This is the 11th post in a series detailing my efforts to construct a RepRap Mendel, an open source 3D printer. For the rest of the build series, see here.

Introduction:

When I last left off things were almost done. The printer had run into a hotend issue, but that had been corrected. The mechanical systems were assembled and verified, the extruder was thoroughly tested and worked correctly, and I ran through some software debugging to allow the print head proper motion and operation. Things were looking up.

Buuuuut, the plastic, while extruded correctly, was not sticking to the printbed, but rather was simply being dragged along with the printhead on the table surface. Further investigation suggested that the print surface needed to be heated for the plastic to stick to the build surface correctly. The Mendel-Parts kit I originally ordered included parts to build a heated printbed, so I set out to build and test this printbed.

Heated Printbed Assembly:

The Mendel-Parts heated printbed is essentially 4 power resistors that are bolted to the bottom of the aluminum build surface. Power is then run through these resistors, heating up the surface. There is no control over the build surface (so if you wanted to, for example, kill power to the heated printbed when the printer’s not in use, you need to do this manually by disconnecting the power supply), but by setting the power supply voltage with some small fuses, a handful of temperatures can be achieved for printing.

Right away, some obvious problems could be seen with the Mendel-Parts approach. For one, the holes in the aluminum printbed were not countersunk. As such, the provided bolt heads would stick up above the print surface, colliding with the extruder head. Additionally, the holes in the power resistors for mounting them were smaller than any of the bolt widths provided in the Mendel-Parts kit. In any event, Mendel-Parts’ errors were corrected, and the build platform was assembled. I would say I was surprised by this lack of thought and professionalism, but I would be lying.

The heated platform assembled, it was time for testing. Rob came over and brought his multimeter which featured a thermocouple for temperature measurements, and I hooked up the build platform to the provided power supply. The platform temperature rose to approximately 100 degrees celsius…

…and within a few minutes, began to issue smoke. Visual inspection revealed that one of the power resistors was in fact boiling. Power was killed. Rob and I spent a few minutes in wonderment at the idea that Mendel-Parts had sent us a poorly designed, non-functional solution (and yes, that is sarcasm for those keeping score), and started thinking about other printbed solutions for the RepRap.

MDF Printbed:

There was some discussion on the RepRap IRC channel, and the consensus was that a build platform using medium density fiberboard (MDF) might do the trick. The aluminum in the printbed conducted heat away from the target area too quickly, perhaps, and some of the original RepRap designs had gotten successful results using MDF. MDF is an easily machineable, common material found at most hardware stores, so I set off for Home Depot in search of a piece.

MDF in hand, the printbed size and hole positions were measured out and traced on the board surface in pencil. The sheet was then cut and drilled, and the new MDF printbed was installed. As a bonus for creating my own printbed, I could do away with the slot that’s featured on the RepRap printbeds. This slot exists for legacy reasons and has no purpose in stepper motor-based extruder designs, and omitting it increases the maximum printable area somewhat.

The new printbed installed, it was time to try a print again. I fired up the printer, heated up the extruder, connected to it via my Macbook, and… no change! The plastic was still sliding around the MDF surface with the printhead. It was at this point that a RepRap channel IRC commenter scrutinized my hotend, and made the observation that I had screwed the heating block all the way up the nozzle thread. This was giving the plastic some room to cool prior to touching the heatbed. If anything was going to work, this was going to have to be fixed.

The RepRap Mendel build saga is ongoing, so stay tuned for future updates! In the meantime, be sure to check out the other posts in the build series, and check out the latest photos from our flickr set.