Unless the material being extruded has a very high viscosity it will ooze from the nozzle in between extrusions. There are several settings in Slic3r to which can help to remedy this.
The retraction settings, found in the
Printer tab, tell the printer to pull back the filament between extrusion moves. This can alleviate the pressure in the nozzle, thus reducing ooze. After the subsequent travel move the retraction is reversed to prepare the extruder for the next extrusion.
Length - The number of millimeters to retract. Note that the measurement is taken from the raw filament entering the extruder. A value of between 1 and 2mm is usually recommended. Bowden extruders may need up to 4 or 5mm due to the hysteresis introduced by the tube.
Lift Z - Raises the entire extruder on the Z axis by that many millimeters during each travel. This can be useful to ensure the nozzle will not catch on any already laid filament, however it is usually not necessary and will slow the print speed. A value of 0.1mm is usually sufficient.
Speed - The speed at which the extruder motor will pull back the filament. The value should be set to as quick as the extruder can handle without skipping steps, and it is worth experimenting with this value to find the quickest retraction possible.
Extra length on restart - Adds an extra length of filament after the retraction is compensated after the travel move. This setting is rarely used, however should the print show signs of not having enough material after travel moves then it may be useful to add a small amount of additional material.
Minimum travel after retraction - Triggering a retraction after very short moves is usually unnecessary as the amount of ooze is usually insignificant and it slows down the print times. Set the number of millimeters minimum distance the nozzle must move before considering a retraction. If the printer handles ooze well this can be increased to 5 or 6mm.
Retract on layer change - Movement along the Z axis must also be considered when dealing with oozing, otherwise blobs may occur. It is recommended to leave this setting on.
Wipe before retract - Moves the nozzle whilst retracting so as to reduce the chances of a blob forming.
Additionally there are several settings in the
Only retract when crossing perimeters (Infill) - Tells Slic3r to only retract if the nozzle will cross the threshold of the current island being extruded. Slight ooze within the walls of a part are not seen and can usually be accepted.
Avoid crossing perimeters (Layers and perimeters - Advanced) - Will force the nozzle to follow perimeters as much as possible to minimise the number of times it must cross them when moving around, and between, islands. This has a negative impact on both G-code generation and print times.
Seam position(Layers and perimeters - Vertical shells) - This setting determines the start point of perimeter loops, and thus the position of the potentially visible vertical seam on the side of the object. The available options are:
Random- This will choose a different point for each layer, thus making the seam less noticeable.
Nearest- This will try to choose a concave non-overhang vertex so that the seam is hidden in the inside of the concave angle. If no concave non-overhang vertices are available, it will choose a convex non-overhang vertex. If none are available, it will choose a non-overhang vertex. The choice among the candidates is operated so that the start point is the nearest to the previous extruder position. So this option will optimize for short travels.
Aligned- This will use the same logic as
Nearestfor finding the candidates, but it will choose the one that is nearest to the start point of the previous layer. This will ensure the seam is mostly aligned throughout the whole object.
See also section : Sequential Printing for another technique which can minimise strings forming between objects.