Clogging happens when material or the filament itself blocks the nozzle or the Bowden/PTFE tube of a 3D printer. While it also happens with regular filaments like PLA and ABS, clogging is more frequent when working with composites and exotic filaments like wood or metal-filled filament.
Some of the most common reasons for clogging are using wet filament, wrong size filament, or incorrect printer settings.
Filament absorbs moisture when not stored properly. Printing with wet filament will very likely produce prints with layer separation, holes, or cracks in them. Filament also swells up when it absorbs moisture. Swelling can add as much as 0.5 millimeters to the filament diameter which can jam the printer.
To avoid extrusion problems from wet filament, store your filament in a dry box or airtight containers. Add silica gel to absorb the excess moisture in your container.
Wrong Filament Size
Filament comes in two diameters. This is either the 1.75 or the thicker 2.85-millimeter diameter. Printers use one or the other, never both. Trying to use the smaller 1.75mm diameter filament in a 2.85mm printer or the larger 2.85mm filament in a 1.75mm printer will certainly clog the printer.
The simple fix here is to use the correct size filament for your printer.
Incorrect Printer Settings
Slicer settings are one way to make sure your prints go smoothly. Printing at the wrong temperature, speed, or shell thickness can waste material on a failed print or jam your extruder. Make sure to print using the right settings for whatever filament you are working with. Source
This is particularly important if you regularly print with different filaments. Switching between materials has a higher likelihood of clogging than when you are printing using the same settings for the same filament all the time.
Temperature is very often the most critical setting. If the temperature is too high the filament will burn and not melt. The molten plastic will then clog up the print head. On the other hand, if the temperature is too low the cold filament can just as easily block the print head as well.
Poor Quality Filament
Using cheap material can be very tempting. While it might seem like a cost-effective solution, especially if you are printing in high volumes, it can turn out to be an expensive solution if that cheap filament has extrusion issues.
Cheap materials contain several unknown additives that dilute the filament to make it cheaper, to begin with. Ideally, you should never buy a large quantity of filament without testing it to see how it performs first. Source
Sometimes the problem is not at the nozzle. In this case, the issue could be with your Bowden tube or PTFE tubing. The filament may have fused with your tubing or the tubing could just be worn out.
If you print in a workshop there is a high chance your filament comes into contact with dust or debris. This could be the dust from grinding or sawing metal or wood. The dirty filament can just as easily clog up your printer as wet filament does.
Your printer will struggle to fully melt dirty filament when you try to extrude it. Storing your filaments in airtight containers not only keeps them free of moisture but dirt free as well.