order to use the printer.
Small SLA printers are just beginning to hit the market, available in the $2,500-3,500 range. The consumable for printing, the photosensitive resin, is more expensive than filament for FDM printing as well. The most popular of consumer-grade SLA printers, the FormLabs Form 1, has resin that sells for $149 per liter.
Selective Laser Sintering
Simultaneously the most flexible and the most expensive type of 3D printing commonly used, selective laser sintering (SLS) printing is similar to stereolithography in that it uses lasers to solidify a loose substrate. In SLS printing, the printing substrate is a powder, and the printers use high-energy lasers, rather than UV. The high-energy lasers selectively fuse sections of a powder together, a new layer of powder is deposited on top of the sintered layer as the entire print bed drops, and the lasers do another pass, fusing the single-layer of powder to the already solid layer below. Thus prints are completed layer by layer, exactly as in the other printing technologies that we covered, except the end product is a solid object that’s been drawn by lasers, encased in all of the powder that wasn’t fused.
This method provides total support for the print in question, so nearly any imaginable geometry can be printed using SLS printing. It is also possible to use any material for SLS that is capable of being powdered and fused with heat, including thermoplastics, covered in my previous post, as well as steel, aluminum, titanium, and other metals and alloys. Prints produced in this way are very nearly as strong as solid-cast parts, which means that it’s possible to 3D print mechanical parts that are directly usable in engineering projects via SLS printing.
Layer height and resolution in SLS printing is completely determined by the resolution of the powder being fused, but is typically on par with SLA printing, averaging around .1mm layer heights. Another similar technology is electron beam melting (EBM) that uses high energy electron beams to melt powdered metals in order to produce 3D objects. The use of electron beams allows for even higher precision than lasers, allowing for up to .05mm layer heights, which is nearly unheard of by any other method.
Laminated Object Manufacturing
The last specific type of 3D printing that I’ll describe is, in my opinion, particularly clever. Laminated Object Manufacturing takes thin materials like paper or plastic sheets, cuts them to a specific shape, and then uses adhesive to glue one layer to the next. The most well known of these types of printers is manufactured by a company called MCor Technologies. Their printer uses normal, ordinary copy paper as its substrate, cutting one sheet at a time into the appropriate shape for the given layer, and then using paper glue to laminate the individual layers together. The high-end model of the MCor printer includes a full-color inkjet printhead inside, to allow for full color 3D prints to be created from very inexpensive raw materials; literally paper, ink, and glue.
Other 3D Printing Types
Numerous other 3D printing technologies are available, many that are patented and limited to a single company. For example, 3D Systems uses a type of 3D printing methodology that they call Color-Jet Printing (CJP) that uses two different materials that are combined using a sort of high-end inkjet printer in order to create the solid end-product. This patented process allows them to print in materials like food-grade ceramic. 3D Systems also makes a 3D printer that is capable of printing in sugar, called the ChefJet, and the high-end model, the ChefJet Pro can print edible 3D models in full color.