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SLA and Polyjet: What’s the Difference?

Because of all the constant developments, there's a lot about 3D Printing that can be confusing, such as when to use SLA over PolyJet. This should clear things up a bit.

Last month, a new company called Rize Inc. came out with a new 3D printing process that leverages patented new materials that essentially eliminate post processing. It’s bad enough the industry has as many names as the Internet of Things. (I vote for calling it Rapid Additive 3D Printing for Manufacturing Prototypes and call it a day) But now there’s more materials to research?

There’s ABS, ABS plus, Digital ABS, PLA, PPSF, ULTEM, SLA, Ceramics, Titanium, Kryptonite, Adamantium, and even chocolate. (Only two of these are fictitious). It’s an exciting time where there’s a printer and plastic for just about any design you can imagine, but this modern embarrassment of material riches can get a little overwhelming at times.

We want to help you build your knowledge base layer by layer, like a 3D Printer would, so let’s start with some of the basics.

You’re probably aware that, along with materials, there are several different methods of 3D printing. There’s Fused Deposition Modeling, better known as FDM, Direct Metal Laser Sintering (DMLS), PolyJet and Stereolithography, or SLA.

FDM works by extruding a melted thermoplastic filament via a head onto the build area in thin layers.

In a DMLS process, a bed of powdered metal is already in the build area, and a Ytterbium fibre laser micro-welds each layer.

Those two would be difficult to confuse, but SLA, developed by 3D Systems, and PolyJet, created by Objet, now part of Stratasys, are much more similar in approach and application. In our sister publication Industry Week, a Stratasys engineer, Andrew Graves explains the nuances of the related, yet distinct options:

On the surface, it appears that stereolithography and PolyJet are identical twins. They both use UV energy to cure liquid photopolymer, after all. But a closer look reveals they’re more fraternal than identical.

Despite sharing a similar printing foundation, they use different print methods to achieve a fully cured product. The divergent build styles mean these methods are not always suitable for the same applications, making it important to understand the differences between the two processes. Chief among those are resolution and size, manufacturing speed, materials and applications.

Here’s some key features of each to help you decide which is best for your application:


How it works: PolyJet works like an inkjet printer, squirting photopolymer droplets mixed with water-soluble support material, onto the build platform. While a 2D printer ejects its build area, a piece of paper, and starts again, the PolyJet cartridge is tailed by UV light to cure the layer.

Quality: The photopolymer is applied, again and again in .00063 to .00118-inch layers. Graves writes that “PolyJet prints in the finest layer resolution of any 3D print technology.”

Speed and Size: The finer resolution means the process could take a while, and is the reason why Graves recommends 5-to 6 inches cubed as the ideal design size for PolyJet.

A multi-material model created by the J750 Printer.

Materials: Because the jetted photopolymers can be jetted from different cartridges, a PolyJet model can comprise several different materials. One model can have separate parts that are rigid and transparent, while other are flexible and opaque. The Stratasys J750 3D Printer can use six materials at once, drawing from a pallet of more than 360,000 colors.


Image: Stratasys

How it works: SLA was invented by the “Father of 3D Printing” Chuck Hull in the early 1980s and later co-founded 3D System. The company explains its process “utilizes a vat of liquid photopolymer resin cured by ultraviolet laser to solidify the pattern layer by layer to create or ‘print’ a solid 3D model.

Once the model is complete, the platform rises out of the vat and the excess resin is drained. The model is then removed from the platform, washed of excess resin, and then placed in a UV oven for a final curing.”

Quality: The high is .002 to .004 in., and the standard is .005 to .006 in. 3D Systems says SLA “offers the most accurate type of fit/form prototype for the verification of any design before committing to your chosen production route.”

3D Systems' SLA material, Accura ABS Black

Speed and Size: Because SLA directs UV light to cure the parts via a rapidly moving mirror, it can end up building larger parts faster than PolyJet. The maximum size for either is 25x25x21 in. This could create a saving of up to 70% versus PolyJet, notes Graves, who manages Stratasys’ Stereolithography systems and operations.

Materials: SLA materials can mimic the properties of materials such as ABS, polycarbonate and polyprophylene, but cannot match the J750 in range in of materials or colors. For larger functional prototypes, the process does excel, and can economically create investment casting patterns, which don’t require tooling as lost wax patterns do.

An intricate model might be good for specific, custom model to be used by heart surgeon, for example, but SLA would most likely be the better option for constructing a functional prototype.

Here’s Graves’ conclusion:

Although they share many similarities, stereolithography and PolyJet should not be used interchangeably. When it comes to choosing between the two, a project’s application should be the deciding factor for any user. Force-fitting one process for all applications will likely leave your team with more hand labor — in other words, longer build times and higher costs. It could also mean a part that doesn’t meet the performance or aesthetic expectations.

TAGS: 3D Printing
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