Laying it on the line

I decided to investigate the conditions necessary for multiple layers of HDPE filament to stick together so I wrote a little Python test script to extrude 20mm squares stacked on top of each other. From my graphs in equations-of-extrusion I chose an output rate of 3mm/s which gives a filament diameter of about 1.2mm. That only requires about 60% PWM which I thought was not too stressful for a 5V motor running from 12V. I set the heater temperature to 200°C. Here is the first run :-



The first two layers look reasonable and then we are into basket work! The z-axis was raising 1.2mm between each layer but, although the nominal filament diameter should be about 1.2mm, the sides were not growing at the same rate. That meant the filament was dangling allowing it to wiggle around. Next I reduced the z-increment to 1.1mm :-



Better, the first four layers are OK this time, so obviously I tried z-increment 1.0mm next :-



Much better! What is happening is that the filament is no longer cylindrical. Each layer is about 1.0mm high and 1.4mm wide. It could be due to gravity but I think it is more to do with being bent through 90° as it comes out the end of the nozzle.

The fact that the filament weaved about when the nozzle was too high made me think that the feed rate might be too fast so I did a taller test with the XY travel 20% faster :-



Another basket case! What is happening here is that there is not enough material so the filament slumps down and holes start appearing.

I went back to the original feed rate and did a couple of 20mm high tests to check consistency :-



These are actually incredibly strong in the vertical direction. I can stand on one and it takes my full weight. Here is a video of the one on the right being made, the middle section is sped up 8 times :-



I also ran a test at 160°C to see if the filament would still weld to the layer below. It did but it did not stick to the foam board.

As you can see the main defect is that the bottom corners curl up. This was completely expected from the work Forrest published here: Ten-layers-with-no-curling, so next I will try his solution of laying down a raft first.

Another defect is that the filament width varies in waves. These seem to be related to the rotation of the extruder drive screw. You can hear the motor labouring more on part of the revolution. I think it is because something in the drive is a bit eccentric but more investigation is required.

Brush off

HydraRaptor was using a knife to remove excess filament from the extruder :-



It always cut the filament OK, but it was random whether the loose bit fell off or stuck to the far side of the nozzle. The soundtrack of a video I saw of a commercial FDM machine said that they use a brush. I thought I would need a wire brush for 200°C but then I reasoned that, if the nozzle passed through fast enough, the high specific heat capacity of plastic might mean that it would not have time to melt. I decide to give it a try with an old electric toothbrush head :-



It does seem to work quite well. Here is a video of it in action :-



The scrap of filament sometimes stays stuck to the brush but subsequent passes eventually knock it off.

When I was using HydraRaptor for milling I had a tray around the table and a plastic skirt to protect the mechanism of the precious XY table from loose plastic chips. When I moved on to FDM I thought these would not be needed because it is a lot less messy. Actually I was wrong as HDPE chips are appearing, presumable from inside the extruder, and the filament offcuts sometimes ping off from the brush. I have therefore refitted the tray and skirt.

Taking up the slack

I had a problem with my HDPE filament getting unwound from its reel. Because my extruder is attached to the z-axis, the filament gets pulled off the reel as the z-axis descends, but when it rises back to the home position there was nothing to take up the slack. Also the springiness of the HDPE makes it want to unwind. It needs a constant back tension to take up the slack and keep the filament on the reel.

My first idea was to attach a small DC motor to the roller to provide a backwards pull. As the motor would be permanently stalled I would have had to limit the current to something reasonable. After some thought I came up with a much simpler solution. I wound some picture cord around the roller and hung a weight from it. As the filament unwinds it lifts the weight. The weight is also tethered to the top of the machine, so once it gets to the maximum height it stops. The reel is only a friction fit on the roller so it starts to slip at that point. When the axis ascends again the weight falls and winds the reel backwards, taking up the slack. There is enough travel on the weight to cover the full z-axis travel, even when the filament has been used down to the inner diameter of the reel.