I have upgraded my RapMan with a heated print-bed and needed a modified z switch holder (the z switch normally is mounted directly on the print bed, which is not a good idea if it is heated). I tried Laszlo's one, it works, but I had problems with it. It is attached to one of the 3 screws the print bed is hold by. These 3 screws are equipped with a spring mechanism, allowing the print bed to back down if the print head by error tries to go down below the print bed surface.
When attaching Laszlo's z switch holder, this spring has no effect any more. Laszlo says he doesn't have the problem and I think the reason is that I have a RapMan version 3.1 while he has a previous version. My RapMan z stage platform has threaded holes for the 3 screws I was talking about which seem to be the reason for problem.

Anyway, I designed a different solution, where the z switch is not attached to the print-bed but to the z stage platform instead. This also makes fine-tuning the print-bed/extruder distance easier. You can do that with the 3 screws now and you get direct feedback about the new distance. Before you had to turn the distance screw at the print head and run a home sequence to get the new adjusted distance.

With this new mechanism the z switch is not pressed directly, but there is a arm which presses it. The arm reaches up where the distance regulation screw comes down during the home sequence.

Here you see my RapMan with the heated print-bed and the new z switch mechanism in the left back corner:

Here a couple of close-up photos. In case you wonder, I added a bit of duct tape because I needed a M3 screw to hold the arm and the only one I had left was a bit too short, so the duct tape holds it in place :-)

The OpenSCAD and STL files for the 2 printed parts can be found on thingiverse.

It is already a couple of weeks since my heated print-bed from László arrived. I was busy so I didn't find time yet to update the blog, but finally here is my experience with building and installing it.

The print-bed is made of aluminium. It's heated from the bottom side using power resistors. The print is done on a Kapton-covered steel sheet which lies on to the of the aluminium bed and which is hold in place by powerful magents.

The bottom side has threads to attach 9 power resistors and pits where the magnets will find it place. First I covered it with Kapton tape, cut out holes for the resistors (I only used 5 in the beginning) and the magnets:

Then I added the magnets.
U/D stands for up/down for the orientation of the magnets.

László recommends to add iron bars on top of the magnets. This makes the magnetic force stronger. Neodym magents don't stand heat that well, but short-cutting them with the iron bars also seems to improve this.
I decided to mount all 9 resistors I ordered at that point, so I added the remaining ones too:

Then I wired the resistors. For a start I was only using 6 of them, this is why the middle 3 are not conencted. Here the 3 mounting screws an the z-switch holder are already added as well.

I tested the bed in this configuration, but it was not getting hot enough. Even with all 9 resistors it hardly reached the temperature where ABS sticks to the steel sheet, which should be at about 100-110°C.

So the next step was to add heat insulation. To prepare that I added the heat-resistant insulation for the connecting wires. I chose not to wire all the resistors together but have two groups with 6 and 3 resistors to have more possibilities later.

Then I covered the bottom side with the rock wool. I also attached a thermistor to the bottom side of the bed, so there are now 5 wires coming out there.
Shouldn't have done this in our living room really, it's quite a mess. I'm lucky I have an understanding wife.

OK, now the bed was ready to be installed. Removed old bed, replaced it with the new one, attached the power transformer, covered the steel sheet with Kapton.
I wasn't happy with the z-switch holder, so I replaced it with a newly designed one (more in an extra post later).

Turned out that the temperature now was ideal for printing ABS. I now always switch the print-bed on about 5 minutes before the print and leave it on for the whole print. With all 9 resistors connected (all parallel) the power transformer which comes with the kit is a bit too weak by specification. It gets a bit hot, but it works fine. I'll probably add a fan for it later.
I didn't measure the temperature, but the ABS sticks very well on the Kapton. I didn't have to do anything (like sanding) to improve the adhesion.

Here's a finshed print (lots of strings, but that's not the fault of the print bed). I'm using the scraper which came with my RapMan to remove the steel sheet (it's not that easy, the magnets are really strong). I added small pieces of card-board on both sides of the scraper so that I do not burn my fingers.
When the print is cooled down a bit you can remove it from the sheet.

Right now am really happy with the print bed. Its a bit costly, but it is nicely made and very flat. And it probably cannot be made cheaper really. You would have to go for a different approach like a circuit board, but I haven't seen one which covers the full RapMan print area.

I do not sand the Kapton, so the bottom faces of my prints are really nice, flat and shiny. I did not have any warping issues any more when using it.
I didn't print really big objects yet, this is one of the next things I will test, but I do not expect problems.