Flexible 3D printing filaments are 3D printing's answer to rubber. The flexibility, elasticity and durability of vulcanised rubbers are extremely desirable properties for a wide range of applications. Unfortunately, rubbers are not usable for 3D printing as they are thermoset plastics, meaning they can only be melted once. 3D printing requires materials to be thermoplastic so that they can be repeatedly reheated and melted, without just burning. This is where flexible 3D printing filaments enter.
Flexible filaments combine rubber like flexibility with the printability of a thermoplastic. Flexible filaments can take many forms, whether they are are copolymers of plastics and rubbers like TPE and TPU, PLA that has been chemically soften or plastic polymers that are inherently flexible, like Nylon. Each type of flexible filaments has distinct advantages and disadvantages that will depends on the material and how it has been processed.
There are some common advantages to printing with most flexible filaments. The first is the ability to withstand mechanical stress and deformation due to the filaments flexibility. Flexibility also gives the filament excellent impact resistance and the ability to dampen vibrations. There are also some disadvantages to printing with flexible filaments though.
One of the biggest disadvantages of printing with flexible filaments is that their flexible nature is inherently unsuitable for being pushed by the extruder gear. Imagine trying to push a piece of spaghetti from the back and you will see the problem! If not printed correctly, this may lead to the filament bending and clogging inside the printer. This is much less of a problem now as in the last few years manufacturers have greatly improved the design of printers to reduce or eliminate any gaps that the filament might squeeze out of after the extruder gear. For older printers you can consider designing an attachment to guide the filament.
Another problem that is commonly observed when 3D printing with flexible filament is that it is particularly prone to stringing and oozing, it is also not very good at bridging between parts. These problems can be reduced a number of ways. A reduced print speed is often necessary to improve quality; regardless of speed, it is particularly important to make sure the feed rate is very consistent. This can be further helped by reducing layer height, which requires a reduced print speed. Optimising the print temperature is also important to prevent stringing and oozing, try incrementally reducing your print temperature but make sure it doesn't disrupt your layer adhesion!
Due to its flexibility and tendency to bunch up in the printer, flexible filaments are quite sensitive to sudden, quick movements like nozzle retraction. It is often recommended to reduce your retraction speed or even completely disable it when first starting out. As well as reducing bunching, this will also make it easier to optimise your print speed.