Banding

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Banding is a common artifact that describes a periodic pattern in layers that should be even. It is unpleasant to see as it gives a rough texture to the prints, and it also reduces the bonding strength between layers, making the object prone to delamination.

Banding bad.jpg Banding good.jpg
Object with banding Object without banding

There are a number of causes for banding, and they depend on various levels of hardware imperfections. In order to have a completely banding-free print, all aspects have to be taken into account.

Contents

[edit] Z Axis drive mechanism imperfections

Unexpected movement in the z-axis drive mechanism is, intuitively, the most important source of banding, as it coincides with movement the Z (vertical) direction. The Solidoodle uses a conventional threaded rod and nut for translating the circular movement of the stepper motor into linear movement in Z direction, but this tends to suffer from two separate but interacting sources of error: backlash and wobbling. Backlash--unexpected vertical movement--results from play in the interface between the nut and leadscrew. Wobbling--unexpected lateral movement--results from a leadscrew that is not perfectly straight or centered on the axis of rotation.

[edit] Backlash

Backlash is the defined as the play between adjacent moveable parts. In the context of banding, backlash refers to the play between the threads of the z-axis leadscrew and nut, and corresponding distance the bed can freely move up and down due to this play. Because of this looseness, not every movement of the screw corresponds to an immediate and equal movement of the nut. For instance, when the threads are touching during movement in one direction, if the direction is then reversed, the threads will have to move by the amount of backlash before actually having an effect on the nut (and on the bed). This effect is also known as "hysteresis", and is most apparent when the bed is instructed to change directions in the middle of a print, as when the "Lift Z" option in slic3r is active.

Even without reversing the bed direction, however, backlash can still introduce imprecise z-positioning that influences or exaggerates banding in general. When combined with wobble (below), this effect can cause classic sinusoidal banding. As a primary cause with readily available free solutions, backlash should be the first problem to be addressed when attempting to reduce both random and periodic banding. A number of backlash-reduction devices are available, including a jam nut, or a spring-based preload.

[edit] Wobbling

Wobble.jpg

The Z-rod is never perfectly aligned or straight, and this imprecision can cause the leadscrew "wobble", exerting a lateral force on the nut. While this force can result in some slight movement in the XY direction (minimized on the Solidoodle by fixed guide rods), its primary effect is to cause the nut to "unseat" and rise up from its lowest position and move within the range of any uncorrected backlash (because the thread tooth profile is cut at an angle). Additional nonlinearities in the nut movement can be introduced from the manufacturing process of the leadscrew. The result of both effects is that every layer will have a slightly different thickness. Given the same flow rate, some layers will become wider and other layers thinner, creating banding that is most pronounced with small layer heights, and that repeats with a period equal to the thread pitch of the leadscrew.

Wobbling effects can be addressed by replacing the leadscrew with a higher quality rod (like an ACME leadscrew), replacing the leadscrew and nut with ones with thinner, finer-threaded versions (like an M5) or by using firmware compensation to counteract any nonlinearity in the z-axis movement.

[edit] Slicing/XY axes imperfections

Backlash in the XY axes can also cause banding when the layers do not start from the same point. This banding is different from z-wobble in that layers are shifted rather than shrunken or enlarged, so that a peak on one side of a print is matched with a trough on the opposite side. With slic3r, this can occur when the feature "randomize starting points" is enabled. In this case, simply disabling the feature or using a firmware compensation (already included in the official Solidoodle firmware) will help.

[edit] Temperature swings

Unstable temperature swings of the hot end can also cause a type of banding whose spacing varies with the width of each layer. To fix it, calibrate your heating settings with the PID tuning procedure.

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