How many times have you heard the word “retraction” before? For many prints, using portrait and setting it up properly is an obligation in order to achieve a result that meets our expectations.
Retraction is the command that, if used properly, allows us to make multiple prints at once without leaving the shadow of the passage, in the form of a thread that joins the various prints.
The option to retract the filament has been introduced in 3D printing to eliminate, or at least contain, the problem of stringing. This manifests itself with the production of wires in the displacement between 2 points of the print where the nozzle makes a “jump in the vacuum”.
The loose material continues to drip, even when pressed by the weight of the material above. These microwires are formed during the movement and have the impression of being a spider web.
Later we will talk about how to set the retraction, but now I would like to mention the 2 problems that some incorrect setting values can create.
Retraction: Filament abrasion problem
The first is the abrasion of the filament, a subject already dealt with in this article. This problem occurs for various reasons. We may have set the retraction speed too high. Or a pressing series of retractions, a preload not compressed enough or even, if our wire pusher pinion does not grip the filament well, it can slip and abrade the filament. In all these cases the filament is worn out and can even fail to be extruded. If the case we will see near the extruder the typical filament dust, to witness the abrasion. In addition to a decrease in the wire cross-section.
Retraction: Extrusion Blocking
The second case of error due to incorrect settings leads to total stoppage of the extrusion. This occurs with excessive retraction distances. Imagine setting over a centimeter of retraction, mistakenly thinking that increasing the distance the dissolved filament does not fall. The hotend is not a vacuum system, so the retraction has no chance of “sucking the filament”. The only purpose is to eliminate the thrust and gravitational pressure from the liquefied material in the nozzle. Too high retractione values can lead the wire in transition between solid and viscous too high, at the radiator. Here the half loose material will solidify quickly, occupying all the available space. This will produce a swelling in the diameter, which will no longer allow the normal flow. A thorough cleaning of the nozzle and the throat will be necessary.
As promised we start talking about retraction and how to set its values.
Let’s start by saying that the parameters to set to eliminate stringing problems are 3. Two are obvious, they are retraction distance and retraction speed. The third has nothing to do with retraction, but can make it easier to achieve the result: the speed of movement.
We have already mentioned that this is a fundamental parameter. Its function, however, is simply to remove pressure from the already liquefied material. This must tell us that it is useless to set enormous retraction distances, already with a minimum will be achieved.
The parameter “retraction distance” changes according to the type of extruder used. With the Bowden extruder, being far from the hot end, a higher value must be set than with the direct extruder. In the case of Bowden I recommend a retraction distance of 6.5 mm, while for direct extruders 3 or 4 mm are sufficient. As mentioned before, I do not recommend going beyond 8 mm as a measure. Beyond 8mm there are only problems!
This is perhaps the basic parameter to get to have satisfactory prints. The speed of retraction determines the time it takes for the pressure to be removed. The shorter the time, the better the result: with limits. A speed value of 25 mm/sec is a good starting point for any material used. You can also push at higher speeds but I recommend not to exceed 45mm/sec, or we risk to find ourselves with the filament abraded.
Considering what has been said up to now, that the stringing is the result of the casting of the melted plastic, it is immediately clear the importance of having a sustained speed of movement. Higher speed in the displacements equals less time to “walk in the vacuum”. This means less chance of forming cobwebs. Depending on the possibility of the printer I recommend a speed of the displacements from 120 to 150 mm/sec.
Now you know exactly how to adjust the retraction and just a few tests will be enough to find the ideal values of your printer. Of course, the type of material used also affects how much to force with shrinkage. PETG is the material that tends to drip most of all and therefore will need to be pushed ad hoc settings. Then, in a hypothetical classification of the filaments would appear the Nylon and the PLA. They need retraction, but not in a pushed way. Finally we find the ABS, which in normal conditions almost does not drip.
However, it can happen, by printing very viscous materials, that the retraction is never enough, or on the contrary by printing elastic materials that can not be used, or almost. Is there a way to avoid stringing even if I don’t use retraction?
There are a couple of tricks, let’s see!
Minimize retraction by setting the right infill pattern
First of all, if we are printing a soft material, not using retraction could be good idea. It could give problems, statistically, in proportion to how many retractions it has to perform. For this reason, the Cura slicer software, with version 3.0, has inserted an infill that minimizes the number of retractions required. This allows, even if we are printing a soft material (e.g. TPU), to leave the retractions activated for when it is necessary, without having to resort to them often. The particular infill’s pattern is called a buttoned cross and requires 1/5 of the retractions in use for the plots used so far.
Strings, strings and more strings!!!
If you are dealing with a material that leaks filaments, even if you set the maximum allowed retraction parameters do not despair! We can order the nozzle to always remain within the printing area. In this way we will avoid “jumps in the vacuum”. Which is the main cause leading to the creation of strings.
Simply by activating the combing mode (new in Cura 3.0). This mode requires that the paths are within the printed area, then “supported”. The minimum “burr” that will be created inside the model will be easily covered by the deposition of the next perimeter layer or infill.