What are the reasons for my 3D prints having large numbers of strings between parts of a layer?
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Stringing is often a result of too-high a temperature, or insufficient retraction. When there is highly liquid filament in the nozzle tip, it can adhere to the remainder of the print while dripping as the nozzle moves, leading to a thin string of the filament forming. As further travel moves are performed in each layer, this turns to a web.

The high temperature causes filament to be very liquid, causing it to move downward in the nozzle chamber easily, as opposed to having to be extruded forcefully due to viscosity. The temperature setpoint of 210 was high enough to cause this to happen.

A second possible cause, insufficient retraction, can also be blamed for this issue. Retraction is a process in which the extruder reverses its movement to pull filament back up the hotend, preventing it from dripping at the tip, and forming a string. Most slicers will allow specifying a numeric value in millimeters of filament to be retracted. Remember that printers with Bowden tubes between nozzle/hotend and extruder motor will require increased retraction and priming (extrusion when starting to print after a retract-and-move). Note that too much retraction can cause other problems, such as insufficient plastic in the hotend chamber at the start of the next printing move, which can cause gaps and other issues.