The infill in a print is important for the structure of your final model. If it is completely removed, the shell of the print becomes flexible and can be easily pressed in. If the filling is very thin, it might as well just not be there at all. Here are some tips on how to avoid it.
Infill print speed
It makes sense to print the filling faster than the shell as the infill should not be seen by anyone, and therefore the diminished precision doesn't really matter. The problem is that your printer may have difficulty spitting out enough plastic to form strong fillings. The solution is to set the overall speed down, or otherwise find the setting specified for the print speed of the infill.
Infill extrusion multipier
Most slicers have a setting for the extrusion multiplier specific for the infill part of the model. Pushing more filament through the extruder can help keep the hotend fed at higher speeds. Slicers tend to call settings different things and in this case, you can find the settings here;
- SimplyPrint slicer: Inside > Fill print factor
- Cura: Custom > Infill > gears> Enable infill line multiplier
- Prusa slicer: Printer settings > Advanced > Extrusion width > Infill
- Simplify3D: Edit process settings > Infill > Infill extrusion width
The first place you can see signs of a stuffed nozzle is in the infill. Due to the slightly higher speeds, there is higher pressure in the hotend which has difficulties getting out if something is stuck in the nozzle.
It is a good idea to clean the nozzle with a so-called "cold pull", which is performed by heating the nozzle up to the print temperature of the material mounted to the printer. When hot, push some filament through before setting the printer to cool down. When it reaches 50 degrees, the heat can be turned back on and the filament can be pulled until it releases from the hot end and takes impurities out of there. Do the same thing another 2-3 times to be sure that everything is out of there. For the best results, use filament such as nylon or PETG for this as they are a bit more slippery than standard PLA.