The extruder retracts filament in order to prevent excess ooze when moving between sections of the print. Issues can arise on certain filaments since this can increase grinding. When retraction is turned off, prints can get a very hairy look and will leave cobweb like strands connecting sections.
The settings involved with retraction are the speed at which the material travels, minimum distanced traveled before retracting, minimum amount of material extruded before retracting, and Z hop when retracting. Z Hop refers to the amount the extruder raises when retracting. This is set to a minimal amount so that small prints are not knocked off.
Each material will require different tweaks to these settings. Materials that are more prone to grinding and snapping will require slower retraction with more caution applied in each of the other areas.
Too little retraction leaves hairy, cob web looking prints with a lot of cleanup and an ugly surface quality. This can also lead to very thin walls if the printer does not extrude enough after retraction. Finally, an extruder may ooze too much when having too little retraction, leaving ugly bulges.
Too much retraction can cause the surface quality of a print to decrease because the extruder may over compensate when starting the print again, leaving a small buldge where it returns. The printer will then have difficulty moving over that section on the next layer. Too much retraction can also lead to grinded filament, as mentioned above.
Following printing guides for specific filaments, such as those we provide on our website, along with trial and error with your specific printer extruder setup, will help you to get these settings right.