Some like it hot

Over the last couple of weeks, I’ve got the printer to a point where I can send it G-Code commands in the reasonable expectation of it carrying them out.  At the moment, the print head only has a pen mounted in it, so I can check its positioning, but it will draw things on a piece of paper.  There’s a rather poor quality video of it doing a calibration test print (or should that be a test plot?) here:  There is still a little bit of levelling to do on the glass plate which acts as the print bed, and I need to come up with a good strategy for calibrating the print head position, but it’s ready for the next step.

Now that I’m finally reasonably confident that the printer is capable of enough accuracy and repeatability, it’s time to make it actually squirt plastic.  For this I need a roll of plastic filament, a ‘hot end’ to melt it and deliver it through a tiny nozzle, and a mechanism to push the filament through the hot end (an extruder).  This is a delta configuration printer, and one of its distinguishing features is that the print head is capable of moving very fast.  This is helped by the low weight of the print head.  To keep the weight low, it is desirable not to mount the extruder on the print head (as most cartesian printers do) because it has fairly heavy stepper motor in it.  Instead, the extruder is connected to the print head by means of a flexible PTFE tube through which the filament is pushed, in a Bowden cable-like arrangement.

The hot end parts have arrived.  I’ve gone for a popular design, the J-head Mk V.  In an exploded view, it looks like this:


Though it looks a bit complex, it’s actually very simple.  The shiny aluminium cuboid/cone combination is the hot part.  This has a big hole in it in which to mount a power resistor or a heating element (the photo shows both) and a small hole in which to mount the tiny glass bead  thermistor.  The heating element heats the hot part up, and the thermistor measures the temperature.  Both are linked to the controlling arduino, so that the hot end temperature is maintained as close a possible to the optimum for melting the plastic in use.  the beige machined cylinder in the centre of the picture serves as a holder for the hot nozzle, a guide for the filament and as a thermal insulator.  it’s made out of a high-temperature plastic known as PEEK.  The purpose of the slots is to help airflow, so that the mounting end stays cool enough to attach to the print head (which is, in my case, made out of plastic which will melt at the temperature of the hot end) while allowing the hot end to deliver plastic-melting heat the the filament before it gets squirted out of a really tiny (0.35mm) hole.

Rob has very kindly printed the plastic parts for the extruder mechanism:


It’s fascinating to see just how far removed we are from conventional manufacturing techniques.  The body of the extruder is a mass of complex shapes, involving curved and tapered surfaces.  It would be a nightmare to make with conventional machinery.  With 3D printing, of course, the shape complexity is almost irrelevant.  If it’s a valid 3D shape, it can be printed (with some minor limitations, like overhangs).   I’m using Airtripper’s design for an extruder, because there is no need to reinvent the wheel.  It requires only three bought-in parts: a couple of small bearings and a toothed pulley which grips and drives the filament.  This last part I have had to order from Denmark, and it seems to be taking some time to arrive.  Watch this space.