Flow Rate

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Overview

We often get asked how we calculate our flow rate.

We do not use your slicer-generated E (flow) values or advance/retract commands. This means we do not use your slicer-generated wxtrusion width variations, filament width variations, flow rate multipliers, or any of that.

What do we use? We use the X/Y/Z positioning and speed, and we use the temperature and cooling. We do not use your flow commands - except to determine which are and are not printing moves.

We calculate the flow rate for each and every move based on the following parameters:

  1. Nozzle (extrusion) width, as specified on the head (or, optionally, in gcode); should match the extrusion width in your recipe and gcode.
  2. Layer thickness (height), as specified in your gcode. Width times Height tells us the Cross Section of your print path.
  3. Print speed, as specified in your gcode. Volume divided by print speed tells us how many nl per second to dispense.
  4. Pulses/nl, as specified on the head (or, optionally, in gcode). Pulses/nl times nl/second tells us how many pulses per second to dispense.
  5. Feed Rate %, as specified on the head (or, optionally, in gcode). Feed Rate % is a direct modifier of your other calculations; 1.00 = no modification; 0.90 means 10% less flow, etc.
I just had to type this out again for another customer, so I will save that text here:
To determine flow we consider:

1. Path Width, as specified in the nozzle diameter paramenter on the data for that print head (NOT path width in the slicer) [unless you manually hard code a value in your gcode];

2. Path Height (or Layer Thickness), as specified in your gcode by an M756 command with an S (slice thickness) value (NOT on the print head);

- 1 x 2 - these two define the a cross section to fill during this move.

3. Print Speed, as specified in your gcode by a G1 command with an F (feed rate) value (which should be set by your slicer, but may be edited afterward);

- 1 x 2 x 3 - cross section times linear speed gives us volume per unit time (in nl/sec) to be dispensed.

4. Pulses per Nanoliter (actually, pulses for 10 nanoliters), as specified on the data for that print head (NOT path width in the slicer) [unless you manually hard code a value in your gcode];

- 1 x 2 x 3 x 4 - volume per unit time times pulses per unit volume gives us how fast in pulses per second to dispense.

5. Feed Rate %, as specified on the data for that print head (NOT path width in the slicer) [unless you manually hard code a value in your gcode];

- 1 x 2 x 3 x 4 x 5 - pulses per second is modified by this feed rate percentage (default of 1) to account for over/under sized filament or slippage due to gummy material.

We also determine when we are transitioning between printing and non-printing moves, and prime (advance) and unprime (retract) per the variables stored on your print head (or as you have manually specified in your gcode).

GCode

So, how can you specify this in GCode?

One of the things that a slicer does is generate extrusion values along with the moves, so that printers know how much material to deploy (or not to deploy any) during a move. For example:

G1 X107.310 Y122.630 E1.45876

This line tells the printer to do a normal speed move from the present location to position X=107.310 and Y=122.630 while extruding a certain amount E1.45876 of material. However, on Hyrel machines, the material feed rate is calculated from the data stored on the print head, and is not taken directly from the gcode.

We do a simple boolean check on the G1 move to determine if it is a printing move or not (if it has an E value or not), and we calculate our own flow based on path width, layer height, pulses per nanoliter and the FR Scale which are stored in the head data. We also calculate our own prime/unprime settings based on the data displayed and stored on the print head.

The default setting for the EMO-25 is 1.76 pulses/nl and 1.0 feed rate. In gcode, this is:

M221 S1 P1.76 T12

This tells the printer to do a Set tool values of Material Flow Rate Multiplier (formerly called Feed Rate Adjustment or "fudge factor") S1 times Pulses per nanoliter P1.76 for the Tool at Yoke 1, Position 2 T12.

This is sourced in as a default when you start the job, but can be changed programmatically in the gcode as follows:

 G1 F1200            ; set print speed to 1200 mm/min (20 mm/sec)
 G1 X10 Y10 Z0.25    ; move to start of line 1
 M221 S1 P1.76 T12   ; set extrusion rate to 1*1.76 on yoke 1, tool 2 (base value)
 G1 X60 Y10 E1       ; 1st printing move
 G1 X10 Y20          ; move to start of line 2 
 M221 S1.1 P1.76 T12 ; set extrusion rate to 1.1*1.76 on yoke 1, tool 2 (10% greater than line 1)
 G1 X60 Y20 E1       ; 2nd printing move
 G1 X10 Y30          ; move to start of line 3
 M221 S1.2 P1.76 T12 ; set extrusion rate to 1.2*1.76 on yoke 1, tool 2 (20% greater than line 1)
 G1 X60 Y30 E1       ; 3rd printing move

To give this more depth, here is exactly how we calculate the flow rate.

At the start of a job (when you click "Run Job"), Repetrel transmits settings via gcode commands to the STM407 Motion Controller, based on the data you have displayed for that print head. For our example, I have a MK1 head loaded in slot 2:

 M6 T12 O2 X0 Y0 Z0
 M721 S10000 E300 P80 T12
 M722 S10000 E300 P105 T12
 M221 S1.0 T12 P0.80 W0.5 Z0.3

Let's break this down:

M6 T12 O2 X0 Y0 Z0 - sets the offsets for tool 2 (normally zero unless you have heads cooperating on a print, but you could program them for their distance from center; T2 is centered on newer printers, and 35mm off center in the +X direction on older printers. On all printers, T3 35mm off center in the -X, T1 is 70mm +X, T4 is 70mm -X.

M721 S10000 E300 P80 T12 - sets the UNPRIME values, in rate (S), min dwell time (E), and pulses (P) for slot 2 (T)

M722 S10000 E300 P105 T12 - sets the PRIME values as above;

M221 S1.0 T12 P0.80 W0.5 Z0.3 - sets up your flow data: S1.0 = adjustment value of 1.0 (100% of the pulses per nanoliter) T12 = tool at yoke 1, slot 2 P0.80 = pulses per nanoliter W0.5 = extrusion width Z0.3 = Z layer thickness

Now, any or all of these variables may be updated with a new value by a subsequent M221 command; all values are persistent unless/until updated to a new (possibly 0) value later on in the code - or by adjusting the S or P values live on the print head.

Immediate Commands

While M721 S10000 E300 P105 T12 will set up the prime values for the print head, it will not cause these values to be executed.

M721 I1 will cause that print head last addressed (or M721 T12 I1 if you want to specify) to perform the prime at this point in the code, with the values previously stored for that head.

Likewise, M722 I1 will cause the print head to do an unprime.