In the last few years flexible filaments have become widely available to the 3D printing community. These filaments open up a huge number of design options, and present us with a completely new set of material properties to help us with our printing. Some uses of flexible filaments include; ‘soft’ hinges and tactile interfaces, such as buttons.
There are lots of different companies that offer flexible filaments made from various different materials. Most flexible filaments are TPEs (thermo-plastic elastomers) and most of these are TPU (a type of TPE), these have a wide range of material properties.
An easy and common way of grading different flexible filaments is by applying the Shore hardness test, using the ‘Shore A’ scale. This scale is often used to compare the hardnesses of different rubbers. 3D printer flexible filaments are usually graded from 75A to 100A. The Shore hardness scale goes from 0 to 100, and 75A is around the low limit in stiffness for a filament to be extruded in an FDM 3D printer – a similar hardness to the sole of a shoe. 100A is equivalent to the hardness of a shopping trolley wheel.
It is harder to print softer flexible filaments (which are lower on the Shore hardness scale) for two main reasons:
- Flexible filaments have less rigidity than stiffer materials like ABS and PLA. So they are harder to push through extruder as they tend to bend and buckle. This is why it is much harder to print with flexible filaments while using a Bowden extruder (though possible).
- The rubbery flexible materials are often ‘grippier’, and so there is more resistance between the filament and the extruder as it is pushed through the hot end. This can result in more blockages and uneven printing.
Here are some of the top flexible filament brands out there:
NinjaTek’s SemiFlex Filament: This filament is much stiffer than NinjaTek’s original NinjaFlex. It has a Shore hardness of 93A which is at the higher end of the stiffness scale, meaning it is fairly easily extruded.
Recreus’s Filaflex TPE: Filaflex is very soft compared with other filaments, so you may have more difficulty printing with it. Very low print speeds are required, and it can be helpful to modify your printer with a low friction PTFE tube feeding into the extruder, and a specialised extruder for printing flexible filaments. It’s Shore hardness rating is low, at about 82A. It is however, very elastic, with about a 6 times elongation.
Polymakr’s Polyflex Filament: This is a TPU filament, and is also fairly stiff, with a Shore hardness of 90A – 95A. Polyflex is also only available on 750g spools with a limited range of colours.
taulman3D’s PCTPE filament: This material is a co-polymer of TPE and Nylon, making it stiffer than TPE filaments. It has a Shore hardness of around 100A, making it much less flexible than some other filaments, but allowing it to be printed faster and more consistently.
FormFutura’s FlexiFil TPC: This is also a fairly stiff filament with a Shore hardness of about 93A. It is made from co-polyester, and prints similarly to SemiFlex and Polyflex. FlexiFil only comes in 500g spools and has a very small range of colours.