Design rules & detail resolution for SLS 3D printing
The following tutorial is a detailed guide with charts, graphs made by EOS GMBH as an "Investigation of detail resolution on basic shapes and the development of design rules." The work is by and based on a dissertation by Dominik Sippel.
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Make sure your walls are above 0.7mm
One of the most common reasons Strong & Flexible parts get rejected is because the part include walls below 0.7mm. Parts below 0.7mm tend to be fragile, and more difficult to handle, more prone to breaking. To check your walls, you can download Netfabb Basic. Click on your part, then click on the ruler icon at top.

Prevent walls warping on larger model
Large models with thin walls may warp depending on the geometry of the part. Because this material is flexible, bigger parts with thin walls will drop due to gravity, so you will get best results if you scale the width of your walls so it is better aligned with the size of the longest dimension. For example, say that your part is x mm long (longest dimension). If x < 117mm, then the min wall should be 0.7mm to prevent bad warping. If x > 117mm, then for each mm above 117, you should add 0.006mm to the 0.7mm. In other words, try to maintain around a 1 to 166 ratio between your longest dimension and your minimum wall thickness.

Print lines or "Stepping"
Depending on the geometry of your model and orientation in the print tray, you might see print lines, or “stepping” phenomena on your model, which is a natural artifact of 3D printing using the SLS process. SLS printing works by printing layer by layer, and while our layers are around 0.12mm thin, there is a "step" between each layer, much like a staircase. Certain areas of your model may show this effect more than others, particularly on curved areas. This is also more obvious in smaller items such as model trains or items with gentle curves such as spheres. We make every effort to minimize the print lines in your model, but they are part of the process and cannot be completely eliminated. Please keep this in mind and refer to the images below for a visual guide of what these lines look like. Choosing Polished Strong & Flexible is an option to wear down the print lines somewhat, but you may also lose some of the finer detail in smaller models.

Thicker models are more difficult to bend
Flexibility depends on the structure and design of the model. The thicker you make something, the less flexible your product will be. A spring made with 1mm thick wires will be able to compress and elongate rather easily. A spring with 3mm thick wires will take a bit more effort, and won’t compress if you drop it from a meter high (about 3 feet).

Polished WSF will wear and dull some of the sharper details
The polishing media (little 5mm diameter cylindrical pellets) can take off up to 0.1mm of the surface. For small details, this can rub away the sharp corners of your details and make them less crisp.

Thicken your engraved/embossed text to make it easier to read.
Though 0.2mm detail will show up in this material, we find that text of 0.5mm will show up the most clearly. If you want crisper text, try bumping your detail thickness to 0.5mm.