Priming The Hot End

Recap: I've built a Prusa Mendel 3-D printer from a kit I purchased from nwreprap.com.  I'm in the process of calibrating the printer in preparation for the first print.

When a print starts, the filament needs to be positioned inside the hot end in such a way that when the extruder is instructed to feed the filament, the molten filament exits the hot end right away.  If the molten filament does exit the hot end right when the extruder is activated, what tends to happen is that by time it actually exits, the hot end might be several layers above the surface of the heat bed.  When the distance between the hot end and the surface below it exceeds a certain amount, instead of adhering to the surface of the heat bed, the molten filament will curl into a messy blob.

So we need to advance the filament into position and actually feed some filament through the hot end.  For this we'll use Pronterface.  You'll also need a pair of pliers or tweezers to remove the excess filament at the end of this step. This step assumes you've downloaded all of the software, installed the Arduino driver, uploaded the firmware and have a good understanding of Pronterface.

• Pull the idler with your thumb, you should see the hobbed bolt.  Insert the filament into the opening so that it passes the hobbed bolt.  

• Start Pronterface

• Check the port, set connection speed to 250000 bps and press "Connect" to connect to the Printer

• Raise the hot end at least 25 mm above the heat bed

• Turn on the hot end

• Wait for the hot end to heat up to full temperature

• Activate the extruder to feed the filament until you see the molten filament exit the hot end.

• Turn off the hot end - click the "Off" button next to "Heat:"
• remove the excess filament with pliers or tweezers.

Now your filament is positioned for printing.  If you saw the filament feed in but nothing came out, it's possible that the hot end did not get hot enough to melt the filament.  Unfortunately what can happen is the hobbed bolt will grind away at a small section of the filament and it will no longer feed the filament.  Try pulling the filament out by hand and starting over.