TPU 3D Printing-is it as hard as everyone makes out?

Summary

1. What is TPU filament?
2. What makes TPU a challenge to print?
3. Can TPU be printed on any FDM 3D printer?
4. Do I need any special slicer settings to print TPU?
5. Printer settings for successful printing in TPU.
6. Conclusion
7. FAQS

What is TPU filament?

TPU or to properly name it, Thermoplastic Polyurethane, is a flexible material with a slightly higher printing temperature than the PLA that just about everyone is used to. It is a very tough material resistant to abrasion, oils and greases, is quite stable at low temperature and is very elastic. Widely used in industry from cable insulation to tyres for caster wheels. This makes it a  very versatile printing medium for our humble home printers. TPU filament commonly comes in two stiffnesses 85 and 95 shore, shore being the industry standard measurement of hardness for rubber or rubber like materials. Essentially the higher the number the stiffer, or harder, the material is. Rubber bands are around 30 shore, tap washers about 85 and skateboard wheels 95.

What makes TPU a challenge to print?

TPU’s strength as a material is also its weakness when it comes to trying to print it with an FDM printer. It’s flexible. If you cut a 30cm length of PLA and holding one end poke something with the other end it doesn’t bend much. A 30cm length of TPU won’t even support itself against gravity and if used to push something will bend rather than go where you want it. This is particularly the case with 85 shore TPU which has the stiffness of wet spaghetti! Feeding the filament through the extruder into the hot end with enough force to print it and not have it buckle and try to escape is the main challenge with this material and there are a few other niggles we’ll get to later.

Can TPU be printed on any FDM 3D printer?

The most common misconception I deal with is that TPU can only be printed with a direct drive extruder. This is where the extruder sits directly on top of the print head. It’s true this style of printer does have a very simple path for the filament to travel and this gives it limited opportunity to squirm its way out of the mechanics. However TPU can be printed successfully from a bowden type printer, where the extruder sits atop the machine and feeds the filament down a tube to the print head, but will need more care in the settings of the slicer and a bit more trial and error with the printer to get it spot on.

Do I need any special slicer settings?

Yes, and this is where the real work is done in getting good prints with TPU. For this article I will be referring to settings in Ultimaker Cura but these are basic settings and are common among most of the popular slicer programs.

Turn off retraction

TPU doesn’t like to be jerked back and forth, particularly the softer versions, as it would rather slither out of the side of the extruder than be pushed into the hot end. If you want to try retraction with TPU keep it short and slow. This is probably the cause of most problems with printing this type of material.

Set all printing speeds to the same speed

Yep, everything except the first layer which should be around 20mm/sec. Be prepared for your prints to be long and slow. Recommend speeds of 30mm/sec for the main printing speed increasing slowly until the print quality starts to drop off. For the softer filaments be prepared to go down to 10mm/sec! The idea behind this is to have the printer provide the same pressure on the filament all the time, slow and smooth is the way to go. Travel speeds won’t have a direct effect on the filament but a faster travel speed can help to minimize the stringing caused by little or no retraction.

Set combing inside the model

Combing is the path the nozzle takes to get to the starting point of the next layer. With no retraction going on there will be a bit of oozing from the nozzle on travel moves. Combing inside the model will help keep the oozing inside the print, particularly on simple designs. Combing inside the model increases the length of travel and will impact on the overall speed of the print. I consider it to be a good tradeoff for a good looking print.

Brim setting

A brim is a good idea to help keep the model on the build plate particularly if there is not much model touching the build plate. Be warned though that TPU is very strong and the brim won’t snap off as it will with PLA. some trimming with a sharp knife should be expected.

Coasting

This setting won’t help you to print with TPU but will improve the look of the finished product. Coasting is how far the nozzle moves while depositing filament without the extruder pushing any more through. As the extruder pushes filament into the hot end a small amount of pressure is built up in the nozzle. The pressure is relieved by the retraction process pulling the filament out of the nozzle for a short time. With retraction settings off there is still pressure in the nozzle when it moves to start the next layer and this can lead to pronounced Z seams. Increasing the coasting time and volume can help to minimize the seam and help alleviate some more of the stringing.

Only print one item at a time

With no retractions the nozzle is going to ooze on travel moves as discussed. It’s also going to ooze between models on the build plate. I learnt this the hard way trying to speed up the process of printing multiple seals. The seals printed perfectly but were all attached to each other by a wall of strings. TPU by its nature is hard to trim with a knife and all but impossible to sand. A costly lesson well learned.

Printer settings for successful printing in TPU

Slicer settings

Slicer settings are only half of the story when it comes to printing in TPU. The slicer tells the printer what to do but there are a few steps to ensure a good print that have to be made on the machine to be able to carry out those instructions to produce a print to be proud of.

First layer adhesion

First layer adhesion is where people often become unstuck with 3D printing. Setting up for TPU is no different. In all probability your printer, set up correctly to print PLA and with the settings outlined, will print TPU with no problem but it is worth checking. Start to print a model with TPU, any will do, but keep the speed down to 20mm sec. Watch how the printer lays down the first layer, a torch used here will enable you to see better particularly with dark filament on a black build plate. Nozzle too close and the line will be transparent with curled edges, too high and the filament will not stick to the build plate and may be dragged about by the nozzle. Just right will see a consistent bead well adhered to the build plate. Pushing on the side of the line won't dislodge it but you can pick it off with a fingernail. All pretty standard stuff except that if you print TPU onto a textured build plate with the nozzle set too low there’s a good chance you won’t get the print off the bed! One of my test prints ripped out a hole in a Buildtak covered build plate when I tried to remove it because the nozzle was set too low! Another costly lesson to learn.

Print out a Temp tower

If you’ve never heard of one then it’s a small model with two towers, a couple of small overhangs and 25mm of bridging space between them. They are usually marked with the temp and material they are designed for. As it’s printed it changes the temperature of the nozzle for every block of the tower, usually hotter to colder. Every printer is different, even two off the same assembly line will be slightly different 200 degrees on one machine might well be 205 on another. Once you have printed out the tower, using the slicer settings above, one of the most obvious things will be the look of the print, hotter is usually shinier than cold, bridging is usually better when cold as is detail. Now try to break the tower. Where it breaks is where the layer bonds are weakest, where it holds together the strongest. The balance of strength, bridging, overhangs and looks is where you decide it to be and this is the temperature that you should print on your machine. Other people with the same model printer as you may have different settings but that works for their machine, not yours.

Extruders

Extruders have a very strong spring in them to grip the filament firmly. TPU is soft so will naturally deform into the serrated wheel in the extruder so doesn’t need as much force to push it along even in bowden tube style machines.  As long as it’s not distorting or ripping the filament up then no problem. If you find your extruder skipping then you are most likely trying to print too fast.

Conclusion

Does TPU earn its reputation as a hard material to print? On the balance of it I would say yes. If you’ve not had your printer for long, exclusively printed PLA and are not particularly familiar with slicer settings then I guess it would give most people in this situation quite a bit of trouble. Having to print at such slow speed may be anathema to those with new high speed machines but I remember when 30mm/sec was high speed! It is definitely counter intuitive to turn off the settings that would normally produce a better result in other filaments and I can see how this would lead to frustration as increasing those settings will only make the situation worse. Coupled with the often sparse information given by the filament manufacturer, usually a temperature range and a suggested speed would make this filament troublesome.

The advice given for both slicer and printer is that which I have had most success with on multiple machines both direct drive and bowden, but by all means play around with the settings your printer may well print it faster or with massive fast retractions.

FAQS

1. What can I print with TPU?

TPU is an excellent material for projects that need to flex. The most commonly printed items are phone cases. Bicycle grips, tyres for radio controlled cars, even shoes are suitable objects to print.

2. Is TPU better than PLA?

No. TPU and PLA serve different purposes in 3D printing. TPU is flexible and durable, making it ideal for items requiring elasticity, like phone cases and gaskets. PLA, on the other hand, is rigid and easy to print, suitable for decorative objects and prototypes. Choose TPU for flexibility and durability, and PLA for ease of use and stiffness.

3. Why is TPU so hard to print?

TPU is challenging to print due to its flexibility, which can cause issues like filament bending and jamming. It requires precise temperature control and slower print speeds to ensure smooth extrusion. Additionally, TPU's flexibility can make it difficult to maintain consistent filament flow, so proper printer setup and tuning are essential for successful prints.

4. What are the drawbacks of TPU?

TPU is not suitable for models with small details and its elasticity and abrasion resistance make post processing difficult, as does its resistance to most chemicals. Its slow printing speed may also be off-putting to some.

5. Can I use TPU to print items for use outdoors?

TPU degrades quickly under UV light so it’s not really a good choice for outdoor use.