How much flexibility do you want - if only a little more flexible then try nylon - an amazing material. Or nGen from colorfabb. If you want it as flexible as a rubber band then go for ninjaflex. There are lots of flexibilities to choose from. The basic measurement of flexibility is called the "modulus" e.g. young's modulus or tensile modulus, etc. They all basically measure the same thing. High values are stiff materials. These values are published by the manufacturer.
Semi flex is possible to print with the UM2+ or even the UM2 with a modified feeder (lRobertI feeder with a guide works with no issues).
I made a small tutorial regarding this a while ago. It's for the Ultimaker 2 but can be applied to the 2+ also (just don't change the feeder on the 2+).
Regarding the type of filaments, i tried: semiflex, cheetah, Ngen Flexible,Ultimaker flex, Ice Flex, WillowFlex with no issues on several Ultimaker printers 2, 2+, 2 with bondtech...
I changed all my printers over to Bondtech extruders and have no problem with flexible materials. The current standard U2 and U2+ extruders have challenges with flexible materials. This is not the cheapest solution but it works.
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