3 Tips for 3D Printing with a ProJet 660 Printer

Do you have an interest in printing with full color sandstone on the ProJet 660 3D printer? If so, be sure to take note of these 3 tips before sending your model out for 3D printing as your print could look significantly better and be significantly stronger!
To be clear, there are some other guides out there on the internet that you can find regarding sandstone 3D printing and those are all terrific starting places, but they are not quite enough. You can find these at places like Shapeways and Sculpteo on their Full Color Sandstone material pages.
Vainglory, Skaarf, Tail, Eucl3D, color sandstone
Eucl3D has been perfecting the art of making the most beautiful color sandstone prints possible and we'd like to share what we have learned from it all. This is by no means a complete list, but we will devote many future blog posts to more specificity with each of these points and more, but let's dive in! 

1. Color

Color and geometry work together to create the final product - the two support each other. Having the best of both worlds is of course the perfect recipe to make an awesome-sauce print, but you don't need to be a master to make that happen.
Chunder, WFTO, Eucl3D, full color sandstone, 3D printing
In this section we'll cover the color side of things and after that we'll cover making great geometry.
The most important thing to understand about the ProJet 660 printers is that they use CMYK ink to produce the colors. This is different from the RGB colors used by your computer. The conversion from RGB to CMYK is unfortunately not quite 1-to-1, the resulting color is different, and that's the color that will be emblazoned on your model forever! Clearly something needs to change. 
The image below shows a very simplistic texture map being brightened with temperature, along with the golds and blues getting a color boost of saturation. Originally this would print as a fairly dark and dull model, but with just a few changes we've improved the printed result.
The best modifications to make are with color saturation and brightness:
Brightness: We work with video game models which often depend on the in-game lighting to make colors in the textures stand out - the best way to resolve dark textures is to either bump up the brightness or the temperature of the image. Temperature will give a more natural light glow rather than the white light of brightness alone. 
Saturation: In the RGB to CMYK conversion colors tend to be more dull and so it's important to ensure extra-vibrant colors are being fed into the ProJet 660. The new Windows 10 image editor has great features for making these adjustments - including a color boost option so you can affect only certain ranges of colors.
Unfortunately there is no set method for how far to take this adjustments, but you can always tell how much is enough once you look back at the originals.

2. Smoothing

One of the really great things about the ProJet 660 machines is that it can handle just about the best level of detail that a powder based printer can. You will often run into the printer showing the base mesh because the polygons/triangles are too coarse. 
With that in mind, any humanoid, animal, organic, or really anything not robotic will benefit immensely from taking the time to smooth the meshes out.
Armello, King, Eucl3D, 3D print, sandstone
This isn't always applicable of course, it depends on what you are trying to print, but knowing that anything coming out of the ProJet 660 is more or less made to just be looked at and enjoyed - feel free to make changes solely for the purpose of improving how it looks. 
For example, the turbosmooth modifier in 3DS or smooth mesh in Maya do pretty good jobs of this! Each mesh modeling program has their own command for this operation. Take a look at the King model in the image above from Armello - notice how his fur, clothing, breastplate, and face all look so clean and realistic. If we hadn't smoothed the mesh it wouldn't have looked nearly as nice and clean as it does now!

3. What is strong, looks good too

No matter what you are trying to print - you don't want it to break. This is priority number one when it comes to 3D printing!
The guidelines I mentioned above go as far as to tell you what the Projet 660 is capable of printing - BUT - not what is structurally capable. The sandstone material is strong, but brittle - this means that it is not bendable and if a feature is just too thin, it could fracture and fall off. 
Like before, there is no real right answer to this issue but we can still go a long way to remedy it. Short of running a full blown structural analysis like a mechanical engineer might when designing a part, we can still pretty easily deal with one of the biggest culprits - stress concentration factors. Take a look at the chart below.
stress concentration factors
What this chart is showing is that when you have an abrupt thickness change you can get a magnification of the potential that a break will occur. Think of the blade of a sword at the hilt or a gun barrel sticking out of the receiver - these are often thin objects already and then combine them with a sharp break point at the joint. 
The chart is describing that if you can blunt that abrupt change with a nice round, or a bevel, then the likelihood of failure can go down rather significantly! This is a pretty simple mesh editable solution to what is a serious issue. No doubt that sword and gun barrel will need to be thickened up above even the 3.5-4mm in addition to fixing the abrupt change.
Ultimately, one of the things that Eucl3D has come to discover is that when an art object is strong it is also often aesthetically pleasing. Think about those levitating street artists and how almost unnerving you feel with not knowing how they are "floating". Really it's coming from a discomfort in knowing that they or their cane shouldn't be that strong. Get rid of that discomfort in your prints! 

ProJet 660 printers have limits, but boy can they shine

At Eucl3D, our engineers are artists too, and we've developed engineering work around's to the ProJet 660's limitations to still produce the most artistic and beautiful color 3D prints on the market!
- Your Friendly Neighborhood Printability Engineer
Stressconcentration chart from: