Although PETG is becoming more and more popular among 3D printing hobbyists, in my experience, it tends to bridge more than PLA. Bridging is the horizontal filament extrusion between two pieces that creates a bridge! Many factors cause bridges to deform and sag from the bottom and create ugly loops. It is a common problem.
In this article, we are going to find some useful tricks on how to fix PETG bridging and improve the print quality.
(A bridging test tower source: 3Dprinting.com)
How To Fix PETG Bridging
The first and most common solution is to increase fan speed/cooling. As the filament gets hotter, it becomes more dilute so that gravity can pull it down easier and ruin the bridge. To avoid this, use the 100% fan speed and check if the airflow passes over the bridging area and keep an eye for improvements.
This way, the molten filament gets cooler as it leaves the hot nozzle. The cooled filament is much more viscous and can withstand gravity forces.
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. In this case, viscus filament can clog the narrow passage of molten plastic. You could start with 100% fan speed and decrease it step by step if you encountered any problem.
(PETG Bridging Example From soliforum.com)
Find The Best Printing Speed
High and Low printing speeds have different effects on bridge quality. When the extruder does fast travel, the melted filament’s internal tension will increase. It can also eliminate the chance of over extrusion, which we’ll cover later. The two effects can help to bridge and speed up the process.
On the other hand, fast printing speed can reduce layer adhesion and prevent them from squishing together. This way, It can have adverse effects on the bridge and overall printing quality. It may even cause new issues. Hence, breakneck speed isn’t a solution.
Another theory is to print as slow as possible. In this case, the melted filament won’t have notable tension though it has time to soften and squish together. The squished string have more contact area
(Printed Temperature Tower From reddit.com)
Minimize Extruder Temperature
The actual nozzle temperature is something that users usually forget to calibrate. Because printing with overheated plastic is much more comfortable and improves layer adhesion. In contrast, the hot filament can be watery and damage the bridges, overhangs, and even first layers (another typical issue named elephant foot).
Extruder temperature’s impression is similar to cooling fan effects. Both change the filament temperature, so low extruder temperature acts as a powerful cooling fan. It makes the filament more solid and reduces its adhesion at the same time. In contrast, the high temperature causes the plastic to become liquid so that it sinks more.
In conclusion, Extruder temperature should be adjusted to a value that has all of the advantages. This way, the output will be at its best quality. The best value varies for different filaments. Even distinct brands or spools may react differently.
For example, two manufacturers may use different additives for their PETG filament to get unique properties. Thus, for one case, PETG bridging is minimum in 235oC while the other works best in 250o C.
The easiest way to find the right temperature for your 3D printer is to print a temperature tower. A temperature tower is ready to print an object that reveals the behavior of filaments. It starts with a base temperature and changes it in every stage to visually find the best range.
Once printing the tower finished, It would be best to use the lowest temperature that print quality is acceptable. By doing so, extruder temperature won’t cause a problem for the bridges anymore.
Keep The Layers Thin
Thin layers mean more contact area and adhesion. They can also reduce voids and affect strength. Thinner layers can also improve print accuracy, so it’s best to decrease layer height to make them lighter. 3D printing at a sharp angle is much more comfortable with several thin layers instead of using a thick round filament string. This is the fact that most 3D printer users forget to change.
Can You Change The Geometry?
One approach to solve PETG bridging is to eliminate bridge positions. There are several ways to do it. If the part is big and complex, you can divide it into individual pieces. In this wise, the bridging areas can be changed, and the slicing softwares can find them easier. However, sliced pieces should later be glued together.
Another wise solution is to change the orientation of the design. A slight rotation on the z-axis may increase airflow over the print and drastically improve its quality. Turns related to the x and y-axis can also eliminate the bridge. However, for some projects, these changes are not practical options.
In some cases, changing the design’s shape is possible. Think about a way to change the basic shape and check if you can remove long bridges. Again, this option isn’t always viable though it’s good to be aware of it.
(Over-Extrusion Source: all3dp.com)
Decrease The Flow Rate
When the nozzle wants to print a bridge, it’s essential to set the filament quickly. Otherwise, the excess molten filament will flow from the nozzle, and it won’t have enough time to cool. This way, the bridge will look like solidified lava. Decreasing the flow rate in your slicer setting can make a steady flow of filament.
Believe it or not, Even expert 3D printer users find it hard to print a bridge longer than one centimeter. Thus most of them prefer to use various supports as a foundation.
Using supports help with bridges and overhangs, but there are caveats to using them. It’s a given that printed surfaces contacting with supports are rougher than the other. Removing supports can also become tricky. However, increasing the gap between supports and the print can solve this problem.
Dry Your Filaments
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. In my experience, PETG is in the middle of this spectrum. However, wet PETG can have a significant impact on print quality.
The first and most effective way to know if your filament is wet 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 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, live in a place with a relative humidity lower than 30%, and you put your PETG 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 different 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. However, a little moisture won’t create bubbles and popping sounds, and mechanical property changes will be minor.
PETG doesn’t absorb too much water; Especially if you keep them in a low humidity place, there won’t be severe changes. As a rule of thumb, if the relative humidity is around 30% or less, there is no need to dry PETG. For example, if you live in Nevada, Arizona, New Mexico, or Colorado, the humidity level is ok.
However, it is highly recommended to dry other materials like Nylon. So drying filament is an excellent skill to have as long as you use hydrophilic filaments.
How To Dry PETG
We learned how wet filament could affect filament quality and PETG bridging. Now it’s time to dry it! There are some easy ways to do it. One option is to use a filament dryer. Filament dryers are specifically designed to dry 3d printer filaments. These devices dry filaments with different methods; They usually heat the filaments a bit and dry them in a few hours. Some dryers have small holes and allow the filament to pass and go to a Bowden tube and stay dry as the print goes on. However, they are expensive and mostly used by professionals.
The easiest option is to use an oven! All you need to do is just put the filament spools into the oven, set the temperature, and wait a few hours. The more you leave the filaments, the more they dry. This method has some cons.
The oven’s temperature is usually not accurate enough, and there is a possibility of fusing the entire spool. If the stove gets overheated, the filaments may pass their glass transition temperature, and release gas, so always use a margin of safety when drying with your oven. I personally don’t like to put hot plastics where I cook my food!
The last and best option is to use food dehydrators. They are designed for drying fruits, but they use the same temperature range to dry fruit as we use to dry filaments! So they provide more accurate temperatures than ovens. You can get a food dehydrator for under 40$ and use it for drying filament.
They usually have meshing and removal layers that can be changed to fit one or more filament spools. Keep in mind that you need to check their interior dimensions and see if they have enough space for filament spools.
Now let’s assume you pick a food dehydrator. How much time does PETG need to be dried? And what is the best temperature for drying it? PETG should be dried at a temperature of around 65 oC for more than four hours. However, you can use a temperature of less than 65 oC with more time.
The filament temperature should never go above its glass transition temperature, and the glass transition temperature for PETG is 88 oC. After that, the filament will start deforming. After drying some filament spools, you can put them into sealed plastic bags or boxes to keep them dry.