First, you need to make sure that you're starting from a good place. If you haven't already tuned your 3D printer, I suggest using Mark Gonsowski's cura settings that you can find here. You may need to tune retraction speed & distance, the extrusion multiplier, and temperatures depending upon your printer and the material you print with.
Too Long, didn't read?
Print slowly (40mm/s) with small layer height (perhaps 0.1mm) inside of an enclosure and use your best adhesion control. Print at the same temperatures at which you normally print your filament. Print framed lithophanes so that their long dimension goes in the same direction that your bed moves. If you're still running into 3D printing problems, then come back and read more closely.
Common errors that occur when 3D printing a lithophane include 1) visible layer lines, 2) layer separation and cracking, and 3) warping and curling. Fortunately, there are ways to reduce or eliminate all of these problems.
Visible layer lines are hard to completely eliminate. They occur for a variety of reasons (listed in order of likelihood) such as inconsistent extrusion, layer height, wide layers, and vibration. Still, they are possible to reduce and here is how:
1) Extrude consistently. Inconsistent extrusion is probably the main cause of layer lines in your 3D printed lithophane. Inconsistent extrusion can be caused by many things including 1) partially clogged nozzle, 2) temperature fluctuations, and 3) inconsistent friction. If you have a bowden extruder, you need to make sure that there is no gap between your bowden tube and the nozzle. If there is a grap there, you can get a partial clog if filament gets squished and caught between the tube and the nozzle. Temperature fluctuations in your nozzle can cause inconsistent extrusion. If the temperature of the nozzle fluctuates outside of the printing range of your filament then you really need to work on this problem by tuning your PID coefficients. Inconsistent friction occurs when your filament roll gets caught while rotating or the filament rubs against your 3D printer while it's fed to the extruder.
2) Lower layer height. This is usually people's first go-to. It helps, but will increase print time.
3) Thinner layer widths. Using a smaller layer width reduces the absolute error of that layer width. For example, say that your layer widths vary by 20%. If you have a layer width of 0.6mm then that 20% variation means your lithophane thickness could be off by 0.12mm, while if you had used a 0.3mm layer width then the variation would be 0.06mm.
4) Compliant frame. You might need to stiffen up your printer by adding some structural support. You may have better luck if you rotate your lithophane, so that it is lined up with the motion of the bed.
5) Smaller nozzle and more time. 2) and 3) above are both easily achievable, but may require a smaller nozzle and more time.
Plastics have a high coefficient of thermal expansion (CTE). Due to the high CTE of plastics, temperature gradients in your 3D print will cause stress. Additionally, the difference in the CTE of your plastic and the CTE of your print bed will cause stress as the print bed tries to expand a different amount than the printed part and the two push and pull against each other at their interface. These effects result in the plastic part separating from the print bed and curling up at corners where the stress is the highest. Okay, that's a fun lot of theory, but what can we do to eliminate the problem?
1) Increasing the adhesion between the print and the print bed is of great importance. To increase adhesion, I clean my print bed before each print by wiping it down first with a small amount of acetone and then cleaning that up with a small amount of isopropyl alcohol. Acetone disolves ABS and PLA (given large quantities of acetone liquid) to remove particulates that would get between the plastic you're laying down and the print bed. Isopropyl alcohol doesn't leave resident and can be used to clean up what was left behind by the acetone. Any remaining isopropyl alcohol simply evaporates and leaves you with a beautifully clean print bed. Note that acetone is flammable and can hurt your skin. Look up and employ proper safety precautions before and while using acetone.
2) The set-point temperature of your print bed is worth tuning. Note that I said "set-point temperature" of your print bed and not "the temperature" of your print bed. Different printers will require different set-points simply because some printers have more insulation between their sensor and the surface of the print bed than other printers. More thermal resistance (like glass + PEI sheet which is what I use) means that the actual bed temperature will be further below the set-point temperature than if you had less thermal resistance. Additionally, the top of the print may be masking tape, a sheet of PEI, bare glass, or a number of other things. Regardless of the surface you use, I recommend that you start with the filament manufacturer's bed temperature but then tweak it to find a sweet spot that's perfect for adhesion.
3) Bed leveling is also strongly at play here. The more level your bed, the better your adhesion.
4) Rafts or brims can improve adhesion. I like to print with a raft. It's easy mode. I don't want to do things the hard way. Does it use more plastic? Yes, but I value plastic less than my own time. These days plastic is cheap, and a raft takes much less plastic than a failed print. Also, rafts save you from having to perfectly level the bed and perfectly hone in your z-offset, because all of the z-offset error is taken out in the raft instead of giving your print elephant's toe.
Layer separation and cracking happens for the same reasons that cause warping and curling (discussed above), so I won't go back into it. The difference is that we're now concerned with adhesion between a new layer of plastic and the layer below it. How do you reduce and prevent layer separation and cracking?
1) Lower layer height. A smaller layer height forces the 3D printer to firmly press new plastic onto old plastic. This pressure helps bond the two layers together.
2) Hotter nozzle temperature. A hotter nozzle will extrude a hotter plastic. The hotter the new plastic is, the hotter the already laid down plastic will become when they're pushed together, and the stronger the bond between the two layers will be.
3) Heated chamber. This is not a standard feature for most affordable 3D printers. If you're able to afford this luxury, good for you! I expect you already know how to use it. A heated chamber keeps the air around the print at a higher temperature which makes the print more isothermal (small temperature gradients). Lower temperature gradients mean lower thermal stress and less layer separation.
The look of your lithophane is also greatly affected by the filament that you use and the light source that you use. Not all "white" filaments are actually the same color of white. They all have a unique tint to them which will affect the look of your lithophane.
Likewise, the lighting that you select for your lithophane will affect its appearance. Obviously, a red backlight will make your filament look quite a bit different than a green or white backlight. There are several different types of lights and they are quantified by their color temperature. The color temperature refers to the temperature in Kelvin that a black body would need to be in order to emit a spectrum of colors following the Stefan-Boltzmann law. The practical relationship between temperature and color is nicely summarized at http://www.westinghouselighting.com/color-temperature.aspx.
For now, this is simply something to become cognizant of. I plan on comparing different filaments and lighting in the future, but I haven't yet.
Got any awesome suggestions for how to improve lithophane 3d prints? Contact me at [email protected]