Tweaking Prusa i3 With Bondtech Mosquito Upgrade To Use Ninjaflex

Printing too fast is one of the most common issues when printing with flexible filaments. When trying to feed more material then what the hotend can melt the soft Ninjaflex will be compressed and be pushed against the walls of the filament path. This will result in under extrusion and your parts will most likely look something like this:

… while your filament will look like this: 

Ninjatek’s recommending print speeds:
Top and bottom layers: 10-20 mm/ sec (600-1200 mm/ min)
Infill speeds: 15-35 mm/ sec (900-2100 mm/ min)

We suggest using Ninjatek’s recommended settings to start with. 
Set Perimeters and Infill Speed to 20mm/s. It is possible to go higher, even double the speed in some applications, but ensure everything is tuned-in before you try to print faster.

It is commonly recommended to increase the extrusion multiplier when using flexible filament. The main reason is related to flexible filament softness. Ninjaflex will deform when pressed by the drive gears, and the distance between the center of the filament and the center of rotation of the floating gear gets shorter. This is the reason we release the tension of the thumb screw. However increasing the multiplier is not always the solution. 

Extrusion calibration should be done every time the filament is changed and may be also necessary to do it from spool to spool even with the same type of filament. A good extrusion calibration will get you the optimal Extrusion Multiplier.

First step is to measure the filament. Use a caliper and measure the filament on a couple of different places to calculate the average diameter.
Take into consideration that flexible filaments tend to be slightly oval when doing your measurements. Take the average diameter to get closer to the effective diameter. We’ve done this and turns out the filament we are using is about 1.69 mm in diameter. This value is inserted into the Diameter field in the Filament Settings tab of your slicer (see image below).

Download This Cube And Calibrate The Extrusion Multiplier

Download the STL file of this 25 mm cube to print as sample part in the extrusion multiplier calibration process. You can use other models you may have. Models with flat walls will be easier to measure. Follow this procedure:

1. In PrusaSlicer add the cube to your Plater;
2. In Print Settings > Layers and Perimeters, select Spiral Vase Mode (single perimeter);
3. Accept for the app to make the necessary adjustments necessary to print Spiral Vase;
4. In Print Settings > Advanced,use 0,45mm as Default Extrusion Width;
5. In Filament Settings > FIlament, set the Extrusion Multiplier to 1.
6. Print the calibration sample
7. Calculate the average wall thickness by measuring the wall at least 1mm above the bottom layer;
8. Divide your Extrusion Width by that number to reach the calibrated Extrusion Multiplier..

For example, if:
Extrusion Multiplier: 1
Extrusion Width: 0.45
Average Measured width: 0.53
New Extrusion Multiplier = 0.45/0.53 ≈ 0.85

Use the calculated value as Extrusion Multiplier in the Filament Settings.

It is worth mentioning that it’s hard to measure the exact thickness of 1 perimeter of Ninjaflex. This procedure will get you somewhat close to where we want to be. Fine tuning is required to optimize the results.

Remember the values we use depend on the temperature, speed, filament condition, thumbscrew tension and more. Your values are unlikely to be the same!

Ninjatek’s recommended extruder temperature:
225°C to 235°C

We want a high enough temperature to melt the material and get a good flow but keep the stringing to a minimum without getting poor layer adhesion.

Download Temperature Calibration Model

Perform a temperature calibration test to find the best value for your filament.
To do so, download and 3D print this simple temperature tower model by clicking the download icon.

To change the the temperature when printing we will add some custom g-code.

In PrusaSlicer got to Printer Settings > Custom G-code > Before layer change G-code

Here you can add

;BEFORE_LAYER_CHANGE
G92 E0.0
;[layer_z]{if layer_z == 0.2}M104 S245{endif};
{if layer_z == 7.2}M104 S240{endif};
{if layer_z == 14.2}M104 S235{endif};
{if layer_z == 21.2}M104 S230{endif};
{if layer_z == 28.2}M104 S225{endif};

To fine tune the temperature you can change the increments.
(remember to remove or comment these rows when finished)

We found that 235°C was the sweet spot for our applications when using a vanadium nozzle. If you use a brass nozzle you can normally lower the extruder temperature by 5-10 degrees.

Try measure the width of the temp tower on different sections. You will notice how different temperature relates to the flow of the filament. You should not use this when calibrating your extrusion multiplier, once you are satisfied with the temperature do another calibration test!

Ninjatek’s recommended bed temperature:
From room temperature up to 40°C

Ninjaflex will stick very well to the build plate so we can use a low temperature to decrease the bonding and to protect the PEI surface. We use 40°C in our applications.

(Remember to use a Spatula or a Scraper to remove the filament and do not pull it straight up.)

It’s not always a good idea to cool down the material fast. If the filament cools down faster than recommended, you will have problems with layer bonding when printing with Ninjaflex.

Start with the fan turned off and slowly increase it if you need to, depending on the geometry of the model you are printing.

Here is an example with the Benchy. The layer print time of the chimney is very short, so in combination with slowing down the print speed we can add some part cooling to get a better shape.

The common advice when printing with flexible is to just turn of the retraction. However, the right amount of retractions could relieve pressure in the hotend and create a better flow, and sometimes it is required for a stringless print.

When tuning your printer with flexible materials start with this setting turned off. This will make it easier to troubleshoot as you have one less parameter to consider. Once you have all your settings done you can slowly increase the retraction settings.

High retractions should not be a big problem with the bondtech extruder since we have a narrow filament path and there is no risk of the filament slipping by the drive gears. Here you can see the difference of a print with 0 retraction vs 5.5mm retraction at 35mm/s retraction speed.

Using 0mm Retraction at 35mm/s

Using 5.5mm Retraction at 35mm/s