4.3 KiB
This document provides information on tuning the "pressure advance" configuration variables for a particular nozzle and filament. The pressure advance feature can be helpful in reducing ooze. For more information on how pressure advance is implemented see the kinematics document.
Prerequisites
In order to tune the pressure advance setting the printer must be configured and operational. The tuning test involves printing objects and inspecting the differences between objects. In particular, the extruder E steps and nozzle temperature should be tuned prior to tuning pressure advance.
Tuning pressure advance
Pressure advance does two useful things - it reduces ooze during non-extrude moves and it reduces blobbing during cornering. This guide uses the second feature (reducing blobbing during cornering) as a mechanism for measuring and tuning the pressure advance configuration.
Start by changing the extruder section of the config file so that pressure_advance is set to 0.0. (Make sure to issue a RESTART command after each update to the config file so that the new configuration takes effect.) Then print at least 10 layers of a large hollow square at high speed (eg, 100mm/s). See docs/prints/square.stl file for an STL file that one may use. While the object is printing, make a note of which direction the head is moving during external perimeters. What many people see here is blobbing occurring at the corners - extra filament at the corner in the direction the head travels followed by a possible lack of filament on the side immediately after that corner:
This blobbing is the result of pressure in the extruder being released as a blob when the head slows down to corner.
The next step is to set pressure_advance_lookahead_time to 0.0, slowly increase pressure_advance (eg, start with 0.05), and reprint the test object. (Be sure to issue RESTART between each config change.) The goal is to attempt to eliminate the blobbing during cornering. (With pressure advance, the extruder will retract when the head slows down, thus countering the pressure buildup and ideally eliminate the blobbing.)
If a test run is done with a pressure_advance setting that is too high, one typically sees a dimple in the corner followed by possible blobbing after the corner (too much filament is retracted during slow down and then too much filament is extruded during the following speed up after cornering):
The goal is to find the smallest pressure_advance value that results in good quality corners:
Typical pressure_advance values are between 0.05 and 0.20 (the high end usually only with bowden extruders).
It is not unusual for one corner of the test print to be consistently different than the other three corners. This typically occurs when the slicer arranges to always change Z height at that corner. If this occurs, then ignore that corner and tune pressure_advance using the other three corners.
Once a good pressure_advance value is found, return pressure_advance_lookahead_time to its default (0.010). This parameter controls how far in advance to check if a head slow-down is immediately followed by a speed-up - it reduces pointless pressure changes in the head. It's possible to tune this - higher values will decrease the number of pressure changes in the nozzle at the expense of permitting more blobbing during cornering. (Tuning this value is unlikely to impact ooze.) The default of 10ms should work well on most printers.
Although this tuning exercise directly improves the quality of corners, it's worth remembering that a good pressure advance configuration can reduce ooze throughout the print.
Finally, once pressure_advance is tuned in Klipper, it may still be useful to configure a small retract value in the slicer (eg, 0.75mm) and to utilize the slicer's "wipe on retract option" if available. These slicer settings may help counteract ooze caused by filament cohesion (filament pulled out of the nozzle due to the stickiness of the plastic). It is recommended to disable the slicer's "z-lift on retract" option.