Extrusion printing is a notoriously messy process. The same material properties that allow for bridging, overhangs and good adhesion lead to nozzle buildup and print errors. This is a particular problem with multi-material prints where the inactive nozzles often ooze material. The H-Series tool changer moves these inactive nozzles out of contact with the part, so the oozed material doesn’t contaminate the print. However, the unknown volume of lost material still presents a problem when that printhead is used for the next operation. Purge walls and prime pillars can help to prep the printhead before it is brought into contact with the part, however some of the lost material inevitably builds up on the outer surface of the nozzle and makes its way to the print. For this reason, we felt an effective and repeatable nozzle cleaning system was critical to the function of the H-Series machine. We prototyped many different configurations of wipers and brushes, both passive and active. The problem was always that the cleaning action got worse over time. Melted plastic would build up on the bristles, often transferring back onto the nozzle in the next pass. Flexible wipers had similar issues and often did not offer enough abrasive action to remove the melted plastic.
The H-Series cleaning system starts with the nozzle design. An extended cylindrical nose and non-stick plating provide a surface that is well-suited for automated cleaning. A solenoid driven pincer mechanism (shown in the image above) is used to close a pair of blades radially over the nose of the nozzle and scrape material axially away from the nozzle tip. This offers a very repeatable and low maintenance cleaning action. With the turret tool changer, the cleaning step is done at a secondary U-axis position with the nozzle horizontal. This way no X or Y travel is lost to make room for the cleaning location.
The video above shows a typical H-Series cleaning cycle:
Active nozzle is first lifted vertically off the print.
Bed is then moved to its Y-max position to prevent any chips from falling into the build volume.
Turret is indexed to bring the next tool into the “prime” position and the prime surface is moved outward into close clearance with the end of the nozzle.
Material is extruded against the prime surface to make up any oozed material and fully charge the hotend. This creates a puck of extruded plastic surrounding the nozzle.
Extruder retracts a set amount and the turret is indexed again to move the nozzle into the cleaning position.
Pincer mechanism is activated to pull the puck axially off the end of the nozzle.
Turret is indexed to bring the nozzle into the printing position and the print is resumed with the extruder first making up the retracted volume from the prime step.
Nozzle cleaning adds very little time to tool changes, so it makes sense to perform it every time a tool is called up. This results in clean, precise finished products.