xLCD enhanced contrast modification  

xLCD enhanced contrast modification

This "mod" applies to any Prusa printer using the xLCD board. It's quite risky because it involves irreversibly disassembling components that Prusa themselves didn't assemble. Also I had a huge problem, so don't follow this as a guide unless you're willing to experiment.

One thing that bothered me with the LCD screen from my MK4 was the very poor contrast. It reminded me of the look of an old resistive touch-screen due to internal reflections. Looking closer, it appeared that behind the glass digitiser was an air gap before an anti-glare polarizer of the actual LCD.

All phones and most modern touch-screens don't have this problem because they fill this air-gap with adhesive. This adhesive is completely clear and has a similar enough Index of Refraction that it pretty much eliminates any reflective surfaces. In the case of a phone it also increases the strength of the whole assembly.

My plan to "fix" this issue was to remove the digitiser, which I believed to be glued only under the black mask, and then re-apply it using a UV-cure Liquid Optically Clear Ahesive (LOCA) 

One major concern I had was that this assembly wasn't designed for optical bonding, and thus there would be nothing stopping the liquid adhesive from seeping into the backlight assembly of the LCD...

Actual execution:

1. Removal of digitiser: this went about as easily as I thought. There was only a thin band of glue between the digitiser and the LCD. One thing that almost caught me out was that the digitiser is actually two quite thick pieces of glass, I tried to separate these in my first attempt.
   The first two glass elements are part of the digitiser and should not be separated from each other. The digitiser should be separated from the white frame.
   After checking the datasheet of the digitiser which showed me the error in my approach, I tried again.
   Using gentle heat from a heat gun with a length of enameled wire, I was able to 'cut' though a corner, like this:
   (In reality, this was far messier, the wire gummed up with glue and snapped a few times)
   After the corner was free and the display was warm (~50-60c), I was able to use my own built-in keratinous spudgers to lift the digitiser free. The strings of glue were cleaned up with IPA.
   
2. Sealing of the display to prevent liquid adhesive from getting into the backlight. I ran a tiny bead of solvent-based viscous glue around the inside edge of the white frame between the frame and glass.
   This did not work, and I had a complete disaster. Liquid adhesive seeped into the backlight from the cable entry. I spent an hour washing out adhesive from between the many backlight diffuser layers. If I was to try this again, I would have used an adhesive plastic sheet to create a mask around Any more ideas welcome.
3. Apply adhesive. I'm not going to describe it in a useful way. There are many guides on YouTube showing how to use UV-Cure LOCA for phone repair, but the process is basically:
   1. Clean the surfaces to be adhered
   2. Place a line of the
   3. Pop any bubbles using a needle
   4. Very, very slowly lower one side of the digitiser panel onto the glue. Gently hold this while supporting the other side
   5. Very, very slowly lower the other side while the glue spreads
   6. Apply a gentle pressure to encourage the glue into any uncovered areas.
   7. Wipe away any excess that oozes out.
   8. Gently hold the display in place and cure with UV light source for ~10 mins.
   9. Rinse any spills with IPA.
4. Done

Before:

The black mask (usually not visible when the display is mounted in the case) is very obvious due to the diffuse reflection of my deliberately positioned overhead light. This is almost the worst of both worlds: you get poor contrast due to the matt polariser of the LCD, and you get the specular reflection due to the front glass element of the digitiser.

After:

The diffuse reflection is entirely eliminated, the black mask almost blends into the LCD when off. It looks like a more premium phone-like touch-screen.

Despite my backlight disaster, the screen works completely fine after cleaning (except one tiny defect in the backlight due to some trapped dust, but this is usually not visible.