photograph of calibrating flow rate with digital calipers

Calibrating Filament Flow Rate


Calibrating the flow rate within Cura, PrusaSlicer, or Simplify3D for each filament is as crucial as calibrating the extruders stepper motor.

So what is the filament flow rate exactly?

Namely the filament flow rate is the rate as which the 3D printer extrudes the molten filament. Naturally 3D slicer software don’t know what the individual printers requirements are. Especially as they can change from the same manufacture and same model of 3D printer. Hence calibrating the filament flow rate is a crucial step in 3D printing.

But why does filament flow rate change so much?

Fundamentally there are two sides of this answer. Firstly involves calibrating the extruder stepper motor steps, so the firmware knows exactly how much solid filament is pushed through the extruder. Next comes the characteristics of the filament itself. Moreover the additives, type of plastic used and so on. But when a slicer program like Cura, PrusaSlicer, or Simplify3D calculate the g-code. They don’t know exactly how much molten plastic is being pushed through the nozzle. Consequentially dimensional accuracy and line widths all go out of the window, as there is no benchmark to calculate from. Hence by default 3D printing slicers set the flow rate to 1.0 or 100%. Naturally for many instances this is either too much, or on the odd occasion too little. Thus the need for calibrating the filament flow rate.

But how do you know if the flow rate is too much or too little?

Incorrect flow rate is easily spotted and is often referred to as over or under extrusion. Additionally it can leave visual marks on the print, such as fattened or thin lines and walls. Furthermore it can leave blobs and blips on the 3D printed models.

In addition, if the bed is correctly levelled and the initial Z height is correct. Then a poor 1st layer, or the print quality deteriorating from the 2nd layer is more often than not, a case of flow rate inaccuracy. But as previously mentioned flow rate is the 2nd step and calibrating the extruder steps is paramount.

If you have not already done so, it is worth calibrating the PID for the hot end. Followed by calibrating the filament extruder motor steps. Once they are completed they are unlikely to change again. Next for each filament you purchase, even the same type and from the same manufacture, needs the flow rate calculated.

How do you calibrate flow rate?

screenshot an empty cube required for calibrating a 3d printers filament flow rate

Without a doubt you are going to need a method of accurately measuring the thickness of an object. Moreover digital callipers are great for this. But ensure the callipers are able to read down to 0.01mm.

  1. Firstly you need a plain walled object such as a 20mm cube without lettering. Then import it into the slicer of choice.
  2. Next from within your slicer check the line width is not on automatic. Additionally make a note of the line width, for example Cura maybe 0.4mm. But PrusaSlicer is likely to be 0.42mm.
  3. Remember to reset the flow rate in the slicer to either 100% or 1.00 if the slicer uses a multiplier.
  4. Now from within the 3D slicer set bottom layers to 1.
  5. Set the top layers to 0.
  6. Set the wall count to 2.
  7. Set infill to 0.
  8. Finally slice and export the cube to the SD card.

Measuring the calibration cube

With the calibration cube printed and cooled down, measure the thickness of the walls on all four sides with the callipers.

A point of note, I personally like to measure each wall in three spots, the middle and slightly off the centre on both sides. But away from the corners, as the rounded edges can cause discrepancies. Furthermore for ease of use, I like to record the measurements in a spreadsheet so that I can get an average as well as the recorded measurement. As shown in the below screenshot.

screenshot of spreadsheet for calibrating flow rate of 3d printer filament

But did you notice the first wall is not constantly the same width when measuring the flow rate on the test cube? Moreover this is down to a very slight loss of material when retracting, and then moving up to the next layer.

What do I mean by the first wall? I call the first wall the point that the printer starts the next layer of the print. Furthermore I call the last wall the final extrusion just before the next layer.

So how would you then measure the flow rate?

Importantly if you are experiencing a similar inaccuracy than ignore the previous walls reading and go by the average of the last wall.

Adjusting the flow rate

With the new measurement recorded we now need to calculate the new flow rate. Consequentially to make life simple, I have created an online flow rate calculator for everyone to use including myself. However don’t use the up and down arrows. Instead enter the numbers manually. Furthermore don’t forget to bookmark the link above, for quick access in the future.

Online 3D Filament Flow Rate Calculator

screenshot of the filament flow rate online calculator from Make N Print

Without a doubt I find the previously mentioned flow calculator quicker and easier to reach then a spreadsheet most of the time. Importantly it is simple to use. Although some slicers record the flow rate differently, it all comes down to the same calculation. Moreover Ultimaker Cura uses a percentage, for example 100% to represent the flow rate. While Simplify3D uses an Extrusion Multiplier, where 1.00 represents 100%.

As a result of the previously mentioned loss of material, in this example I will be using the last walls measurement to adjust the flow rate. Furthermore I reset the flow rate back to 100% in Ultimaker Cura, so using the flow calculator the Current Flow Rate will be 1.00. Next is the Required Width option, in this example the line width in Cura is 0.4mm and printed with two walls. Resulting in an expected width of 0.8mm. Finally we insert the recorded measurement into the Actual Width and the New Flow Rate will be automatically calculated using an Extrusion Multiplier.

A point of note, the New Flow Rate in the example imagery below of 1.026 would give a new flow rate of 102.6% for use in Ultimaker Cura.

screenshot of the new filament flow calculated using the Make N Print online flow rate calculator.

While your results are likely to be very different from the example, for many the actual new flow rate is likely to be under 100%.

Test Again

screenshot of adjusting the flow rate within Ultimaker Cura 3D printer slicer.

Importantly with the New Flow Rate adjusted within the slicer, re-slice and reprint the empty test cube in the exact same way as you did the first one. Once completed remeasure the wall widths and if required use the calculator to gain the new flow rate and repeat the process. Finally when the width of the measured walls is spot on, then for many this guide for calibrating the filament flow rate is complete.

But how do I fix the loss of material when retracting, and when it moves up to the next layer ?

screenshot of fixing the first thin wall after retractions

Without a doubt fixing the slight loss of filament when moving and retracting is a game of patience. But it is a simple game. Moreover in most 3D printer slicers is an option for an extra prime amount. Which helps to solve this issue. However its a case of trial an error and on this particular 3D printer, I started of a value of 0.1 mm3 and worked my way up. Importantly each time I reprinted a flow rate calibration cube, I measured the first wall again. Naturally I over stepped the mark, and then I started working my way back down in 0.01 increments until it was perfect.

Finally after this I remeasured all the walls again and adjusted the flow rate again if it was required.

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