A new heater and better printing.

When I was ordering the parts for my reprap last year I accidentally ordered some thermistors that were only rated to 150°C rather than the 300°C of the recommended part.  I can’t remember if it was just an oversight or I was swayed by the fact that they were only £0.58 each as a opposed to £3.99.

After realizing my mistake I ordered the correct part but assembled an extruder using the thermistor at hand out of both curiosity and impatience.  The results of initial testing were quite good (see this post) and the initial prints seemed acceptable.  I decided after printing Adrian’s geared driver it would be a good time to rebuild the heater of the extruder using the glass bead thermistor.

Photo 1: The charred remains of a thermistor 100°C outside its comfort zone.

After removing the resistor from the charred heater (pictured above in Photo 1) one of my main concerns with the heater was that the polyimide tape was often melting and giving off fumes.  As I was only heating to 220°C at most according to the extruder control board then either my tape was not the Kapton™ tape it said it was or the thermistor was reading the wrong temperature.  I’m not sure that the wrong temperature was due to the bad thermal contact with the nozzle or the fact that I was operating outside the recommended temperature range.  The new thermistor is much much smaller (0.8mm diameter (Ø) as opposed to 6.3mm Ø) and so should be much closer to the heater barrel and able to achieve much better thermal contact.

Once rebuilt, tested, the extruder firmware reprogrammed for the new thermistor look-up table, and the sanguino motherboard reprogrammed for the new extruder drive I started printing some test parts.  The test parts are from a design I’ve started for an adjustable hub for use in feeding filament into the reprap.

Photo 2: The Hub-o-matic

I had found that with the previous heater I was able to extrude reliably at temperatures around 200°C – 220°C. With this new thermistor in place I needed to heat the system up to 250°C to get comparable results.  I hope to get a thermocouple sensor at some point to confirm the actual temperature of the nozzle.  My preliminary testing at temperatures below 100°C show the tip of the nozzle to be about 50°C below the temperature read at the thermistor.  I am unsure if this is due to sensor error or simply heat dissipation along the heater barrel.

Once up and running though I did get some parts printed, but only after a couple of runs that ran out of steam half way through.  The photos below show examples of the good and the bad prints.  To get a sense of scale the spindle is M8 threaded rod and the screws holding the struts to the bearing holders are all M3x20.  The bearings a 608ZZ (AKA skateboard bearings).  The usual problem I find with my prints going wrong is that they print perfectly for the first part and then the extruder just runs out of heat and the extrusion becomes quite lumpy.  This I normally try to fix by increasing the temperature.  This works to an extent but costs you in quality as the extruder begins to ooze uncontrollably or simply extrudes too much filament.

Photo 3: A good print of a 60mm x 10mm x 10mm strut for the Hub-o-matic
Photo 4: A bad print of a 60mm x 10mm x 10mm strut for the Hub-o-matic

After the initial prints I built up a second heater barrel using the same type of thermistor and confirmed the temperature behaviour and performance to be the same.

The new extruder driver certainly has plenty of torque and I feel it could almost push through the filament cold.  I must admit I had my doubts about the design when I first looked at it and printed out all the parts.  There didn’t seem to be enough parts to hold all the bearings in place, and the 55 tooth gear seemed a little loose and unconstrained in its positioning.  After building it and threading the filament through I am impressed.  The brass insert from Conrad Electronics really does grip the filament well and the gears mesh beautifully. It is also worth noting that I initially overlooked the last instruction in the build, to use some silicone grease on the gears, but it does make a dramatic improvement in the gears ability to mesh nicely, smoothly and quietly.

I did deviate from Adrian’s design slightly. As my motors have a 2mm Ø cross drilling on the end of the shaft, instead of filing the end of the motor shaft flat, I drilled a 2mm Ø hole through my drive gear and used a spring tension pin to retain it.  I’ve also made one other change to Adrian’s design.  I can’t stand using glue or epoxy for something like this so I retain my PTFE thermal barrier using two M3x20 screws through the base and thermal barrier (perpendicular to the direction of the filament and heater barrel).  This allows me to swap out extruder barrels quickly without cutting any tape when they give trouble or become blocked.

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