Tuesday, 26 August 2008

Bearing fruit

My wife has been asking me to make something to prop up the overladen branches of our dwarf apple tree for a few weeks now. I put it off while I was set up for ABS because I knew I did not have enough to finish my Darwin. Now that I have switched the machine to HDPE it is no problem, but it is now a few days late as one large branch has already snapped off!

We have lots of plastic covered metal poles so all I needed to do was make some Y-shaped end pieces. My first attempt went a bit chaotic while making the arms: -

I wasn't watching it but I figured it got too hot when doing the small pieces so I made the arms thicker.

Better but still very rough, it should look like this :-

I cleaned it up with a penknife and it was functional but it felt more whittled than extruded.

I made a couple more with even thicker arms but I was around to observe what was going wrong this time: -

When building the curved arms Enrique's software switches to 100% fill because it decides part of the layer is two layers from an outer horizontal surface, which a thin sliver down each side is. That would not be a problem in itself but because I have the infill overlap option set it ends up with slightly too much plastic on the 100% layers. As the height increases this excess builds up until the nozzle is actually submerged in the object while it is building it. Amazing that it manages to make anything resembling the correct shape!

What really needs to happen is that if the infill overlap parameter is set then the head needs to lay down the infill slightly faster so that the amount of plastic is still correct. I ran into the same problem with ABS when making an object with 100% fill.

I made a fourth version with the infill overlap set to zero and it was a lot better: -

Still very blobby but all the blobs are down to extruder overrun and easier to carve off. Overrun is worse with HDPE because it seems to be a more non Newtonian liquid than ABS. I.e. it compresses and expands more than ABS does, so when the extruder stops it oozes for longer.

I haven't tried anything to stop ooze yet. Simply stopping the extruder before the end of the line like the RepRap host does should improve it and is easy to do. Reversing the motor drive should also help. Simply stopping causes the extruder flow rate to fall exponentially but backing up a little should stop it completely in a finite time. The shaft encoder can then be used to go back at full speed to where it was before it backed up. There will still be some ooze without a valve but I think it could be a lot better.

Here is the final version cleaned up: -

And here is the tree with four crutches installed although only three are visible from this view though: -

Monday, 25 August 2008

Back to black

I have used up all of the 5lbs of ABS that I bought from the RRRF with just a couple of plastic parts left to make to complete my Darwin. I bought 2Kg of ABS from Tempatron but it is too oval to fit my current extruder, so it will have to wait until I build the new extruder for my Darwin. In order to complete that extruder I have had to go back to using HDPE. I always intended to make the filament guide for it out of HDPE anyway, because it is a lot more slippery than ABS, PCL and PLA.

It is quite a while since I did any work with HDPE and I didn't print many modeled objects with it before I switched to ABS, just lots of test blocks. In the meantime I have rebuilt and tweaked my extruder quite a lot and changed the way I do rafts. It took me a few attempts to dial in the parameters to get reasonable print quality.

In the end the result is not bad. The object on the far right is an ABS version, the one next to it is the best HDPE version.

The test piece is an alternative solution to the problem Vik mentions here: new-x-carriage-hot-off-reprap.

It replaces the y-belt clamp that holds a piece of filament (which carries the cables to the extruder) and also provides the bearing surface missing from the "no support" version of the Darwin x-carriage. You can see the gap above the bar created by the teardrop shaped hole below: -

Here is the new HDPE piece installed: -

It also fixes the fact that the screw hole pitch of the belt clamp does not match the holes in the carriage. HDPE is very good for making bearings so I intend to remake all the bearing inserts in it when the ABS ones wear out.

The first thing to do is get the raft temperature correct to make it stick, but still be peelable from the chopping board. I start with the temperature at a value I know will be too low and go up in steps of 10°C until it sticks enough, but not too much.

The next thing I do is get the first layer temperature right so that the object can be separated from the raft. I found that quite hard to control with ABS and much more so with HDPE. I increased the temperature until the outline started sticking properly and found a small test object was still peelable. When I made larger objects they were stuck fast. I normally use a small penknife to separate stubborn objects but after stabbing my fingers three times I resorted to a chisel! I think some of the variability is down to changes in ambient temperature and bed temperature. The XY table of HydraRaptor is made of several Kg of aluminium, which acts as a heatsink for the motors. They run a lot cooler than my Darwin's motors despite having more power through them. The table slowly warms up to about 30°C, about 10°C above ambient. 10°C is enough to make all the difference between sticking and not. I probably need to measure raft surface temperature and adjust the first layer accordingly. Another thing to try would be to do the first layer outline hotter than the infill.

I dropped the build speed from 16mm/s to 8mm/s for three reasons: -
  • HDPE puts maximum strain on the extruder because it is the most viscous at extrusion temperature so it is both the hottest and the highest pressure.
  • Heat builds up in the object limiting the minimum size that can be made without inter-layer pauses, something I have not implemented yet.
  • HDPE likes to cut corners and not go where it should and I think going slower helps the accuracy.
I also had to set the infill overlay parameter to 0.5 in Enrique's software to make the infill join to the outlines correctly.

A few things that are different about HDPE:
  • Extruder ooze creates blobs rather than strings with my current nozzle. They are harder to remove because they are a lot thicker. Faster head movement on my Darwin should drastically reduce this effect.
  • The brush I use to wipe the nozzle works perfectly with HDPE but not very well with other plastics. This is because HDPE extrudes in a straight line and is not very sticky. In contrast ABS tends to curl upwards and sticks well to the nozzle.
  • HDPE spans gaps a lot tighter than ABS does because when it is stretched it remains under tension whereas ABS doesn't.
  • Holes always come out smaller than they should be, but with HDPE this effect is worse. Dr Bowyer published a correction formula based on there being too much material on the inside of circles but I get contractions an order of magnitude greater. I think it is related to how much I stretch the filament but I need to do some more work on it.
I then made the extruder clamp in HDPE. It is a bit warped of course but it doesn't really matter. The raft was strained upwards at the bottom left corner, not surprising as this is one of the longest pieces I have made.

Here it is cleaned up :-

The large hole for the PTFE barrel is undersized and my PTFE stock is oversized so I think I will have to turn it down to get it to fit. I drilled out the other holes.

Similarly the filament guide is warped on the underside but it is only the top side that needs to be flat.

As predicted it is slippery and my oval ABS glides through it very well.

I just have to make the metal extruder parts now and that completes the mechanical build of my Darwin.

Sunday, 24 August 2008

Alternative Z-endstop

I prefer my machine to home the z-axis away from the workpiece so that homing is always a safe operation, regardless of what state the machine is in. The standard endstop bracket allows the opto to be mounted on the horizontal rails but for a bottom endstop it needs to be mounted off one of the vertical posts. I designed a new bracket and tab for this: -

The tab enters the opto from the side which gives the best resolution. Here it is installed :-

It can be mounted anywhere on the vertical post so could be used as a top endstop as well but I don't see the point of stops at both ends.

The way I calibrate HydraRaptor's z-axis for FFF is that I get the head somewhere near the table and measure how far away it is with a rule to get rough calibration. I then instruct it to go to 3mm above the table. I roll a 3mm bright steel rod under the nozzle and jog the axis up and down in s/w until it just touches. This has to be done with the nozzle fully warmed up to the working temperature because the PTFE expands about 0.5mm.

The calibration drifts on HydraRaptor because the frame is made from wood so the weather affects it, something that Darwin should not suffer from.

The files are here: www.thingiverse.com/thing:124.