Strong & Flexible plastic

What is the border between flexibility and strength? What minimal thickness can have wall to be strong?

It all depends on the material. Here are the guidelines for Strong & Flexible Plastic
https://www.shapeways.com/materials/strong-flexible-design-gu idelines

For example, is it possible to print pulley belt from Strong & Flexible plastic? What dimensions can it have?

It is most definitely possible to print a belt pulley in any material available here. Failure of the pulley will depend on several other factors: speed, power, size, misalignment, belt type, etc.

Asking whether a material is strong enough is a bit vague. For example...

"Are bricks a strong building material?"

"Yes, if you're building a house. No, if you're building the Eiffel Tower."

WSF is sintered nylon, as the material pages will tell you. Expect it to perform to a lower standard than solid (bar stock, sheet, or molded) nylon of the same variety. Part geometry will contribute significantly in determining if the part is strong or weak, pliable or brittle, soft or hard.

I need to print this from S&F plastic (chain and gears):


What are the smallest dimensions can it has?

I need gears with diameter about 1.5-2 cm and thickness about 0.4-0.5 cm and space between gears about 30 cm. What chain can be printed for this?

The info you need is all on the Materials page:

Min Wall Supported: 0.7mm
Min Wall Free: 0.7mm
Min Wire Supported: 0.8mm (Unpolished) · 0.9mm (Polished & Dyed)
Min Wire Free: 1.0mm
Max Bounding Box: 650x350x550mm (White) · 230x180x320mm (Black) · 150x150x150mm (Polished & Dyed)
Min Escape Hole: 4mm (1 hole) · 2mm (2 holes) · Big models require multiple holes
Interlocking or Enclosed Part?: Yes
Multiple Part per STL: Yes
Clearance: 0.5mm
Accuracy: ± 0.15mm, then ± 0.15% of longest axis

Your chain will have to obey all these, and the critical factor will probably be the Clearance. Also you might end up with problems with the limit on the number of triangles, depending on how you design and how many links you want.

Have you considered using a toothed belt instead? Some motor cycles use these, so they must work and the geometry is much simpler than the set up you propose. It might work in wsf (or might not, the belt would have to be thin and could not bend too tightly) or you could try the new material.

You should be able to get all your information from the guidelines. If it's meant to go at the speed of your animation, you shouldn't have a problem.

I read guidlines, but it didn't give me answer to a question "When S&F plastic will be strong and when will be flexible?"

I thought about toothed beld, but I don't know is it possible to print it with S&F plastic and what dimensions it must have to be flexible.

I need about 5X speed than in animation.

The thicker you make WSF, the less flexible it will be. It's comparable to polypropylene in it's strength and flexibility. You can push it and pull it, it wont break easily. It will bend though but will more than likely spring back into position with force removed.

What is the Min Escape Hole?

You mean the requirements or definition?
The requirements are Min Escape Hole: 4mm (1 hole) · 2mm (2 holes) · Big models require multiple holes
Escape holes are for hollow models, to remove support material. In the case of WSF, the support material is unsintered Nylon.