This is the 12th 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:

Last time, the RepRap Mendel was nearly working, with the exception that plastic was not sticking to the build platform. After attempting a variety of build platform materials and options, an astute IRC channel member noted that the heater block on my hotend was screwed all the way up the nozzle threading. As such, the plastic had a couple of centimeters and seconds to cool before reaching the printbed. In order for 3D printing to be done successfully, this issue was going to have to be resolved.

Not Again!

Last time I broke the hotend on my printer, I redesigned a bracket plate for the printer that improved the hotend mounting for my extruder considerably. As such, the hotend was considerably easier to remove this time around, and even didn’t require me to completely disassemble the X-carriage this time. Unfortunately, due to thermal expansion or a number of other possible explanations, the block was secured to the threads of the nozzle and would not budge. In attempting to unscrew the heater block, I broke one of the leads off the power resistor. Craaaaaaaap.

And now we wait:

I ordered a new hotend from Mendel-Parts and shifted my attention to other things. A week and a half later, I was wondering where my hotend was; checking Mendel-Parts revealed that they had decided to take my money and not ship my hotend. Awesome…

3 weeks of waiting and backlash from extremely angry customers later, my hotend arrived. I constructed it, lowered the heater block so the conical section of the nozzle was just exposed beneath it, and installed it.

If You Can’t Take the Heat…

Rob came over, and the extruder worked for a little while, but after about 45 minutes of operation we were forced to kill power to the device (no emergency stop button yet). After doing so, we found that the hotend refused to heat up anymore. Further forensics revealed that the power resistor was open-circuited. After tapping the power resistor out of the extruder block, we found the following:

The power resistor had been boiled and was destroyed. Successfully tapping the power resistor out of the last failed hotend revealed the same result: the extruder resistors were getting baked.

So what happened? The documentation on both the Mendel-Parts product page and RepRap Wiki page flaunted the fact that the Generation 6 electronics have an input voltage range of 12-24 volts, and as such I was powering the electronics at 18.5V. The electronics dump the unregulated input voltage across the extruder resistor, so 18.5V was being applied across the resistor. The resistor shipped from Mendel-Parts is 7.5 ohms and is rated for 5W, so at 18.5V, (18.5)^2/7.5 = 46W is being dissipated across the 5W resistor. The resistors couldn’t handle that level of power dissipation; cracked, and swelled inside of the extruder resistor holes, making them nearly impossible to remove. To fix this, I installed a new power resistor, and reduced the supply voltage to 12V. This did the trick, and the hotend was ready for action. I performed an extrusion test, and was quite pleased to see the plastic stick to the MDF build surface:

Well, sort of stick to the build surface. The ABS would stick to the surface if held there for a length of time, but for shorter time frames, sticking was still a no go. It was time to once again examine the build platform in hopes of getting this thing working.