Flexible 3D printed materials have a lot to offer us. Rather than just make rigid things, flexible materials can let us make wearables, shoe soles, clothing and other things such as buttons. Flexible materials such as FlexPLA, Polycaprolactone and some Nylons have been used. Most popular on the desktop are TPUs (Thermoplastic polyurethane, which may be called TPE depending on where you live). These thermoplastic elastomers are very tough and flexible and have been pioneered through Ninjaflex and Filaflex. These kinds of 3D printing materials have been used for slippers, orthotics and many soft things. Flexible materials and structures greatly expand the realm of the makeable with desktop 3D printers.
Issues with Flexibles
For many desktop 3D printers, flexible materials pose a problem, however. Bowden tube based systems have a lot of difficulty printing flexible materials. TPUs tend to get stuck or not be able to be pressed through the Bowden tubes for example. Removing the Bowden tube or replacing it may be a way around this. You can also try to put oil in your Bowden tube (which sucks, don’t do it). Having an extruder on your print head pulling the filament through instead of pushing it from the back also solves the problem. These direct drive extruders which are close to or on the print head itself ameliorate this issue while typically they make your printer a bit slower. Direct Drive also may lack accuracy to a certain extent but suffer less from under extrusion. With flexibles, however, most Bowden based systems will tend to have issues but can be upgraded with specific extruders for flexibles. Bondtech and Recreus have extruder upgrades for bowdens. Gyrobot, the number one expert on flexible 3D printing materials, has an excellent guide on how to 3D print flexible filaments.
A form fitting variable density insole developed by Steve Wood, better known as Gyrobot.
For most people, however, flexible materials are a part of their repertoire, not something they want to spend all of their time on. For most people the annoyance of printing flexible materials is simply too much or they don’t see a need to stray from the rigid materials. For those of us who want to stick to regular filament and have difficulties extruding flexible filaments, Ben Kromhout and Lukas Lambrichts illustrate some other possibilities. They designed patterns that let you make flexible structures with regular PLA. By developing specific patterns one could make objects flexible in predetermined ways. Variations in pattern could make a product deform in one way only for example or deform in one particular area much more so than in others.
TPEs (Thermoplastic Elastomers) are a more well worn path towards flexible materials. There are different types of these materials; some of them are styrene block copolymers (SBCs) but others can be thermoplastic vulcanized rubbers (TPV) or copolyester elastomers (COPE). TPU is one thermoplastic elastomer which is a block copolymer made from polyurethane (ester or ether). TPE and TPU as terms are widely used interchangeably even though TPE is actually a much wider group of materials. The term elastomer is also sometimes generally used and this is just any polymer that is elastic.
Thermoplastic Elastomers and TPUs are very popular bulk plastics. You could find varying TPEs in your shoes, roof, window frames or car. The TPE group is widespread with lots of materials used in it for the core compounds as well as in fillers. Some of these materials contain styrene and many harmful chemicals. Others have been certified safe in many applications. Generally many TPU materials are unsafe when burned and emit toxic fumes when burned. Polyurethanes themselves, however, are also comprised of a wide group of materials with differing safety and toxicity levels.
Due to the nomenclature confusion and the widely differing varieties of these materials, I would recommend extreme caution when printing with or using something that is only generally identified as a TPE or TPU because it could be just about anything. Generally with any material I would recommend a fume hood or enclosure (Yes, just because it’s PLA doesn’t mean it is safe). If we compare it to types of food, then saying PLA would be akin to saying, “this is tomato soup.” You can still put a lot of stuff in tomato soup, including poison and it would still be tomato soup. If you have a nut allergy then tomato soup could still kill you because someone could have put nuts in it. There are lots of different tomato soups and all contain tomatoes but they may contain much else besides. Saying that something is a TPU would be like identifying it as a soup. There could be just about anything in it, but we would expect bouillon and perhaps a vegetable. Saying something is a TPE is similar to identifying something as a hot food. Generally, however, these materials have high abrasion resistance and elasticity.
A popular polyurethane-based TPE for 3D printing is Recreus’ Filaflex. Filaflex is tough and probably has the strongest part strength of the TPEs used for 3D printing. The company is also working on its own hot ends and extruders to make it easier to 3D print their materials. NinjaTek’s Ninjaflex is an alternative TPU material developed by parent company Fenner Drives. They also have Cheetah. This material has been optimized to print much faster and much more easily on many different desktop machines. 3D printer companies have now also come out with their own TPUs, with Airwolfreleasing Wolfbend TPU filament. Taulman has a Plasticized Copolyamide TPE, which has been optimized to print easier than other TPEs.
Copolymers are in the COPE TPE group and consist of materials such as ABS, SBR (Styrene Butadene Rubber), polystyrene butadiene styrene (SBS) and HIPS. For most of these materials, caution is suggested since styrene and other fumes might emerge. Also these materials are also often just called a “copolymer” which may refer to many different materials. So your copolymer is like a box of chocolates, you never know what you’re going to get. Materials such as Bendlay are made to be easier to process than ABS and with less warping.
A copolyester is a modified polyester. You can for example add my favorite Scrabble word: Cyclohexanedimethanol and ethylene glycol to terephthalic acid to get what is a much more pronounceable PETG. PETGs such as colorFabb Ngen Flex mimic Flex PLA in being easier to print and more rigid while having flexible properties. Meanwhile, generally PETGs have good chemical resistance. PETG materials are made by lots of companies and have differing formulations to be competitive with each other.
PCL (polycaprolactone) is a very tough flexible bioabsorbable, biodegradable and compostable material. PCL prints at 120 degrees, much lower than we normally 3D print things. It will require extra model cooling to print well also. The Tg is low compared to other materials and the material can be heat set. This means that it can be printed flat for example and then heated to be formed on your body for a form fitting medical brace. Although it is very flexible, PCL is mainly used for 3D printing things such as scaffolds, trachea and drug loaded bioabsorbable implants (Disclosure: I work for 3D4Makers which makes PCL filament).
Flex PLA or Soft PLA
Flex PLA filaments are filaments that are mainly comprised of PLA but have added fillers and plasticizers to make them more flexible. Generally much easier to print than TPE/TPUs, these materials are also harder and not as flexible. Whereas the TPE/TPU materials easily find their original forms, this may be limited with Flex PLAs. Flex PLA tend to have less chemical and abrasion resistance and will typically be less soft. If you need something to be stretchy or very soft then the TPUs will tend to be better for you. If you have a Bowden setup, however, then these materials will be far easier to print. With Flex PLAs you can use variations in infill structure and percentage to increase and decrease your flexibility, similar to what was done with regular PLA above. It is difficult to say anything about the properties of these materials and their safety since it is such a general term. Depending on the material, it could be any kind of filler or material mixed in with PLA to any degree.