Cura Retraction Extra Prime Amount – Explanation & Usage

While retraction is our primary line of defense against filament oozing, which is known to create common issues that can easily ruin print quality, such as stringing and blobs, there can be cases where the material is still lost during travel moves, even when the filament is retracted, potentially leading to under-extrusion as a result.

In this article, we will explain the purpose of the retraction extra prime amount feature in Cura, discuss when it would be suitable to use this feature to improve your prints and take you through the process of configuring this feature as optimally as possible to gain the maximum benefit out of it.

What Is the Purpose of the Retraction Extra Prime Amount Feature in Cura?

We can consider the retraction extra prime amount feature in Cura to be pretty uncomplicated in terms of functionality, as all it does is push an additional volume of filament specified by the value (in cubic millimeters) of this parameter on top of the length of filament that’s already primed (which is equal to the retraction distance) under normal circumstances.

cura retraction extra prime amount parameter

While getting the 3D printer to prime more filament than it has retracted can sound counterintuitive at first, as this would technically lead to over-extrusion, at least in a perfect world, the need for this feature comes from the fact that retractions are usually not perfect, just as many other things in the process of 3D printing, which causes some of the material to be lost regardless as the printhead moves to its next destination after retracting the filament.

cura retraction extra prime amount explanation

To avoid under-extrusion in such a case, which can lead to all sorts of issues from gaps in the prints to poor layer adhesion, using the retraction extra prime amount feature becomes the key to getting your 3D printer to compensate for the material that’s lost during movement, and ensure that the nozzle receives enough plastic to complete the extrusion path correctly.

When to Use Retraction Extra Prime Amount?

As the retraction extra prime amount feature compensates for lost filament by pushing extra filament during priming, it’s critical to ensure that you only use it in cases where you can clearly observe that under-extrusion occurs after retractions and priming to avoid pushing more filament than necessary and creating over-extrusion issues.

underextrusion after retraction example
Source: dgrat @ Stack Exchange (CC BY-SA 4.0)

To find out whether this is the case for you, the first thing you will need to do is carefully track any extrusion that takes place after a retraction, travel move, and priming sequence, with the most common examples being paths where the nozzle has to cross walls, as your printer will have to retract filament even if features that reduce the number of retractions, such as combing, is enabled.

Additionally, if the Retract at Layer Change feature is active in Cura, layer changes are also something to pay attention to, since this feature, as the name suggests, will cause your 3D printer to retract filament whenever the nozzle needs to reposition itself for the extrusion of the next layer.

When observing these extrusion paths, you will specifically want to focus on the very first portion, as less filament being primed than required will present itself in the form of the start of each extrusion path being thinner than the rest of it, which will practically guarantee that you will need to use the retraction extra prime amount feature.

Once you have the end product at hand, you can also look for the appearance of gaps and holes on the print, as once the plastic is dry and the print is fully completed, it’ll be easier to notice the presence of such flaws that occur as a result of under-extrusion.

Finally, as it may not be possible to reliably spot whether the under-extrusion is explicitly taking place after retractions in every case, depending on the 3D model you’re printing, we highly recommend running a test print where you can see this effect more comfortably before making adjustments, such as this retraction test model, which involves plenty of retractions for you to observe.

retraction test model travel move paths

Bonus Tip: This feature will be especially beneficial if you’re printing with TPU due to it becoming necessary to reduce both retraction distance and speed drastically to ensure that the filament doesn’t end up clogging the extruder, which in turn causes your 3D printer to lose more material to oozing, and as a result, make it necessary for extra material to be primed to compensate for this loss.

How to Optimally Configure Retraction Extra Prime Amount?

Similar to recognizing when it would be necessary to activate retraction extra prime amount, it’s also vital to configure this feature correctly, as using a too-low value will cause the under-extrusion issue to remain unresolved, and using a too-high value will cause your 3D printer to over-extrude after retractions & priming.

The recommended starting point for the retraction extra prime amount value is nozzle size cubed (nozzle size)^3, which you can further adjust based on your findings once you complete a test print for the first time with this value, preferably with the retraction test model we have mentioned earlier, or anything that’s similar.

For instance, if you’re using a 0.4 mm nozzle (the standard size for many 3D printers out of the box, such as the very popular Ender 3), the retraction extra prime amount you should be starting off with is 0.064 mm^3 (0.4 * 0.4 * 0.4).

On the other hand, if you’re using a bigger nozzle, such as a 0.6 mm one, the starting point retraction extra prime amount value for you will be 0.216 mm^3 (0.6 * 0.6 * 0.6) instead.

Our recommendation for further adjusting the retraction extra prime amount value once you have the test print at hand would be to increase or decrease it by 10% at a time, based on whether you’re observing symptoms of under-extrusion or over-extrusion, and to keep testing until you find the optimal value.

In the case of under-extrusion, you would specifically be looking for signs such as the start of the extrusion paths that take place after retraction & priming being thinner than usual and possibly the appearance of the gaps on the print, which are trademark signs of under-extrusion after retraction.

gaps between layers after retraction
Source: user3779413 @ Stack Exchange (CC BY-SA 4.0)

On the other hand, in the case of over-extrusion, the main sign you should be paying attention to is the appearance of blobs and zits at the locations where priming takes place, which should be pretty straightforward to spot, and potentially stringing alongside it if the value you’re using is really high.

blobs and zits on 3d print example
Source: OrElse @ Stack Exchange (CC BY-SA 4.0)

Finally, we would highly recommend re-adjusting this value (along with the usual printing temperature, bed temperature, etc.) whenever you bring in a new spool of filament, even if it’s the same type but a different brand, as each brand of filament behaves differently, and a misconfigured retraction extra prime amount value can easily lead to under-extrusion or over-extrusion issues that would be harmful to the quality of your prints.

Is It Possible to Use a Negative Retraction Extra Prime Amount Value?

Even though entering a negative value for retraction extra prime amount goes directly against the purpose of this feature on paper, which is to compensate for material that’s lost during the process of retracting the filament and moving to the next destination, it is actually possible to do this, with cases where it’s a feasible thing to do as well.

negative retraction extra prime amount value example

When you enter a negative value into the retraction extra prime amount input box, Cura will end up subtracting this volume of filament from the amount of filament that your 3D printer usually primes (which is equal to the retraction distance under normal circumstances) and practically cause the amount of primed filament to be less than the amount of retracted filament.

In most cases, this would directly lead to the occurrence of severe under-extrusion issues, especially considering that material already lost to oozing on its own makes under-extrusion a commonly encountered problem, and priming even less filament on top of this will make things even worse.

That being said, in rare cases, getting your 3D printer to prime less filament than what it retracts can be the thing that quickly saves you from the nozzle leaving blobs if such an issue explicitly takes place after the nozzle travels to its next destination and starts priming the filament, which is definitely something to test out.

Conclusion

The retraction extra prime amount feature in Cura is definitely a handy tool to resolve any under-extrusion issues that may stem from using retraction, especially when printing with flexible filament due to it not being possible to retract flexible filament quickly enough in most cases.

That being said, as a misconfigured retraction extra prime amount value, specifically when it’s higher than it should be, can lead to over-extrusion, ensuring that you have configured the parameter correctly is critical to avoid a scenario where it does more harm than good for the quality of your prints.