TPU Printing Issues: How To Print Flexible Filaments?

3D Printing hobbyists use 3D printers to create innovative designs out of various filament materials. Most users utilized rigid plastics like PLA and ABS filament with suitable properties for their projects.

However, as a new category of flexible filaments like TPU was introduced, it brought unique characteristics to the table that were not available in the past.

Materials like Polylactic Acid (PLA) or ABS have low abrasion and impact resistance. They have limited flexibility, so printing objects like tires, bumpers, bushings, or any similar item out of rigid filaments isn’t a good idea.

On the other hand, flexible plastics are the material of preference for these purposes. As the name states, they are flexible, highly wear-resistant, and can withstand severe impacts. However, these materials are not without their shortcomings.

Printing flexible filament is Significantly more challenging than rigid ones. Most importantly, the highly flexible ones tend to buckle, string, and thus lower the print quality.

In the following, we’ll learn that TPU filaments are in the more rigid range of flexible filaments. However, TPU printing issues are still quite typical!

TPU, TPE, Rubber, What Are They?

I’m sure you’re quite familiar with rubbers! For decades, they were almost the sole material for creating elastic objects in various industries. You can find this material in automobile tires, garden hoses, footwear, and belt.

To shape rubber, engineers use a process called vulcanization; they heat it and pour it into a mold until it cools down and gets the desired shape. However, once the rubber cools down, you can’t melt it again to change its shape.

As you know, 3D printers need to heat up and melt filaments to change their shape. Hence, 3D printing rubber isn’t an option. If you heat them, they’ll catch fire!

In conclusion, engineers decided to produce flexible plastics that can be remelted. Nowadays, we recognize these plastics as, Thermoplastic Elastomers, or TPE. They are the rubber alternative for 3D printers and have similar properties. TPE filaments consist of several subgroups, and Thermoplastic Polyurethane (TPU) is one of them.

TPE group filaments are very similar though they have different quantities of hardness, strength, and elasticity.

One of the critical properties of every material is its hardness, and it’s mainly measured as ‘Shore Hardness’ for flexible substances. For example, TPEs can have a shore hardness value from 20 Shore OO (very soft) up to 85 Shore D (hard). Furthermore, TPE filaments used in 3D printing have more of a limited range.

TPU filaments are often more rigid. This trait makes TPU filaments easier to use because they remain elastic at low temperatures and have higher abrasion resistance than other TPEs.

3D printing TPU doesn’t have a meaningful difference from other flexible filaments. The principles and recommendations are the same. 

In each case, you need to find the best printing parameters and calibrate the machine. It’s Because different filament materials, brands, and even different spools of the same company may need rematched setups.

Here we mainly focus on the TPU printing issues though you can apply these principles for similar cases. 

You can also find the most common problems and their solutions at the end of this article.

How to solve TPU Printing Issues? The General Answer

There are a few rules that should be executed to ensure a good print.

It would be best to check the items one by one to see how each one affects your results. Note that 3D printing flexible materials need lots of calibration, trial, and error. Anyway, the key factors are:

Adjust Extruder Temperature

The best extruder temperature based on filament brand can be anywhere between 200 to 250°C. An easy way to adjust this parameter is to start a test print with a mid-range value and see the results. For example, you can first set the extruder temperature to 230°C and print a small object and analyze the output.

The high temperature usually increases stringing. This phenomenon shows itself as hairy strings between various surfaces of the print. 

High temperature can have some advantages too. It can improve filament adhesion and increase overall strength. This way, the filament also becomes more fluid, and it makes the extrusion occur smoothly.

On the other hand, As the temperature goes down, the filament will melt much harder. It can cause problems for the extrusion gear, nozzle, and inter-layer adhesion. This way, overall part strength drastically drops.

Low extruder temperature eliminates strings but increases the chance of nozzle clogging, filament buckling, and more.

Hence, extruder temperature should be adjusted to avoid the problems mentioned above.

Level The Printing Surface

An unleveled bed is the first and most common cause of TPU printing issues. Warping is the most common outcome of an unleveled print surface.

In general, every 3D printer uses a different leveling mechanism. Some have four or more knobs under their print surface, which can change one corner’s height. Also, there may be other knobs for adjusting the whole bed’s size at the same time.

On the other hand, various bed-level sensors automatically level the bed for you and fix its warpage. You can always upgrade your 3D printer with a professional bed-level sensor like Bltouch if your 3D printer already doesn’t have one. Using the BLtouch is worth it.

Manual leveling usually starts with bringing the nozzle to one corner of the print bed. The next step is to adjust the surface’s height to the point that a standard paper can barely pass through the gap.

You can use the paper though some people prefer to have their calibration tools like business cards. After finishing one corner, you should go for the remainings corners. After some trial and error, you can reach a decent leveled bed.

Adjust Z-Offset

Another factor that ensures an excellent 3D printing experience is z-offset. It’s the distance between the Z-axis home position and the nozzle’s tip.

This parameter is equal to the gap between the print surface and the hot end. Keep in mind that before adjusting your z-offset, you need to make sure that your bed is leveled correctly. Having a suitable z-offset and a leveled bed is the key to a perfect first layer.

The ability to adjust this parameter can also open new doors to your projects. By learning the process, you can directly print on top of another object or pause the print midway and embed something into it.

There are mainly two methods for adjusting the z-offset, and learning each one has its unique benefits. You can either use slicers or changing the output G-code.

To change the offset using G-code, you first need to send the nozzle home. The relative G-code command is “G28 Z0”. After doing so, it’s time to adjust the z-offset, and its code is like “G92 Z0.3”. In this case, the nozzle will now treat the 0.3 as the new zero, so it will start printing 0.3 millimeters lower; thus, the gap decreases.

Using the z-offset parameter in your slicer is much more comfortable. It’s more than enough when it comes to solving the first layer curling problem. This parameter’s exact location varies from the slicer to slicer, but it’s typically easy to find.

However, This parameter works contrary to the G-code command direction, As a z-offset value in a slicer increases the gap.

Bed leveling and calibration of the Z-offset are the keys to solve most TPU printing issues.

Slow and steady

It’s a given that slow printing speed results in higher surface quality. Decreasing print speed can avoid under-extrusion and improve filament adhesion.

Anything between 15 to 25 mm/s can be a good printing speed.

Calibrate Retraction Settings

Suppose the printer’s nozzle is traveling from one point to another. One approach to avoid sinking of melted filament is to retract it a few millimeters. Retraction can drastically reduce filament strings. Thus, setting a suitable retraction distance value can eliminate some of your printing issues.

There is no perfect global retraction distance though the best value can be anywhere between 0.2 to 8 millimeters.

As a rule of thumb, Bowden tube extruders need more retraction than direct-drive extruders. They are usually more suitable for printing flexible filaments.

If you saw stringing, consider increasing the distance until you get better results.

Use Adhesives

Most 3D printer users try different adhesives for their projects. While there are many products on the market and each one promises the best results, some are more popular among the community.

Blue painter and Kapton are two beginner-friendly options. To use them, you’re going to need to put an even layer on your print surface. You can also set a thin layer of glue stick on top to increase adhesion.

Another option is to use hairspray on the bed directly. While this method is sufficient most of the time, you should clean it after each print. There are other decent options like nano polymers and PVA glue sticks that you can choose from.

Dry Your Filament

Wet filaments are another cause of TPU printing issues. It’s a good habit to dry 3d printer filaments before putting them into work.

Some filament materials are more sensitive to moisture, and some are less affected by it. Different TPU brands show various behaviors, so it’d be best to dry them before work.

The first and most effective way to discern whether your filament is wet or not is to listen to the extruder sounds. If you hear a noise like popping and cracking, it’s mostly because of wet filaments.

The next sign is the appearance of random bubbles inside the melted filament after it leaves the nozzle. These are the apparent signs of wet filaments.

However, if the moisture level is low, It may only affect the printing quality by stringing, oozing, and rough bridges.

For example, if you live in a place with a relative humidity lower than 30% and put your TPU filament spools in a room, you may only see more stringing without extra bubbles or a noisy nozzle. So, low humidity won’t affect the printing quality that much.

Depending on the amount of absorbed water, humidity can cause numerous problems at different levels. The least problem you can face is to have more stringing around edges and corners. Surface quality may fall, and the layer adhesion drops. Hence, it will affect the overall print quality.

Drying filament is an invaluable skill to have if you’re using hydrophilic filaments.

Power Up Cooling Fans

A great way to increase surface quality and reduce strings is to increase fan speed/cooling. As the filament gets hotter, it tends to stretch and create hairy effects. To avoid this, use the proper fan speed and check whether the airflow passes over the printing area and keep an eye for improvements.

This way, the hot filament gets cooler as it leaves the hot nozzle. The cooled filament is much denser.

However, fan cooling can cause additional problems. Cold filament has less interlayer adhesion, so that it will decrease print strength. Nozzle clogging is another possible issue; viscus filament can clog the narrow passage of molten plastic. You can start with 100% fan speed and decrease it step by step if you encounter any problems.

Most Common TPU Printing Issues
Problem	Definition	Typical Solution
Stringing	Strings of melted filament are widely seen on the printed surfaces.	Use the cooling fans to solidify the melted filament. Decrease Extruder temperature. Improve retraction distance.
First Layer Doesn’t Stick To Bed	The first layers of hot filament don’t have enough adhesion to stick to the printer’s bed	Add proper adhesives to the print surface. Increase extruder temperature. Slow down the printing speed.
Extruder Jamming/Clogging	3D printer can’t properly extrude filament. There may be fluctuations in the feed rate, or the nozzle completely stops the process.	There are several reasons for this issue. Decreasing print speed usually helps though some extruder upgrades can help the process based on your 3D printer model.
Blobs And Oozing	Contained moisture in the filament starts to vaporize as it becomes hot in the hot-end.	Dry your filament. You can do it by using an oven, filament dryer, or a food dehydrator.