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A Glossary of 3D Printing Terms
INTRODUCTION
This glossary of terms is being written as part of our A Solid Foundation for 3D Printing series. It will be populated with terms as articles are posted.
Corner Swell
Corner swell is a term for a common “bulge” that can occur at corners. It is undesirable mostly because it negatively affects how well structural prints fit together, but it also gives prints an unprofessional appearance.
A typical single-wall example of corner swell is shown here, viewed from above:
Fundamentally, corner swell is caused by continued extrusion while the printer slows down to navigate the corner. Excessive slowdown can be due to settings addressed in the article Corner Quality. The causes of continued extrusion will be explored here.
A printer’s propensity for corner swell lies in the compression of the filament between the extruder’s drive gear and the nozzle. Corner swell occurs when excess pressure in the filament system causes plastic to continue to extrude when the nozzle slows down at a corner, resulting in a physical bulge.
Like any material, when plastic filament is pushed from behind (extruder) into a wall of resistance (the nozzle), it will compress a small amount under the force. This is described roughly by Hooke’s Law, relating the applied force (F) and the material’s stiffness (k) to the resulting distance that it will compress:
Proportional distance = k*F
Let’s look at the three parts of this equation. First, this is a proportional relationship. Printers with long filament paths (i.e. Bowden systems) will have more distance of filament to compress than printers with short filament distances (i.e. direct drives, or flying extruders for deltas). As a result, in theory, Bowden systems would be slightly more prone to corner swell than a direct drive printer that is otherwise exactly the same with identical settings. However, in real life, Bowden-drive printers have lighter moving components, which reduces ringing, and allows the printers to spend less time slowing down for the corner (this relationship is explored in Ringing).
Also, material choice factors into this. Because of the compressed material behind them, “squishier” materials like TPU or TPE will continue to be pushed out from a nozzle during corner slowdowns, making them prone to corner swell. On the other hand, a filament like PLA, which is rather stiff and does not compress much, will not make a corner bulge out because the filament has not compressed as far under the same pressure.
Finally, the force on the filament also factors into this. Higher backpressure and thus higher extrusion force (perhaps from printing at low temperatures or high speeds) will cause greater compression of the filament, which will manifest itself in increased corner swell.
For applications of these ideas (and more in-depth solutions to ringing) see Corner Quality,
Ringing
Ringing (also known as ghosting) is a visible print issue where a short series of vibrations follow every sharp surface detail. On complex models with plenty of sharply defined edges and details, this “ghost” of each detail is quite distinctive. The term “ringing” comes from the cause of the issue: that the printer physically vibrates for a short time after the corner, due to the sharp force necessary to stop the nozzle’s movement once it reaches the corner. Typically, the wavelength and amplitude of the oscillations remain approximately the same from layer to layer, so the visible marks from the vibrations stack up vertically, creating the vertical lines that follow each sharp detail.
A typical single-wall example of ringing after a simple 90 degree corner is shown here:
Fundamentally, the vibrations that cause this issue come from lateral forces involved in rapidly starting and stopping the moving components. The lower the rigidity of the nozzle positioning system, and the higher the forces, the higher the ringing’s amplitude will be and the farther it will last along the print.
Rigidity
Sometimes a printer’s frame is not as stiff as it could be, due to improper material selection, poor bracing, breaking parts, or even just some loose fasteners. This allows the printer’s frame to deflect farther under the forces applied to it during a sharp direction change. Another source of excessive flexibility is sloppiness in any moving assemblies; for example, bearings might not be rigidly fixed to carriages, smooth rods can be undersized or loosely mounted, or belts can be too loose or too springy. When a printer is well-designed and tightly assembled, the nozzle’s position will not deflect much under sudden forces, so ringing will be low.
Forces
The other factor that contributes to ringing is the force used to make the printer change directions at a corner. The weight of the moving parts factors into this quite a bit: when a moving carriage is heavy, it applies a lot of force to the printer’s belts, and applies a lot of torque to the motor. This causes belts to stretch a little more when printing a sharp corner, which introduces a source of “springiness.” The increased torque applied to the motor also pushes the rotor slightly past where the stepper driver is commanding it to be; the restoring force will cause a small amount of oscillation as well. But when the moving components are lightweight, the forces necessary to make them change direction quickly are smaller, which translates to lower sudden forces on the components responsible for their motion. Lower forces mean lower springy displacements (as per Hooke’s Law) which results in less ringing.
There is one more factor that determines the forces applied at corners, and that is the deceleration applied by the motors. This topic is dealt with in the article Corner Quality because it’s mostly software-based, and it is strongly tied to the issue of Corner Swell.
Damping
The final main factor that influences the appearance of ringing is the amount of damping (relative to the energy stored in the printer’s oscillations). When the printer changes directions quickly and the positioning system is slightly deflected under the forces involved, the frame and movement components store a small amount of energy in their oscillations the way a spring does. Ringing ceases when the energy in that oscillation mode has been dissipated by internal friction in the printer. For instance, when a carriage is “bouncing” along its axis due to a springy belt, its energy is dissipated by the friction in its linear bearing system. While it might seem good to have as little friction as possible, a small amount can have the positive effect of helping to shorten the time that the ringing persists after each corner.
For applications of these ideas (and more in-depth solutions to ringing) see Corner Quality.