Testing the precision of Elegoo Mars – Volume 3: New view on the Z-axis problem

In the previous post, I observed squashing of the prints in the Z-direction near the build plate (roughly first 20 layers). I have discussed the problem on the Elegoo group on facebook and some of the people suggested it might be related to viscosity or surface tension of the resin itself – when a thin layer of resin forms it can either push the build platform away from the print due to viscosity or it can pull the platform closer due to the surface tension. Therefore, I run some experiments.

I appended the results of my experiments to the table starting with sample 6 (direct link to the table):

There are several interesting outcomes. First, when I introduced a 40-second delay before exposure (sample 7) I was expecting to get things better if the problem is caused by the viscosity of the resin, which is pushing the build plate away. However, I would say it had nearly the opposite effect.

So I tried the opposite – to increase the drop speed. I observed no difference in the result. It might be caused by not actually increasing the speed – there might be a software limit in the firmware and I haven’t measured the actual speed.

The last experiment is the most interesting one. I tried to print the full print bed of the material with the staircase:

The object around the staircase is only 1.5 mm tall, the staircase is 3 mm tall. The first layers of this sample (number 9) have the height of the first layers nearly good (it might be related to the precision of leveling), however, once the object ends, it gets flat. See the graphs in the table and a photo:

I have then tried to print just a frame around the build plate, however, the results were the same as in previous experiments. Therefore the problem is not related to the lead screw (as it happens practically on an arbitrary height) and occurs whenever a large surface area of a layer is printed. It means that printing on supports is not the solution to the problem! Imagine printing a box – when the flat bottom ends, the walls near the bottom get distorted. Tilting the prints could help, however, it requires supports and on many of my planned parts, it is undesirable to put the supports on the sides as they are functional surfaces which should not be distorted (or it might be too much work to polish them up).

Interestingly enough there is one more observation – the layer height distortion happens only when the dense layer ends – in my test case the layers between 0-1.5 mm have a rather good thickness. From the table, it seems like the effect takes place once there is a vertical gap between large surface area on a build plate and the FEP film between 0.1-0.5 mm. However, I have no explanation of the phenomenon yet. Last what I struggle with is the fact that if it would be simple FEP film deflection, it wouldn’t cause a change in the total object height. I would expect the object to have the correct height as the other layers will be a little taller. This leads me to an idea of actually losing steps on the stepper driving the Z-axis. But I have not verified yet.

Do you have some observation, ideas? Please let me know! If you try to reproduce my experiments, let me know!

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