Learning Through Educational Tools: Bridging

Bridging is the process of extruding a string of plastic over a gap. Typically it is across two points, in a straight line. Several strings of plastic cross a gap, allowing further layers to be placed on top. This process can be tricky, due to being influenced by nozzle temperature, air movement, and gravity (just to name a few factors).

Nozzle temperature influences how quickly the filament cools once it leave the nozzle. With bridging, you want it to cool fairly quickly, since you don't want your filament drooping as it crosses the gap. A successful bridge should be a straight line between two points, almost seeming to defy gravity.

Gravity plays a big role in why we have to consider settings in overhangs and bridging. Gravity will pull filament down that is still pliable. Hot filament is still pliable. That is why some times we need to consider air flow. Usually part cooling fans are used, in order to blow air onto the extruded filament, in order to cool it quickly. The more targeted the air flow, the better gaps and overhangs will work.

Of course, too much air flow can be problematic, particularly if it either cools the plastic too quickly or blows the plastic out of alignment. The latter isn't as likely, but the former can be problematic, since layer adhesion is important. Layer adhesion is influenced by temperature. The plastic coming out of the nozzle has to be hot enough to melt together with the layer below it. But it also has to be cooled enough for it not to cause drooping (Yellow in image below).

Targeted cooling is important, since having air flowing on the nozzle just causes the heating element to work harder, while only marginally helping cool the filament. Targeting the extruded filament helps cool the filament, and makes the machine work efficiently. Fan speed also comes into play, since it controls how much cooling there is. Which can help or hinder, depending on the operation.

The picture above shows several issues. Overhangs that are way too steep (Purple), temperatures that are way too high, insufficient retraction (Red), and several other problems. Basically, I need to dial in the settings for this filament. As discussed in the overhangs section, speed and temperature can influence the finished product. One other consideration is retraction. it is the process of pulling the filament back. Therefore creating a suction that slightly sucks the melted plastic back in the nozzle. This is done to prevent oozing during travel moves. You can see this in the little outcroppings in the above image (Red).

Distance can also influence how well bridging works. Bridging relatively short gaps of 15mm is fairly easy, since there is less of a chance for drooping. You can see that there is a little drooping around 65mm, with a fair amount more at 85-110mm. I cleaned up some of the parts that drooped, so this doesn't fully represent the outcome at different distances.

Even so, every bridge came out better than I expected. Particularly the 110mm gap, that is quite a distance to bridge. I plan to try reprinting this in the future, with the settings better dialed in. I will either update this post, or make a whole new post, depending on if what I learn from the process.