Saturday, 11 June 2011

Testing ABS on Mendel Parts Heated Bed

It has been suggested that you can't print ABS on a RepRap Mendel built with the current kits supplied on, since they include a power supply for the heated bed that only delivers 70W at the highest voltage, and that isn't enough to produce temperatures high enough to keep the parts stuck down.

Recently, I have been trying to challenge that claim with a few tests of my own. Firstly I taped some cotton wool under each of the heating resistors to provide a bit of insulation, hoping that would conserve some of the heat, and then pre-heated the printbed using an iron again, turning the thermostat up to the setting for cotton and leaving it for a few minutes, which brought the bed up to a temperature of around 110C.
Kapton tape hugs makeup removal pads over heating resistors under the printbed.

By doing this and then removing the iron, I could easily print a few layers of ABS onto the bed and test my settings for the plastic by trying to print a standard Mendel vertex joint:

I soon found of course that the extruder was putting down too much plastic using my settings for PLA, since ABS is easier to extrude with its lower friction, and the pinch wheel bites into the plastic slightly more as it is softer. I found after a few tests that I could get good results by setting the flow rate at 60% of what I used with PLA.

However, once I had a good print going, the first layer of plastic did very slowly start to peel away from the printbed, as I could hear happening with an ominous clicking sound for a couple of minutes, before it finally gave way enough that the extruder head pushed it around, and it moved so far that the print would be ruined thereafter.
Notice the gap from the outline, and if you look closely, the circle that was just being printed out of line.
This occurred about 2/3 the way through the print, and after I aborted, with the heating still on, I measured the bed temperature at an average of 77C.

I have to head out now so can't do any more tests today, but I suspect that I could be able to get successful ABS prints out of the bed by insulating it further, as with another test I did yesterday by simply leaving a cotton cloth over the top of the bed for insulation, when I measured the temperature between the cloth and the bed it was in the nineties.

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