Monoprice Select Mini V2 retraction/stringing problem

Question

I have a Monoprice Select Mini V2 and I’ve recently been having issues with little strings that are thick and sturdy sticking off of models:

I used Ultimaker Cura v3.4.1 to slice this model: Basic Retraction test using Hatchbox Black PLA filament

And the following settings:

• Layer Height: 0.1313 mm
• Wall Thickness 1.05 mm
• Infill: 22%
• Extruder Temp: 200 °C
• Bed Temp: 55 °C
• Retraction Distance: 4 mm
• Retraction Speed: 40 mm/s
• Speed: 50 mm/s
• Z-Hop When Retracted: Off
• Supports: Off
• Adhesion: Brim

I’d appreciate any advice on how to fix this problem.

Answer 1

The Monoprice Select Mini V2 uses a Bowden style extrusion system. Bowden extruders compress the filament in the tube where the gap between the filament and inner tube diameter allow for the filament to buckle slightly and compress causing a pressure build-up in the hotend. Without retraction this implies that the filament will start to ooze out of the hotend once you stop extruding.

From your settings can be seen that you are using retraction. The question now arises whether you retract enough? Furthermore, this stringing can be reduced by setting the correct settings for e.g. print temperature, retraction, coasting, travel speed. It is up to you to find the correct settings; test print objects like calibration prints (like in the image you showed) help you with finding the correct settings.

Apparently, the retraction you use still allows for some filament to ooze out of the hotend as shown in your image. This means that prior to the travel move there is still some pressure left in the hotend. A parameter that reduced the pressure prior to the travel movement is called "coasting"; basically you define to stop extruding while the head is still completing the printing moves prior to the travel move. It is advised to explore this setting. Other options to investigate further include lowering filament print temperature or increase travel movement.

Answer 2

What I ended up doing was using the settings from the cat.gcode that came with the printer. It has a retraction of 10 mm specified, which I thought was a bit much, but if it works, it works.