95 lines
4.3 KiB
Markdown
95 lines
4.3 KiB
Markdown
This document provides information on tuning the "pressure advance"
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configuration variables 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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Prerequisites
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=============
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In order to tune the pressure advance setting the printer must be
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configured and operational. The tuning test involves printing objects
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and inspecting the differences between objects. In particular, the
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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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should be tuned prior to tuning pressure advance.
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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 measuring and tuning the pressure advance configuration.
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Start by changing the extruder section of the config file so that
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pressure_advance is set to 0.0. (Make sure to issue a RESTART command
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after each update to the config file so that the new configuration
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takes effect.) Then print at least 10 layers of a large hollow square
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at high speed (eg, 100mm/s). See
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[docs/prints/square.stl](prints/square.stl) file for an STL file that
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one may use. 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 set pressure_advance_lookahead_time to 0.0, slowly
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increase pressure_advance (eg, start with 0.05), and reprint the test
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object. (Be sure to issue RESTART between each config change.) The
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goal is to attempt to eliminate the blobbing during cornering. (With
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pressure advance, the extruder will retract when the head slows down,
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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 0.05 and 0.20 (the high
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end usually only with bowden extruders).
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It is not unusual for one corner of the test print to be consistently
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different than the other three corners. This typically occurs when the
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slicer arranges to always change Z height at that corner. If this
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occurs, then ignore that corner and tune pressure_advance using the
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other three corners.
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Once a good pressure_advance value is found, return
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pressure_advance_lookahead_time to its default (0.010). This parameter
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controls how far in advance to check if a head slow-down is
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immediately followed by a speed-up - it reduces pointless pressure
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changes in the head. It's possible to tune this - higher values will
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decrease the number of pressure changes in the nozzle at the expense
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of permitting more blobbing during cornering. (Tuning this value is
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unlikely to impact ooze.) The default of 10ms should work well on most
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printers.
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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 can reduce ooze throughout the print.
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Finally, once pressure_advance is tuned in Klipper, it may still be
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useful to configure a small retract value in the slicer (eg, 0.75mm)
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and to utilize the slicer's "wipe on retract option" if available.
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These slicer settings may help counteract ooze caused by filament
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cohesion (filament pulled out of the nozzle due to the stickiness of
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the plastic). It is recommended to disable the slicer's "z-lift on
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retract" option.
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