5.1 Slicer Calibration

To get top quality prints, you will need to invest some time in calibrating your slicing profiles to suit both your printer and choice of filaments. Fortunately, the process (presented in Section 5.1.4) is simple and straightforward, though it does require a basic understanding of your slicing software.

5.1.1 Filament Diameter

The need to measure your filament diameter before preparing a model for printing cannot be stressed enough. It is important that you have use of a caliper with which to measure the diameter of your filament.1 Filament sold as 1.75 mm filament may in fact be 1.68 or 1.85 mm in diameter. However, when preparing models for printing, especially large models, it is critical that the slicing process know the actual diameter of the filament. Otherwise too much or too little filament will be extruded, as the slicer determines the amount of filament needed based upon the expected volume per millimeter of raw input filament. If the actual diameter is larger than expected, too much plastic will be extruded and “over-extrusion” results. And, if the diameter is smaller than expected, too little plastic will be extruded and “under-extrusion” results.

With a calibrated slicing profile and measured filament diameter, very smooth top surfaces can be achieved when printing. The blue tag print of Figure 5.1 is such an example.

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Figure 5.1: Smooth surface finish achieved through careful calibration and measurement

Over and under-extrusion has a cumulative effect: it will not impact small models nearly as much as it will large models, which may fail to properly print. Over-extrusion can jam the extruder when the buildup of plastic becomes so much that there is no longer a small gap between the nozzle and the previously printed layer. A printed model with a deficit of plastic — under-extrusion — may be too weak to serve its intended purpose. Indeed, it may even break apart while being printed or removed from the build plate.

5.1.2 USB vs. SD Card

If possible, prints should always be run from an SD card. Printing over USB introduces additional sources of problems and may result in prints with small bumps. Accelerated printing runs fast enough that, if your computer is busy with other tasks, there may occasionally be small delays introduced to the serial communications. These delays can cause the printer to pause for very brief periods of time and produce small bumps or blemishes — the result of filament oozing out of the idle extruder.2

When calibrating your printer, avoid this additional complication and print from an SD card. If you are ever printing over USB and see printing defects you do not expect, then attempt the same print from an SD card and see if that leads to an improvement.

5.1.3 Know Your Defects

Before launching into a discussion of tuning, you should first be familiar with three particular print defects which, incidentally, occur with or without accelerated printing and are not unique to Sailfish. There are other defects as well, but familiarity with these three is helpful as they may appear on your calibration boxes. You may not be used to seeing them when printing at low speeds (e.g., 40 mm/s), but at higher speeds, accelerated or otherwise, they will be more likely to manifest.

5.1.3.1 Infill telegraphing

Infill telegraphing, such as that seen in Figure 5.2, often occurs when printing with a single shell and under-extruding. It may also be caused by a hot extruder nozzle dragging over or pushing into the layer below it, as can occur when over-extruding. Make sure that you correctly measured the filament diameter and supplied that value to your slicer when slicing. Also, be sure to calibrate your extruder as outlined in Section 5.1.4.

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Figure 5.2: Infill telegraphing through the print’s perimeter

Any overshoot caused during infill direction changes may also cause this effect, in which case an extra shell will help hide it. If excess momentum is implicated, then the same techniques used to reduce corner ringing (Section 5.1.3.2), may also be applied here.

A short term fix may be to slice your model with an additional shell or two.

Note that Figure 5.2 is an extreme case — to the point where the infill itself is visible between some of the perimeter’s layers. On some of the layers where the infill is not directly visible, ripples caused by it or the nozzle pushing against the inside of the perimeter may still be visible.

5.1.3.2 Ringing

At higher print speeds, when the motion of the build platform or extruder makes a sudden change in direction, a ringing vibration may occur.3 Since sudden direction changes are often associated with turning a corner in the print, this effect is often referred to as “corner ringing”.

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Figure 5.3: Ripples caused by ringing

Although quickly dampened, this ringing nonetheless impacts surface finish, as can be seen in Figure 5.3. Ringing is particularly noticeable on long, flat surfaces and appears as ripples which rapidly diminish in amplitude. Like infill telegraphing, this can occur with non-accelerated printing as well as accelerated printing. The advantage of accelerated printing is that acceleration can be used to mitigate the issue while still attaining high print speeds away from corners by lowering the maximum X and Y speed change values, the maximum X and Y accelerations, or any combination of the two. For non-accelerated printing, the only recourse is to print the entire model at slower speeds.

Also note that the mechanics of a given printer may contribute to ringing effects. If you are experiencing problems with ringing, then research postings about ringing in online forums and newsgroups relevant to your make of printer. You can also try rotating your model 45 before slicing.

5.1.3.3 Stepper clipping

When infill telegraphing and corner ringing are not present, you may see some very faint rippling along flat faces when you look carefully in the right lighting and from the right angle. Figure 5.4 shows a print done on a Thing-o-Matic in ABS plastic which illustrates this effect.

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Figure 5.4: Faint ripples caused by stepper clipping

This faint rippling may be caused by clipping of the sinusoidal control signals to the stepper motors. Thing-o-Matics and Cupcakes with stepper motors from before May 2011 are especially vulnerable to this issue. Unfortunately, later models suffer as well, as evidenced by the model shown in Figure 5.4 which was produced on a Thing-o-Matic with post May 2011 stepper motors. Ed Nisley described and analyzed this in his May 2011 blog posting, Thing-o-Matic: MBI Stepper Motor Analysis, found at softsolder.com. At low speeds, this clipping produces a faint repeating pattern of about 25 Hz. At much higher stepper speeds, the pattern has a frequency of 50 Hz. Hardware replacement is needed to nearly eliminate the problem: stepper motors with winding resistances at or below 2 Ohms, sufficient pull in torque, sufficiently low inductance, and stepper drivers capable of handling the motors (e.g., Pololu).

Significantly slowing down the exterior printing speed may help mitigate the effects of stepper clipping. On well-designed and tuned printers, this is often the one remaining print quality issue. On suboptimal or untuned printers, it is still present but hidden by more visible printing defects.

5.1.4 Calibration Box

To achieve quality prints, start by ensuring that you can print a decent calibration “box” whose top is nice and flat. Producing a respectable box involves calibrating a slicing profile to your printer and choice of filaments. So, until you can print a good calibration box, there is little point in worrying about other printing defects you may be experiencing. Here is the step-by-step procedure for accomplishing this calibration:

1.
Obtain a model for a 10 mm high box which is 20 mm on a side. ReplicatorG contains as its first example this calibration box: look under the Examples section of the File menu. It is the 20mm_Calibration_Box.stl. Alternatively, Thing #2064 at thingiverse.com contains the calibration box as the download file 20mmbox.stl.
2.
Use calipers to measure the diameter of the filament with which you will be printing.
3.
With the calibration box model in your slicer, slice it at a 0.3 mm layer height, 100% infill, and using the diameter of the filament you just measured.4 It is critical that you use 100% infill and that you measure the diameter of your filament and input that to the slicer.
4.
Print the box.
5.
Carefully examine the top surface of the box. While it is easy to see if the top is convex, you may need to use a straight edge to gauge how flat or concave the top is.

(a)
If it is nice and flat, then you are done!
(b)
If it is convex, then too much plastic was extruded and your printer is over-extruding. Configure your slicing profile to put out slightly less plastic. How you will do this depends upon which slicer you use. For ReplicatorG, increase the “filament packing density” in the Dimension plugin. For MakerBot MakerWare and Desktop, increase the “feedstockMultiplier”. For Simplify3D, reduce the “extrusion multiplier”. Only change the value in small increments, such as 0.05.
(c)
If it is slightly hollow (concave), then too little plastic was extruded: your printer is under-extruding. Decrease the filament packing density (ReplicatorG), decrease the feedstockMultiplier (MakerWare and Desktop), or increase the extrusion multiplier (Simplify3D).
6.
Go back to Step 3, reslicing, reprinting, and re-evaluating the result.

If you happen to have two extruders, it is recommended to do this calibration once for each extruder. Then keep distinct slicing profiles for each extruder: one for the right extruder and another for the left extruder.

Once you can print a nice calibration box, you are ready to get back to printing. Keep in mind that this calibration process should be repeated for different type of plastics. At issue is the differing hardnesses of the plastics used. The pinch gear in your printer’s extruder feed mechanism bites into the plastic filament. The depth to which it bites depends upon the hardness of the plastic. And the deeper the bite, the smaller the effective turning radius of the gear. With smaller turning radius, less filament is fed per rotation of the extruder stepper motor. This calibration is primarily to address your extruder’s handling of these variations in hardness. For example, ABS is significantly softer than PLA and so significantly different adjustments may be needed for ABS versus PLA. This will, of course, depend upon the geometry of the pinch gear and how capable it is of biting into the filament.

1A digital caliper is easiest to use although a dial or vernier caliper will work just as well. You will need to produce measurements in units of millimeters, mm. Consequently, calipers which directly read in those units is preferred. Otherwise, you will need to convert to units of mm.

2If the computer is not sending commands over USB to the printer, then even features such as deprime (Section 4.5) and slowdown (Section 4.6) will not help prevent the formation of blemishes.

3In severe cases, there may be actual overshoot caused by the extruder carriage’s momentum carrying it farther than intended. At issue is that the printer has no feedback mechanism and thus does not know whether the maximum rates of acceleration and deceleration with which it has been configured are actually achievable.

4If using ReplicatorG with Skeinforge-50, then do not use hexagonal infill. There is a bug in SF-50’s hexagonal infill which will result in irregular infill; use grid rectangular or line infill instead