Support, who needs it?

I think the original Darwin design assumed it would have a support material extruder, so some of its parts require support material to be made. I.e. they have overhangs that are more than 45°. Vik Olliver and Steve DeGroof subsequently modified the parts requiring support material so that they can be made without it. That allowed Vik to replicate his Darwin without a support extruder. STL files for the modified parts can be found here.

At the time I was making the Y-motor-bracket for my Darwin, the modified file was missing, so I decided to see what would happen if I tried to make the unmodified version. I expected the result to be a mess.

Here is the original file, it has a recess in the bottom to fit the shape of the motor and stud coming out of the side at right angles: -



Here is the modified version to reduce overhangs to 45°: -



The problem with this is that it doesn't fit the motor properly. I think Vik was using a different motor.

To my surprise the original version came out fully functional. It is a bit messy, some of the outline was extruded into mid air and had to be cut off, but the infill managed to build out and recover the correct shape after a couple of layers.





It makes me think we might be able to build out into fresh air simply by going slowly and with a fan to cool the filament.

I built the modified version of the X-motor-bracket, but that has no recess as well so the motor did not fit it. To fix that I made a washer to replace the recess. This was simply a slice off the Y-motor-bracket: -



It is 1.6mm thick, which is four layers with my preferred settings. It seems to do the job. I had to use 20mm bolts rather than 15mm to mount the motor. The pulley is normally mounted 2mm from the end of the shaft so moving to 0.4mm from the end should compensate for the washer.



It should also be possible to use this washer on the other motor brackets. I uploaded it to the wiki page.

Infill and warping

Now that I can create blocks with different infill densities I decided to experiment to see what effect it has on HDPE warping.

I have been using a standard test shape and a jig made of three nails to make comparative measurements.



I measure from the middle nail to the base with a pair of digital calipers and subtract the distance to a rule placed across the nails. The figure I get is an average of the amount each end warps upwards. Not very precise because the base is warped the other way as well.

The block is 40 x 10 x 20mm because you need about 40mm length before the warping becomes big enough to measure and 20mm height is about where things start to straighten out. Bigger shapes warp more but obviously take a lot longer to make. Each one of these takes about an hour including making the raft, extruding the block, separating it from the base and measuring it.

The block is held flat while it is stuck to the bed of the machine by the raft. It warps when I remove it. I have only recently noticed that it warps even more when left overnight, so some of my previous tests are not that accurate. For example I was quite pleased when I first produced this extruder sized block :-



But here it is again photographed some days later :-



Not easy to compare because of the angle but the uplift at each end probably increased from about 0.5mm to 1mm. It implies to me that HDPE creeps when under prolonged strain, not a very good engineering property. That is the main reason PTFE fails in the extruder.

I made the test blocks with different infill densities and left them overnight before measuring them :-



Here are the results: -

Density	Warp
20%	0.44 mm
25%	0.79 mm
33%	0.47 mm
50%	0.47 mm
100%	0.53 mm

The 33% value looks totally anomalous but that is because I tried a thicker base. Its base is 3mm of 100% fill including the raft, whereas all the other tests begin the sparse fill on the first layer above the raft.

I also tried 1mm filament 50% fill which gave 0.42mm warp showing that not stretching the filament does not give any improvement.

Conclusions: well sparser fill reduces the warping slightly. A thicker base, rather than resisting warping, actually contributes to it. I must point out that once you get less than 50% fill the object is considerably weaker than a solid block.

Finally here is a longer example, which illustrates how warping gets worse the larger the object is. This is 100 x 10 x 20mm with 20% fill. The first time I made it it lifted the raft away from the base. I got round that by increasing the raft temperature by 10°C to get a stronger weld. It was then quite hard work removing it and it caused some damage to the PP bed.



The 40mm section in the middle is only warped by 0.19mm but the ends are well over 1mm. That shows that you cannot compensate for the warping with a crowned bed because it is not a constant curvature. One could probably scan the shape of the base and lay down support material with the inverse curve. I expect it would then pull itself flat.

In my next experiment I will try filling the sparse blocks with polyurethane two part thermoset plastic.

Filling in

I have been experimenting with various infill patterns. Here is a 40 x 10mm block made with 0.5mm filament at 50% fill: -



For simplicity I used alternating horizontal and vertical lines rather than diagonal. The layer height is 0.4mm so the width is about 0.6mm and so are the gaps. A couple of things that weren't obvious to me at the beginning were: -

The first and last lines of the fill must be adjacent to the outline so that the U turns on the alternate layer above have something to rest on, otherwise they curl upwards or downwards and don't bond to the outer skin. That means adjusting the gaps slightly to make the overall width correct. When the fill is 100% I adjust the filament width slightly to exactly fill the interior. Easy enough with a rectangular object but probably not with an irregular polygon.

The fill lines probably should line up with the those two layers below so that the intersections form a solid column of filament from top to bottom, otherwise some sag may be expected. Again trivial for rectangles but could get tricky to generalise.

Here is 33% fill, i.e. the gaps are about twice the filament width: -



This is 25%. Notice how, although the filament is laid down in a perfect square wave, when it shrinks it pulls itself to the first harmonic. A physical low pass filter!



And here is 20%: -



I found that when putting a lid over the top it struggled with an infill this sparse, so I settled on 25% as the limit for making closed boxes.

All the above are done with filament stretched to 0.5mm. When extruding through a 0.5mm orifice, left to its own devices the filament would be about 1mm due to die swell. I decided to try the same pattern with 1mm filament, i.e. with no stretching: -



As you can see the filament holds the square wave better but what is not obvious is that without stretching it sags a bit in the gaps where it is not supported from below. So some stretching is beneficial, when it comes to spanning voids, but it does increase corner cutting.

As I mentioned before, with my old nozzle, I could extrude 0.5mm filament at 16mm/s. This is what happens with the new one which has an exit hole which is too shallow: -



One unfortunate characteristic of FDM is that errors tend to be cumulative. What I mean by that is if, for example, the U turn of the zig zag fails to bond to the outer wall then that causes the next layer to have nothing to rest on, so that fails as well. The defect then propagates all the way up the object. With 100% fill, any errors tend to have less effect on the layers above.

Rather than slow down my experiments I decided to go to 0.75mm filament at 7mm/s until I make a new nozzle. Here is a 50% fill: -



I also added a bit of overlap between the fill and the outline at the u-turns to get a better bond.

So does the infill density affect warping? I made several test blocks and it looks like the answer is not much. However, I have come to realise that the warping takes hours to fully develop after the object is removed from the base so I will leave them overnight before attempting to make measurements.