The LCD used for printing on Elegoo Mars is RGB – there are three subpixels – red, green and blue. Since the backlight LED produces a quite narrow range of wavelengths (peaking around 405nm), only the blue filter passes the light. That means that 2/3 of the backlight power is wasted to the LCD. Also, it means that only 1/3 of the pixel is exposed and the rest is hardened only via exposure bleeding – the effect we, on one hand, want to eliminate, on the other hand, it is essential for properly working screen. After my modification, which removed the protective glass, you can see on my prints under a microscope the effect I mentioned – 1/3 of the voxel is nice and sharp, the remaining 2/3 are smudgy.

There were recently announced printers with monochromatic LCD. They feature low exposure times (around 2 seconds) with less power than Elegoo Mars. However, their LCDs have poor resolution.

So I was wondering – would it be possible to turn Elegoo Mars LCD to a monochromatic one?

The LCD stack up

First, I examined the LCD. I figured out that the two most outer layers are the polarizer. They are not glued, they hold similarly like to protective glass. Then there is a bottom plastic film with the pixel electrodes (I am sorry for not providing pictures, however, I was not able to take them from the microscope). Then there is a glass layer – on one side (the top side) it is covered by ITO – a conductive clear coating. On the bottom side, there is a layer of color filters. Between the glass and plastic film is a layer of liquid crystals, more specifically between the plastic film and the filters.

One important part – the ITO layer is connected to the plastic film via a conductive ink – see photo below.

The procedure

Having experiences with disassembling LCDs before, I know how to separate the glass from the plastic film. Just use a fresh Xacto knife and slide it between the layers, move along the perimeter and the layers separate:

You can nicely see the liquid crystal. I used a rubber spatula to save as many crystals as possible from the glass. Then, I started to scratching of the filters. It was pretty easy using a knife. I decided to leave the black border – it probably provides correct spacing between the layers – but I am not sure.

You can see that the glass got much more transparent. Then I cleaned it properly and put the LCD back together. Unfortunately, I was not careful enough and I got some dust particles to the crystals. Therefore, I got polarizing patterns on the LCD:

I connected the LCD to mars and run an exposure test. To my surprise, the LCD was working – you can clearly see the test pattern:

So – it is possible to do! To fix the issue, I separated the layers again, cleaned them both with IPA using a paper towel, put liquid crystals I harvested from an old computer screen between them. I got LCD without dust particles, but with strange, colorful, patterns:

Unfortunately, the LCD stopped working. I have no idea why, just a few theories:

• there are different type of crystals and they are not compatible,
• by cleaning the LCD with IPA and paper towel I corrupted the pixel electrodes,
• or I destroyed micro scratches in the surface – see the video by Applied Science about the scratches
• or something else.

Also, I don’t know what caused the colorful effects after using the crystals from a big LCD. Do you have any ideas? Please, leave a comment.

To do more experiments, I am looking for screen donors – I look for people willing to send me their LCDs with dead pixels. Doing these experiments on brand new displays seems to be wasteful to me. So if you have such a display and you live in Europe, please contact me!

In one of the recent posts I printer a solder paste stencil. The print is challenging as it contains small details, is thin and prints directly on the build plate. Even I use the same exposure on the first layers as did on the other layers, the first layer showed some exposure bleeding.

The hypothesis is that the silver surface of the build plate reflects the light, which cures the resin, and therefore more resing gets cured. I was thinking about getting really dark black paint and to paint the build plate. One of my readers, Zemerick (thank you!), brought up that Vantablack cannot be used (as it is a fragile forest of carbon nanotube), but Black 2.0 or 3.0 is just acrylic paint with a lot of pigment. So I got Black 3.0 and painted half of my build plate:

The paint can get be scratched easily (you can see the test scratches), so I was afraid it would peel of. To my surprise – it didn’t. But the test print didn’t stick to painted plate:

Reason for that? The paint is soluble in resin. I got leaking black pigment to my vat, the paint after a 5-minute print was really soft. I tried 2 more prints with roughly the same result.

Since the prints peel off, there is not much to compare and I cannot say if it helped or not.

Conclusion? None. I still think UV absorbent surface finish on the build mitigates the exposure bleeding. However, I am not sure how to create one. Anodizing would stick but is still too reflective. I could use spray paint, but not sure if it is dark enough and if resin will stick to it. I probably wait until the replacement build plate will be available and then (maybe) try some more experiments.

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:

Results

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).

Conclusion

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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After investigating the Z-height inaccuracy on the Elegoo Mars, and applying the official motor replacement sent to me by Elegoo (they have the best user support!) I was left with one unanswered question – what causes the last, quite small (roughly 2 %) linear error on the models’ height?

I started to measure the movement of the arm using an indicator, however, all the measurements looked good. I even measured the original screw using an optical microscope (see the raw data). It is pretty good – it features practically no jitter and only small linear error (0.2 mm/150 mm) which could be easily compensated in software.

Then I printed a large staircase (see photo below) which revealed the fact I was missing – the error is not linear. The printer prints some levels higher and some lower. The cause is in the combination of the long printing arm and the Z-rail. The Z-rail provides good guidance in the direction of the axis movement, however, it does not preserve the parallelity of the carriage and the axis. It means the carriage can “wobble a little bit” during the movent which translates into an observable difference in the Z-height on the end of the long arm (see illustration below). The reason I did not get the problem with an indicator was that I was measuring too close to the screw. My bad – there’s not plenty of places you can mount the indicator on the printer and I went with the easiest one…

There is no easy fix, however, as tuning Elegoo Mars become my hobby in the last few months, I decided to rebuild the Z-axis. You can see the results below:

I machined the new axis column and mounted two linear rails there. The columns are scraped so they are perpendicular to the display – currently, the perpendicularity is withing 0.03 mm/150 mm. I am not sure if it is an improvement over the original rail as I forgot to measure it. Also, the new rails were shifted so the linear rails are in a plane of the screw – this arrangement should minimize the wobbling of the carriages caused by the screw pushing to them.

I also decided to switch to a ball screw instead of the original one. Precision was not the main reason here – the original screw is pretty good and also featured practically no backlash when I used a casted nut. However, the casted nut has a too tight fit and squeaked occasionally. It was also an opportunity to use a proper screw housing with proper bearing. It also allowed me to mount the motor using a flexible coupling, thus to mitigate resonances from the motor. I used 1204 screw with appropriate FK10 housing.

I also changed the stepper driver to Trinamic TMC2025 – it supports StealthChop – a special current chopping, which allows the motor to move practically silently. I also changed the fan for quiet one.

Note that the modification are not final yet – some prototyped 3D printed components need to be properly machined (this is mostly as a proof-of-concept) and the wiring needs to be tidied a little.

Results

The most noticeable improvement is the reduction of noise during printing. The printer is quieter than many laptops. I have also verified the printer produces square models that fit nicely together – and most importantly have the correct height. However, there is minimal or no improvement in the surface finish – it stays pretty much the same (as there is not much to improve anyway). So if you are printing minis and not functional pieces do no bother with such tuning.

If you are interested, you can download the CAD files for modification: https://a360.co/36D6BHS.

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After publishing yesterday’s post I observed a strange thing – the lid of my Elegoo Mars came off during printing. The encoder I mounted on top of the Z-axis bounced it off. That was strange, how? And then it hit me. I have never, ever measured backlash of the bearings of the lead screw. I have only measured the backlash of the screw itself. Well, I think a video is worth more than a thousand words:

There is a play roughly 2 mm in the housing of the lead screw! I disassembled the printer:

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The cause is clear – even the motor has a lead screw as shaft and therefore, you would assume it is designed for axial load, it is not. There are ordinary ball bearings (no axial nor angle contact bearings) and most importantly – there is a spring washer tensioning the bearings – just like in an ordinary stepper designed for purely radial load. This is, in my opinion, a clear failure of the motor manufacturer MOCOC TECH. Also, there is another source of problems – the silencer – as the screw is mounted in the motor and the rubber silencer is soft. The silencer probably prevents from resonating with the top plate of the printer and also creates a flexible element which can compensate for the axial misalignment of the screw and the nut.

When you combine this flexible play in the screw with my observation about forces present during printing, you get imprecise print height – up to the size of the play of the screw. It can shrink or squeeze your layers arbitrarily.

What is the solution? There are three solutions in my mind:

• dirty & cheap – get two M8 washers, put them in the motor’s rotor, remove the spring washer and tightened motor body screws carefully to slightly tension the ball bearings. Also, remove the silent block. This is a solution for roughly 3 CZK. Warning: This is a dirty solution. Deep groove ball bearings are not designed for axial load nor tensioning. Also by removing the silent block, you remove flexible element which could compensate for misalignment of the screw and the nut. Your risk shorter life of the bearing, screw, and nut. On the other hand, the speeds and axial loads on Elegoo Mars are not that big, so you might be OK for years with this solution.
• better solution – get a pair of axial bearings F8-19G and use them instead of the ball bearing. Pretention them either as in the previous case or with a hard spring washer.
• The best solution – build separate housing for the screw with contact angle bearings and connect the motor via flexible shaft coupler. This solution provides noise reduction using the silencer. However, when you try this it might be worth it to rebuild the Z-axis to use a linear rail as the Elegoo solution of the Z-axis has a high effect-to-cost ratio, however, I can measure about 0.2 mm of play when I apply reasonable forces by hand.

As the G8-19G bearings are not available at my local store and I had to order them, I applied the dirty & cheap fix to find out what improvement can I get. Spoiler: a huge one.

I printed the test pieces from my previous posts (volume 1, volume 3, volume 4) and the problem practically disappeared. Compare the real size of the test piece before and after:

The full dataset can be found in this table (new measurements are from sample 10).

Most notably what changed is that if an error is introduced in a layer, it is compensated by the others. Therefore absolute precision is preserved. See that all the test pieces got practically the same height.

If you look at test piece 11, you’ll see it is quite distorted. It is the sample surrounded by a full plate of material. There was noticeable distortion, however, it was different compared to the previous cases. The overall test piece height was preserved, but the layers surrounded by material were a little bit higher. Just like first layers of other pieces. This is probably due to the effect I described in volume 4 (recommend reading before continuing). The effect is that the resin is viscous and as the build plate sinks, it has to push away the resin. When I introduced the delay to allow the resin to flow away and to settle the build plate in place I got much more precise pieces. On the simple pieces, even the first layers got the correct height. On the pieces with extra material, the distortion is still there, however it is much less significant. I believe by introducing even longer delay, we can get much more precise (I plan to do this experiment).

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What struggles me is that instead of 3 mm I got 2.9 mm – pretty constantly. Therefore, I printed another staircase – 0.5 mm steps, 15 mm in total height (sample 15). I also got less – 14.7 mm. Currently, I have no idea what is this caused by – it not a constant error (not coming from incorrect bed leveling) and it is too large for shrinkage (2 % – epoxy or polyurethane resins have shrinkage less than 0.5 % and I don’t expect printer resin to be that different). Maybe tensioned bearings with misaligned screws cause step loses on the stepper. I am also not sure the error is linear – I’ll have to run many more tests. Any ideas what could it be caused by?

On the topic of lost steps – before the first print I releveled the build plate. When the print started, the build platform started to move down as expected. The build plate touched the bottom of the VAT and the stepper still continued – by the sound it clearly lost some steps. I am sure I have leveled my bed correctly. I leveled it against empty VAT. Is it possible the printer FW moves the platform a little bit below the zero point to pretention the Z-Axis to mitigate the problem with flexible housing of the screw? I don’t know, but this also something I would like to explore in the future.

After all, even there are still open questions I consider my Elegoo Mars to be used as I intended when buying it – to produce precise functional mechanical components.

Since the last post I have made many more experiments regarding the Z-issue on Elegoo Mars.

First, I tried to mount an indicator to the Z-axis. I mounted it in the middle of the arm carrying the print bed and printed my test model. To analyze the results, I aimed my phone camera to the printer to capture the measurements. I performed both dry and an actual print run. You can find the whole, uncut footage of the experiment here (warning, it is really boring):

Then I took the footage and put the numbers in a table (direct link to the table):

Continue reading “Testing the precision of Elegoo Mars – Volume 4: More observation, no solutions” →