Heated Printbed


My current 3d Printer from Bitsfrom Bytes, the rapman 3.1 has a bit of a problem to print bigger objects in ABS. ABS shrinks when it cools so if you build bigger objects, they begin to deform, detaching their edges from the print bed and eventually ruining the whole print.

A common solution for this problem is a heated printbed which keeps the ABS at a temperature where it doesnt shrink too much, until you finished the whole object. Luckily i noticed that http://blog.arcol.hu/ sells a heated printbed which should be a drop-in to the Rapman machines. So i ordered one and it got delivered a few days ago.

Its a nice kit, consisting of a purpose milled alu plate, heating resitors, a steel plate, magnets to hold the steelsheet in place, kaptontape to cover the sheet and the neccessary transformer. I was able to assemble this kit in one evening, but got into trouble when i tried to mount the heated bed to the machine. The Z-switch holder, (which you have to print yourself), just didnt fit to the place where it thought it should go. But after some fiddeling and some modifications, i got everything mounted.

The next day i tried to print some test rafts to see how level the bed is. The Bed heated fine, but my extruder just wasnt able to make the printed ABS stick to the kapton covered steel sheet. I tried with lowering the printhead even more, but this got me into the trouble, that the printhead would move the kapton covered sheet out ofplace, when it comes from the heat-up postition to the start of printing. I then tried to adjust the bed itself higher while it was already printing, to see if the ABS would stick better if it is pressed more into the Bed. I tried different heights, from only slightly flat ABS string to complete skipping, where you can only see a film of filament printed. Unfortunatly nothing made it stick better, even the filament film from skipping could be removed with some slight pressing with a fingernail.

I then tried to heat the bed really long, so it surely is at maximum temperature before is start with printing. This made it stick a little bit better, but its still not really useable, as later layers easily rip it off again.

Now i am a bit confused, i read at several places that ABS would stick very well to Kapton tape, but it just wont work for me ? Is my kapton tape somehow wrong/old, or the temperature of the heated bed too low ?

I heard on the chat rooms, that blue painters tape would stick better on lower temperatures, so thats what i will try to test on monday. Also i plan to buy a temperaturemeter, so i can know for sure how hot the bed is.