I wrote several posts on the precision of the Z-axis and gaining the precision in the XY direction on Elegoo Mars. However, last week I wanted to print models that feature narrow rectangular holes – roughly 0.2 x 0.3 mm. I thought it will be trivial – I have well explored the problem of exposure bleeding and wrote a tool for its compensation.
However, my first experiments were a complete failure. I used Elegoo standard grey and Siarya Fast grey – both without acceptable results. See the photo below (left first results, right expected results). Practically all the holes were rounded and closed in the bottom. No matter how big compensation I used, I got the same result. And when you think about it, it makes sense – the exposure bleeding rounds the sharp corners. If I compensate for it, I might get the correct dimensions of the holes, but I cannot get sharp corners.
I thought – the holes are only 5 pixels wide, maybe I am running at the edge of what the printer is capable of. Then I took of the resin vat and looked at the pattern the display is showing. It showed a nice, sharp, crisp image of what I wanted to print. Therefore, the limitation is not in the display itself. I took one of my old broken displays from the printer and start to examine it.
There is protective glass on the display (see photo below). The glass is not glued to the display, in fact, it is similar to a glass you can put on your smartphone. It is not held by an adhesive, rather it is some kind of electro-static bond (I actually don’t know the physics behind sticking the protective glass on your smartphone). So it is easy to remove. I thought this glass contains the top polarizer, but it is not the case. This glass is 0.7 mm thick and provides a gap between the resin and the displayed patter itself. This gap provides a space for the beam of light to spread and therefore, to reduce the effective resolution of the display – well – it is one of the significant causes of exposure bleeding.
The Modification of the Printer
So I decided to try an experiment – remove the protective glass and print without it. The good news is there is a piece of glass underneath the display which provides mechanical support for the display (the glass is there to probably protect the top polarizer from scratches and also from resin leaks not to strengthen the display).
To print directly on the display there is a need to lift it by the thickness of the glass. I actually decided to lift the display 0.2-0.3 mm above the vat bottom to ensure my FEP film lies directly on the display – the FEP film stretches over the display. To do so, I printed some spacer I put between the pink base plate and the supportive glass.
Since I removed the protective glass with a black outside frame, some of the UV-light was passing around the display. I solved it by putting a thin aluminum tape around the display.
And then the modification was done. Below you can see the vat sitting on top of the display:
I poured resin in and started printing. As you could see in the introduction, the modification helped a lot. See microscope photos below (I am sorry for the poor quality, but I haven’t created a photo shooting jig for an optical microscope). On the left the original one, on the right after modification. One thing you note immediately – you can clearly see individual pixels/voxels after modification. When you look by eye, the prints are matte, not glossy – this is probably due to the “pixelated” surface. In the prints, I used only one bottom layer. You can see that the other layers are nice and sharp, however, the first layer (despite I used only 15 seconds for exposure) is bloated. This is probably caused by the light bouncing off the build plate. But this is only a speculation and I would appreciate the opinion of the others.
I also printed the AmeraLab test piece. The result is wonderful. You can see that even the smallest strands on top the building got printed (however they got bent during washing).
I am pleased with the results of this modification. However note that this modification has its downsides – if your resin leaks, you can probably say bye to your display. You can scrape the resing with a razor from the protective glass, however, I think you cannot do the same with the polarizer – the resin will stick to it much better and the polarizer is soft, compared to the glass, so no razor.
Also, this mod adds accuracy, however, it removes nice smooth, glossy surface finish many Mars users are used to. So you get detail for I am not sure if worse, but definitely different surface finish.
I haven’t done tests that measure the amount of the exposure leaking but I expect much fewer problems with it, maybe it will be reduced under a measurable amount. I am also left with an open question on how to deal with the exposure bleeding of the initial layer which is significant even in case I use the same exposure as for regular layers. It would be interesting to put an extremely dark coating on the build plate. However, even if I got vantablack or similar, I don’t know how to protect the paint from the prints and during scraping the prints.
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