Understanding variable layer heights for quick or higher quality 3D prints.

If you read our Exploring 3D column in Issue 020, we discussed the importance and benefits of learning Slicing techniques. This highlighted some of the more advanced features of your slicer, including processes in order to print using a variable layer height.

In this issue, we are going to show you how and when you should use this variable layer height technique.

WHY IS A VARIABLE LAYER HEIGHT USEFUL?

The layer height is, for the most part, an important factor in the appearance of your 3D printed item. With a fine layer height of say, 100-micron or less, the individual layer heights will be very small and give the appearance of a smooth object. However, this fine layer height also takes substantially longer to print, compared to a much coarser layer height. In fact, the relationship is fairly linear. If a part takes one hour to print at low resolution of 300 microns, the exact same part at 100-micron will take about three hours.

On top of this, not all surfaces benefit equally from printing at higher resolutions.

Take for example the images shown below, which use the Cali Cal calibration and benchmarking print from Dezign on Thingiverse: https://www.thingiverse.com/thing:1545913

The difference in layer height here is quite obvious. You can see the black Cali Cat was printed at a 200-micron layer height while the grey Cali Cat was printed at 100-micron.

It becomes harder to notice this when we orient the part slightly differently, as the layer lines don’t catch the light quite as much.

Several other factors can also affect the visibility of layer lines. Round or curved objects on the vertical plane become very obviously marked with horizontal striations. This can be seen on the octopus image shown below, which was printed at a 300-micron layer height.

This Octopus design by McGybeer can be downloaded from Thingiverse: https://www.thingiverse.com/thing:3495390

At this height, it took 1 hour and 44 mins to print using 7.31m of PLA, and the curved surfaces show very significant banding.

To rectify this, we can print it at a finer resolution which will make the banding significantly smaller.

In the image shown below we can see that the layer striations have been significantly reduced resulting in a much smoother appearance and better-finished model. However, this came a significant cost.

At this resolution, it took 5 hours and twenty mins using 9.76m of PLA. This means it took 3.6 hours longer to print compared to the 300-micron version and used 2.45m more PLA.

Note: In this case, we did use a raft which will add some time and filament cost, however, this was done to reduce failures at such a fine resolution where the small tentacle pieces would lose bed adhesion and the print would fail. As such we suggest at 100-microns or lower a raft is mandatory.

This is where clever use of processes can be used to help you print great prints without taking as much time as printing the entire print in high resolution.

For this, we are going to use the three most popular 3D printing slicers. Cura 4.0 which is free software you can download from: https://ultimaker.com/en/products/ultimaker-cura-software

And Simplify3D 4.1 which can be purchased here: https://www.simplify3d.com/

and finally, Slic3r which can be downloaded for free from here: https://slic3r.org/

Before we start we want to look at the model in a 3D viewer to identify which areas need to be printed at the lower layer height to improve their appearance and which won’t really matter. I find the best place to do this is in your slicer. Slice the model to 300-micron and use the built-in viewer to inspect the part overall.

While changing the layer height, look for large gaps between the end of one layer and the start of the next. These large gaps can easily make your part look quite rough and inorganic.

The top of the head will certainly benefit from printing at a lower layer height as its roundness provides quite large striations.

Like with the head, the tentacles have a large round top that shows significant striations when compared to the rest of the model.

Lastly, the top of what I think is supposed to represent the mouth or maybe nose will look quite misshapen when printed at a 300-micron layer height. As such printing this section at 100-micron will greatly improve the look of this feature.

These are the 3 main areas which will detrimentally affect the look of the print when printed at 300-micron layer height. Other sections of fine detail such as the eyes could also benefit but are not quite as obvious.

Now we have identified which areas we want to print at a higher resolution we need to convey this information to our slicing software.

Since there are a number of different slicers we are going to look at arguably the three most popular; Cura, Slic3r and Simplify3D.

Simplify3D:

With Simplify3D you need to import the model and orient it on your build platform as you want it to print.

Process 1 will be your starting process, so you simply click on “Edit process settings” and set the starting values.

Since the first layers of this model are not adversely affected by a high layer height, process 1 can be printed at 300-micron. For simplicity we just use the “auto configure for print quality” drop-down and select “Fast”.

Note: You can also use this time to change speeds, temperatures and infill etc. however, since we are only interested in layers at this point we can just change the Auto-configure setting.

From here you want to go to the advanced tab and under Layer Modifications check the “Stop printing at height" check box.

Since the first 3mm of the print can be printed fine at 300-micron lets set the stop height to 3mm. This means if we were to slice this model as it is, the print will only print the first 3mm. It also means it will finish the model with a fully enclosed top layer which we don’t want, so be sure to go to the Layer tab and select 0 top solid Layers.

Now what we need to do is have the next part of the print, print on top of this base. To do that we need to add another process. This is done by clicking the add button in the bottom right of the Simplify3D workspace.

This will add a second process to the build list. In this process, we want to print at the much finer resolution or 100-microns to shrink the gaps between layers.

To do this double click on the process 2 and change the Auto-Configure option to high. However, we also need to go to the Layer tab and instruct the slicer not to print a bottom or top solid layer as these layers will simply sit on top of the previous layers.

We also need to tell the slicer to start this print from the previous 3mm point and print until the top of the tentacles which is about 9mm from the base of the print.

To do this go back to Advanced and under Layer Modifications tell the slicer to start printing from 3mm and stop printing at 9mm.

You’re going to want to verify that each layer is going to print how you expect it to print. The best way to do this is to click the Prepare to Print button and select all of the relevant processes.

This will provide you with a detailed view of the print. You can easily see the transition from 300 to 100-micron layers. Take the time to make sure these transitions are where you want them, simply adjust the start and stop distance to get the desired results.

The next step is to add another process to print from the previous 9mm endpoint until the next desired transition point. This point is the top section of the nose or mouth of the octopus which starts around 14mm from the base.

Note: Make sure you enable top layers on the final print as failing to do so will cause the top to be very thin and unsupported.

You want to continue adding processes which start and end at your chosen heights until the entire print is included in the processes. For this model, we used the following settings.

When printed in this configuration the print takes 3 and a half hours using 9.2 metres of plastic, a time saving of 1.4 hours compared to using a 100-micron layer height.

As you can see in the image on the right, the transition from the 300-micron first layers to the 100-micron top of the tentacles. This could be smoothed out by adding a 200-micron intermediary layer, however, given the process required to add this it’s usually avoided.

Whilst the process to implement variable layer heights is quite complex and time consuming in Simplify3D this slicer gives you the most control over the process.

Slic3r:

The next slicer to test is the open source Slic3r program. I was amazed to learn that unlike Simplify3D, Slic3r has the option to automatically adjust the layer height dependent on your model. They call it Adaptive slicing and it is simply activated via a checkbox in the Print Settings tab.

From here you can adjust the Adaptive Quality slider which adjusts the overall quality between 0% giving the thickest printable layers over the entire object and 100% which provides the thinnest possible layers. Using the preconfigured settings, the results in the preview look like this:

With Slic3r the transition between layer heights is less obvious compared to Simplify3D. It is very conservative, often choosing a higher resolution where it isn’t overly needed, such as the bottom of the tentacle as shown below.

However, with tweaking of the Adaptive Quality slider, you can easily adjust this level of conserveness to match your own. With that said, its single button press implementation is very impressive and the function works as desired albeit without the level of customization that one gets with Simplify3D.

Cura:

Amazingly Cura has also just released an experimental feature called “Adaptive Layers” which allows the slicer to automatically slice the model at varying layer heights to improve the look, all at the press of a button This is still an experimental feature and as such isn’t quite perfect yet. In some cases, it chooses to increase the resolution in places that you may not personally desire and with no way to manually change this, it is significantly less customizable compared to other slicers. However, it’s impressive to see free slicers producing such advanced features at the push of a single button. This will allow users who don’t necessarily want to tweak every print to rapidly and easily get much of the benefit of variable layer printing without much effort.

To enable the automatic adaptive layers in Cura 4.0 you need to open the print settings drop down tab and scroll to the bottom of this menu. Here you will see the experimental tab. Click it and you will see the following options.

Adaptive layer maximum variation is the maximum allowable layer height difference from the base height. i.e. if we start at 300-micron or 0.3mm and the maximum is Δ.2mm the maximum resolution can be 0.1mm or 100-micron.

After slicing the automatic adaptive layers process in Cura and printing it on our Cocoon Create 3D printer, it took about 4 hours and used 6m of plastic. The print came out looking great, although some settings unrelated to the adaptive layers need a tweak for this printer on the Cura slicer.

FINAL THOUGHTS

All in all, a fair bit of time and a little plastic can be saved by carefully setting up your prints to utilise variable layer heights. This process can be made simpler with slicers such as Cura and Slic3r. Slicers make this very user friendly with their one button setup, however, they lack the customisation offered by Simplify3D. With that said, get in and give variable layer heights a go and see for yourself the time saving benefits of this simple technique.