*3.2.6 Grinding*

In FDM printers, a knurled ring drives the filament toward the hot end. If it loses grip on the filament, a small amount of material is removed from the filament's surface. If this happens for some time, then a groove forms in the filament thread, making transportation to the hot end difficult or impossible. Aggressive retraction settings can be the cause.

### *3.2.7 Overextrusion/underextrusion/missing extrusion*

There are multiple causes for a missing extrusion: clogging, nozzle cake, or grinding. A camera, mounted at the nozzle's height, is used by many authors to monitor if the specified extrusion amount is matched.

### *3.2.8 Overheating*

A central concept of extrusion-based 3D printing is the use of heat to extrude the raw filament. Overheating happens when a new layer is deposited on a lower layer, whose temperature has not cooled down to a certain level. Uneven printed parts may occur as a result (see **Figure 3**).

### *3.2.9 Curling*

Another temperature-related defect caused by overheating is curling, which becomes apparent on topmost corners. This is similar to warping, but on the top instead of the bottom.

### *3.2.10 Pillowing*

The exhibition of blisters or undesired holes in the topmost layers is termed pillowing and is caused by overheating. It can be detected the earliest after the last layer has finished and cooled down.

*Development of a Quality Gate Reference Model for FDM Processes DOI: http://dx.doi.org/10.5772/intechopen.104176*

#### **Figure 3.** *Effects of overheating on print.*
