Idiot's Guides: 3D Printing (2015)
What Is 3D Printing?
Types of 3D Printers
In This Chapter
· Stereolithography and its subset, digital light processing
· Creating colorful models with powder bed printers
· Printing strong parts with MultiJet printers
· Melting particles with selective laser sintering
· The reason for fused filament fabrication’s popularity
So far in this book, I’ve talked about 3D printing in generic terms that apply to most 3D printers. But in practice, not all 3D printers work in the same way. To move forward with this book, I need to pick a particular 3D printing process that I can dig deeper into.
There are many kinds of 3D printing technologies on the market today, and each of them operates differently. Most of these processes are used specifically for a certain application or a particular material. I don’t have the space to cover every process here, so instead I go over the most common types of 3D printing in use today, plus talk about why fused filament fabrication (FFF) printing is the go-to type for many hobbyists.
If you recall from Chapter 2, this is the original type of 3D printing process developed by Charles W. Hull and his company 3D Systems in the ’80s. Stereolithography (SLA) works by shining a UV laser onto a vat of UV-curable photopolymer resin. The laser is focused on the resin to produce the 2D cross-section of each layer on a build platform, and then the platform is slightly lowered in order to form the next layer.
SLA, aside from having the proven history of being the first 3D printing process, has some key advantages which make it a popular choice even today. An SLA printer can print parts with relatively smooth surface finishes compared to other types of 3D printing, with almost completely unnoticeable differences between layers. This quality is possible because of the high precision of the UV laser, which also allows it to create parts within very tight tolerances.
The quality and precision of SLA printers makes them ideal in professional research and development situations, where prototype parts need to very closely resemble their mass-produced counterparts. The surface finish also has a high-enough quality that SLA printed parts can be used as masters for creating molds.
However, the quality of an SLA printer comes at a high price. High-end consumer model SLA printers have started entering the market recently, but they’ve mostly been used only by businesses for creating prototypes. The cheapest SLA printers start in the thousands of dollars and can be more than a hundred thousand dollars for professional models.
Aside from the cost of the printer itself, expensive resin is also needed as the material to create parts. This resin can cost anywhere from $80 to $200 per liter, and is much more expensive than the plastic filament used in most consumer 3D printers. The way SLA printers create parts in a vat also means that, for conventional designs at least, hollow parts will be filled with resin. This can add to the cost of the material and to the weight of the part.
In some cases, the printed part requires additional curing as well. This is dependent on the printer and resin being used, but is still a factor. Curing the part generally requires a separate device and additional time.
Digital Light Processing
Digital light processing (DLP) 3D printing is very similar to SLA printing; in fact, it’s considered a subset of SLA printing. DLP printing works almost exactly like SLA printing, with a vat of photopolymer resin that is hardened layer by layer.
The primary difference between DLP printing and traditional SLA printing is the method used to shine light onto the resin. If you’re familiar with video projector systems, you may have already heard of DLP in the context of movie projects. The technology is used to project light (in the form of images) onto a screen.
This same technology can also be used to cure the layers of resin in a DLP 3D printer. Instead of projecting a video, a DLP printer projects light in the shape of each cross-section on the resin. This allows a DLP 3D printer to cure an entire layer at once, a big advantage over the single-point laser used in conventional SLA printers.
DLP 3D printers are capable of curing an entire layer of resin in just a few seconds. This speed allows them to print very quickly compared to other types of printers.
By curing an entire layer at once, DLP printers dramatically reduce the time it takes to print a part. Additionally, the size, shape, and complexity of each layer have no effect on the time it takes a print. Aside from the obvious advantage of being able to print large and complex parts, this also means that many duplicate parts can all be printed at once without adding any print time compared to a single part.
However, DLP 3D printers tend to have the same downside as standard SLA printers: cost. While the price of resin is usually about the same as SLA resin, the printers themselves are more expensive. DLP projectors are still complex and expensive pieces of technology, and adding them to a 3D printer is not a cheap proposition.
Powder Bed Printing
A handful of specific printing processes could all be considered powder bed printing. All of these types of printers have one thing in common: a powder material is spread on a bed, and a liquid binder is used to solidify it in layers. The differences come from the following:
· The type of powder material used
· The type of binder used
· The method used to inject the binder into the powder
The types of powder and binder used can vary based on need, and have evolved over the years since power bed printing was invented. It can be anything from simple plaster powder that is solidified by injecting water (like the original binder jetting process developed at MIT and sold by Z Corp.) to advanced resin or epoxy combinations that replicate common engineering materials.
One of the most common and popular of these powder bed printers on the market today is the inkjet type. They use a print head similar to what is used in common household inkjet printers, but instead of printing ink, they print a binder onto the powder.
There are two primary advantages that powder bed inkjet printers have over other types of printers: support material isn’t necessary, and they have the capability of printing multicolor parts. They don’t need additional support material because the powder itself can support overhangs while the part is being printed.
The leftover powder can be reused, so not much material is wasted. However, cleaning the powder off the part isn’t a small job. It can be a time-consuming task, sometimes made even more difficult by the fragility of the part produced by some printers (and depending on the material). Removing the powder without damaging the part in those cases takes finesse and care, but the other major benefits can make that inconvenience worthwhile.
By using multiple inkjets in the print head, each with a different-color binder, parts can be printed in multiple colors. Colors can even be combined, similarly to inkjet printing on paper, to create a wide spectrum of colors.
The ability to handle overhangs with ease and to print colorful parts has made powder bed inkjet printers very popular for creating architectural models. Usage for architectural models is also a result of the relative ease of building large powder bed printers, which aren’t limited to small sizes like other technologies. This allows architecture firms to print large full-color models of their building designs for presentation purposes.
Most powder bed printers have a major downside though, which is that prints tend to be brittle and weak (though this problem has largely been solved with newer materials and processes). This often necessitates the use of additional postprint treatments just to make the part suitable for handling. The powder bed also introduces the same problem that is common with printers that use vats of resin: you can’t print a hollow part without a way of removing the unused material inside before it’s completely sealed.
MultiJet Printing (MJP) , also called PolyJet Printing, is another 3D printer technology that uses an inkjet-style print head and can print in multiple colors. But unlike powder bed printers, MJP printers don’t use powder beds; instead, they print UV-curable resin directly onto the print bed. MJP printers add a UV light to the print head, which instantly cures the photopolymer resin as it’s deposited.
MultiJet Printing and PolyJet Printing are examples of different companies using similar technologies under different names. MultiJet Printing is used by 3D Systems, while Stratasys uses PolyJet Printing. Both work in approximately the same way but use different names for trademark, patent, or marketing reasons. When looking at 3D printers, keep in mind that the same type of printer can go by multiple names.
Because they use resin instead of powder, MJP and PolyJet printers can produce strong and usable parts. This gives them a distinct advantage over powder bed printers, which produce relatively weak parts.
MJP printers handle overhangs by printing a gel support material where necessary. The support material can be easily washed away without damaging the part, making the support removal process easier and faster than more traditional approaches.
The disadvantages of MJP and PolyJet printers are the cost of the printer itself and the cost of the resin. The resin cost is on par with the resin used in other types of printers. However, to print in multiple colors or materials, different types of resin are required. In the long term, this shouldn’t add any significant cost, because the total amount of resin being used should be roughly the same. But in the short term, it can be a large upfront cost to get outfitted with a range of resin types.
The printers themselves are priced similarly to other professional 3D printers, which of course are still too expensive for most consumers. Being a new technology, MJP printers haven’t yet matured enough to reach more affordable prices that would allow them to be purchased for home use.
Selective Laser Sintering
Selective laser sintering (SLS), direct metal laser sintering (DMLS), and selective laser melting (SLM) all work in basically the same way. They use high-power lasers to actually “sinter” or melt powdered particles together into a solid mass.
Like power bed inkjet printers, they use a container that is filled with powder layer after layer. After a new layer of powder is added, the laser heats the part cross-section and then moves onto the next layer. As usual, the process is repeated until the part is finished.
However, unlike other powder bed printing methods, SLS printing doesn’t require any kind of binder. That means any material that can be powdered and melted or sintered with a laser can be used, such as various metals, ceramics, plastics, and even green sand.
Green sand is a specially formulated material used for sand casting. Sand casting is used to produce metal parts by pouring molten metal into a mold made of sand. The mold is normally produced by forming it around a positive master part; however, 3D printing the mold removes the need to first create a master part.
These are the only commonly used 3D printers capable of printing metal parts, which gives them an obvious advantage. In some cases, the printed parts can even be as strong as an identical part manufactured by traditional means. This makes SLS-created parts particularly unique and especially useful.
Unfortunately, SLS, DMLS, and SLM 3D printers are all very expensive at this point, to the point that they’re mostly only used in a handful of industries that require the specific capabilities they offer. In most cases, a capable multiaxis CNC mill is more economical than an SLS printer, so companies that purchase them don’t usually do so without a strong necessity.
Fused Filament Fabrication
All of the technologies I’ve talked about so far in this chapter have been too expensive or too specialized for consumer use. So what process are you likely to use at home? That’s where fused filament fabrication (FFF) comes in. This process takes a string of thermoplastic (called filament), melts it, and deposits it on to the print bed.
FFF printers are highly economical, making them popular for consumer use. The affordability of the printers themselves is due primarily to the low cost of the individual parts which make up the printer and the fact that they don’t need to be precisely built to function well. Most of the other printer types I’ve discussed are difficult to build and have to be finely tuned to produce good results.
On the other hand, FFF printers can be built in a garage with common hand tools and still print well. Mechanically speaking, they’re also relatively simple and easy to understand. No complex laser or optical systems are needed, because they operate on mechanical principles.
If you’ve ever used a hot glue gun, you can probably understand how an FFF 3D printer works. The filament is comparable to the glue stick, the hot end is similar to the heated nozzle of the glue gun, and the extruder works like the trigger system that pushes the glue stick into the hot nozzle (although extruders work with continuous rotation).
The FFF 3D printer process, then, is like using a hot glue gun to draw a square, and then another square on top of that one, and then another square on that one, and so on. The 3D printer is doing it much more precisely than you could by hand, of course, and it’s doing it with plastic, but the general idea is the same.
While the affordability of FFF 3D printers makes them ideal for consumer use, the technology is used for professional 3D printers as well. Professional versions work in exactly the same way as consumer models, just with greater precision and sometimes with additional features added.
The cost advantage doesn’t stop at the printer itself either. The filament used in FFF printers is the most inexpensive material available for any 3D printer out there. This is partially due to the low-tech nature of the filament, but it’s also a result of the competition in the consumer market driving prices down. The popularity of FFF printers has also ensured rapid development of both the printers themselves and the filament used with them.
While FFF printers are popular mostly because they’re affordable and easy to understand, unfortunately, they probably yield the lowest-quality prints of any of the 3D printer types discussed in this chapter. The filament extrusion method of layering plastic is inherently imprecise. And, while a great deal of effort is being put into improving the quality, FFF printers still lag behind the others.
While the quality is lower than other printers, FFF printers can print a range of thermoplastics. Many of those thermoplastics are popular for engineering work because of their superior mechanical properties. Parts printed with materials like ABS and nylon are very strong, and are usable as soon as they’re finished printing.
Another downside is the time it takes an FFF printer to create a part. Because of the weight being moved around, FFF printers can only accelerate so fast. This makes small features slower to print. Because the plastic is melted as it’s being deposited, it’s also necessary for it to cool before printing another layer on top of it. Most of the time, it’s already cool by the time the next layer is printed. But if the layers are very small, an FFF 3D printer will either need to pause between layers or risk deforming the part due to the heat.
That heat also provides the underlying cause of the single biggest disadvantage of FFF printers: warping. Because thermoplastics expand and contract as they’re heated and cooled, the part actually changes shape slightly as it’s being printed. If some layers cool before the others, this will result in the part warping or even cracking.
An FFF-printed part showing warping and cracking.
How extreme this warping is depends on the particular thermoplastic being used. Acrylonitrile butadiene styrene (ABS), for example, is especially prone to warping and cracking. Other materials minimize warping, but none of them seem to be able to avoid it entirely.
Printer manufacturers, in an effort to avoid warping and cracking issues, have introduced a number of ways to try and control it—heated beds, enclosures, and heated print chambers. These methods are mostly related to managing the temperature of the part to try and keep it consistent throughout the print. These work fairly well and almost completely eliminate the problem in some materials. Right now, however, it still can’t be avoided entirely.
Some of the methods of reducing warping, such as heated build chambers, are patented technology. This means that small 3D printer manufacturers can’t use those methods without making a deal with the patent owner. This is part of the reason that some of the features seen on professional 3D printers aren’t available on consumer printers.
Though warping problems and relatively low print quality are certainly considerable disadvantages, there is no denying the low cost of FFF 3D printers. They are by far the most popular type of 3D printer on the consumer market, and virtually all printers in use by hobbyist and home users today are FDM/FFF 3D printers. Therefore, it’s extremely likely that this will be the type of printer you’ll be purchasing.
For that reason, throughout the rest of the book, I’ll be talking about 3D printing in the context of FFF 3D printing. Many of the principles apply to other types of 3D printing as well, but the FFF process is what I’ll be focusing on.
The Least You Need to Know
· There are many types of 3D printers on the market, each with their own advantages and disadvantages.
· While SLA, DLP, powder bed, MJP, and SLS printers print high-quality parts, their cost keeps many hobbyists from buying them.
· FFF 3D printers are the most popular for consumer use because of their affordability.