I'm not familiar with formfutura flexifil but ninja flex is very flexible. Similar to the material in a rubber band. They also have a "cheetah" filament that is much less flexible.
Somewhere hopefully these companies publish the shore hardness and/or the modulus (it's the same modulus - modulus of elasticity, youngs modulus, tensile modulus, etc.).
Higher numbers for hardness or modulus are stiffer. Lower numbers are stretchier/bendier.
I tryed this one too. I also tryed Layfomm and gellay. Those one were very soft but layer adhesion is not that good.
In fact, there are flexible only when wet. There is some PVA and another plastic in those filament. When you print it, it is not soft at all. After that, you have to rinse it to remove the pva and it become soft. The layer adhesion is very week when it is wet and if you make a closed volume with infill, it is hard to remove the water inside. The pression can break the object while you deform it (if water inside) and if you tryed to extended on the z axe (because of layer).
When it dry, it is no more soft and you have to put it in water again for hours (like a sponge).
For my project, I really needed soft material (with a shore hardness near 00-25). I print molds in PLA or ABS and cast soft silicon in it. It works very well.
I think it depends on what is your project. The water things is really hard to use in a project I think :/
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