144 lines
6.9 KiB
Markdown
144 lines
6.9 KiB
Markdown
This document provides information on tuning the "pressure advance"
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configuration variable for a particular nozzle and filament. The
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pressure advance feature can be helpful in reducing ooze. For more
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information on how pressure advance is implemented see the
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[kinematics](Kinematics.md) document.
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Tuning pressure advance
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=======================
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Pressure advance does two useful things - it reduces ooze during
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non-extrude moves and it reduces blobbing during cornering. This guide
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uses the second feature (reducing blobbing during cornering) as a
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mechanism for tuning.
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In order to calibrate pressure advance the printer must be configured
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and operational. The tuning test involves printing objects and
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inspecting the differences between objects. It is a good idea to read
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this document in full prior to running the test.
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Use a slicer to generate g-code for the large hollow square found in
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[docs/prints/square.stl](prints/square.stl). Use a high speed (eg,
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100mm/s) and a coarse layer height (the layer height should be around
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75% of the nozzle diameter). It is fine to use a low infill (eg, 10%).
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Prepare for the test by issuing the following G-Code commands:
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`SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500` and
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`SET_PRESSURE_ADVANCE ADVANCE_LOOKAHEAD_TIME=0`. These commands make
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the nozzle travel slower through corners and they emphasize the
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effects of extruder pressure.
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For the first print use a pressure advance of zero by running
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`SET_PRESSURE_ADVANCE ADVANCE=0.000`. Then print at least 10 layers of
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the test object. While the object is printing, make a note of which
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direction the head is moving during external perimeters. What many
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people see here is blobbing occurring at the corners - extra filament
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at the corner in the direction the head travels followed by a possible
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lack of filament on the side immediately after that corner:
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This blobbing is the result of pressure in the extruder being released
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as a blob when the head slows down to corner.
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The next step is to increase pressure advance (start with
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`SET_PRESSURE_ADVANCE ADVANCE=0.050`) and reprint the test object.
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With pressure advance, the extruder will retract when the head slows
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down, thus countering the pressure buildup and ideally eliminate the
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blobbing.
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If a test run is done with a pressure advance setting that is too
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high, one typically sees a dimple in the corner followed by possible
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blobbing after the corner (too much filament is retracted during slow
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down and then too much filament is extruded during the following speed
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up after cornering):
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The goal is to find the smallest pressure advance value that results
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in good quality corners:
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Typical pressure advance values are between 0.050 and 1.000 (the high
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end usually only with bowden extruders). If there is no significant
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improvement after gradually increasing pressure advance to 1.000, then
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pressure advance is unlikely to improve the quality of prints. Return
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to a default configuration with pressure advance disabled.
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Although this tuning exercise directly improves the quality of
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corners, it's worth remembering that a good pressure advance
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configuration also reduces ooze throughout the print.
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At the completion of this test, update the extruder's pressure_advance
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setting in the configuration file and issue a RESTART command. The
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RESTART command will also return the acceleration, cornering speeds,
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and look-ahead times to their normal values.
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Important Notes
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===============
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* The pressure advance value is dependent on the extruder, the nozzle,
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and the filament. It is common for filament from different
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manufactures or with different pigments to require significantly
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different pressure advance values. Therefore, one should calibrate
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pressure advance on each printer and with each spool of filament.
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* Printing temperature and extrusion rates can impact pressure
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advance. Be sure to tune the extruder
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[E steps](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#E_steps)
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and
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[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
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prior to tuning pressure advance.
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* It is not unusual for one corner of the test print to be
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consistently different than the other three corners. This typically
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occurs when the slicer arranges to always change Z height at that
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corner. If this occurs, then ignore that corner and tune pressure
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advance using the other three corners.
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* Check for warping at the corners during the test prints (the corners
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detaching from the bed and rising a small distance upwards during
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the print). If one corner appears warped then ignore that corner
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when tuning. If significant warping is seen throughout the test then
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typical solutions are to reduce the slicer's first layer speed,
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adjust the bed temperature, and/or to use the slicer's brim feature.
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Pressure advance itself is unlikely to impact warping, but this
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tuning test is sensitive to it.
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* If a high pressure advance value (eg, over 0.200) is used then one
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may find that the extruder skips when returning to the printer's
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normal acceleration. The pressure advance system accounts for
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pressure by pushing in extra filament during acceleration and
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retracting that filament during deceleration. With a high
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acceleration and high pressure advance the extruder may not have
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enough torque to push the required filament. If this occurs, either
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use a lower acceleration value or disable pressure advance.
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* The pressure_advance_lookahead_time parameter controls how far in
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advance to check if a head slow-down is immediately followed by a
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speed-up - it reduces pointless pressure changes in the head. It is
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recommended to follow the steps above so that it is set to zero
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during tuning and to use the default (0.010) during normal prints.
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It is possible to tune this setting - higher values will reduce the
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amount of pressure change in the nozzle during cornering, but
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setting it too high can cause blobbing during cornering. (Tuning
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this value is unlikely to impact ooze.) The default of 10ms should
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work well on most printers.
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* Once pressure advance is tuned in Klipper, it may still be useful to
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configure a small retract value in the slicer (eg, 0.75mm) and to
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utilize the slicer's "wipe on retract option" if available. These
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slicer settings may help counteract ooze caused by filament cohesion
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(filament pulled out of the nozzle due to the stickiness of the
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plastic). It is recommended to disable the slicer's "z-lift on
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retract" option.
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* Configuring pressure advance results in extra extruder movement
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during move acceleration and deceleration. That extra movement is
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not further constrained by any other other configuration parameter.
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The pressure advance settings only impact extruder movement; they do
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not alter toolhead XYZ movement or look-ahead calculations. A change
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in pressure advance will not change the path or timing of the
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toolhead nor will it change the overall printing time.
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